ALBERT

Notes: Abstract Rocks of the glaucophane-schist facies are widely though irregularly developed in the Franciscan formation of California. Minerals critical of the facies are lawsonite, aragonite, jadeite and omphacitic pyroxenes associated with quartz; amphiboles of the glaucophane-crossite series are almost ubiquitous. The most widely distributed rock, occurring over areas of many square kilometers, is jadeite-lawsonite metagraywacke, commonly veined with aragonite. More spectacular, but occurring mainly in isolated blocks are coarse-grained glaucophane-lawsonite Schists of many kinds. Commonly, but by no means invariably, they are closely associated with bodies of serpentinite. Also common in the vicinity of serpentinite masses are blocks of amphibolite and eclogite. All the metamorphic rocks are considered to be Franciscan sediments and basic volcanics metamorphosed and metasomatized in the deep levels of a folded geosynclinal prism. Experimental data on the stability fields of jadeite-quartz, aragonite, and lawsonite show that the glaucophane-schist facies represents metamorphism at pressures of between 5 and 10 kb and temperatures of 150–300° C. Such conditions could develop at depths greater than 15 km provided a very low geothermal gradient (10°/km) were maintained. The metagray-wackes are considered to represent a regional response to such conditions. The role of serpentinites in glaucophane-schist metamorphism is discussed in terms of a tentatively proposed model: — In very deep levels — perhaps at depths as great as 30 km, bodies of hot ultramafic magma develop restricted aureoles' in which temperatures of 400–600° C are maintained fer perhaps 100–1000 years. The products of metamorphism, which also involves desilication under the influence of the ultramafic magma, are eclogite and amphibolite. Later, and perhaps at higher levels serpentinization of the now solid ultramafic masses (near 400° C), causes renewed metamorphism at lower grades. Marginal development of glaucophane Schists and prehnite and hydrogarnet rocks, and retrogressive alteration of eclogite and amphibolite to glaucophane-schist assemblages is attributed to this period.

Notes: Abstract Peak metamorphic temperatures for the coesite-pyrope-bearing whiteschists from the Dora Maira Massif, western Alps were determined with oxygen isotope thermometry. The δ18O(smow) values of the quartz (after coesite) (δ18O=8.1 to 8.6‰, n=6), phengite (6.2 to 6.4‰, n=3), kyanite (6.1‰, n=2), garnet (5.5 to 5.8‰, n=9), ellenbergerite (6.3‰, n=1) and rutile (3.3 to 3.6‰, n=3) reflect isotopic equilibrium. Temperature estimates based on quartz-garnet-rutile fractionation are 700–750 °C. Minimum pressures are 31–32 kb based on the pressure-sensitive reaction pyrope + coesite = kyanite + enstatite. In order to stabilize pyrope and coesite by the temperature-sensitive dehydration reaction talc+kyanite=pyrope+coesite+H2O, the a(H2O) must be reduced to 0.4–0.75 at 700–750 °C. The reduced a(H2O) cannot be due to dilution by CO2, as pyrope is not stable at X(CO2)〉0.02 (T=750 °C; P=30 kb). In the absence of a more exotic fluid diluent (e.g. CH4 or N2), a melt phase is required. Granite solidus temperatures are ∼680 °C/30 kb at a(H2O)=1.0 and are calculated to be ∼70°C higher at a(H2O)=0.7, consistent with this hypothesis. Kyanite-jadeite-quartz bands may represent a relict melt phase. Peak P-T-f(H2O) estimates for the whiteschist are 34±2 kb, 700–750 °C and 0.4–0.75. The oxygen isotope fractionation between quartz (δ18O=11.6‰) and garnet (δ18O=8.7‰) in the surrounding orthognesiss is identical to that in the coesitebearing unit, suggesting that the two units shared a common, final metamorphic history. Hydrogen isotope measurements were made on primary talc and phengite (δD(SMOW)=-27 to-32‰), on secondary talc and chlorite rite after pyrope (δD=-39 to -44‰) and on the surrounding biotite (δD=-64‰) and phengite (δD=-44‰) gneiss. All phases appear to be in nearequilibrium. The very high δD values for the primary hydrous phases is consistent with an initial oceanicderived/connate fluid source. The fluid source for the retrograde talc+chlorite after pyrope may be fluids evolved locally during retrograde melt crystallization. The similar δD, but dissimilar δ18O values of the coesite bearing whiteschists and hosting orthogneiss suggest that the two were in hydrogen isotope equilibrium, but not oxygen isotope equilibrium. The unusual hydrogen and oxygen isotope compositions of the coesite-bearing unit can be explained as the result of metasomatism from slab-derived fluids at depth.

Notes: Abstract Based on lithological, structural and geophysical characteristics, the Proterozoic Grenville Orogen of southern Ontario and New York has been divided into domains that are separated from each other by ductile shear zones. In order to constrain the timing of metamorphism, U-Pb ages were determined on metamorphic and igneous sphenes from marbles, calc-silicate gneisses, amphibolites, granitoids, skarns and pegmatites. In addition, U-Pb ages were obtained for monazites from metapelites and for a rutile from an amphibolite. These mineral ages constrain the timing of mineral growth, the duration of metamorphism and the cooling history of the different domains that make up the southern part of the exposed Grenville Orogen. Based on the ages from metamorphic minerals, regional and contact metamorphism occurred in the following intervals: Central Granulite Terrane: Adirondack Highlands: 1150 Ma; 1070–1050 Ma; 1030–1000 Ma Central Metasedimentary Belt: Adirondack Lowlands 1170–1130 Ma Frontenac domain 1175–1150 Ma Sharbot Lake domain ca. 1152 Ma Flzevir domain: 1240 Ma; 1060–1020 Ma Bancroft domain: ca. 1150 Ma; 1045–1030 Ma Central Gneiss Belt: ca. 1450 Ma; ca. 1150 Ma; 1100–1050 Ma Grenville Front Tectonic Zone ca. 1000 Ma. Combination of mineral ages with results from thermobarometry indicates that metamorphic pressures and temperatures recorded by thermobarometers were reached polychronously in the different domains that are separated by major shear zones. Some of these shear zones such as the Robertson Lake shear zone and the Carthage-Colton shear zone represent major tectonic boundaries. The Grenville Orogen is made up of a collage of crustal terranes that have distinct thermal and tectonic histories and that were accreted laterally by tectonic processes analogous to those observed along modern active continental margins. The subsequent history of the orogen is characterized by slow time-integrated cooling rates of 3±1°C/Ma and denudation rates of 120±40m/Ma.

Notes: Abstract Sodic amphiboles in high pressure and ultra-high pressure (UHP) metamorphic rocks are complex solid solutions in the system Na2O–MgO–Al2O3–SiO2–H2O (NMASH) whose compositions vary with pressure and temperature. We conducted piston-cylinder experiments at 20–30 kbar and 700–800 °C to investigate the stability and compositional variations of sodic amphiboles, based on the reaction glaucophane=2jadeite+talc, by using the starting assemblage of natural glaucophane, talc and quartz, with synthetic jadeite. A close approach to equilibrium was achieved by performing compositional reversals, by evaluating compositional changes with time, and by suppressing the formation of Na-phyllosilicates. STEM observations show that the abundance of wide-chain structures in the synthetic amphiboles is low. An important feature of sodic amphibole in the NMASH system is that the assemblage jadeite–talc ± quartz does not fix its composition at glaucophane. This is because other amphibole species such as cummingtonite (Cm), nyböite (Nyb), Al–Na-cummingtonite (Al–Na-Cm) and sodium anthophyllite (Na-Anth) are also buffered via the model reactions: 3cummingtonite + 4quartz + 4H2O=7talc, nyböite + 3quartz=3jadeite + talc, 3Al–Na-cummingtonite + 11quartz + 2H2O=6jadeite + 5talc, and 3 sodium anthophyllite + 13quartz + 4H2O=3 jadeite + 7talc. We observed that at all pressures and temperatures investigated, the compositions of newly grown amphiboles deviate significantly from stoichiometric glaucophane due to varying substitutions of AlIV for Si, Mg on the M(4) site, and Na on the A-site. The deviation can be described chiefly by two compositional vectors: [NaAAlIV]〈=〉[□ASi] (edenite) toward nyböite, and [Na(M4)AlVI]〈=〉[Mg(M4)MgVI] toward cummingtonite. The extent of nyböite and cummingtonite substitution increases with temperature and decreases with pressure in the experiments. Similar compositional variations occur in sodic amphiboles from UHP rocks. The experimentally calibrated compositional changes therefore may prove useful for thermobarometric applications.

Notes: Abstract The oxygen isotope ratios of various minerals were measured in a granulite-grade iron formation in the Wind River Range, Wyoming. Estimates of temperature and pressure for the terrane using well calibrated geothermometers and geobarometers are 730±50° C and 5.5±0.5 kbar. The mineral constraints on fluid compositions in the iron formation during retrogression require either very CO2-rich fluids or no fluid at all. In the iron formation, isotopic temperature estimates from quartz-magnetite fractionations are controlled by the proximity to the enclosing granitic gneiss, and range from 500° C (Δ qz − mt=10.0‰) within 2–3 meters of the orthogneiss contact to 600° C (Δ qz − mt=8.0‰) farther from the contact. Temperature estimates from other isotopic thermometers are in good agreement with those derived from the quartz-magnetite fractionations. During prograde metamorphism, the isotopic composition of the iron formation was lowered by the infiltration of an external fluid. Equilibrium was achieved over tens of meters. Closed-system retrograde exchange is consistent with the nearly constant whole-rock δ 18Owr value of 8.0±0.6‰. The greater Δ qz-mt values in the iron formation near the orthogneiss contact are most likely due to a lower oxygen blocking temperature related to greater exchange-ability of deformed minerals at the contact. Cooling rates required to preserve the quartz-magnetite fractionations in the central portion of the iron formation are unreasonably high (∼800° C/Ma). In order to preserve the 600° C isotopic temperature, the diffusion coefficient D (for α-quartz) should be two orders of magnitude lower than the experimentally determined value of 2.5×10−16 cm2/s at 833 K. There are no values for the activation energy (Q) and pre-exponential diffusion coefficient (D 0), consistent with the experimentally determined values, that will result in reasonable cooling rates for the Wind River iron formation. The discrepancy between the diffusion coefficient inferred from the Wind River terrane and that measured experimentally is almost certainly due to the enhancement of exchange by the presence of water in the laboratory experiments. Cooling rate estimates were also determined for iron formation retrograded under water-rich conditions. Application of the experimentally determined data to these rocks results in a reasonable cooling rate estimate, supporting the conclusion that the presence of water greatly enhances oxygen diffusion.

Notes: Abstract Högbomite has generally been considered to be a rare accessory phase in metamorphic rocks. While investigating high-grade peraluminous metamorphites in the Benson Mine District, Adirondack Mountains, New York and the Manitouwadge Massive Sulfide District, Ontario, Canada, we have found several högbomite occurrences and believe that högbomite is more widespread in high-grade aluminous rocks than previously recognized. At Benson Mine, an iron-rich högbomite (Hög) occurs with K-feldspar-magnetite (Mt)-ilmenite (Ilm)-biotite-almandine (Alm)-sillimanite (Sil)-quartz (Qz)-hercynite (Hc)-corundum (Cor)-rutile (Ru). At Manitouwadge, Fe -Zn högbomite is found with gedrite-cordierite-staurolite-hercynite-magnetite±quartz ±ilmenite±rutile±biotite±cassiterite. Because composition varies with structure type, it is essential to determine the structure of högbomite utilized in specific reactions. Högbomite from Benson Mine has an 8H structure type, while that at Manitouwadge has a complex mixed structure. Both are more iron-rich than previously reported högbomites, and their composition can be approximated by the ideal formula Fe5Al16TiO30(OH)2. Proposed reactions for 8H-högbomite are Hög=Ilm+Hc+Cor+V, Hög=Ru+Hc+Cor+V, Hög+Ru=Ilm+Cor+V, and Hög+Ilm=Ru+Hc+V. These reactions can be combined with the experimentally determined reactions Alm+Sil=Hc+Qz and Ru+Alm=Ilm+Sil+Qz to derive reactions in the system FeO-Al2O3-TiO2-SiO2-H2O that limit the stability of the assemblages Hög+Alm and Hög+Sil. Oxidation-sulfidation reactions define a wedge-shaped stability field for högbomite that is closed on the high f S2 side.

Notes: Abstract Omphacite is a common mineral in greenstones, metasediments and related Franciscan rocks of the glaucophane schist facies. It also occurs in late veins cutting amphibolites, glaucophane schists, eclogites, greenstones, and occasionally metagraywackes. It is apparent that this mineral is stable under glaucophane schist facies conditions in rocks of a suitable bulk composition, and is not restricted to the eclogite facies. Association with albite, quartz and lawsonite, and late veining of omphacite veins by aragonite indicates that pressures necessary to form omphacite are reasonably close to those calculated from an ideal solution model.

Notes: Abstract Recently published thermodynamic and experimental data in a variety of chemical systems have been evaluated to derive Gibbs free energies for hedenbergite and pyrope. These were used to calculate the geobarometric equilibria Hedenbergite+Anorthite=Grossular+Almandine +Quartz: “HD barometer”, Diopside+Anorthite=Grossular+Pyrope+Quartz: “DI barometer”. We have compared the pressures obtained from these equilibria for garnet-clinopyroxene-orthopyroxene-plagioclasequartz assemblages with the geobarometer Ferrosilite+Anorthite=Almandine+Grossular+Quartz: “FS barometer”. Pressures calculated for 68 samples containing the above assemblage from a variety of high grade metamorphic terranes indicate that, in general, the HD and DI barometers yield values that are in good agreement with the FS barometer, and that the three barometers are generally consistent with constraints from aluminosilicate occurrences. However, in some samples the HD barometer yields pressures up to 2 kbar greater than constraints imposed by the presence of an aluminosilicate phase. Relative to the FS barometer, the HD barometer overestimates pressure by an average of 0.2±1.0 (1σ) kbar and the DI barometer underestimates pressure by an average of 0.6±1.6 (1σ) kbar. The pressure discrepancies for the HD and DI barometers are likely to be a result of imprecision in thermodynamic data and activity models for silicates, and not a result of resetting of the clinopyroxene equilibria. The relative imprecision of the DI barometer relative to the FS barometer results from overestimates of pressure by the DI and FS barometers in Fe-rich and Mg-rich systems, respectively. Application of the HD and DI barometers to high grade Cpx-Gt-Pg-Qz assemblages yields pressures that are generally consistent with other petrologic constraints and geobarometers. It is concluded that the HD and DI barometers can place reasonable constraints on pressure (±1 kbar relative to the FS barometer) if not extrapolated to mineral assemblages whose compositions are extremely far removed from the end member system for which the barometers were calibrated.

Notes: Abstract The oxygen isotope compositions of coesite, sanidine, kyanite, clinopyroxene and garnet were measured in an ultra-high pressure-temperature grospydite from the Roberts Victor kimberlite, South Africa. The δ18O values (per mil v. SMOW) of each phase and (1 σ) are as follows: coesite, 8.62 (0.31); sanidine, 8.31 (0.02); kyanite, 7.98 (0.08); pyroxene, 7.63 (0.11); garnet, 7.53 (0.03). In situ analyses of the coesite with the laser extraction system are δ18O=9.35 (0.08), n=4, demonstrating that the coesite is homogeneous. The coesite has partially inverted to polycrystalline quartz and the pyroxene is extensively altered during uplift. The larger scatter for the mineral separate coesite and pyroxene data may be due to partial reequilibration between the decompression-related breakdown products of these two phases. The anomalously high δ18O value of the grospydite (δ18Owholerock=7.7‰) is consistent with altered oceanic crust as a source rock. Temperature estimates from a linear regression of all the data to three different published calibrations correspond to an equilibrium temperature of 1310±80°C. The calculated isotopic pressure effect is to lower these estimates by about 40°C at 40 kb. The estimated temperature based on Al−Si disorder in sanidine is 1200±100°C and that from Fe−Mg exchange thermometry between garnet and clinopyroxene is 1100±50°C. Given the large errors associated with thermometry at such high temperatures, it is concluded that the xenolith equilibrated that 1200±100°C. Pressure estimates are 45±5 kb, based on dilution of the univariant equilibria albite = jadeite + coesite and 2 kyanite + 3 diopside = grossular + pyrope + 2coesite. Zoning in the outer 20 μm of the feldspar from Ab0.8 to Ab16 indicates rapid decompression to 25 kb or less. The isotopic temperature estimates are the highest ever obtained and combined with the high degree of Al−Si disorder in sanidine require rapid cooling from ultra-high temperatures. It is inferred that the xenolith was sampled at the time of equilibration, providing a point on the upper Cretaceous geotherm in the mantle below South Africa.

Notes: Abstract Pyrope and quartz are stable with respect to aluminous enstatite and sillimanite at 1400 °C, 20 kb and at 1100 °C, 16 kb. The phase boundary limiting the coexistence of pyrope and quartz towards lower pressures is probably slightly curved. A slope of 15 bars/°C at 1400° and of 10 bars/°C at 1000 °C has been estimated from the experimental data. Between 1050 and 1100 °C the curve is intersected by the kyanite-sillimanite phase boundary. The calculated slope of the reaction aluminous enstatite + kyanite ⇌ pyrope + quartz is negative (ca. 18–25 bars/°C). The existence of a negative slope has been demonstrated experimentally. Experimental evidence indicates that the assemblage aluminous enstatite and sillimanite is metastable with respect to sapphirine + quartz at high temperature. The invariant point involving the phases pyrope-sapphirine-aluminous enstatite-sillimanite-quartz is estimated to occur at 1125°±25 °C and 16±1 kb. A model phase diagram for the silicasaturated part of the system MgO-Al2O3-SiO2 has been constructed. The position of three invariant points in this system has been estimated on the basis of presently available data.