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  • Springer Science + Business Media  (4,158)
  • MDPI Publishing
  • 1975-1979  (4,158)
  • 1
    Publication Date: 1979-06-01
    Description: This paper was originally submitted as a background essay to an early meeting concerned with the development of the GARP Climate Dynamics Sub-Programme. It was an attempt to put the scientific problems into a context which, for the benefit of administrators, leads to a fairly natural structuring of relevant research activity. In its specific references to GARP, the essay has been somewhat overtaken by events - but the concepts are still relevant at this time when it remains unclear as to how the “climate problem” is to be solved. See ‘Terminology’ at the end of this article for definitions of specialized terms. ©1979 D. Reidel Publishing Company
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  • 2
    Publication Date: 1979-06-01
    Description: A large scale numerical time-dependent model of sea ice that takes into account the heat fluxes in and out of the ice, the seasonal occurrence of snow, and ice motions has been used in an experiment to determine the response of the Arctic Ocean ice pack to a warming of the atmosphere. The degree of warming specified is that expected for a doubling of atmospheric carbon dioxide with its associated greenhouse effect, a condition that could occur before the middle of the next century. The results of three 5-year simulations with a warmer atmosphere and varied boundary conditions were: (1) that in the face of a 5 K surface atmospheric temperature increase the ice pack disappeared completely in August and September but reformed in the central Arctic Ocean in mid fall; (2) that the simulations were moderately dependent on assumptions concerning cloud cover; and (3) that even when atmospheric temperature increases of 6–9 K were combined with an order-of-magnitude increase in the upward heat flux from the ocean, the ice still reappeared in winter. It should be noted that a year-round ice-free Arctic Ocean has apparently not existed for a million years or more. ©1979 D. Reidel Publishing Company
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  • 3
    Publication Date: 1979-06-01
    Description: The growing season is considered by some to be a simple and yet significant indicator of the impact of hemispheric temperature variations at the local level. Yet, the effect of the use of different definitions of the growing season has never been determined. In the present paper, time series of the length of differently defined growing seasons at four Wisconsin stations are compared. The results show that their lengths have fluctuated in a variety of patterns over the past 80 years. Two growing seasons which showed a significant trend did not agree on its direction. The reason for this disparity is that trends in maximum and minimum temperatures are not necessarily of the same sign at different times of the year. These findings suggest that the length of the growing season is not the simple climatic indicator it has been assumed to be. ©1979 D. Reidel Publishing Company
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  • 4
    Publication Date: 1979-03-01
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  • 5
    Publication Date: 1979-03-01
    Description: Sunspot structure is measured by taking the ratio of the area of the umbra to that of the penumbra. The time dependence of this umbral/penumbral ratio is remarkably similar to the record of the Northern Hemisphere surface temperature anomalies. On a year-to-year basis the correlation is 0.57, which is significant at better than the 0.01% level. It is postulated that a change in the umbral/penumbral ratio is proportional to a change in the solar luminosity and that long-term variations in the luminosity lead to corresponding changes in the climate of the Earth. ©1979 D. Reidel Publishing Company
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  • 6
    Publication Date: 1979-08-01
    Description: In highly oxidized Mn- and Al-rich layers of the low-grade metamorphic metasediments north of Salmchâteau, Belgium, three types of viridine porphyroblasts can be distinguished: (a) single-phase porphyroblasts, (b) composite porphyroblasts with a core consisting of very fine-grained kaolinite, which is interpreted as a pseudomorph after andalusite, and a rim of viridine and, (c) composite porphyroblasts with a viridine core, a first mantle of kaolinite (as a pseudomorph after andalusite) and a second rim of viridine. These grains form an assemblage with muscovite, paragonite, Mg-chlorite, piemontite, braunite, hematite, quartz, rutile, and apatite. In the single-phase porphyroblasts the chemical composition of the core, (Mn _0.51 3+ Al_0.44Fe _0.04 3+ )Al(O/SiO_4), grades continuously into (Mn _0.79 3+ Al_0.15Fe _0.06 3+ )Al(O/SiO_4) of the rim. The outer zones of the analyzed grains are thus richer in kanonaite-component, MnAl(O/SiO_4), than the original kanonaites described recently by Vrána et al. (1978) from the type locality in Zambia. Taking into consideration also other chemical data of viridines of the Salmchâteau locality there exists a complete solid solution series in the system Al_2SiO_5 (And)-MnAlSiO_5 (Kan)-FeAlSiO_5 at least from And_61Kan_37(FeAlSiO_5)_2 to And_15Kan_79(FeAl-SiO_5)_6 for the P-T conditions verified here (≦ 400 ° C/1–2 kb). The oxygen fugacity of the rocks is estimated to correspond to f _O2 of the MnO_2/ Mn_2O_3-buffer or to exceed it. Single-phase and composite porphyroblasts of the andalusite-type minerals contain S-shaped internal structures, which, in the marginal parts of the grains, are always oriented perpendicular to the schistosity planes. Together with the zoning of the composite grains these structures give clear evidence, that the andalusite formation is preceded by viridine growth in suitable rocks high in manganese. Thus, the viridines of the cores are formed under P-T conditions of less than 360–380 ° C/1–2 kb. The recurrent availability of Mn to form the kanonaite-rich rims in all three types of porphyroblasts may be explained by Mn release out of the recrystallizing hematites along the schistosity planes during decreasing temperatures. ©1979 Springer-Verlag
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  • 7
    Publication Date: 1979-08-01
    Description: An empirical calibration of the spinel-olivine geothermometer (Evans and Frost, 1975) is attempted and applied to parageneses of lherzolitic rocks. In ultramafic complexes, most of the derived temperature estimates range between 700 ° and 850 °C, and appear generally lower than those given by other geothermometers, based upon Ca or Al contents of coexisting pyroxenes. A comparison of the different thermometric data in two well studied occurrences, the Ronda ultramafic complex and the San Carlos xenolith field, suggests that differential rates of diffusion and recrystalization may account for the large range of temperatures (between 700 ° and 1200 °C) determined by various mineral geothermometers in peridotites. The equilibrium exchange of Mg and Fe between spinel and olivine would be the fastest one and would continue effectively during the cooling down to relatively low temperatures, while other exchange reactions used as geothermometers in peridotites, like partitioning of Ca and Al in coexisting pyroxenes, are blocked at higher temperatures. ©1979 Springer-Verlag
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  • 8
    Publication Date: 1979-08-01
    Description: Thermodynamic parameters for the reaction: Mg_2 Si_2 O_6 = Mg_2 Si_2 O_6 hfill \ enstatite clinopyroxene hfill \ in the system CaO-MgO-SiO_2 have been deduced from phase equilibrium and enthalpy of solution data. From the regular solution theory, the seventeen currently available reversed experimental compositions of coexisting enstatite and clinopyroxene, presumed to be ordered diopside, lead, by a statistical regression, to the following best fit parameters: Δ H o=6.80 kJ Δ S o=2.75 J/K W _H Cpx =24.47 kJ (regular solution enthalpy parameter) W _V Cpx =0.105 J/bar (regular solution volume parameter). The derived parameters are not significantly affected by the (necessary) choice of W Opx in the range 20–50 kJ. The above values are in very good agreement with deductions from the solution calorimetry on synthetic CaMgSi_2O_6-Mg_2Si_2O_6 clinopyroxenes of Newton et al. (1979), which also places bounds on possible departures from the optimal values of these parameters. The calorimetric data may also be interpreted in terms of a Bragg-Williams cooperative-disordering model (Navrotsky and Loucks, 1977), in which diopside-structure clinopyroxene and a ‘relaxed’ low-Ca clinopyroxene (‘Fe-free pigeonite’) approach each other in composition, structural state, and stability with increasing temperature. The Δ H o parameter deduced from the regular solution theory is reinterpreted as the enthalpy change of enstatite to Mg_2Si_2O_6 pigeonite; the Δ H o of the transformation of enstatite to the diopside structure would, in this case, be considerably larger than 6.8 kJ. The curvature of the enthalpy of solution data, explained by the regular solution theory in terms of M 2-site energetics (involving W _H cpx ), is reinterpreted as due to disordering and ‘relaxation’ in the Navrotsky-Loucks model. Although the regular solution theory with the best-fit parameters accounts for all of the reversed enstatite and diopside compositions to within 18 ° C, and is a convenient representation of the phase equilibria for purposes of geothermometry, the disordering model is, at the present level of knowledge, equally valid and allows for a region of stability of two coexisting clinopyroxenes. ©1979 Springer-Verlag
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  • 9
    Publication Date: 1979-08-01
    Description: Sulfide minerals in amounts up to 3 vol% are found in basal, chilled marginal zones of two layered peridotite-pyroxenite-gabbro sills in the Early Precambrian Deer Lake Complex, northcentral Minnesota. The sulfides occur interstitially to silicate minerals, and consist of pyrrhotite with minor exsolved cobaltian pentlandite, chalcopyrite, gersdorffite, and marcasite±pyrite as an alteration product of pyrrhotite. The basal chilled units (3–6 m) of the sills are divisable into three zones based primarily on textures. The lowermost zone is an equigranular basalt, whereas the overlying zone is characterized by skeletal, spinifex-like actinolite after clinopyroxene. The upper zone of the basal margins contains elongate and swallow tail plagioclase, and is barren of sulfide minerals. Electron microprobe analyses of sulfide minerals and modal data suggest that sulfide bulk compositions at 1,100−1,000 ° C represent a pyrrhotite solid solution and a coexisting Cu-rich sulfide liquid. Cooling of the Cu-rich liquid and low temperature transformations are thought to have produced chalcopyrite or chalcopyrite plus pyrrhotite. The sulfide minerals have reequlibrated to temperatures near 300 ° C or less. Analyses of sulfur content and δ34S values suggest that assimilation of sulfur from adjacent country rocks was the principal mechanism responsible for anomalous concentrations of sulfides in the basal chilled margins. The distribution of sulfides in the peridotite-pyroxenite-gabbro portions of the sills, and calculations of settling rate preclude an origin involving gravitational settling of immiscible droplets through the magma body. ©1979 Springer-Verlag
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  • 10
    Publication Date: 1979-06-01
    Description: The formation and stabilities of Fe2+-NAT (NaTi_1/2Fe2+ _1/2Si_2O_6) and Mg-NAT (NaT_1/2Mg_1/2 Si_2O_6) clinopyroxene end-members in peralkaline nepheline syenites of a shonkinite-nepheline syenite suite in the Tertiary ultramafic alkaline Gardiner complex, East Greenland is described. Based on the composition of coexisting aegirines and Ti-minerals it is suggested that the formation of Mg-NAT is favoured by high f _O2 and high Nds - and TiO_2-activity. The instability of ilmenite at a late magmatic stage leads to the breakdown of a peralkaline acmite-ilmenite oxygen-buffer. Lack of buffering results in strong enrichment of aegirines in Fe2+-NAT. Up to 8% TiO_2 and 20% Mg-NAT and 20% Fe2+-NAT is recorded from these aegirines. The most evolved compositions show decreasing NAT and increasing Ac. The breakdown of sphene at a late magmatic to subsolidus stage results in increasing Wo in coexisting aegirines. Based on comparison with other occurrences it is concluded that NAT endmembers form at low pressure, but not necessarilly at low T., and that high f _O2 favors Mg-NAT formation and low f _O2 Fe2+-NAT formation. ©1979 Springer-Verlag
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  • 11
    Publication Date: 1979-06-01
    Description: The water-undersaturated melting relationships of a mafic, peralkaline, potassic madupite (with about 3% H_2O as shown by chemical analysis) from the Leucite Hills, Wyoming, have been studied at pressures up to 30 kb. At low pressures (〈5 kb) leucite is the dominant liquidus phase, but it is replaced at higher pressures by clinopyroxene plus olivine (〈5–7 kb), clinopyroxene (7–12.5 kb), clinopyroxene plus minor spinel (12.5–17.5 kb), and clinopyroxene alone (17.5–〉 30 kb). At all pressures there is a reaction relationship with falling temperature between melt, olivine and probably clinopyroxene to yield phlogopite. Apatite is stable within the melting interval to pressures above 25 kb. Electron microprobe analyses demonstrate that the clinopyroxene is diopsidic, with low aluminium and titanium contents. Pressure has relatively little effect on the composition of the pyroxene. Phlogopite is also aluminium-poor and has only a moderate titanium content. The experimental results indicate that madupite is not the partial melting product of hydrous lherzolite or garnet lherzolite in the upper mantle and it seems improbable that it is derived by melting of mantle peridotite with a mixed H_2O-CO2 volatile component. Madupite could, however, be the partial melting product of mica-pyroxenite or mica-olivine-pyroxenite in the upper mantle. It is pointed out that the chemistry of some potassium-rich volcanics may have been affected by volatile transfer and other such processes during eruption and that experimental studies of material affected in this way have little bearing upon the genesis of potassic magmas. Finally, the experimental results enable constraints to be placed upon the P-T conditions of the formation of richterite-bearing mica nodules found in kimberlites and associated rocks. Maximum conditions are 25 kb and 1,100 ° C. ©1979 Springer-Verlag
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  • 12
    Publication Date: 1979-06-01
    Description: The increase in metamorphic grade toward the Hackett River gneiss dome indicates that the structural dome is also a metamorphic culmination. In pelitic rocks east of the dome, the prograde sequence is chlorite, biotite, staurolite-cordierite, sillimanite. To the southwest the sequence is andalusite-staurolite-cordierite, sillimanite. In quartzofeldspathic gneisses which are closer to the dome than the sillimanite isograd, kyanite occurs as corroded relics, cross-cut by sillimanite and rimmed by cordierite and plagioclase. The pelites were metamorphosed under regional low-pressure conditions at the same time the quartzo-feldspathic rocks underwent higher-pressure metamorphism. A lateral variation in geothermal gradient during metamorphism is postulated to account for the change in facies-series. High heat flow beneath oceanic crust produced the low-pressure assemblages in the pelites; higher-pressure assemblages formed in a region of suppressed isotherms around a relatively cool, proto-continental trondhjemitic body. Low-pressure conditions were imposed on the kyanite-bearing gneisses during continuing metamorphism and diapiric uplift. Spatial association of late Archean kyanite-bearing rocks with early Archean sodic proto-cratons has also been noted in the Churchill and Superior Provinces of Canada as well as the Rhodesian Craton. ©1979 Springer-Verlag
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  • 13
    Publication Date: 1979-06-01
    Description: Malignites from the Poohbah Lake complex of northwestern Ontario, Canada are melanocratic cumulates. Cumulus pyroxene and apatite are poikilitically enclosed in a groundmass of large plates of intercumulus orthoclase and nepheline. Nepheline-feldspar fingerprint-like intergrowths occur. Nephelines are commonly zeolitized and pyroxenes altered to aggregates of biotite and/or garnet by deuteric alteration. Pyroxenes are weakly zoned from Di_71 Hd18Ac_11 to Di_63Hd_22Ac_15, and are similar to the least evolved pyroxenes of other alkaline rocks. Nephelines all have compositions within the Morozewicz-Buerger convergence field and feldspars have a limited compositional range from Or88 to Or95. Perthites are absent. Inconsistancies in the usage of the terms malignite and juvite are discussed and it is considered that a non-genetic petrographic classification of nepheline syenites leads to the obscuration of a group of potassic nepheline syenites, characterized by the presence of nepheline plus orthoclase which are typically associated with saturated to over-saturated alkaline rocks, contain pseudo-leucite or nepheline-orthoclase intergrowths, are emplaced in mobile belts and are not associated with rocks of the ijolite-carbonatite suite. A genetic classification of nepheline syenites is suggested and it is proposed that; (1) mafic-rich nepheline syenites be referred to as mela-nepheline syenites ( sensu lato ) rather than as malignites; (2) the term malignite be used for magmatic potassic nepheline syenites characterised by the presence of nepheline plus a single potassium-rich feldspar (orthoclase or microcline) and devoid of exsolution perthite under subsolvus conditions; (3) the metasomatic malignites and juvites of ijolite-carbonatite complexes be referred to as varieties of fenites. ©1979 Springer-Verlag
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  • 14
    Publication Date: 1979-06-01
    Description: Basic inclusions of two types occur in a kimberlitic diatreme at Kayrunnera in northwestern New South Wales. Type I inclusions comprise assemblages of clinopyroxene+garnet+rutile±plagioclase ±quartz±K-feldspar±scapolite±sphene±apaite. Type II inclusions have assemblages of clinopyroxene +garnet+kyanite+quartz±plagioclase and are lower in Ti, total Fe, and higher in Al and have a higher Mg/Mg+gSFe ratio than the Type I inclusions. Experimental and theoretical data indicate that both inclusion types equilibrated at between 850–900 ° C and 18–23 kb. Due to their low concentrations of incompatible elements, the Type I inclusions are considered to represent a basaltic melt derived from an Fe-rich mantle source rock, and not to be the product of fractionation. The Type II inclusions are believed to represent cumulates which formed from a basaltic magma. The presence of sulphur rich scapolite in the Type I inclusions extends the range of P-T conditions from which this mineral has been reported thus adding further credence to the hypothesis that it may act as a stable repository for S and CO2 in the crust and upper mantle. ©1979 Springer-Verlag
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  • 15
    Publication Date: 1979-06-01
    Description: Forty lavas from the Azores Islands have been analyzed for 87Sr/86Sr ratios, major elements, first transition series metals, and LIL elements. The samples belong to the alkali basalt magma series but range from transitional hy-normative basalts from Terceira to basanitoids from Santa Maria. Differentiated lavas include both typical trachytes and comenditic trachytes and comendites. Major and trace element concentrations define smooth trends on variation diagrams, and these trends can be related to phases crystallizing in the rocks. Systematic interisland differences are also apparent in these variation diagrams. LIL element concentrations in island basalts are roughly twice as high as those in tholeiites from the adjacent Mid-Atlantic Ridge which transects the Azores Plateau. 87Sr/86Sr ratios in lavas from 6 of the 9 islands range from 0.70332 to 0.70354, a range similar to that found in tholeiites from the Mid-Atlantic Ridge transect of the Azores Plateau. This suggests that lavas from these islands and this portion of the Mid-Atlantic Ridge may be derived from a similar source. However, lavas from the islands of Faial and Pico have 87Sr/86Sr ratios up to 0.70394 and ratios in Sao Miguel lavas range up to 0.70525, suggesting basalts from these islands are derived from a chemically distinct source. Differences in the average LIL element concentrations of the least fractionated ridge tholeiites from the Azores Plateau and alkali basalts from the islands result from differences in extent of partial melting and residual mineralogy. The alkali basalts are derived by roughly half as much melting as are the tholeiites. Trace element concentrations in Azores peralkaline lavas preclude their derivation by partial melting of peridotitic mantle or basaltic crust; rather the data suggest they are produced by fractional crystallization of a basaltic parent. ©1979 Springer-Verlag
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  • 16
    Publication Date: 1979-06-01
    Description: In the Ultevis District, Northern Sweden, viridine occurs in a potassium rich, microcline-bearing leptite together with quartz, plagioclase, phlogopite, opaques, and a retrogradely formed muscovite. The formation of viridine in this rock may be described by a dealkalization process of microcline, in which opaque minerals are involved. A diffusion of K+ and H_2O out of the centres of viridine formation and of H+ into these localities, which is a prerequisite of such type of reaction, may have been favoured by a contemporaneous migmatization of the rock, since the granite-like melts can be regarded as sinks for potassium and water. The following decomposition of viridine is initiated by the reverse reaction. Texturally this is indicated by small microcline seams along the contacts between viridine and quartz grains. The final alkalization of the viridine-quartz assemblage to form muscovite reflects increasing K+/H+-ratios and H_2O-contents of the vapor phase and/or decreasing temperatures. The potassium and water amounts necessary for the viridine decomposition reactions may have been generated by the cooling of the granitic liquids. An intergrowth of sillimanite and the muscovite can be described by a degenerated reaction. It can be interpreted as giving way to an equilibration of the alumosilicates und the actual metamorphic conditions, that is here P , T , $$a_{{ ext{H}}_{ ext{2}} { ext{O}}} $$ , a _ k +, a _ h + and a _ M n 3+. The viridine investigated has the lowest Mn-contents recorded so far for andalusite-type minerals with viridine optics (1.3–2.2 mol-% Mn_2SiO_5-component). Since no andalusite with lower Mn-contents occurs in the rock and since no other Al_2SiO_5-phase formed at this metamorphic stage it may be assumed that the incorporation of manganese stabilizes the andalusite structure under the P -, T -, $$a_{{ ext{H}}_{ ext{2}} { ext{O}}} $$ −, a _ k +-, a _ h +- and a _ M n 3+-conditions verified in this particular leptite. The temperature of viridine formation is estimated to be about 600 ° C, the decomposition of viridine +quartz into the muscovite-sillimanite assemblage may have happened between 650–500 ° C at 6–2.5 kb. ©1979 Springer-Verlag
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  • 17
    Publication Date: 1979-06-01
    Description: We report the results of noble gas analyses (Ne, Ar, Kr, and Xe) of a transitional basalt from the East Pacific Rise; a mantle xenolith, alkali basalt and trachyte from Guadalupe Island; and a basalt and icelandite from Isla Tortuga. The results for the East Pacific Rise basalt and comparison with the noble gas compositions of glassy mid-ocean ridge tholeiites indicate that the heavy noble gas patterns of these basalts can be accounted for by mixing of a juvenile reservoir with an atmosphere-related reservoir. This mixing may take place during hydration of the glassy basalts. Comparison of the noble gas compositions of these submarine basalts with those of the subaerial oceanic extrusives studied suggests that subaerial extrusives may provide noble gas samples which are less contaminated with air gases than do submarine extrusives. Our results for Guadalupe and Tortuga basalts and their differentiates provide evidence for the exsolution and loss of a gas (fluid) phase accompanying or subsequent to fractional crystallization at shallow depths. The gas loss probably took place prior to extrusion and was apparently rapid, since it did not fractionate the heavy noble gases. ©1979 Springer-Verlag
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  • 18
    Publication Date: 1979-12-01
    Description: Microprobe analyses of 26 natural sapphirines from 17 localities indicate that the predominant chemical substitutions in this mineral occur along the solid solution joinVI(Mg,Fe)2++IVSi4+=VI(Al, Fe)3++IVAl3+. Chromium and manganese are minor substituents. Evidence for the substitution Si⇄Al+1/2Mg+1/2 vacancy is absent within the limits of analytical error. A partitioning scheme based on electrostatic charge balance considerations has been devised permitting calculation of Fe2+ and Fe3+ from total iron content. Results are in good agreement with previous Mössbauer studies which indicate Fe3+ is sometimes in octahedral and/or tetrahedral coordination. Distribution coefficients for Fe2+-Mg exchange equilibria between sapphirine-spinel and sapphirine-orthopyroxene are similar for most mineral pairs and suggest that most of the assemblages equilibrated at about the same temperature or that the exchange reactions are insensitive to temperature. Compositions of synthetic sapphirines as a function of temperature and pressure are qualitatively predictable from crystal chemical considerations. Changes in sapphirine composition along the MgSi= AlAl solid solution join toward more aluminous compositions stabilize the sapphirine structure at high temperatures and low pressures. The limited extent of MgSi=AlAl solid solution observed in natural sapphirines appears to be related to the requirements of geometrical fit among octahedra and tetrahedra in the almost idealized cubic closest-packed anion framework. ©1979 Springer-Verlag
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  • 19
    Publication Date: 1979-12-01
    Description: The crystal structure of a P 2_1/ a polymorph of sapphirine ( a =11.286(3), b =14.438(2), c =9.957(2) Å, β=125.4(2) °) of composition [Mg_3.7Fe _0.1 2+ Al_4.1- Fe _0.1 3+ ]IV[Si_1.8Al_4.2]IVO_20 was refined using structure factors determined by both neutron and x-ray diffraction methods to conventional R factors of 0.067 and 0.031. respectively, for F _obs〉2σ. The results of the two refinements agree reasonably well, but a half-normal probability plot (Abrahams, 1974) comparing the two data sets indicates that the pooled standard deviations of the atomic coordinates have been underestimated by a factor of two. The structure of sapphirine, solved initially by Moore (1969), consists of cubic closest packed oxygens with octahedral and predominantly tetrahedral layers alternately stacked along [100]. The layer in which 70% of the octahedral sites are occupied has an Mg-Al distribution characterized by Mg-rich octahedra sharing edges mainly with Al-rich octahedra. Mean octahedral bond lengths correlate well with Al occupancy determined by neutron site refinement if the relative number of shared octahedral edges is taken into account (see Table 1). The predominantly tetrahedral layer has 10% of the octahedral sites occupied by Al and 30% of the tetrahedral sites occupied by Al-Si in the ratio 2.33∶1. There are single chains of Al-Si tetrahedra parallel to z with corner-sharing wing tetrahedra ( T 5 and T 6) on either side in the (100) plane. The mean T -O distance is highly correlated with Al occupancy, X_Al, as determined from the neutron site refinement: $$langle T - O angle = 1.656 + 0.105X_{Al} (r2 = 0.995).$$ Details of the neutron refinement are summarized below. ©1979 Springer-Verlag
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  • 20
    Publication Date: 1979-12-01
    Description: The anorogenic igneous rocks of Mull consist essentially of a lava pile of predominantly basaltic composition, cut by an intrusive complex. The basement consists of Precambrian metamorphic rocks of the Moine Series underlain by Lewisian gneiss. The intrusive complex contains a significant proportion of granitic intrusions which can be ascribed to three successive centres of activity, Centres 1–3. We report new major and trace element, including rare earth element analyses, 87Sr/86Sr ratios and δ18O values for a comprehensive collection of granitic rocks from the 3 centres. The δ18O values range from +4 to −6 indicating variable extent of interaction between the rocks and heated meteoric groundwater. However, correlations of δ18O with other major and trace element data and 87Sr/86Sr ratios are uniformly low, apart from Fe_2O_3. It is thus unlikely that the interaction of the rocks with meteoric water has systematically altered the chemical (including Sr isotope) characteristics. The chemical and Sr isotope data reflect magmatic values and can therefore be used to comment on the petrogenesis of the granitic rocks. These data indicate that there are important differences between granitic rocks of the centres with Centre 1 forming one distinct group and Centres 2 and 3 a different group. For a given SiO_2 value, the Centre 1 granites have higher Na_2O, MgO, P_2O_5, TiO_2, Sr, and V and lower Al_2O_3, MnO, Zn, Zr, and Y than those of Centres 2 and 3. In addition, the Centre 1 granites have lower REE contents and higher Ce_N/Yb_N ratios than those of Centres 2 and 3. Granites from all three centres have Eu anomalies, those of Centre 3 being generally greater (Eu/Eu* = 0.66–0.10). Finally, there are important Sr isotope distinctions between the three Centres; calculated initial 87Sr/86Sr ratios for the Centre 1 granites (using 58.2Ma) range between 0.71366–0.71646 (average 0.71530) and have a general correlation of 87Sr/86Sr with 87Rb/86Sr. The initial 87Sr/86Sr ratios of the Centre 2 granites range from 0.70663 to 0.70868, but the 87Sr/86Sr data do not define an isochron. Finally, data for the Centre 3 granites define a Rb-Sr wholerock isochron with an age of 58.2±2.5 Ma and an initial 87Sr/86Sr ratio of 0.71003 ±36. Both the chemical trends and isotopic data for the Mull granites can be interpreted in terms of contrasted origins for the granitic rocks of the two groups. The relatively ‘primitive’ chemical composition and high initial 87Sr/86Sr ratios of the Centre 1 granites indicate a substantial crustal contribution and we consider that these granites formed by a combination of partial melting of Lewisian basement together with some magma derived by fractional crystallization of basaltic magma. In contrast, the chemical and isotope data for the Centre 2 and 3 granites are consistent with formation dominantly by fractional crystallization of basaltic magma, together with a relatively small proportion of crustal contamination. A model is proposed which emphasises that acid magmatism in Mull is a consequence of the rise and crystallization of basic magma into continental crust. Granite magma has formed both by partial melting and by fractional crystallization and both of these events probably occurred under ‘open system’ conditions. ©1979 Springer-Verlag
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  • 21
    Publication Date: 1979-10-01
    Description: Subsolidus and melting reactions involving calcic plagioclase in pelitic assemblages in the K-Na-Ca model system occur at higher temperatures than their K-Na counterparts. For the most calcic plagioclase compositions observed in high-grade pelitic rocks (An_25-An_40) the equilibria are rarely extended by more than 30 ° C above those in KA1O_2-NaAlO_2-Al_2O_3-SiO_2-H_2O, although all discontinuities in facies inferred for the K-Na system are continuously displaced when they involve Ca-bearing plagioclase. The maximum pressure-temperature overlap between muscovite dehydration and initial melting reactions occurs in the pressure range of 4–6 kbar between about 640 ° and 720 ° C. This provides optimum conditions for anatectic melt generation in felsic rocks of the appropriate compositions progressively metamorphosed in kyanite-sillimanite facies series. Progressive regional metamorphism at pressures of 2–4 kbar, corresponding to andalusite-sillimanite facies series, shows little overlap between muscovite dehydration and initial melting reactions. Consequently anatectic melt generation in andalusite-sillimanite facies series would require the participation of biotite in dehydration-melting reactions. Felsic intrusive rock in andalusite-sillimanite terranes could have risen upward from their anatectic sites in high grade kyanite-sillimanite facies series at depth. Many andalusite-sillimanite facies series terranes culminating in migmatites could represent upward movement of kyanite-sillimanite facies series rocks to shallower depths with uplift rates faster than cooling rates. ©1979 Springer-Verlag
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  • 22
    Publication Date: 1979-11-01
    Description: Geochemical investigations in the Utralanama Block, an intermediate pressure granulite facies terrain in the Arunta Block, central Australia, has revealed several anomalous features, not consistent with the depletion of granitophile components generally considered to accompany granulite facies metamorphism. However, other geochemical features are indicative of depletion. The mean K_2O for the Utralanama Block is exceptionally low relative to most other granulite facies terrains, but Rb contents are comparatively high. Consequently, the mean K/Rb ratio is relatively low for granulite facies terrains as is the mean Ba/Rb ratio, whilst mean K/Sr and Rb/Sr ratios are much higher than usual for such terrains. Only the K/Ba ratio shows equivalent values to depleted terrains elsewhere. Comparison of these ratios for the three main compositional groups of rocks in the Utralanama Block reveals that for mafic rocks all the above ratios are characteristic of extreme depletion, whereas, for all but the K/Ba ratio, mean ratios for the pelitic rocks, and to a lesser extent for the quartzofeldspathic rocks approach normal crustal values or values for metasomatic rocks. The abnormally high Rb/Sr ratios of these rocks compared to average crustal rocks suggest, however, that metasomatism is the cause of the anomalous geochemical features of the Utralanama Block, and this is supported by field and microstructural evidence. Thus, Rb/Sr ratios appear to be useful indicators of metasomatism where no gross mineralogical or microstructural evidence for metasomatism is obvious, and under such conditions the K/Ba ratio may be more reliable than the K/Rb ratio for indicating prior depletion of the terrain. ©1979 Springer-Verlag
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  • 23
    Publication Date: 1979-10-01
    Description: In the metabasites of Val Chiusella, metamorphic assemblages are present, corresponding to the glaucophane schist facies, i.e. garnet glaucophanites to omphacite-garnet glaucophanites, as well as to the eclogite facies, i.e., glaucophane eclogites, eclogites, and omphacite felses. Both groups of assemblages are divided by the critical reaction 1 zoisite +1 glaucophane ⇆ 1.2 omphacite+0.8 garnet+0.7 paragonite +1.4 quartz+0.8 H_2O. From textural evidence it is clear that in the investigated area this reaction proceeded to the right according to a prograde metamorphism. Correspondingly, K garn-cpx _D(Fe/Mg) values of coexisting garnet-omphacite pairs in the glaucophane schist facies assemblages are higher than in the eclogite facies assemblages and reflect a temperature increase from about 450 ° C to about 550 ° C at minimum water vapour pressures of 12 to 16 kb. ©1979 Springer-Verlag
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  • 24
    Publication Date: 1979-10-01
    Description: Seventeen granitoid samples from batholiths in central Chile (33 °–34 ° S) have been analyzed for trace element content. The samples range in age from Paleozoic to Tertiary, and in rock type from quartz diorite to granite. In general, compared to andesites from central-south Chile these rocks are more siliceous with lower abundances of compatible trace elements and higher abundances of incompatible trace elements. However, Upper Tertiary granodiorites have important geochemical similarities, such as highly fractionated rare-earth element (REE) distributions relative to chondrites, to some modern andesites in this region; e.g., Marmolejo. Similar highly fractionated REE distributions are also common in the cores of zoned intrusive sequences in the Sierra Nevada of the western U.S. Based on limited sampling of central Chile Cretaceous and Tertiary plutonics, there may be a west to east increase in light REE/heavy REE ratio and in Sr content. Compared to the Upper Tertiary granodiorites, the Paleozoic granodiorites have similar REE abundances but lower Sr, Sc, Cr, Co, and Ni contents. ©1979 Springer-Verlag
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  • 25
    Publication Date: 1979-03-01
    Description: Analysis of the annual blue crab catch in Chesapeake Bay for the years 1922–1976 shows that there are variations with periods of 18.0, 10.7, and 8.6 years. Analysis of Philadelphia air temperatures shows periods of 17.5, 9.8, and 7.4 years. The periods of 18.0 and 17.5 years agree with the 18.6 year period of the Earth-Moon-Sun tidal force, and the periods of 8.6 and 7.4 years agree with the 8.8 year period of the Earth-Moon-Sun tidal force, within experimental error. The periods of 9.8 and 10.7 years, for the temperatures and crabs, respectively, are probably related to the 10.5 year sunspot cycle within experimental error. ©1979 D. Reidel Publishing Company
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  • 26
    Publication Date: 1979-03-01
    Description: The possibility of controlling atmospheric carbon dioxide accumulation and attendant climatic effects from fossil-fuel burning by diverting a fraction of the combustion product and injecting it into the deep-ocean, as proposed by Marchetti, is analyzed using an atmosphere/mixed layer/diffusive deep-ocean model for the carbon cycle. The model includes the nonlinear buffering of CO2 at the air/sea interface, and considers the long term trends associated with consuming an assumed fossil-fuel reserve equivalent to 7.09 × 1015 kg carbon as a logistic function of time as in the projections of Siegenthaler and Oeschger, except that atmospheric carbon dioxide levels are computed for five alternate strategies: (a) 100% injected into atmosphere, (b) 50% injected at oceanic depth of 1500 m and 50% into atmosphere, (c) 50% injected at sea floor (4000 m) and 50% into atmosphere, (d) 100% at 1500 m depth and (e) 100% at sea floor. Since no carbon leaves the system, all runs approached the same post-fossil fuel equilibrium after several thousand years, C _ a ∼- 1150 ppm, almost four times the pre-fossil fuel value (∼- 300 ppm). But the ‘transient’ response of these cases showed a marked variation ranging from a peak overshoot value of 2800 ppm in the year 2130 for 100% atmospheric injection to a slight decrease to the pre-fossil fuel 300 ppm lasting till 2300 with a subsequent slow approach to equilibrium for the 100% deep-ocean injection. The implications of these results for an oceanic injection strategy to mitigate the climatic impact of fossil-fuel CO2 is discussed, as are the ingredients of a second generation carbon cycle model for carrying out such forecasts on an engineering design basis. ©1979 D. Reidel Publishing Company
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  • 27
    Publication Date: 1979-03-01
    Description: Multidisciplinary teamwork can create efficient and effective assessment tools and supply information required for inputs in management decisions. In this paper, illustrations are provided to indicate that much information can be gained from using climatological data to assess the impact of the variability of weather and climate on agriculture. Climatic data have been collected and archived for many areas of the world. These data must not go unused. ©1979 D. Reidel Publishing Company
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  • 28
    Publication Date: 1979-03-01
    Description: Wheat stripe rust ( Puccinia striiformis West.) epidemics are confined predominantly to the Pacific Northwest in the U. S. A. because of climate. This disease was frequently reported until the late 1930's and then virtually absent until the late 1950's. Since the severe epidemic in 1961, stripe rust has been frequently severe on winter wheat and has caused losses in susceptible cultivars in many years. Because of the unusual history of stripe rust in this region, the possibility that climate variability affected the pattern of rust occurrence was investigated. Meteorological data for seven locations in Oregon, Washington, and Idaho were analyzed. In 1961–1974 for the Columbia Basin locations, January and February temperatures averaged 1.20° C higher than during the period 1935–1960; however, April temperatures averaged 1.28° C lower in 1961–1974 than during the earlier period. Monthly precipitation averages have not varied more than 12.7 mm in any month. Between 1961–1974, December snowfall almost doubled over that in 1935–1960; snowfall in February decreased over 50% from the earlier period. Data was computed on a seasonal basis since 1901 and considered in respect to stripe rust epidemics. Since 1961, above-normal winter and below-normal spring temperatures have increased the frequency and severity of stripe rust epidemics in the Pacific Northwest. The direction of temperature and precipitation trends varied with the time period considered. How the climate variability which has occurred may have affected winter wheat growth and yields is postulated. Studies such as this should be useful to researchers modelling crop-yields, agronomists evaluating results from field experiments and to researchers studying fluctuations in pest populations. ©1979 D. Reidel Publishing Company
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  • 29
    Publication Date: 1979-03-01
    Description: The quantitative relation between mean monthly temperatures and rainfall has been investigated for a hundred years period at Jerusalem. It was found that a decrease of 1°C in the mean monthly temperature is associated with an average increase of 13 mm in monthly precipitation, on the seasonal scale. The findings are consonant with previous results pointing to an increase of about 100 mm in annual rainfall for a decrease of 1°C in the seasonal temperature. Such coefficients may, to a first approximation and with some qualifications, be used in estimating rainfall in earlier periods in the Middle East, and to evaluate hydrological effects and potential risks in future centuries. ©1979 D. Reidel Publishing Company
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  • 30
    Publication Date: 1979-03-01
    Description: Two areal scales were used to determine the effect of land size on wheat yield estimates from climatic data. The larger scale was the state of Oklahoma and the smaller scale included five crop reporting districts within the state. Two multilinear regression models were developed. One used unadjusted, and the other square-root adjusted, climatic data. Any comparative advantage of district modeling over a state model was judged upon the correlation coefficients of the model and its estimation capability over a five year trial period. When state and district models were compared in estimation capability, the state model achieved more accurate yield estimates of district wheat yields than did the individual district models. ©1979 D. Reidel Publishing Company
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  • 31
    Publication Date: 1979-06-01
    Description: Aenigmatite, sodic pyroxene and arfvedsonite occur as interstitial minerals in metaluminous to weakly peralkaline syenite patches in alkali dolerite, Morotu, Sakhalin. Aenigmatite is zoned from Ca, Al, Fe3+-rich cores to Ti, Na, Mn, Si-rich rims reflecting the main substitutions Fe2+Ti4+⇌Fe3+, NaSi⇌CaAl and Mn2+⇌Fe2+. Aenigmatite replaces aegirine and ilmenite supporting the existence of a ‘no-oxide’ field in $$f_{{ ext{O}}_{ ext{2}} }$$ — T space. In one case aenigmatite has apparently formed by reaction between ilmenite and arfvedsonite. Titanian aegirine (up to 3.0 wt% TiO_2) and Fe-chlorite may replace aenigmatite. Sodic pyroxene occurs as zoned crystals with cores of aegirine-augite rimmed by aegirine and in turn by pale green aegirine containing 93 mol% NaFe3+Si_2O_6. Additional substitution of the type NaAl⇌CaFe2+ is indicated by significant amounts (up to 6 mol%) of NaAlSi_2O_6. Arfvedsonite is zoned with rims enriched in Na, Fe and depleted in Ca which parallels the variation of these elements in the sodic pyroxenes. The high peralkalinity of the residual liquid from which the mafic phases formed resulted from the early crystallization of microperthite (which makes up the bulk of the syenites) leading to an increase in the Na_2O/(Na_2O+K_2O) and (Na_2O+K_2O)/Al_2O_3 ratios of the remaining interstitial liquid which is also enriched in Ti, Fe, and Mn. Bulk composition of the melt, $$f_{{ ext{O}}_{ ext{2}} }$$ , temperature and volatile content were all important variables in determining the composition and stability of the peralkaline silicates. $$f_{{ ext{O}}_{ ext{2}} }$$ in the residual liquid appears to have been buffered by arfvedsonite-aegirine and later by the arfvedsonite-aenigmatite and aenigmatite-aegirine equilibria under $$f_{{ ext{O}}_{ ext{2}} }$$ conditions of a ‘no-oxide’ field. An increase in $$f_{{ ext{O}}_{ ext{2}} }$$ , above that of the alkali buffer reactions, is inferred by an increase of Ti and Mn in aenigmatite rims. The latest postmagmatic vapour crystallization stage of the syenites is marked by extremely low $$f_{{ ext{O}}_{ ext{2}} }$$ which may have been facilitated by exsolution of a gas phase. Low $$f_{{ ext{O}}_{ ext{2}} }$$ is supported by the replacement of aenigmatite by titanian aegirine, and the formation of rare Ti-rich garnet with a very low (ΣTi4++Fe3+)/(ΣTi+ΣFe) ratio of 0.51, associated with leucoxene alteration of ilmenite. ©1979 Springer-Verlag
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  • 32
    Publication Date: 1979-06-01
    Description: Thermodynamic calculations, modified after Nicholls et al. (1971), which relate the activity of silica in a lava to the temperature and pressure conditions at which the lava could be in equilibrium with a mantle mineral assemblage, have been extended to H_2O-bearing magmas by using published experimental data to derive the dependence of $$a_{{ ext{SiO}}_{ ext{2}} }$$ on the weight fraction of H_2O dissolved in a magma. A petrogenetic grid has been calculated which gives the P-T conditions under which a magma with a given $$a_{{ ext{SiO}}_{ ext{2}} }$$ at its liquidus at 1 atm could equilibrate with a mantle mineral assemblage containing olivine (ol) and orthopyroxene (opx) for different amounts of H_2O in the magma at its source. This grid is in good agreement with the results of experimental studies as summarized by Green (1971) and Brey and Green (1975). The results show that the pressure at which a given magma composition can equilibrate with ol + opx increases for increasing amounts of H_2O dissolved in the magma at depth. In addition, experimental data have been used to calculate the effect of olivine crystallization and removal on the $$a_{{ ext{SiO}}_{ ext{2}} }$$ in the residual liquid to assess the effect of low-pressure differentiation on $$a_{{ ext{SiO}}_{ ext{2}} }$$ . The results show that if 20 % olivine is added to a basalt magma, its calculated pressure of equilibration with ol+opx increases by ∼4–5 kbar for a given temperature. The calculated effects of olivine removal and H_2O addition on $$a_{{ ext{SiO}}_{ ext{2}} }$$ are reasonably consistent with the silicate mixing model of Burnham (1975). Thermodynamic calculations of this type may be useful for assessing the internal consistency of certain experimental data, and in extrapolating the results to other magma compositions. The application of these calculations to determining the possible depth of origin of natural lavas appears to be limited primarily by the difficulty in determining $$_{{ ext{SiO}}_{ ext{2}} }$$ in a lava at its liquidus temperature. ©1979 Springer-Verlag
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  • 33
    Publication Date: 1979-06-01
    Description: Secondary Ca-Al silicates, including andradite-grossular garnet, epidote, pumpellyite, and prehnite are shown to be extremely widespread as low-grade alteration products of granitoid biotite. All may occur within metadomains on the scale of a single biotite pseudomorph. They all generally have a lensoid habit parallel to the host biotite cleavage, which may, or may not be deformed. A cleavage or parting inherited from the host biotite cleavage is usually present. Prehnite, pumpellyite and epidote are unusually Fe-rich: 100 Fe/Fe + Al for prehnite and pumpellyite reaching 19.1 and 49.1 respectively and epidote Ps_36-Ps_48. Garnet compositions are relatively uniform averaging approximately andradite_56 grossular_35 hydrogrossular_7. Correlation of prehnite Fe3+ with host biotite Fe3+ and oxidation state support the textural evidence that prehnite in this paragenesis often replaces the host biotite. Sericitization of plagioclase and commonly a complete lack of hornblende indicate that plagioclase (An_50-An_15) is the chief source of Ca. Chlorite and muscovite (whose compositions are both directly related to the host biotite composition), aluminian sphene (Al_2O_3 reaching 7.72%), and K-feldspar (Or_99+) are complimentary to the biotite breakdown reaction. Host rock composition to some extent controls the development of the Ca-Al silicates which do not occur in granitoids whose whole-rock CaO is less than 1%. An aqueous pore fluid with locally varying activities of ions enabled concentration of Ca in micro-metadomains, allowing development of Ca-Al silicates in relatively low-Ca granitoids. Individual phases generally appear to have developed independently due to fluctuation in chemical environment in micro-space and/or T , P fluctuation with time. Textures suggest that grandite may have locally replaced epidote in response to increasing temperature. In granitoids of the Victoria Range, South Island, New Zealand the alteration has probably occurred with P 〈2kb and T 〈 300 °–350 °C. Here alteration can be ascribed to strictly deuteric activity in only one of the two groups of granitoids so affected. ©1979 Springer-Verlag
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  • 34
    Publication Date: 1979-06-01
    Description: High pressure phase transformations for all the mineral phases along the joins Mg_2SiO_4-Ca_2-SiO_4 and MgO-CaSiO_3 in the system MgO-CaO-SiO_2 were investigated in the pressure range between 100 and 300 kbar at about 1,000 °C, by means of the technique involving a diamond-anvil press coupled with laser heating. In addition to the four end-members, there are three stable intermediate mineral components in these two joins. Phase behaviour of all the end-member components at high pressure have been reported earlier and are reviewed here. Results of this study reveal that the three intermediate components are all unstable relative to the end-members at pressures greater than 200 kbar. Ultimately, monticellite (CaMgSiO_4) decomposes into CaSiO_3 (perovskite-type)+MgO; merwinite (Ca_3MgSi_2O_8) decomposes into Ca_2SiO_4(K_2NiF_4-type)+CaSiO_3 (perovskite-type)+MgO; and akermanite (Ca_2MgSi_2O_7) decomposes into CaSiO_3 (perovskite-type)+MgO. Note that the decomposition reactions of all phases studied here result in the formation of MgO. Intermediate Ca-Mg silicates transform to pure Ca-silicates plus MgO, while pure Mg_2SiO_4 transforms to MgSiO_3+MgO. ©1979 Springer-Verlag
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  • 35
    Publication Date: 1979-06-01
    Description: 143Nd/144Nd, 87Sr/86Sr, and REE analyses are presented on a wide variety of Pliocene-Recent volcanic rocks from central Italy. 143Nd/144Nd varies from 0.51214–0.51289 and 87Sr/86Sr from 0.7255-0.7036; while the rare earth elements are characterised by light RE enrichment and a significant negative Eu anomaly. These Italian volcanics are tentatively subdivided into three zones: (1) N. Tuscany where the magmas are believed to reflect crustal anatexis. (2) A central zone in which hybrid (crust/ mantle) rocks have been recognised. (3) A southern zone, south of Rome, where mantle-derived magmas are identified which have been largely unaffected by interaction with continental crust. At Roccamonfina, in zone 3, Rb/Sr and Sm/Nd pseudo isochrons are observed but since the calculated ages are 0.5 and 2.0 b.y. respectively it is argued that a simple isochron model is not applicable and that the data are most easily explained by a recent mixing event within the upper mantle. It is envisaged that this occurred during metasomatism of the upper mantle source region by a fluid that had high 87Sr/86Sr and low 143Nd/144Nd and was enriched in K, Rb, and LREE's but relatively depleted in Sr2+ and Eu2+. ©1979 Springer-Verlag
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  • 36
    Publication Date: 1979-06-01
    Description: Riebeckite-arfvedsonite amphiboles occur in very low-grade metamorphosed doleritic sills at various localities within the Iberian Pyrite Belt. The alkali-amphiboles grew during sub-solidus hydrothermal ‘spilitization’ of basalt associated with submarine massive sulphide ore formation. The riebeckite-arfvedsonite is only very rarely preserved, being converted to albite-chlorite during regional metamorphism. In the South Portuguese zone Hercynian regional metamorphic grade increases in a northward direction from zeolite facies south of the Pyrite Belt through prehnite-pumpellyite facies to the greenschist facies in its northernmost zone. Compositional and mineralogical data indicate a geothermal gradient in the order of 40–50 °C/km. Volcanism in the Pyrite Belt is essentially representative of a bimodal association of twoleiitic to alkalic basalt and dacite/rhyolite. Geochemical data for the Pyrite Belt mafic meta-volcanics contrast with available data for subduction related volcanic suites in orogenic belts but exhibit similarities with the basaltic members of basalt-rhyolite associations found in areas of extensional tectonics. It is proposed that the Iberian Pyrite Belt volcanism represents magmatic activity in an intra-continental basin undergoing rifting during the late Devonian and lower Carboniferous times. ©1979 Springer-Verlag
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  • 37
    Publication Date: 1979-06-01
    Description: An isotopic dating investigation (66 K-Ar and 34 Rb-Sr analyses) provided the geochronological framework for the Alpine events of metamorphism and granitic magmatism on Naxos. The oldest phase of high-pressure/medium-temperature metamorphism, M1, was dated by Rb-Sr and K-Ar analyses of paragonites, phengitic muscovites and muscovites at 45±5 Ma (Middle Eocene). Most of the record of the M1 phase has been eraded by a second phase of medium-pressure/high-temperature metamorphism, M2, which induced a metamorphic zonation with anatectic melting in the highest-grade part, the migmatite dome. Most K-Ar dates of M2 hornblendes, muscovites, biotites and tourmalines range from about 21 Ma in the lower-grade part (biotite-chloritoid zone) to about 11 Ma in the migmatite dome. From the pattern of K-Ar mineral dates it is concluded that the M2 phase took place 25±5 Ma ago (Late Oligocene/Early Miocene) and was followed by a prolonged cooling history until about 11 Ma ago (Late Miocene), when the ambient temperature in the migmatite dome had decreased to below 400−360 °C. A Rb-Sr whole-rock isochron analysis of a granodioritic mass dated the intrusion (and the associated M3 phase of contact-metamorphism) at 11.1±0.7 Ma (Late Miocene), with an initial 87Sr/86Sr ratio of 0.7112 ±0.0001. A local phase of low-grade retrograde metamorphism, M4, probably related to Late Alpine overthrusting, was dated at about 10 Ma (Late Miocene). ©1979 Springer-Verlag
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  • 38
    Publication Date: 1979-06-01
    Description: Eclogite inclusions from kimberlitic diatremes on the Colorado Plateau contain intricately zoned garnet and pyroxene and unusual textures. Detailed electron microprobe traverses for a clinopyroxene-garnet-phengite-lawsonite-rutile assemblage show garnet zoning from Alm_69Gr_21Py_10 (core) to Alm_61Gr_13Py_26 (rim) and pyroxene zoning from Jd_50 (core) through Jd_77 to Jd_55 (rim). Pyroxene cores are Cr-rich in another rock. Sharp compositional discontinuities and zoning reversals are preserved in garnet and pyroxene. Oscillatory zoning occurs in both phases on a 10–20 μm scale, with variations of up to 6% Py in garnet and 15% Jd in pyroxene. Phengite is unzoned and contains 74% celadonite endmember. Skeletal, pyroxene-filled garnet crystals are common in some rocks, and garnets in other rocks clearly began growth as shell-like crystals. Some rocks contain domains of coarse, prismatic pyroxene with very fine-grained, interstitial magnesium silicates. The texture appears to have resulted from crystallization in the presence of a fluid phase, and water pressure is inferred to have equalled total pressure during crystallization. Eclogite formation at high water pressure may reflect subcrustal crystallization. An analysis of error propagation shows that ferrous iron calculations from electron probe data are not meaningful for these jadeitic pyroxenes, and temperature differences between core and rim crystallization cannot be documented. The garnet textures and oscillatory zoning are unusual for metamorphic rocks, and they suggest disequilibrium crystallization after overstepping of reaction boundaries. All data fit a model of eclogite formation during cooling and metasomatism of basaltic dikes intruded into a cool upper mantle, but the results here do not preclude other origins, such as subduction zone metamorphism. ©1979 Springer-Verlag
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  • 39
    Publication Date: 1979-05-01
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  • 40
    Publication Date: 1979-05-01
    Description: A native iron bearing buchite xenolith enclosed in basalt in a native iron bearing lava from Disko is similar in chemistry, apart from volatile components, to unheated Mesozoic to early Tertiary carbonaceous shale sediments from West Greenland, believed to have been the major contaminant and reducing agent causing the formation of the native iron in the volcanic rocks on Disko. The shale buchite has preserved original sedimentary lamination and experienced only a comparatively short-lived heating which allowed only short-range equilibration. The xenolith contains native iron and graphite, while the immediately enclosing basalt does not, and demonstrates the indigenous reducing properties of the shales when heated to high (1,150–1,200 ° C) temperatures at low pressures. Sedimentary clastic iron titanium oxide grains have reacted with the immediately surrounding matrix and completely recrystallized to aggregates of rutile and Al-armalcolite with up to 24 mol% Al_2TiO_5. The Al-armalcolites contain 5 to 11 mol% of the anosovite (Ti_3O_5) component. They show a considerable variation in iron-magnesium ratio due to small-scale variations in bulk chemistry to varying f _O2 and to the effects of progressive melting of the sediment. Glassy veins in the buchite, formed along original fractures in the sediment, contain an assemblage of cordierite, low Ca-pyroxene, plagioclase, ilmenite, ferropseudobrookite-rich armalcolite, iron metal, troilite and acid glass, and crystallized under higher oxygen fugacities than the buchite interior, probably due to influx from the less reduced enclosing basalt magma. ©1979 Springer-Verlag
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  • 41
    Publication Date: 1979-05-01
    Description: Metamorphic rocks at Milos are known in small outcrops beneath the volcanic formations, as xenoliths in the Traphores volcanic breccia and as pebbles in the Paleochora Quaternary deposits. These rocks seem to belong to three different metamorphic units which probably have intricate relationships in the basement: — the “eclogites unit” shows garnetjadeite or garnet-omphacite primary associations, with apparently late crystallization of lawsonite phenoblasts an recrystallization features at relatively low temperatures; — the “glaucophane schist unit” exhibits Jadeite+quartz or glaucophane+lawsonite primary mineralogy; — the “greenschist facies unit” shows low-pressure main assemblages, most generally developed after high-pressure events. The two former units involve tholeiitic meta-igneous rocks, having been spilitized before metamorphism; they generally suffered more or less advanced recrystallization features in the low-pressure field of the greenschist facies. The third unit shows only meta-sedimentary rocks, with the sole exception of one single meta-doleritic pebble having calk-alkaline affinities. Petrological and mineralogical studies, based upon 15 bulk-rock compositions and 178 probe-analysed data points, lead to suspect at least 2, perhaps 3, different metamorphic events rather than one single metamorphic evolution, to account for the 3 U distinguished. From the observation of the mineralogical assemblages and their evolution, the former events (“stages 1 and 2”) could be related to rapid subduction of ocean-floor or back-arc basalts, whereas, during the latter event (“stage 3”), the rocks experienced crystallization conditions involving both decreasing pressures and increasing temperatures. ©1979 Springer-Verlag
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  • 42
    Publication Date: 1979-05-01
    Description: Data from experimental runs, coexisting phases in ultramafic rocks and phenocryst-matrix pairs in volcanic rocks have been used to compile a table of mineral-liquid distribution coefficients for Ti, Zr, Y, and Nb for basic, intermediate and acid melt compositions. These values have then been used to interpret variations of these elements, first in basalts and second, during fractional crystallization from basic to acid magmas. For basalts, petrogenetic modelling of Zr/Y, Zr/Ti, and Zr/Nb ratios, when used in conjunction with REE, Cr and isotopic variations, suggests that: (1) the increase in Zr/Y ratio from mid-ocean ridge to within plate basalts and the low Zr/Nb ratios of alkalic basalts are due to (fluid controlled) source heterogeneities; (2) the low Zr and Zr/Y ratio of volcanic arc basalts results from high degree of partial melting of a depleted source; and (3) the high Zr and similar Zr/Y ratio of basalts from fast spreading relative to slow spreading ridges results from open-system fractional crystallization. Modelling of fractionation trends in more evolved rocks using Y-Zr, Ti-Zr and Nb-Zr diagrams highlights in particular the change in crystallizing mafic phases from island arcs (clinopyroxene-dominated) to Andean-type arcs (amphibole±biotite-dominated). These methods can be applied to altered lavas of unknown affinities to provide additional information on their genesis and eruptive environment. ©1979 Springer-Verlag
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  • 43
    Publication Date: 1979-03-01
    Description: Fresh basalts from the Oceanographer Fracture Zone are petrographically and chemically similar to typical abyssal tholeiites, but are somewhat enriched in large ion lithophile elements, with consistent differences among separate dredge hauls. Olivine compositions are in equilibrium with host basalt for reasonable K _ D values, but some plagioclases are anomalously calcic (e.g., a rock highly differentiated in Mg/Mg+Fe bears some plagioclase of An_86). Ti/Al in clinopyroxene is approximately constant at 1/6, but Ti+Al abundance decreases in the sequence basalt groundmass cpx, basalt phenocryst+microphenocryst cpx, gabbro cpx (samples from adjacent dredges), an effect that may be related to decrease in cooling rate. Least-squares calculations indicate that 45% of magma with the composition of the more primitive sample must be removed as plagioclase, clinopyroxene and olivine (in the ratio 4.4∶2.7∶1, respectively) to obtain the composition of the more differentiated sample. Both samples have excess plagioclase on the liquidus, however, which should not be the case for the differentiated composition if it is produced by fractionation of all three phases. The excess of plagioclase and the anomalous plagioclase compositions indicate that the samples cannot be related solely by fractional crystallization. Additional processes such as magma mixing or plagioclase accumulation (or both) must have effected these differences. ©1979 Springer-Verlag
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  • 44
    Publication Date: 1979-05-01
    Description: Late-crystallised interstitial alkali feldspars and a single epidote from selected Proterozoic dolerites in Sweden have higher initital 87Sr/86Sr ratios, (e.g., 0.709) than the early-crystallised minerals of the same rocks (∼ 0.704); anomalies in Rb and Sr concentrations are also noted. This radiogenically-enriched Sr must originate in the older host rocks of the intrusions. As the contaminated phases often occupy 〈1 % of the dolerite, only an aqueous fluid would have been capable of transporting the contamination through the 99% solid intrusions. Textural association of late feldspars with hydrous alteration products supports this interpretation. Feldspar structural data suggest that most dolerites have been affected by subsolidus aqueous fluids, causing extensive structural re-equilibration in interstitial K-feldspars, as well as occasional metasomatic effects. Anomalies in 87Sr developed only where the fluids interacted with host rocks. ©1979 Springer-Verlag
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  • 45
    Publication Date: 1979-03-01
    Description: A simple modification was made to the reaction vessel designed by Shaw (1963) in order to use it as a hydrogen sensor instead of a hydrogen source, thereby allowing a continuous record of $$f_{H_2 } $$ to be made during an experiment. The advantage of this arrangement is that (reversed) $$f_{O_2 - } T$$ curves for minerals can be generated from a single experimental run. Other applications are possible. This method was successfully used in the determination of the Ni-NiO-H_2O equilibrium between 580 ° and 830 °C at 2 kbar fluid pressure. The corrected 1 atm values are in agreement with electrochemical measurements. An Ag_70Pd_30 hydrogen permeable membrane was used for the experiment and flow rates of hydrogen through the membrane were calculated by monitoring the rate of increase or decrease in pressure with an induction pressure transducer. These flow rates range up to two times greater than those calculated for a pure platinum membrane. ©1979 Springer-Verlag
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  • 46
    Publication Date: 1979-03-01
    Description: The partitioning of Fe and Mg between coexisting garnet and olivine has been studied at 30 kb pressure and temperatures of 900 ° to 1,400 °C. The results of both synthesis and reversal experiments demonstrate that K _ D (= (Fe/Mg)_gt/(Fe/Mg)_OI) is strongly dependent on Fe/Mg ratio and on the calcium content of the garnet. For example, at 1,000 °C/30 kb, K _ D varies from about 1.2 in very iron-rich compositions to 1.9 at the magnesium end of the series. Increasing the mole fraction of calcium in the garnet from 0 to 0.3 at 1,000 ° C increases K _ D in magnesian compositions from 1.9 to about 2.5. The observed temperature and composition dependence of K _ D has been formulated into an equation suitable for geothermometry by considering the solid solution properties of the olivine and garnet phases. It was found that, within experimental error, the simplest kind of nonideal solution model (Regular Solution) fits the experimental data adequately. The use of more complex models did not markedly improve the fit to the data, so the model with the least number of variables was adopted. Multiple linear regression of the experimental data (72 points) yielded, for the exchange reaction: 3Fe_2SiO_4+2Mg_3Al_2Si_3O_12 olivine garnet ⇌ 2Fe_2Al_2Si_3O_12+3Mg_2SiO_4 garnet olivine Δ H ° (30kb) of −10,750 cal and Δ S ° of −4.26 cal deg−1 mol−1. Absolute magnitudes of interaction parameters ( W _ ij ) derived from the regression are subject to considerable uncertainty. The partition coefficient is, however, strongly dependent on the following differences between solution parameters and these differences are fairly well constrained: W _FeMg ol - W _FeMg gt ≃ 800 cal W _CaMg gt - W _CaFe gt ≃ 2,670 cal. The geothermometer is most sensitive in the temperature and composition regions where K _ D is substantially greater than 1. Thus, for example, peridotitic compositions at temperatures less than about 1,300 ° C should yield calculated temperatures within 60 °C of the true value. Iron rich compositions (at any temperature) and magnesian compositions at temperatures well above 1,300 °C could not be expected to yield accurate calculated temperatures. For a fixed K _ D the influence of pressure is to raise the calculated temperature by between 3 and 6 °C per kbar. ©1979 Springer-Verlag
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  • 47
    Publication Date: 1979-03-01
    Description: The Newer Basalts of Victoria contain an abundance of inclusions, the majority being Iherzolites of an accidental origin. During ascent of the host magma a CO2-rich vapor phase ‘boiled’ off and penetrated the inclusions, resulting in ubiquitous fluid inclusions and partial melting along some fractures and intergranular boundaries. Fe+Ni+Cu sulfide globules distributed within the glass and along planes in the primary silicates apparently formed during ascent and originated with a sulfur component in the invading vapor and not with remobilization of free sulfides in the xenoliths themselves. ©1979 Springer-Verlag
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  • 48
    Publication Date: 1979-03-01
    Description: Diorite plutons at Al Hadah Saudi Arabia, which constitute part of the pan-African magmatic sequence (ca. 600 Ma), exhibit extensive development of epidote. The epidote alteration is concentrated along veins and dyke margins, and is characterised by transformation of plagioclase (Ab 67)+hornblende+biotite+quartz to epidote+hornblende+tremolite−actinolite+plagioclase (Ab 99)±quartz. The reactions involve addition of CaO and total Fe_2O_3, depletion of MgO, Na_2O and K_2O, with variable gains or losses of SiO_2. Epidotised alteration products are hydrated and oxidised relative to the diorite precursor. The whole rock δ18O of fresh diorite is + 8.2‰ to + 8.8‰, whereas epidote domains have δ18O whole rock of +9.8‰ to +10.48‰ and negative Δ18O_quartz-plagioclase, implying oxygen isotope exchange with fluids at low temperatures. Epidotisation is considered to have accompanied influx of fluids into plutons during cooling and contraction. The fluids were probably deep formation waters with relatively high Ca2+/Na+ ratios, moving up thermal gradient. ©1979 Springer-Verlag
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  • 49
    Publication Date: 1979-03-01
    Description: The temperature dependence of cell parameters for three disordered, synthetic alkali feldspars (Or_19, Or_38, and Or_100) has been determined up to 1,000 °C. The samples show no change in composition or degree of Si-Al disorder during the experiments. The triclinic-monoclinic inversion in the sample of composition Or_19 occurs at 560 °±10 °C and is accompanied by changes in the rates of expansion of a , b and c ; the rate for a increases and those for b and c decrease above the inversion. The b and c parameters in Or_100 show small decreases with increasing temperature and this may be due to thermal motion effects causing a contraction of cell directions that are fully expanded at room temperature. Calculation of the thermal expansion ellipsoids for the monoclinic phases shows that the major expansion coefficients (α_1) for all three samples are more than an order of magnitude greater than the intermediate (α_2) and minor (α_3) coefficients. Thus the thermal expansion of these phases is dominated by that of α_1 which makes an angle of 22 ± 4 ° with+ a ; this orientation is parallel to that of the short M-OA2 bonds. The thermal expansion mechanism for monoclinic, disordered alkali feldspars may involve tilting within the framework releasing compression along this direction and allowing the M-OA2 bonds to show high expansion rates. The stretching of the crankshaft units, which are parallel to a , may only play a subordinate role in controlling the expansion of the feldspar framework. ©1979 Springer-Verlag
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  • 50
    Publication Date: 1979-03-01
    Description: Fifty-two samples of inverted high-temperature quartz from volcanic rocks were investigated by Guinier-Jago powder diffractometry and differential scanning calorimetry (DSC). Quartz megacrysts from Clear Lake and Cinder Cone, California show a variability of ≃2.5 ° K in their α-β transition temperature ( T _α-β). Quartz phenocrysts and quartz from crystalline rocks give a range of 0.5 ° K in T _α-β. Neutron activation analysis of single crystals demonstrates that Al is the principal impurity (17–380 ppm). Its concentration is inversely correlated with T _α-β. A very small variation was found in the a and c lattice parameters among the specimens of volcanic quartz studied. This variation does not correlate with Al content or transition temperature. Mean values at 22 ° C ( a =4.1934±0.0004 Å, c =5.4046±0.0006 Å) are similar to those of quartz grown at low temperatures. Enthalpy of the α-β transition (Δ H _α-β), obtained over 9.0 ° from DSC runs, is dependent upon sample grain size and for a crushed powder with zero hysteresis ( T _α-β on heating= T _α-β on cooling) is 92.0 ±1.4 cal/mol. In contrast, a single piece of quartz requires Δ H _α-β be 107.7±1.4 cal/mol and has a T _α-β hysteresis of 1.1 ° K. Regression of published data provides equations for the variation of the molar volume (cc/mol) of quartz with v. These equations imply a Δ V _α-β of 0.205±0.031 cc/- mol. Expressions are also provided for the temperature dependence of the thermal coefficient of expansion, α, the compressibility, β, and (∂/gb/∂ T )_p (which is identically -(∂α/∂ P )_ T ). DSC heat capacity measurements over the range 400 to 900 ° K were fitted to extended Maier-Kelley type expressions to give: C_P = 10.31 + 9.116 imes 10{ - 3} T - frac{{1.812 imes 105 }}{{T2 }} hfill \ - { ext{5}}{ ext{.630}} imes 10{ - 2} { ext{ }}frac{T}{{(T - 848)}} - 0.3553frac{T}{{(T - 848)2 }} hfill \ - 0.9011frac{T}{{left( {T - 848} ight)3 }} hfill \ (400{ ext{ to 842}} circ { ext{K), and}} hfill \ C_P = - 318.8 + 0.2532T hfill \ { ext{ + }}frac{{8.687 imes 107 }}{{T2 }} + 0.1603frac{T}{{left( {T - 848} ight)4 }} hfill \ (851 to 900 ° K), which together with the values of Δ H _α−β measured over the range 842–851° K give 7875.3 cal/mol for H_900-H_400. The behavior of α, β, and C _ p as a function of T emphasizes that structural changes which occur at the α−β transition do so over a broad temperature interval. ©1979 Springer-Verlag
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  • 51
    Publication Date: 1979-03-01
    Description: Although remnants of a Permian volcanic arc in the South Island of New Zealand have hitherto been considered to comprise a single volcanostratigraphic unit, recent work in the southern part of the arc has revealed the presence of two lithologically distinct terranes, only one of which is definitely Permian in age. Major and trace element analyses of representative specimens show that these terranes are geochemically distinct. Comparison with modern volcanic arcs indicates that this ancient arc in the South Island was a paired arc system made up of primitive island arc tholeiites to the east and more evolved andesitic rocks to the west. ©1979 Springer-Verlag
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  • 52
    Publication Date: 1979-03-01
    Description: Three contemporaneous yet mineralogically and chemically distinct lava series which range in composition from olivine basalt through basaltic andesite through two-pyroxene andesite, have been analyzed for their strontium isotopic composition. The Mammoth-Modoc lavas produce a mantle isochron which suggests derivation from a source approximately 243 m.y. old. The Medicine Lake shield-forming extrusives produce a minimum mantle isochron; the age of this source is likely greater than 760 m.y. old. Encompassing the entire time span of eruption for the two basalt-andesite series are a number of low potassium high alumina olivine tholeiite lavas which, when analyzed as a coherent group, produce a zero age. Lavas related to these three magma series have been extruded during the last 105 years and cover a limited geographic area. The isotope data suggest a model with substantial heterogeneity in the subcontinental lithosphere and asthenosphere, at least beneath the Medicine Lake Highland portion of the southern Cascade petrographic province. ©1979 Springer-Verlag
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  • 53
    Publication Date: 1979-02-01
    Description: Samples of granitic rock from south-central Maine contain primary igneous minerals altered by hydrothermal fluids. The reaction mechanisms (by which the over-all mineralogical change during the alteration was accomplished) involve several different mineral-fluid reactions at different reaction sites in the rock. The reactions involve both molecular and charged species in solution. The different reaction sites correspond to alteration of different primary igneous minerals. Biotite is partially converted to chlorite+sphene; microcline to muscovite; plagioclase to various combinations of muscovite, epidote, and calcite. The different reaction sites are linked by exchange of ions: some reaction sites produce ions consumed at other sites and vice versa . Physical conditions during the hydrothermal event are estimated from mineralogical and thermochemical data: P = 3,500 (±300) bars; T =425 ° (± 25 °)C. The fluid was characterized by X _CO _2 = 0−0.13; ln([K+]/[H+ ]) = ∼10.0; ln([Ca2+]/[H+]2)=∼9.1; ln([Na+]/[H+]) = ∼ 10.5; Fe/(Fe+Mg) = 0.95. Amounts of secondary minerals in altered rock, when compared to the inferred mineral reactions that formed them, indicate that small but significant amounts (0.01–0.3mol/ 1,000cm3 altered rock) of CO2, H_2O, H+, and K+ were added to the granites by fluids during the alteration, as well as lesser amounts (〈 0.01–0.03 mol/1,000cm3 altered rock) of Mg2+, Fe2+, Fe3+, Mn2+, Na+, and Ti4+. The sole element leached from the granitic rocks during alteration was Ca in amounts 0.1–0.3 mol/1,000 cm3 rock. By estimating the composition of the hydrothermal fluids before and after reaction with the granites and by measuring the amount of material added to or subtracted from the granites during the alteration, the amount and volume of hydrothermal fluid involved can be calculated. Two independent calculations require minimum volumes in the range 100–1,000 cm3 fluid/1,000cm3 altered rock to participate in the hydrothermal event. ©1979 Springer-Verlag
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  • 54
    Publication Date: 1979-02-01
    Description: Magmatic evolution on the active volcano of Agrigan in the northern Mariana Island Arc is interpreted as resulting in the production of calc-alkaline andesites by the fractional crystallization of high-alumina basalt. Basaltic products predominate, but the ratio of andesites to basalts increases with time up to an event of voluminous andesitic pyroclastic ejection accompanied by caldera-collapse; post-collapse lavas are entirely basaltic. Moderate iron-enrichment is demonstrated for the volcanic suite, with indications of a progressive, pre-caldera decrease in iron-enrichment; post-caldera lavas display a return to moderate Fe-enrichment. Overall, the lavas are enriched in the LIL elements (K, Rb, Ba, Sr) and depleted in Ti, Mg, Cr, and Ni. From the oldest to the youngest pre-caldera volcanic sequence, the LIL elements increase 3-6X while Ca and Mg decrease by 50% or more. Approximately constant K/ Rb (430±60) and 87Sr/86Sr (0.7032–0.7034) indicate consanguinity of the basalts and the andesites. Cumulate plutonic xenoliths, common in the lavas, are composed of mineral phases also encountered as phenocrysts. The following order of crystallization is indicated: olivine; anorthite-bytownite; clinopyroxene; orthopyroxene and titanomagnetite. Co-existing xenolithic olivines (Fo_74–83) and plagioclase (An_88–96) are typical of calc-alkaline island-arc assemblages and contrast with assemblages in the tholeiites from the Mariana Trough to the west. The relatively fayalitic composition and low abundances of Ni in olivines and Cr in clinopyroxenes indicate equilibrium with an already-fractionated liquid. These data, along with structural evidence, high Ca in the olivines, and comparison of the observed assemblages with experimental studies, suggests that these xenoliths formed as crystal cumulates at the floor of a shallow (≦ 7 km) crustal magma chamber. Major element modeling studies using the separation of observed xenocrystic and phenocrystic phases from assumed parental liquids reproduce the observed temporal and geochemical variations in the lavas. Trace element modeling parallels this evolution with the exception of Cr and Ni in the andesites. An extensive (16.3 km3) gabbroic body is required by this modeling to be present beneath Agrigan to produce the inferred volumes of the various lithologies preserved in the volcano's evolution. The sum of stratigraphic, geochemical, and isotopic evidence on Agrigan supports the derivation of calc-alkaline andesite by the removal of about 75% solids from a high-alumina basalt accompanied by a process of K and Rb enrichment, such as volatile-transfer. Considerations of 87Sr/86Sr, 143Nd/144Nd, and 3He/4He isotopic data indicate that the source region of these parental liquids lies in the mantle, not subducted crust. In the northern Marianas, the model of a shallow-crustal origin for andesite is preferred over one requiring andesite generation in the deeper mantle and/or subducted slab. ©1979 Springer-Verlag
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  • 55
    Publication Date: 1979-02-01
    Description: Late Cretaceous, granitic pegmatite-aplite dikes in southern California have been known for gem-quality minerals and as a commercial source of lithium. Minerals, whole-rock samples, and inclusion fluids from nine of these dikes and from associated wall rocks have been analyzed for their oxygen, hydrogen, and carbon isotope compositions to ascertain the origins and thermal histories of the dikes. Oxygen isotope geothermometry used in combination with thermometric data from primary fluid inclusions enabled the determination of the pressure regime during crystallization. Two groups of dikes are evident from their oxygen isotope compositions (δ18O_qtz≃+10.5 in Group A, and ≃+8.5 in Group B). Prior to the end of crystallization, Group A pegmatites had already extensively exchanged oxygen with their wall rocks, while Group B dikes may represent a closer approximation to the original isotopic composition of the pegmatite melts. Oxygen isotope fractionations between minerals are similar in all dikes and indicate that the pegmatites were emplaced at temperatures of about 730 ° to 700 ° C. Supersolidus crystallization began with the basal aplite zone and ended with formation of “quench aplite” in the pocket zone, nearly to 565 ° C. Subsolidus formation of gem-bearing pockets took place over a relatively narrow temperature range of about 40 ° C (approximately 565–525 ° C). Nearly closed-system crystallization is indicated. Hornblende in gabbroic and noritic wall rocks (δD_w.r. = −90 to −130) in the Mesa Grande district crystallized in the presence of, or exchanged hydrogen with, meteoric water (δD≃ −90) prior to the emplacement of the pegmatite dikes. Magmatic water was subsequently added to the wall rocks adjacent to the pegmatites. Groups A and B pegmatites cannot be distinguished on the basis of their hydrogen isotope compositions. A decrease in δD of muscovite inward from the walls of the dikes reflects a decrease in temperature. δD values of H_2O from fluid inclusions are: −50 to −73 (aplite and pegmatite zones); −62 to −75 (pocket quartz: Tourmaline Queen and Stewart dikes); and −50 ± 4 (pocket quartz from many dikes). The average δ13C of juvenile CO2 in fluid inclusions in Group B pegmatites is −7.9. In Group A pegmatities, δ13C of CO2 is more negative (−10 to −15.6), due to exchange of C with wall rocks and/or loss of 13C-enriched CO2 to an exsolving vapor phase. Pressures during crystallization of the pockets were on the order of 2,100 bars, and may have increased slightly during pocket growth. A depth of formation of at least 6.8 km (sp. gr. of over burden = 3.0, and P _fiuid= P _load) is indicated, and a rate of uplift of 0.07 cm/yr. follows from available geochronologic data. ©1979 Springer-Verlag
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  • 56
    Publication Date: 1979-12-01
    Description: Pyroclastic fragments of basaltic composition are obtained in a thick metabasaltic layer from the Singhbhum orogenic belt of Eastern India. Field relations indicate the basalts to be submarine lava flows. Chemically, the metabasalts represent an original dry basaltic magma, showing a differentiation trend of Fe-enrichment much like the abyssal tholeiites and the Skaergaard rocks. The fragments are sharply angular in shape and distinctly richer in FeO, MgO, and MnO relative to the enclosing matrices. It is shown that the fragments are least likely to represent solidified lavas. These are best explained as cumulates which were fragmented and erupted with the fractionated melts. ©1979 Springer-Verlag
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  • 57
    Publication Date: 1979-12-01
    Description: δ18O of quartz, biotite, muscovite, garnet, ilmenite or magnetite, K-feldspar, and δD of biotite from prograde metamorphic pelites of the Damara Orogen have been analyzed. The samples were taken from one stratigraphic unit which is exposed in the various stages from low-grade up to high-grade metamorphism with anatexis. Using the fractionation curves experimentally investigated up to now, it can be shown that equilibrium has been reached among the metamorphic assemblages in the low-grade and lower medium-grade metamorphism only. In the high-grade rocks only a partial equilibrium exists between those phases formed at the specific high-grade isogrades, i.e., K-feldspar, garnet, magnetite, or ilmenite, but not between these and the main reacting phase biotite. Biotite in equilibrium with these minerals is found only in anatectic rocks where new biotite crystallized from the melt. From this it is concluded that the18O/16O ratio of biotite is fixed at the time of crystallization and its initial composition is preserved when the temperature increases, unless the biotite recrystallizes. The isotopic temperatures derived from mineral pairs formed at specific isograds are in excellent agreement with temperatures derived from petrological investigations. The hydrogen isotoperatio of biotites decreases with increasing grade of metamorphism. ©1979 Springer-Verlag
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  • 58
    Publication Date: 1979-12-01
    Description: A zircon study has been made on eleven samples of igneous rocks from the Saudi Arabian Craton. Ages of sized and magnetic fractions of zircon concentrates show variable degrees of discordance which seem to result from a very young disturbance that produces linear arrays in the Concordia plot. Model age calculations based on a statistically and geologically reasonable lower intercept produce very consistent internal relationships. The Pan African Orogeny, considered to be responsible for loss of radiogenic argon and strontium from minerals of many rocks, does not appear to have affected the zircon data, even though uplift had exposed the rocks of the Arabian Shield at that time. Tonalite, granodiorite, and crosscutting leucoadamellite bodies in the southern part of the An Nimas Bathylith yield ages in the time range 820−760 Ma. A narrow time range of 660 to 665 million years was indicated for ages of widely separated and compositionally different intrusive bodies all to the east of the An Nimas Bathylith. This work suggests that the younger end of the age spectrum established from regional K-Ar and Rb-Sr measurements may be underestimated, and that magmatic activity could be more episodic than previously assumed. ©1979 Springer-Verlag
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  • 59
    Publication Date: 1979-12-01
    Description: The bulk compositions of the groundmass alkali feldspar from the Hell Canyon Pluton is 0.146mole% albite. The composition of the outermost zone of the oscillatory zoned plagioclase is 0.686 mole% albite, whereas the most calcic cores have a composition of 0.43 mole% albite. The structural state of the alkali feldspar is near orthoclase. Both composition of coexisting feldspars and structural state of the alkali feldspar are nearly constant throughout the pluton. Exsolved albite in the alkali feldspar have a composition of 0.965 mole% albite and the orthoclase host has a composition of 0.032 mole%. Singe crystal X-ray studies indicate that the albite intergrowths are coherent with the host. Equilibrium temperatures derived from the coexisting feldspar average 554 ° C; about 150 ° C, too low for the minimum solidus temperatures for reasonable emplacement pressures (2 kb). If this minimum solidus temperature is assumed, then the alkali feldspar has lost about 0.15 mole% albite. This loss was most likely caused by hydrothermal solutions associated with the crystallizing magma and equilibrated at about 550 ° C. However, based on the coherent albite intergrowths and the orthoclase structure state it can be inferred that the system was relatively free of volatiles below 500 ° C. Final equilibirium between orthoclase host and albite intergrowths occurred at about 311 ° C. ©1979 Springer-Verlag
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  • 60
    Publication Date: 1979-12-01
    Description: The experimental crystallization of staurolite in two mafic compositions at high pressures (24–26 kb) and temperatures (740 °–760 ° C) is reported. This, together with a recent report of staurolite from metamorphosed basic igneous rocks from Fiordland, New Zealand, leads to the conclusion that Staurolite may be an important subordinate phase in the subducted lithosphere. The synthesized staurolites are more magnesian than those occuring in natural metapelitic assemblages and are characterized by relatively high Si, Ti, Mg, Ca, and low Al, Fe. ©1979 Springer-Verlag
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  • 61
    Publication Date: 1979-12-01
    Description: The H_2O content of cordierite is often regarded as incidental to its stability, probably because cordierite has substantial fields of stability at low pressures both in wet and dry experimental systems. In this paper we show that, in contrast, the molecular water content of cordierite has profound effects on many equilibria involving this phase. Mg-cordierite has been modelled as an ideal solid solution of the hydrous and anhydrous end-members Mg_2Al_4Si_5O_18·1.2H_2O and Mg_2Al_4Si_5O_18 respectively. The H_2O-solubility data of Mirwald and Schreyer (1977) fit this model within experimental uncertainty and yield 1 bar enthalpy and entropy changes for the reaction: Mg_2 Al_4 Si_5 O_{18} + 1.2H_2 O = Mg_2 Al_4 Si_5 O_{18} cdot 1.2H_2 O hfill \ cordierite fluid codierite hfill \ of −12,300 cal and −32.87 cal/K. This implies that the partial molal entropy of H_2O in cordierite at 298 K/l bar is almost exactly the same as the molar entropy of liquid water (16.9 cal/K as opposed to 16.7 cal/K) and that the interaction energy of liquid water with cordierite is only of the order of a few hundred calories per mole. Application of the model to the hydrous experiments of Fawcett and Yoder (1966) and Chernosky (1974) yields a value for Δ G _f,298 0 of anhydrous Mgcordierite of between −2,062.71 and −2,074.21 Kcal per mole. This in in good agreement with the calorimetric data of Charlu, Newton and Kleppa (1975) which yield Δ G _f,298 0 of −2,067.03±1.18 Kcal. Water pressure has a considerable influence on the (Mg, Fe) isopleths of coexisting cordierite and garnet, and hence, their use as geobarometric curves. Pressures estimated from the Mg/Fe ratios in the high-Mg range can vary by two kilobars or more, depending on the assumed P_{H_2 O} , with highest estimates for P_{H_2 O} = P_{total} . The stability field of the talc-kyanite “white-schist” assemblage (Schreyer, 1973) is found to expand appreciably as P_{H_2 O} is lowered. Thus the minimum pressure required to form this assemblage can be considerably less than the 10 kb required under conditions of P_{H_2 O} = P_{total} = P _total, as anticipated by Schreyer (1977). The high partial molal entropy of H_2O in cordierite results in small entropy changes coupled with large volume changes in dehydration reactions forming cordierite. This greatly influences the slopes and positions of univariant reactions involving cordierite. The stability of cordierite is promoted to higher pressures in H_2O-bearing systems where none of the cordierite breakdown products is a hydrate. Cordierite-forming reactions from hydrates can have the H_2O released on the relatively low-temperature sides of the reaction curves, an anomalous situation known only in zeolite stability curves. These considerations can have profound effects on model “petrogenetic grids” involving cordierite. ©1979 Springer-Verlag
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  • 62
    Publication Date: 1979-12-01
    Description: K-Ar dating on a suite of volcanic rocks from the island of Principe gives the following chronology. 1. Basal palagonite breccia (30.6 ± 2.1 Ma). 2. Older Lava Series (OLS) basalt (23.6±0.7 Ma) and hawaiite (19.1±0.5 Ma). 3. Younger Lava Series (YLS) nephelinite (5.60±0.32 Ma) and basanite (3.51 ±0.15). 4. Intrusive phonolite (5.32±0.17 Ma, 5.48±0.19 Ma), tristanite (4.89±0.15 Ma) and trachyphonolite (6.93±0.68 Ma) plugs. Phonolites and YLS samples plot on a 5.9±0.3 Ma Rb-Sr isochron. The tristanite-trachy-phonolite suite samples also lie on this isochron. This lends support to the suggestion that the YLS basanite magmas were parental to the phonolites but rules out a similar relationship between the OLS magmas and the tristanite-trachyphonolite suite. The mean initial 87Sr/86Sr ratio for the YLS nephelinites and basanites is 0.70297. The basalts and hawaiites of the OLS show a positive 87Sr/86Sr vs. Rb/Sr correlation which may be interpreted as a 244±43 Ma pseudoisochron. This could be the result of a large-scale heterogeneity generated in the mantle during the early stages in the break-up of Gondwanaland. The mean initial 87Sr/ 86Sr ratio (at 21 Ma) for the OLS (0.70326) is significantly higher than that for the YLS and implies an isotopically distinct mantle source. ©1979 Springer-Verlag
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  • 63
    Publication Date: 1979-12-01
    Description: The temperature dependence of the Mn-Mg distribution between garnet and clinopyroxene, originally proposed by Carswell, was confirmed by Shimizu and Allègre (1978) using ion microprobe and electron microprobe data. High precision electron microprobe analyses of a larger set of 52 Iherzolites from S. Africa and Malaita, Solomon Islands show considerable scatter in the temperature dependence of this distribution, and correlation with the CaO content of the garnet is indicated. A new distribution coefficient is based on the reaction: operatorname{Mn} _{ ext{2}} operatorname{Si} _2 operatorname{O} _6 { ext{ + }}operatorname{CaAl} _{2/3} operatorname{SiO} _4 { ext{ + }}operatorname{MgAl} _{2/3} operatorname{SiO} _4 hfill \ { ext{Mn - pyroxene grossular pyrope}} hfill \ { ext{ }} ightleftharpoons operatorname{CaMgSi} _2 operatorname{O} _6 { ext{ + }}2operatorname{MnAl} _{2/3} operatorname{SiO} _4 hfill \ { ext{ diopside spessartine}} hfill \ It was calibrated against temperature determined from two independent thermometers (Wells pyroxene and O'Neill-Wood garnet-olivine) for Iherzolitic assemblages, and shown to to be sensitive to within + 50 °C for most specimens in the range 900 °– 1,300 ° C. This distribution coefficient appears independent of pressure within the uncertainty of the available data, and has the potential to be a third independent thermometer for use in garnet Iherzolites and possibly eclogites. ©1979 Springer-Verlag
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  • 64
    Publication Date: 1979-12-01
    Description: Densities of 21 silicate liquids have been determined from 1,000 ° to 1,600 ° C. The compositions studied contain from two to eight oxide components and have the following ranges in composition (mole %): SiO_2, 35–79%; TiO_2, 4–36%; Al_2O_3, 5–25%; FeO, 11–41%; MgO, 7–28%; CaO, 7–35%; Na_2O, 5–50%; and K_2O, 4–20%. The compositions thus cover the upper range observed in magmas for each oxide. Precision for each determination of liquid density is always better than ±1%. Volumes/gfw (gram formula weight) calculated from the density measurements and the chemical compositions of the analyzed liquids have been combined with data on 96 silicate liquids reported in the literature. From this data set we derive, by using multiple linear regression, partial molar volumes of the components SiO_2, TiO_2, A1_2O_3, FeO, MgO, CaO, Na_2O, and K_2O at five temperatures. The standard deviation of the multiple regression is 1.8% of the molar volumes, which is considered about equal to the total errors due to compositional and instrumental uncertainties. These derived partial molar volumes have been used to calculate volumes/gfw of natural silicate liquids which are found to agree within 1% of the measured values. No compositional dependence of the partial molar volumes can be detected within the error considered to be typical of the measurements. This is further supported by the close agreement between the calculated volumes of CaMgSi_2O_6 and Fe_2SiO_2 liquids derived from the initial slopes of their fusion curves and their heats of fusion, and the volumes obtained by summing the respective partial molar volumes. The experimental data indicate that silicate liquids mix ideally with respect to volume, over the temperature and composition range of this data set. ©1979 Springer-Verlag
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  • 65
    Publication Date: 1979-12-01
    Description: Electron microscopy of the minerals in, and adjacent to the black veins of the Tenham meteorite reveals that the minerals have undergone varying degrees of shock. The orthopyroxenes of the bulk of the meteorite have been transformed to clinopyroxenes in areas adjacent to the veins, and to majorite in the black vein itself. The majorite, which occurs in both equant and dendritic habit, is associated with a glassy phase from which it crystallized, and with a microcrystalline clinopyroxene aggregate produced from the majorite upon the release of pressure. The olivines of the meteorite are also deformed, and some have been transformed to the high pressure spinel polymorph. The spinel has subsequently partially inverted to the β-phase polymorph. ©1979 Springer-Verlag
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  • 66
    Publication Date: 1979-12-01
    Description: Hydrothermal reversals of the univariant reaction orthopyroxene + clinopyroxene + spinel to garnet + olivine in the system CaO-MgO-Al_2O_3-SiO_2 have been made in the range 900 °–1,000 ° C without intervention of hydrous phases. At 1,000 ° C the equilibrium is bracketed at 16.0±0.5 kbar, and, at 900 ° C, at 15.0±0.5 kbar. A flat but not well-constrained dP/dT slope is inferred. The composition of the garnet has been determined as (Ca_3Al_2Si_3O_12)_0.15 (Mg_3Al_2Si_3O_12)_0.85 at 900 ° C and 1,000 ° C. A dP/dT slope of 4.5±3 bars/K at 1,000 ° C, calculated from enthalpy of solution data, is within the experimental error range and agrees with the flat calculated slopes of Obata (1976) and Herzberg (1978). The large curvature of the reaction in the low-temperature range predicted by these authors is established. Hydrothermal reversal runs on a mixture of natural pyropic garnet and forsteritic olivine and their low-pressure breakdown products were made at 900 ° C. Above 12.5 kbar, garnet + olivine are stable. Below 9.5 kbar, garnet + olivine react to pyroxenes +spinel. Between these pressures, the probable stable assemblage on the bulk composition used is garnet + olivine + orthopyroxene. Thus, the peridotite transition has only been determined within a three-kbar interval. It is inferred from the experimental data that garnet peridotites occurring in amphibolite-granulite gneiss terrains could be formed by deep-crustal metamorphism; a subcrustal origin of the garnet-olivine association is not necessary. Seismic velocity increases at depths of 60–90 km in oceanic lithosphere, which have been considered by some to correspond to the onset of garnet in peridotite, are much deeper than the experimentally determined transition interval at 30–40 km at 900 ° C. ©1979 Springer-Verlag
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  • 67
    Publication Date: 1979-11-01
    Description: Experiments have been carried out on the separation of H_2O-rich vapor phase from a hydrous andesite melt at pressures between 5 and 15 kbar at 1,150 ° C. The pressure at which the vapor phase separates from the melt by isothermal decompression depends on the H_2O content in the melt; for example, 14 kbar for 12wt.% and 8 kbar for 9wt.% H_2O. These values are lower than the solubility of H_2O in andesite melt previously estimated. Extensive decompression to near atmospheric pressure resulted in the formation of pumiceous glass. Vapor phase separated from the melt moves upward and transports significant amounts of alkalies (Na_2O and K_2O), resulting in the depletion of alkalies near the bottom and concentration of alkalies near the top of the container. The maximum concentration observed is 5.0 wt.% for Na_2O and 1.7% for K_2O, compared to the initial contents 3.3 and 1.3 wt.% respectively. The approximate viscosity of hydrous andesitic melt with 7.5–12 wt.% H_2O was roughly estimated to be less than 10 poise. The results of the present experiments imply that when H_2O-rich vapor separates from magma in a magma chamber (or in a conduit) and moves upward, the top of the chamber would be enriched in alkalies while the bottom would be depleted. ©1979 Springer-Verlag
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  • 68
    Publication Date: 1979-11-01
    Description: Fission track ages have been measured for 12 sphenes, 18 zircons and 25 apatites separated largely from Lower Tertiary magmatic rocks of East Greenland, with a few examples from Caledonian rocks. The sphene and zircon ages of Caledonian rocks agree with other radiometric ages but apatite is strongly discordant indicating that these rocks cooled very slowly over a 200 m.y. period. It was not until the Permian/Lower Jurassic that they finally cooled below 100 ° C, possibly as a consequence of uplift and erosion at this time in connection with extensive rifting. No evidence of a Tertiary imprint has been found in these rocks. Layered gabbros, such as Skaergaard, were emplaced at about the same time (ca. 54 m.y.) as the latest plateau basalts. Some evidence of syenitic activity from this period occurs in the Angmagssalik area ca. 400 km to the south but the syenites of Kangerdlugssuaq cluster around 50 m.y. The Gardiner ultramafic alkaline complex and some of the offshore gabbros apparently also were emplaced at about 50 m.y. Late dykes in the Kangerdlugssuaq area were emplaced over a considerable time span (43-34 m.y.) in keeping with their variable petrographic character, and the Kialineq centre was formed at 36.2±0.4 m.y. Intrusions of the Masters Vig area differ in age. Kap Simpson and Kap Parry to the northeast were emplaced around 40 m.y. whereas the Werner Bjerge complex is the youngest igneous activity so far identified in Greenland with an age of 30.3±1.3 m.y. Many apatites give strongly discordant ages of about 36 m.y. and these are concentrated in the area of a major domal uplift centred on Kangerdlugssuaq. The uplift is older than these ages but on field evidence post-dates the basalts. It probably formed in conjunction with alkaline magmatism at ca. 50 m.y. Cooling below ca. 200 ° was slow for these intrusions and was probably controlled by a number of factors including erosion of the dome, high heat flow caused by continuing dyke injection and regional plateau uplift. The last is believed to have taken place about 35 m.y. ago at the time of emplacement of the Kialineq plutons and last dykes. Renewed rapid erosion and declining heat flow at this time led to rapid cooling of the rocks now at the surface to below 100 °. ©1979 Springer-Verlag
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  • 69
    Publication Date: 1979-11-01
    Description: The mobility of the rare earth element (REE) during hydrous burial metamorphism is described from three localities. Comparison of REE abundances in relict domains and metadomains from flood lavas in the Maddina Volcanics, Fortescue Group, Western Australia shows that, relative to the relict domains, REE may be strongly depleted in certain metadomains. Strong variations in La/Yb, La/Sm and Eu may also occur due to postcrystallization secondary processes. Similar comparisons in flood lavas from Mamainse Point in the Canadian Keweenawan Series show that REE are mobile and increase in abundance in metadomains. Spilites from U.S. Virgin Islands also show evidence for REE mobility during low grade burial metamorphism. In this case light REE (LREE) have been preferentially mobilized with heavy REE (HREE) remaining parallel or sub-parallel on chondrite normalized plots. REE analyses from these locations together with an evaluation of published work suggests that the mobility of REE can be described by: 1. gross REE and selective LREE enrichment; 2. REE movement around a primary mean; 3. gross REE depletion; 4. selective REE mobility. Failure to take into account REE mobility may lead to incorrect conclusions concerning petrogenetic models based on altered basic sequences. The coherence of REE with other key trace elements during alteration is also discussed. ©1979 Springer-Verlag
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  • 70
    Publication Date: 1979-10-01
    Description: Petrological mixing problems such as modal analysis, magma mixing, and liquid line of descent calculations, can be solved using the methods of linear programming. If estimates of the standard error of the chemical data are introduced as weights into the set of equations, it is possible to assign confidence limits to the solutions which are obtained and to apply formal statistical tests to geological hypotheses based on the mixing model. This approach is applied to petrological data previously analysed by Wright and Doherty (1970) using a combination of linear programming and least squares methods. It is shown that some of the geological inferences which they drew were based on an overoptimistic assessment of the confidence limits on their solutions, and cannot be regarded as proven. ©1979 Springer-Verlag
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  • 71
    Publication Date: 1979-11-01
    Description: A series of basaltic compositions and compositions within the simple system CaO-MgO-FeO-Al_2O_3-SiO_2 have been crystallized to garnetclinopyroxene bearing mineral assemblages in the range 24–30 kb pressure, 750°–1,300° C temperature. Microprobe analyses of coexisting garnet and clinopyroxene show that K _D(Fe2+/MgG+/Fe2+/MgCpx) for the Fe-Mg exchange reaction between coexisting garnet and clinopyroxene is obviously dependent upon the Ca-content and apparently independent of the Mg/(Mg+Fe) content of the clinopyroxene and garnet. The Ca-effect is believed to be due to a combination of non-ideal Ca-Mg substitutions in the garnet and clinopyroxene. Our data and interpretation reconciles previous inconsistencies in the temperature dependence of K _D − values determined in experimental studies of simple systems, complex basalt, grospydite and garnet peridotite compositions. Previous differences between the effect of pressure upon K _Das predicted from simple system theory (Banno, 1970), and that observed in experiments on multicomponent natural rock compositions (Råheim and Green, 1974a) can now be resolved. We have determined K _Das a function of P , T , and X _Gt Ca (grossular) and derived the empirical relation $$Tleft( {circ { ext{K}}} ight) = frac{{3104X_{{ ext{Ca}}}{{ ext{Gt}}} + 3030 + 10.86Pleft( {{ ext{kb}}} ight)}}{{ln K_{ ext{D}} + 1.9034}}$$ . This empirical relationship has been applied to garnet-clinopyroxene bearing rocks from a wide range of geological environments. The geothermometer yields similar estimates for garnet-clinopyroxene equilibration for neighbouring rocks of different composition and different K _Dvalues. In addition, temperature estimates using the above relationship are more consistent with independent temperature estimates based on other geothermometers than previous estimates which did not correct for the Ca-effect. An alternative approach to the above empirical geothermometer was attempted using regular solution models to derive Margules parameters for various solid solutions in garnets and clinopyroxenes. The derived Margules parameters are broadly consistent with those determined from binary solution studies, but caution must be exercised in interpreting them in terms of actual thermodynamic properties of the relevant crystalline solid solutions because of the assumptions which necessarily have to be made in this approach. ©1979 Springer-Verlag
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  • 72
    Publication Date: 1979-11-01
    Description: U-Th-Pb systematics for zircon and monazite from Massabesic Gneiss (paragneiss and orthogneiss) and the granite near Milford, New Hampshire, were determined. Zircon morphology suggests that the paragneiss may be volcaniclastic (igneous) in origin, and thus the age data probably record the date (minimum of 646 m.y.) at which the rock was extruded. A two-stage lead-loss model is proposed to explain the present array of data points on a concordia diagram. Orthogneiss ages range only narrowly and are clustered around 475 m.y. Data for the granite of Milford, New Hampshire, are scattered, but may be interpreted in terms of inheritance and modern lead loss, yielding a crystallization age of 275 m.y. This is the only known occurrence of Avalonian-type basement in New Hampshire and as such provides evidence for the location of the paleo-Africa-paleo- North America suture. The geochronology also further documents the occurrence of disturbances during the Ordovician and Permian. ©1979 Springer-Verlag
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  • 73
    Publication Date: 1979-11-01
    Description: The phase relations of pyroxenes, amphiboles and associated minerals in metamorphic rocks of the Franciscan Complex can be graphically depicted on a ternary diagram which has at its apices the metamorphic clinopyroxene end members, viz NaAl-NaFe3+-Ca(Fe2+, Mg). Phases are plotted by projection from a constant subassemblage of minerals. This analysis allows interpretation of the effects of pressure, temperature, bulk rock composition and fluid composition on stability of minerals within the Franciscan. Pyroxenes in meta-igneous rocks and metagraywackes have a limited compositional range and fall into two groups: the omphacites, with 50±5% diopside +hedenbergite component; and the jadeitic pyroxenes with 10±5% diopside+hedenbergite. Pyroxenes intermediate between these two groups are unstable relative to assemblages containing Na-amphibole+other minerals. Coexisting pyroxenes and amphiboles in eclogites and associated coarse blueschists comprise equilibrium assemblages, and the proportion of pyroxene to amphibole is a function of rock composition. Eclogites are stable at higher temperature than regionally developed fine-grained greenstones and blueschists in the Franciscan, and at higher pressure than amphibolites. X _H2O fluid is not an important factor in the stability of Franciscan eclogite relative to amphibolite. ©1979 Springer-Verlag
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  • 74
    Publication Date: 1979-10-01
    Description: The chlorine concentration has been determined by a chemical method in 7 quartz tholeiites, 19 alkali olivine basalts, 9 basanitic alkali olivine basalts and 11 olivine nephelinites to be on average 80, 280, 720, and 400 ppm Cl respectively. If these basalts are products of decreasing degrees of partial melting of mantle rocks a regular increase of chlorine is to be expected in this sequence. The actual chlorine abundances are a function of partial losses of gases during rock consolidation and optimum stabilities of sodalite group minerals as major chlorine traps in alkalic basalts. The occurrence of sodalite and sodalite nosean solid solutions has been detected by microprobe in 7 out of 10 alkalic basalt species in grains smaller than 70 μm. Quantitative analyses of 4 sodalite group minerals from the olivine nephelinites are listed. One contains the sodalite and the nosean molecule in a proportion one to one and must be formed above 1,050 °C according to the experimental results of Tomisaka and Eugster (1968) in the respective system. In the majority of the samples apatite contains less than 20% of the total chlorine of the basalts. Biotite as chlorine containing phase (about 900 ppm Cl) is rare. The proportion of chlorine which could be extracted from rock powders by boiling water is small. No general correlation between the element pairs Cl/S and Cl/K could be observed. Excluding tholeiites a tendency of a reversed correlation between chlorine and ‘potential primary water’ (as indicated by the Fe_2O_3/FeO ratio) and between chlorine and silica can be derived.