ALBERT

Notes: Abstract Electron diffraction and electron microscopic evidence is presented for a dynamical and reversible $$P\bar 1 - I\bar 1$$ phase transition in anorthite at T c=516 K. Antiphase boundaries with a displacement vector, R=1/2[111] become unstable at T c, while other antiphase boundary loops with the same displacement vector are formed. These interfaces are very mobile and vibrate with a frequency which increases strongly with temperature. At temperatures considerably above T c, a shimmering effect is observed on imaging in dark field using diffuse c reflections. These observations are in agreement with the interpretation of the high temperature body-centered phase as a statistical dynamical average of very small c type antiphase domains of primitive anorthite. We propose that the c type antiphase domains in primitive anorthite originate from ordered and anti-ordered configurations around Ca2+ ions at (ooo) and (oio) [likewise (zoo) and (zio)] positions. The dynamical model for the transition involves a two-stage mechanism: a softmode mechanism causing the aluminosilicate framework to approach body-centered symmetry, followed by an orderdisorder of the Ca2+ ion configurations. Close to T c, statistical fluctuations set in and breathing motion type lattice vibrations of the aluminosilicate framework cause the configurations around Ca (ooo) and Ca(oio) [likewise Ca(zoo) and Ca(zio)] in the $$P\bar 1$$ configuration to dynamically interchange through an intermediate $$I\bar 1$$ configuration. The dynamical nature of the phase transition in anorthite is comparable to the α — β phase transition in quartz.

Notes: Abstract The products of the transformation of natural (Mg0.83Fe0.17)2SiO4 olivine have been prepared at various high pressures (between 25 GPa and 90 GPa), and high temperature in a laser-heated diamond-anvil cell (DAC). Studies of the high-pressure phases have been made by transmission electron microscopy (TEM), and X-ray microanalysis. The olivine/spinel boundaries exhibit all the characteristics of a diffusionless shear transition, having a finely sheared structure and a constant orientation relationship between the close-packed planes of the two structures ((100)ol∥(111)sp). The TEM observations of zones where olivine (or spinel) transforms into post-spinel phases show that the transformation possesses the features of an eutectoïdal decomposition, leading to a lamellar intergrowth of magnesiowüstite (Mg,Fe)O and perovskite (Mg,Fe)SiO3. With increasing temperature and/or decreasing pressure, the grain size of the high-pressure phases increases and obeys an Arrhenius law with an activation volume equal to zero. (Mg,Fe)O grains exhibit a very high density of dislocations (higher than 1011cm−2), whereas (Mg,Fe)SiO3 grains exhibit no dislocations but systematic twinning. The composition plane of the twins is (112) of the GdFeO3-type perovskite, corresponding to the {110} plane of the cubic lattice of ideal perovskite.

Notes: Abstract The theoretical variation law for the (saturation-free) intensity ratio of a paramagnetic doublet is written in a way which allows one to check easily whether or not experimental data which are intended to be fitted by a (saturation-free) theoretical spectrum or subspectrum follow actually this law. The expression of this law is still simplified by taking the absorber symmetries into account. This is illustrated by two examples: a sheet-silicate mosaic and an olivine single-crystal.

Notes: Abstract The paper reports an attempt to study the topologies of the phase diagram for the MgO-SiO2 system at high pressure and temperature using computer simulation. Phase equilibria at MgSiO3 stoichiometry is investigated, demonstrating that the invariant point Gr+Ilm+Pv is stable at 21.6 GPa and 2270 K. A thermodynamic data base for minerals in the MgO-SiO2 system is established by supplementing the calorimetric data for low pressure phases and equations of state for low and high pressure phases with data calculated from high pressure synthesis experiments. A refined set of standard free energies of formation and phase transformations in the MgO-SiO2 system is presented. The proposed phase diagram covers a wide range of pressure (up to 25 GPa) and temperature (up to 2500 K) and forms the basis for a geochemical interpretation of the nature of seismic discontinuites in the mantle.

Notes: Abstract The enthalpies of solution in molten 2PbO · B2O3 of phases synthesized at one atmosphere in the system MgGa2O4-Mg2GeO4 have been measured. A spinel solid solution, which is stable at 1400 °C from the MgGa2O4 end-member to 27 mole percent Mg2GeO4, shows endothermic heats of mixing of up to 10 kJ/mole at the solubility limit. The spinelloid phase, Mg3Ga2GeO8, is energetically less stable than a mixture of terminal spinel solid solutions (0.73 MgGa2O4·0.27 Mg2GeO4(sp)+Mg2GeO4(sp)), by 3.63±3.64 kJ/mole. This indicates that the spinelloid is a high-entropy phase. The volume of the spinel solid solution, MgGa2O4-Mg2GeO4, shows a positive deviation from Vegard's Law. Modeling of the cation distribution in the solid solution indicates that this ΔV is due to a change in the spinel type from inverse towards normal as the Mg2GeO4 content increases.

Notes: Abstract Recent transmission electron microscopy (TEM) electron diffraction investigation of β-cristobalite has revealed a strong characteristic diffuse intensity distribution. In this work a similar, but less intense, diffuse scattering is reported for synthetic α-cristobalite. This material also displays both tetragonal twinning and characteristic striations correspond to planar boundaries which may exist on any {1, 0, l}tetragonal plane.

Notes: Abstract The concept of short-range interlayer interactions, fundamental to spin-analogue models for polytypism, is examined in the case of sapphirine. Consideration of interactions out to fourth-nearest neighbours provides a rationale for the difference between the polytype suites observed for sapphirine and wollastonite. In each case, the observed long-period structures are consistent with those predicted to be stable by the appropriate mappings onto the axial next-nearest neighbour model. Short-range interaction parameters may also be used to express stacking fault energies. This approach, combined with a simple nucleation-andgrowth model, is used to examine the possibility of metastable generation of complex polytypes in sapphirine. Statistical analysis of defect distributions and frequencies in sapphirine suggests that interactions over several hundred Ångstroms must be considered if the stacking energetics are to be accurately modelled.

Notes: Abstract The vibrational frequencies of a series of splatquenched, olivine glasses spanning the compositional range from Mg2SiO4 to Mn2SiO4 have been determined using both infrared and Raman spectroscopies. The spectra of all glasses show evidence of tetrahedral coordination of silicon (possibly with some slight distortions), and largely octahedral coordination of magnesium. Spectra of Mn-rich glasses indicate that there is some manganese in 4 or 5-fold coordination. The frequencies observed for the fundamental vibrations of the silica tetrahedra are similar to those previously observed for SiO4 groups in both crystalline and glassy orthosilicates. Additionally, there is evidence for a small amount of silicate polymerization in all glasses characterized: vibrations attributable to Si2O7 groups are visible in both infrared and Raman spectra.

Notes: Abstract Electron paramagnetic resonance (EPR) measurements of natural barite BaSO4 have revealed the presence of PO 2 2− and SO 2 − ionic radicals. For the latter a hyperfine structure from the 33S isotope has been detected and measured for the first time. The temperature dependence of the hyperfine interaction (HFI) constant of the PO 2 2− and SO 2 − radicals has been investigated in the 100–400 K range.

Notes: Abstract Trivalent iron is known to occur in amethysts in three different environments: substitutional with an as yet unidentified alkali ion in an adjacent interstitial site on the twofold axis (so-called S 1 center), substitutional with a proton on one of the four oxygen neighbors (S 2 center) and without detectable charge compensation (so-called I center because it was proposed to be in interstitial sites, but its nature is still disputed). The alkali ion in the S 1 center was now identified as Li and the Li and Fe superhyperfine (shf) tensors were determined by electron paramagnetic resonance at low temperatures in samples of unusually high quality. The fine structure and 1H shf structure tensors for the S 2 center with the proton on an oxygen with longer bond distance were evaluated and the geometry of the oxygen coordination was estimated by application of the superposition model. For the I center the 57Fe and 29Si shf splittings were determined and the evidence in favor of assignment to an interstitial site is reviewed.

Notes: Abstract The formulas for thermodynamic functions for minerals are presented, couched in terms of the important thermodynamic variable αKT= (ϱP/ϱT)v, where α is the volume thermal expansivity and KT is the isothermal bulk modulus. Presenting the formulas in this way leads to simplification since αKT as a product varies only slightly with volume, and is close to being independent of temperature at high temperature. Using our equations, we present as examples some computed data in the form of graphs on the entropy, internal energy, Helmholtz free energy, and Gibbs free energy in the high temperature regime (up to 2000 K) and for high compression (up to 0.7), for MgO. For entropy, knowledge of the V, T dependence of αKT is sufficient. For enthalpy and internal energy, the equation of state is needed in addition.

Notes: Abstract The timescale of structural relaxation in a silicate melt defines the transition from liquid (relaxed) to glassy (unrelaxed) behavior. Structural relaxation in silicate melts can be described by a relaxation time, τ, consistent with the observation that the timescales of both volume and shear relaxation are of the same order of magnitude. The onset of significantly unrelaxed behavior occurs 2 log10 units of time above τ. In the case of shear relaxation, the relaxation time can be quantified using the Maxwell relationship for a viscoelastic material; τS = ηS/G ∞ (where τS is the shear relaxation time, G ∞ is the shear modulus at infinite frequency and ηS is the zero frequency shear viscosity). The value of G ∞ known for SiO2 and several other silicate glasses. The shear modulus, G ∞, and the bulk modulus, K ∞, are similar in magnitude for every glass, with both moduli being relatively insensitive to changes in temperature and composition. In contrast, the shear viscosity of silicate melts ranges over at least ten orders of magnitude, with composition at fixed temperature, and with temperature at fixed composition. Therefore, relative to ηS, G ∞ may be considered a constant (independent of composition and temperature) and the value of ηS, the relaxation time, may be estimated directly for the large number of silicate melts for which the shear viscosity is known. For silicate melts, the relaxation times calculated from the Maxwell relationship agree well with available data for the onset of the frequency-dependence (dispersion) of acoustic velocities, the onset of non-Newtonian viscosities, the scan-rate dependence of the calorimetric glass transition, with the timescale of an oxygen diffusive jump and with the Si-O bond exchange frequency obtained from 29Si NMR studies. Using data obtained over a range of frequencies and strain-rates we illustrate the significance of relaxed versus unrelaxed behavior in laboratory experiments on silicate melts. Similarly, using strain-rate estimates for magmatic processes we evaluate the significance of the liquid-glass transition in igneous petrogenesis.

Notes: Abstract The article sets out the main tasks facing researchers studying RTL dating and suggests various ways of accomplishing them, that have been developed in the departments of radiochemistry and geomorphology at Moscow State University. The questions under discussion are the formation of the age RTL lightsum in sedimentary rocks and its functional connection with dose-rate and zeroing of paleodosimeters. Methods of RTL dating developed and used by the authors are discussed in detail. The version the RTL method suggested is illustrated by the data concerning the age of the Dniestr terraces. The investigations lead to the conclusion that natural quartz can be employed as a thermoluminescence paleodosimeter for the determination of ages ranging from tens of thousands to 1–2 million years by using the high temperature peaks of the glow curve.

Notes: Abstract Crystal field spectra have been obtained from K2Co2(SO4)3 at a number of temperatures from 20 K to room temperature. The transition to the 4 T 1g(P) excited state is found to be split at all temperatures owing to the trigonal distortion of the Co site. Below the P213-P212121 phase transition temperature an excess splitting is observed owing to the additional distortion of the site in the orthorhombic phase. This excess distortion is found to be a linear function of temperature with no first order step at T c. Thus on a local scale the transition appears to be second order. The trigonal splitting increases with increasing temperature above Tc; this is not expected from the Speer-Salje model of the transition mechanism, which predicted decreasing distortion of the oxygen octahedra with increasing temperature. We propose that the apparent increase of asymmetry is due to the off centring of the Co atom in the high temperature phase in a more regular environment.

Notes: Abstract Dynamics in minerals at time scales from seconds to microseconds are important in understanding mechanisms of displacive phase transitions, diffusion, and conductivity. High resolution, magic-angle-spinning (MAS) NMR spectroscopy can directly show the rates of exchange among sites, potentially providing less model-dependent information than more traditional NMR relaxation time measurements. Here we use a newly developed high temperature MAS probe (Doty Scientific, Inc.) to observe the exchange of Na+ among the alkali sites in a high-Na nepheline at temperatures as high as 500° C. Observed exchange rates are consistent with correlation times derived from cation diffusivity.

Notes: Abstract The thermoelastic analysis of the possible stable stress-free equilibrium states of growth twins is shown to be the basis for a new technique of geobarothermometry. The detailed study of a cyclic cross-shaped twin in low quartz exemplifies the procedure. To explicitly determine the barothermometric function, the simultaneous dependence upon the external pressure and temperature of the lattice-parameter ratios c/c 0 and a/a 0 of quartz has been calculated; we present a five-parameter, temperature-corrected version of the equations proposed by Thurston (1967) for extrapolating to high pressures the behavior of lattice parameters of crystalline solids in isothermal compression.

Notes: Abstract The electric field gradient (EFG) tensors of 7Li and 23Na in a zektzerite, NaLiZrSi6O15 single crystal have been determined by the nuclear magnetic resonance technique. The quadrupole coupling constants, e2qQ/h and the asymmetry parameters, η, for 7Li are 119 ± 5 kHz, 0.05 and for 23Na 2.42 ± 0.15 MHz, 0.86 respectively. In both cases, the EFG tensors show the two-fold site-symmetries at the Li- and Na-sites. For 7Li, the magnitude of the quadrupole coupling constant and the principal axis of the EFG tensor reflect the large angular distortion of the [LiO4] tetrahedron. The large quadrupole coupling constant of 23Na indicates a high degree of distortion of the Na coordination polyhedron.

Notes: Abstract The dielectric constants and dielectric loss values of BeAl2O4 (chrysoberyl), MgAl2O4 (spinel), Be2SiO4 (phenacite), and Mg2SiO4 (forsterite) were measured at 1 MHz using a two-terminal method and empirically determined edge corrections. The results are: chrysoberyl, κ′ a =9.436, κ′ b =9.071, κ′ c =8.269; spinel, κ′ a =8.18; phenacite, κ′ a =6.28, κ′ c =6.06; and forsterite, κ′ a =6.867, κ′ b =7.392, κ′ c =6.739. The agreement between measured dielectric polarizabilities as determined from the Clausius-Mosotti equation and those calculated from the sum of oxide polarizabilities according to αD(M2M′X4) = 2αD(MX)+αD(M′X2) is ~ 1.0%.

Notes: Abstract 29Si MAS NMR spectra of synthetic akermanite, Ca2MgSi2O7, exhibit a splitting into three or more lines in the stability feld of the low-temperature incommensurate phase. With increasing temperature, these lines continuously merge into a single line which is then characteristic of the high-temperature, melilite-type structure. The data point to the non-equivalence of Si sites in the low-temperature phase and the gradual structural changes as the phase transition temperature is approached.

Notes: Abstract Phase transformations in a natural sample of hedenbergite ((Ca0.93Fe0.61Mn0.34Mg0.08Na0.01Zn0.02Al0.003)Si2O6) have been studied by X-ray diffraction up to 40 GPa at ∼ 1200°C in a diamond anvil cell interfaced with a laser heating system. The starting hedenbergite phase decomposed into a garnet plus γ-spinel and stishovite at ∼ 14 GPa; then into garnet plus stishovite and wüstite at ∼ 18 GPa; and finally into perovskite plus stishovite and wüstite at pressures higher than ∼ 24 GPa. On decompression to 0.1 MPa, all the high pressure phases are retained except for the cubic perovskite, which reverts back into the ɛ-CaSiO3 phase, in accordance with previous reports. Energy-dispersive SEM analyses show that the garnet is present as a calcium-rich ABO 3-type phase. As no garnet phase has been previously observed either in pure CaSiO3 or in pure CaMgSi2O6, it appears that the observed calcium-rich garnet phase has been stabilized by the presence of other cations such as the Na+, Zn2+, Mn2+, Fe2+, Mn3+, Fe3+ and Al3+.

Notes: Abstract The enthalpies of transition at T= 298 K between zinc metasilicate assemblages, measured by molten oxide solution calorimetry, are: $$\begin{gathered} {1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2} Zn_2 SiO_4 (phen) + {1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2} SiO_2 (qz) \to ZnSiO_3 (cpx) \hfill \\ \Delta H = 10.4 \pm 2.4 kJ/mol \hfill \\ \end{gathered} $$ (1) 2 $$\begin{gathered} {\text{ZnSiO}}_{\text{3}} {\text{(cpx)}} \to {\text{ZnSiO}}_{\text{3}} {\text{(ilm)}} \hfill \\ \Delta H = {\text{52}}{\text{.4}} \pm {\text{2}}{\text{.7kJ/mol}} \hfill \\ \end{gathered}$$ (2) For reaction (1), the measured enthalpy is lower than the value 15.7±2.0 kJ/mol from the phase boundary of Syono et al. The P-T slope derived from the calorimetric data has a somewhat greater value than that of the line chosen by Syono et al., but is still consistent with the individual data points in that study. For reaction (2), the measured enthalpy of transformation is in close agreement with the value 54.0±5.1 kJ/mol estimated from the phase boundary of Akimoto et al, and with the value 50.9±3.2 kJ/mol from the phase boundary of Ohtani, which is parallel to and 0.6 GPa lower than Akimoto's. All estimates assume a pressure-independent volume of transition. For the zinc metagermanate composition, measured heats of transformation are: 3 $$\begin{gathered} {\text{1/2 Zn}}_{\text{2}} {\text{GeO}}_{\text{4}} {\text{(phen) + 1/2 GeO}}_{\text{2}} {\text{(qz)}} \hfill \\ \to {\text{1/2 Zn}}_{\text{2}} {\text{GeO}}_{\text{4}} {\text{(dist sp) + 1/2 GeO}}_{\text{2}} {\text{(rutile)}} \hfill \\ \Delta H = - {\text{17}}{\text{.7}} \pm {\text{3}}{\text{.8 kJ/mol}} \hfill \\ \end{gathered}$$ (3) 4 $$\begin{gathered} {\text{1/2 Zn}}_{\text{2}} {\text{GeO}}_{\text{4}} {\text{(dist sp) + 1/2 GeO}}_{\text{2}} {\text{(rutile)}} \to {\text{ZnGeO}}_{\text{3}} {\text{(lim)}} \hfill \\ \Delta H = {\text{14}}{\text{.3}} \pm {\text{5}}{\text{.9 kJ/mol}} \hfill \\ \end{gathered}$$ (4) Raman and infrared spectra of ZnSiO3 clinopyroxene and ilmenite are presented. The entropy change for the clinopyroxene → ilmenite transiton is calculated from the spectra and estimates for the directionally-averaged acoustic velocities, using the Kieffer model for the vibrational density of states. The value ΔS = −12±5 J/mol·K, calculated using a variety of plausible densities of states, agrees with the value from phase equilibria, ΔS = −12.7±6.0 J/mol·K.

Notes: Abstract The uptake of water in quartz at 1.5 GPa total pressure, 1173 K and high water fugacity, over times up to 24 h, has been investigated using a newly developed assembly to prevent microcracking. It is found that the uptake is small, and below the detectability of the presently used technique of infrared spectroscopy and serial sectioning. This observation reflects either a low value for the diffusivity or the solubility or a combination of both, and is in agreement with the observations of Kronenberg et al. (1986) and Rovetta et al. (1986). It brings into question the interpretation of the early experiments on water weakening by Griggs and Blacic (1964) and the recent estimates of the solubility and diffusivity by Mackwell and Paterson (1985). Rults of a combined T.E.M., light-scattering and infrared-spectroscopy investigation of ‘wet’ synthetic quartz before and after heating at 0.1, 300 and 1500 MPa total pressure and 1173 K, strongly suggest that the water in ‘wet’ quartz is mainly in the form of H2O in inclusions, consistent with the solubility being low, possibly less than 100 H/106Si. From these observations, water-containing inclusions appear to play a major role in the plasticity of quartz, while any role of water in solid solution remains to be clarified.

Notes: Abstract The adiabatic single-crystal elastic moduli of a beryllium silicate (phenacite: Be2SiO4, trigonal, have been determined at atmospheric pressure and 22° C by Brillouin spectroscopy. The elastic stiffness moduli in gigapascals are: C 11=341.9 C 33=391.0 C 44=91.4 C 66= 96.9 C 12=148.0 C 13=136.0 C 14= 0.1 C 15= 3.5 Overall, the elastic stiffness moduli for phenacite parallel and perpendicular to the c axis are comparable (i.e., it is almost cubic in its elastic “signature”). The elastic moduli can be rationalized in terms of division of the structure into two types of coordination polyhedra (1Si+2Be) with slightly different stiffnesses, which are linked to form a three dimensional framework. Values of the isothermal bulk modulus and the linear compressibilities, as determined from hydrostatic compression experiments of Hazen and Au (1986), are in good agreement with those obtained here. Combining the two studies indicates a low pressure derivative of the bulk modulus for phenacite.

Notes: Abstract A detailed evaluation of the assignments given to the infrared (IR) vibrations in the lattice stretching region is presented here based on observations of the effects of various chemical substitutions in synthetic analogues of phlogopite, KMg3(AlSi3)O10(OH)2. As in previous studies, this study has confirmed that the 995, 960, and 460 cm−1 vibrations are influenced by Si, the 822 and 760 cm−1 vibrations by Al, the 915 and 725 cm−1 vibrations by Al and Si, and the 592 cm−1 vibration by OH. Contrary to previous studies, it is shown here that the 690, 495, and 375 cm−1 vibrations are strongly linked with Mg and not just Si. The 655 cm−1 band in phlogopite is attributed to an in-plane Al-O vibration rather than an Al-O-Si vibration. As a check on the band assignments made here, IR spectra were obtained for synthetic clintonite, CaMg2Al(Al3Si)O10(OH)2, as well as its chemical analogues and compared with the IR spectrum of phlogopite. The band intensities for the Si-O, Al-O, and Si-O-Mg vibrations changed in accord with the composition of clintonite. The most intense band in clintonite at 660 cm−1 appears to be associated only with Al and is assigned here to a tetrahedral Al-O-Al vibration which must be present, if not dominant, in this mineral. The near coincidence of an in-plane Al-O vibration at 655 cm−1 (phlogopite) and an in-plane Al-O-Al vibration at 660 cm−1 (clintonite) makes the identification of tetrahedral Al-Si order-disorder in trioctahedral layered silicates by IR spectroscopy very difficult. The ratio of the 822/995 cm−1 bands may, however, prove to be very useful for discerning the amount of tetrahedrally coordinated Al in these types of minerals.

Notes: Abstract New data concerning glaucophane are presented. New high temperature drop calorimetry data from 400 to 800 K are used to constrain the heat capacity at high temperature. Unpublished low temperature calorimetric data are used to estimate entropy up to 900 K. These data, corrected for composition, are fitted for C p and S to the polynomial expressions (J · mol−1 · K−2) for T〉 298.15 K: $$\begin{gathered} C_p = 11.4209 * 10^2 - 40.3212 * 10^2 /T^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} - 41.00068 * 10^6 /T^2 \hfill \\ + 52.1113 * 10^8 /T^3 \hfill \\ \end{gathered} $$ $$\begin{gathered} S = 539 + 11.4209 * 10^2 * \left( {\ln T - \ln 298.15} \right) - 80.6424 * 10^2 \hfill \\ * \left( {T^{ - {1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} - 1/\left( {298.15} \right)^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} } \right) + 20.50034 * 10^6 \hfill \\ * \left( {T^{ - 2} - 1/\left( {298.15} \right)^2 } \right) - 17.3704 * 10^8 * \left( {T^{ - 3} - \left( {1/298.15} \right)^3 } \right) \hfill \\ \end{gathered} $$ IR and Raman spectra from 50 to 3600 cm−1 obtained on glaucophane crystals close to the end member composition are also presented. These spectroscopic data are used with other data (thermal expansion, acoustic velocities etc.) in vibrational modelling. This last method provides an independent way for the determination of the thermodynamic properties (Cp and entropy). The agreement between measured and calculated properties is excellent (less than 2% difference between 100 and 1000 K). It is therefore expected that vibrational modelling could be applied to other amphiboles for which spectroscopic data are available. Finally, the enthalpy of formation of glaucophane is calculated.

Notes: Abstract Crystals of the olivine minerals, tephroite (Mn2SiO4) and fayalite (Fe2SiO4) containing manganese(II) and iron (II and trace of III), respectively, were synthesized. Glasses were prepared from these crystalline materials by a splat-quench technique. Measurement of electron paramagnetic resonance (EPR) of all these powdered samples at room temperature show that the g-factors of Mn2+ in both glassy and crystalline environments (geff = 2.004) are the same, although the EPR linewidths (for glass, ΔHpp = 200 G; for crystals ΔHpp = 287 G) suggest less clustering of paramagnetic Mn2+ ions in the glass. Mn2+ probably occupies a distorted octahedral site in the tephroite crystal structure, although a four-fold coordination is suggested from other spectroscopic investigation on this glass. The EPR parameters of Fe3+ in synthetic fayalite glass (geff = 2.01 and 6.00; ΔHpp=150 and 1375 G, respectively, for the high and low field resonances) and powdered crystals (geff = 3.31 and ΔHpp = 900 G) indicated that Fe3+ ion in the crystals, is probably located in a distorted tetragonal site M2 and an axial environment has been proposed in the glassy system.

Notes: Abstract The elastic moduli of single crystals of pyrope-rich garnet and San Carlos olivine have been measured over a 3 GPa pressure range at room temperature. The combination of improved ultrasonic techniques and this large pressure range provide for more reliable characterization of the pressure dependence of acoustic wave velocities than has previously been possible. First and second pressure derivatives of the velocities have been determined within ∼ 1 percent and ∼ 10 percent respectively. The Hashin-Shtrikman bounds for the pressure dependences of the bulk and shear moduli of the garnet used in this study are; K = 173.6 GPa, K′ = 4.93, K″ = −0.28 GPa−1, G= 94.9 GPa, G′ = 1.56, G″ = −0.08 GPa−1 and the Hashin-Shtrikman least-upper bounds and greatestlower bounds for the pressure dependences of the bulk and shear moduli of the San Carlos olivine are K=129.8 GPa, K′ = 4.66, K″= −0.15 GPa−1, G = 77.8 GPa, G′ = 1.93, G″ = −0.11 GPa−1 and K = 129.2 GPa, K′ = 4.63, K″= −0.15 GPa−1 G = 77.3 GPa, G′=1.96, G″ = −0.11 GPa−1 respectively. The determination of the room-pressure elastic moduli of this pyrope-almandine garnet removes the previously observed anomaly in the predictions of systematic treatments of variations of the elastic moduli of garnets with composition. The determination of the second pressure derivatives of the moduli of garnet and olivine illustrates the importance of these terms in extrapolations to higher pressures — with ϱK/ϱP for these crystals being reduced by ∼ 17 percent and ∼ 9 percent respectively over the 3 GPa pressure range.

Notes: Abstract Lattice parameters, and intensities of selected X-ray reflexions, have been measured as a function of temperature for natural leucite, to characterise the phase transformation behaviour. At low temperatures leucite has a large ferroelastic distortion, but the temperature evolution of lattice parameters cannot be explained in terms of a purely ferroelastic phase transition; in particular, the considerable change in volume with temperature implies an additional transition mechanism, which we correlate with off-centring of K-ions in the low-temperature phase, and a collapse of the 〈111〉 structural channels. The transition behavior can therefore be rationalised in terms of two competing mechanisms: (I) Ferroelastic (consistent with the change m3m→ 4/mmm); (II) Volume-changing (consistent with m3m → 4/m). Coupling of the two order parameters QI, and QII gives rise to the intermediate 4/mmm tetragonal phase. Our results confirm the existence of an I4 1/a→I4 1/acd transition, but the non-disappearance of the 200 reflexion at high temperatures implies that the expected transition from I41/acd to Ia3d (cubic) symmetry does not occur. We attribute this to a residual strain field conjugated to the order parameter, due to defects (with possible Al/Si order). Nevertheless, within our experimental resolution, the lattice becomes metrically cubic at 665° C.

Notes: Abstract Using fused SiO2, CaF2, and SrF2 samples with accurately known dielectric constants, we have evaluated the accuracy and precision of two-terminal dielectric constant measurements on small single crystals using empirically determined edge corrections. Values of κ′ at 1 MHz of 3.836±0.05 for silica, 6.814±0.07 for CaF2 and 6.463±0.09 for SrF2 indicate an accuracy and precision of 1.0–1.5% for samples having areas of 0.05–1.0 cm2. Dielectric constants of BeO, MgO, and CaO measured by this technique are: BeO, κ′a=6.87 and κ′c=7.74; MgO, κ′= 9.90; and CaO, κ′=11.95 where κ′a and κ′c are the dielectric constants parallel to the a and c axes, respectively. Dielectric loss measurements on CaO in vacuum between 5–400 K at 10–105 Hz indicate significant dispersion at temperatures higher than 300 K, but the effect of the losses on the dielectric constant is less than 1% at 1 MHz and 300 K.

Notes: Abstract A study of the ESR signal complex in flints was used to examine its origin within the mineral. One component of the ESR spectrum, known to be a trapped hole, was confirmed to be a structural defect of the silica fraction of flint. The relationship of this center to that seen in natural quartz and glass is discussed. The defect has importance for geochronological direct-dating as it is shown to anneal and regenerate in a systematic manner in flints.

Notes: Abstract A swarm of Caledonian microtonalite sheets is host to W-Sn mineralization in SE Ireland. Many but not all mineralized sheets are within a regional shear zone which, however, does not appear to have been a major control on mineralization. Scheelite and arsenopyrite are the dominant minerals within and adjacent veins in greisened tonalite. No wolframite occurs. The intrusions are affected by two alterations, a pervasive one and another which is spatially associated with veins. Both are associated with high metal values. Veining is confined to microtonalite sheets and was initiated at least partly by hydraulic fracturing of the intrusions. Stable isotope analyses indicate that mineral deposition occurred over narrow time and temperature intervals from a hydrothermal fluid of magmatic origin. Methane is important in fluid inclusions in quartz veins and the low oxygen fugacity of the mineralizing fluid was probably critical in concentrating metals and in suppressing wolframite formation.

Notes: Abstract Some Mediterranean bauxite deposits in karst, considered to be allochthonous originating from neighboring silicate rocks, reveal the following common features of fluvial transport and sedimentation: (1) The SiO2 content increases in the direction of the transport; (2) the trace elements indicate the area of origin; (3) the bauxite was deposited in depressions of a slightly karstified coastal plain in a lagoonal or estuarine environment; (4) the karst relief became accentuated after the bauxite was covered by sediments; and (5) the depth of the karstic depressions was controlled by the former groundwater level. Therefore, the thickness of the orebodies increases with the distance from the former shoreline. These factors have an implication on the strategy of exploration. The following examples are taken from central Greece and Montenegro and comparisons are made with the bauxite districts of France, Italy, and Jamaica.

Notes: Abstract New investigations are carried out on the mineralogy and mineral chemistry of sulfide assemblages obtained in samples from one core in the hydrothermally active, southwest basin of the Atlantis II deep, Red Sea. The most abundant sulfide phases are the exsolved intermediate solid solution (ISS) and chalcopyrite. Sphalerite, pyrrhotite, marcasite, mackinawite, and presumably wurtzite are also observed. Two distinct groups of paragenesis were encountered: (a) Intermediate solid solution with sphalerite incrustations and intergrowths, and (b) intermediate solid solution barren of sphalerite intergrowths. The first group is confined to the upper part of the Co zone and the SOAN zone (Bäcker and Richter 1973), and the second is present in the entire core 100-3-7. An optically isotropic chalcopyrite is found for the first time as a natural mineral in Atlantis II, Red Sea. Yet its existence as a novel phase needs x-ray confirmation. It exhibits a lower reflectivity than normal chalcopyrite and is isotropic. Chalcopyrite occurs either as a single phase or in association with tetragonal chalcopyrite. Our investigations indicate that the formation of Atlantis II deposits is a result of complex processes. These processes are characterized by compositional changes in the ore-bearing fluids and the change in sulfur fugacity (especially with depth). The presence of exsolved chalcopyrite lamellae in ISS indicates slow cooling below 450°C. However, it is difficult to understand why the cubic chalcopyrite is not converted to the tetragonal form even though the temperature of transformation lies above 450°C (470° – 500°C, Cabri 1973). The Cu/Fe ratio changes in the exsolved chalcopyrite lamellae from core to rim of the composite grains. The ratio is higher in the rims. This suggests that primary inhomogenous ISS grains formed from solutions with a continuous increase in the Cu/Fe ratio. Slow cooling is also required to account for the exsolution of chalcopyrite lamellae in ISS. The low sulfur content in isotropic chalcopyrite is also suggestive of low fs2. The low S content in the chalcopyrite may be the controlling factor for the sluggish conversion from cubic to tetragonal chalcopyrite. Mackinawite lamellae show the same orientation in ISS and exsolved isotropic chalcopyrite indicating that mackinawite exsolved before the breakdown of ISS. This strongly suggests that mackinawite is stable above 300°C (contrary to experimental results by Zoka et al. 1973). Pyrrhotite was probably formed by the sulfurization of ilvaite. The pyrrhotite grains with several complex successive zones show the sequence of the sulfurization episodes. Metalliferous sediments related to hot brines were discovered in the Red Sea in 1964 (Miller et al. 1966). Since then, several papers have been published on this subject (Degens and Ross 1969, Bäcker and Schoell 1972, Bäcker and Richter 1973, Bignell et al. 1976, Shanks and Bishoff 1977, Weber-Diefenbach 1977, Nöltner 1979, Pottorf 1980, Pottorf and Barnes 1983, Oudin et al. 1984). Complex sulfide phases including intermediate solid solution (ISS), chalcopyrite, and a chalcopyritelike mineral (which exhibits a lower reflectivity than normal chalcopyrite and appears to be isotropic occur in the metalliferous sediments. These phases were found in association with several minerals in different parageneses. In an attempt to understand the origin of the formation of the sulfide-bearing sediments in the Atlantis II deep of the Red Sea, a detailed study of the phase relations of the Cu-Fe sulfide ores of this locality was carried out.

Notes: Abstract The manganese ores of Falotta and Parsettens (Oberhalbstein, Grison Canton, Switzerland) are enclosed in Upper Jurassic radiolarites and overlay ophiolites of Upper Pennine nappes. These ores exhibit the botryoidal microstructures typical of manganese nodules. The mineralogical components of the outerpart of these nodules, which were affected by alpine metamorphism — were first transformed gradually into a poorly crystallized braunite (with a large excess of SiO2). In contrast, the inner part of the nodules is composed of well-crystallized braunite with normal (10 wt%) contents of SiO2. Narrow veinlets with intermediate members of the cryptomelane-hollandite isostructural series crosscut the braunite mineralization, and represent a secondary paragenesis. A third step is marked by the appearance of todorokite and birnessite through the oxidation of the rhodonite veinlets. This is apparently the first observation of primary botryoidal microstructures in these nodules — and the first observation of braunite microstructures in a metamorphic area. The occurrence of undeformed volcanic and sedimentary textures in the upper Platta thrust sheets explains the preservation of these primary structures in these nodules. Moreover, the increase in flattening and intensive schistosity from Falotta to Parsettens may be related with the enrichment of Sr2+ in the intermediate members of the cryptomelane-hollandite series and with the destabilization of the braunite along the veinlets. It must also be pointed out that the Fe/Mn or Mn/Co&+Cu&+Ni ratios, currently used in research on manganese nodules, seem to be inadequate even for the Falotta ore, in which the best-preserved primary microstructures exist. In Falotta as in Parsettens, todorokite and birnessite come from the late rhodonite veinlets and are not related with the primary ore.

Notes: Abstract The Macraes gold-tungsten deposit occurs in a low-angle thrust system in biotite grade Otago Schist. Native gold, scheelite, pyrite and arsenopyrite are found in and adjacent to quartz veins and silicified schist of lenticular reef zones, where the thrust system cuts through graphitic pelitic schist. Mineralization is confined to a shear zone, up to 80 m thick, which is closely sub-parallel to the regional schistosity. Chemical alteration is dominated by silicification, with some addition of Cr and depletion of Sr and Ba. Alteration extends only about 5 m from major veins. Oxygen becomes isotopically heavier away from veins due to temperature decrease as hot fluids penetrated into cooler (250°C?) rock. Graphite within the shear zone rocks has reflectance of 6–7% (in oil), similar to graphite in medium-high grade Otago Schist, and is presumed to be metamorphic in origin. This graphite has acted as a reducing agent to cause precipitation of gold where the thrust system, acting as a conduit for metamorphic fluids, intersects the graphitic schist. The metals were derived from the underlying schist pile which may include an over-thrust oceanic assemblage containing metal-enriched horizons.

Notes: Abstract Sphalerite skeletal crystals in inclusions in chalcopyrite (so-called sphalerite stars) are widely thought to be the result of high-temperature unmixing processes. Detailed observations in the chalcopyrite ores at Aïn Barbar (Algeria) demonstrate that the unmixing theory cannot work in this case. It is here proposed that skeletal crystals are the result of a local supersaturation in sphalerite at the growing fronts of chalcopyrite. This implies a high precipitation rate, estimated to be ca. 10-6 m sec-1, remaining compatible, however, with a recharge in ore-forming components by the flowing mineralizing solutions. In addition to Aïn Barbar, this explanation is probably of more general significance since the kind of relationships described here is by no means restricted to this particular deposit.

Notes: Abstract The Zlaté Hory ore deposits — as an example of volcanogenic, strata-bound, massive, base-metal sulfides occurring in the Devonian formations of the Jeseníky Mts. — are compared with distinctive Phanerozoic types representing this type of ore deposit, i.e., with Kuroko-type, Rosebery-type, Besshi-type and Cyprus-type deposits. The results of comparison performed with fuzzy-linguistic diagnosis indicate close resemblance of the Zlaté Hory deposits to Rosebery-type massive sulfides with regard to primary features; however, individual features point to certain original similarity with Kuroko-type deposits, too. The metamorphic history of the ore deposits studied was similar to that of the Rosebery and Besshi types.

Notes: Abstract Chromite-rich lithologies in both the lower and the critical zones of the Bushveld Complex in the Potgietersrus area display flat, chondrite-normalized, platinum-group-element (PGE) concentration patterns, whereas those of associated sulphide-bearing, but chromite-poor rocks are considerably steeper. The low (Pt+Pd)/(Os+Ir+Ru) ratio in the chromite-bearing rocks is maintained irrespective of the amount of sulphide or chromite in the rock. This feature suggests that the partitioning of the individual PGE into PGE-bearing phases during conditions in the magma under which crystallization of chromite in excess of the normal cotectic amounts was favoured differed from conditions under which an immiscible sulphide liquid separated from the same magma in the absence of enhanced chromite crystallization. These changes in the partitioning coefficients of the individual PGE are considered to reflect changes in the solubility of these elements in response to variations in the intensive parameters in the magma necessary to bring about the enhanced crystallization of chromite.

Notes: Abstract This paper deals with barite from stratiform, karst, and vein deposits hosted within Lower Paleozoic rocks of the Iglesiente-Sulcis mining district in southwestern Sardinia. For comparison sulfates from mine waters are studied. Stratiform barite displays δ34S=28.8−32.1‰, δ18O=12.7−15.6‰, and 87Sr/86Sr=0.7087, in keeping with an essentially Cambrian marine origin of both sulfate and strontium. Epigenetic barite from post-Hercynian karst and vein deposits is indistinguishable for both sulfur and oxygen isotopes with δ34S=15.3−26.4‰ and δ18O=6.6−12.5‰; 87Sr/86Sr ratios vary 0.7094–0.7140. These results and the microthermometric and salinity data from fluid inclusions concur in suggesting that barite formed at the site of mineralization by oxidation of reduced sulfur from Cambrian-Ordovician sulfide ores in warm, sometimes hot solutions consisting of dilute water and saline brine with different δ18O values. The relative proportion of the two types of water may have largely varied within a given deposit during the mineralization. In the karst barite Sr was essentially provided by carbonate host rocks, whereas both carbonate and Lower Paleozoic shale host rocks should have been important sources for Sr of the vein barite. Finally, δ34S data of dissolved sulfate provide further support for the mixed seawater-meteoric water composition of mine waters from the Iglesiente area.

Notes: Abstract Replacement of dolomite rhombs is zone selective in Cracow-Silesian Zn-Pb deposits. The discussed replacement of dolomite by sphalerite occurs as spotted sphalerite ore constituting about one-fourth of the economic sulfide mineralization of the Pomorzany Mine. Replacement is controlled by two factors, i.e., microporosity and chemical composition. The epigenetic host dolomite rhombs are zoned into three main zones, i.e., the outer rhombohedral rim which is hard and Fe rich with remarkable Zn content, the inner zone which is porous and Zn rich but usually low in Fe content, and the core of the dolomite rhombs which is Fe free but may contain some Zn. Zones rich in Zn were the first to be replaced by sphalerite; the same holds true for zones with higher microporosity showing a high degree of crystallographic disorder of cations. The other zones seem to be intact. There are four successive replacement stages, i.e., partial, moderate, intense, and advanced. During replacement the intact parts become more Zn-rich, consequently being replaced completely in the later stages, the greater the zinc content in the host dolomite is, the more reactive it is to replacement by sphalerite. The liberated iron from replaced Fe-rich zones is fixed as finely crystalline pyrite.

Notes: Abstract At Rodalquilar gold mineralization is found in Late Tertiary volcanic rocks of the Sierra del Cabo de Gata and is related to a caldera collapse. Radial and concentric faults were preferred sites for gold deposition. Hydrothermal activity produced a specific alteration zoning around gold-bearing vein structures, grading from an innermost advanced argillic via an argillic into a more regionally developed propylitic zone. Advanced argillic alteration with silica, pyrophyllite, alunite, and kaolinite extends down to several hundred m indicating a hypogene origin. High-grade gold mineralization in vein structures is confined to the near-surface part of the advanced argillic alteration. Fine-grained gold is associated with hematite, jarosite, limonite, or silica. At a depth of about 120 m, the oxidic ore assemblage grades into sulfide mineralization with pyrite and minor chalcopyrite, covellite, bornite, enargite, and tennantite. Two types of fluids from different sources were involved in the hydrothermal system. Overpressured and hypersaline fluids of presumably magmatic origin initiated the hydrothermal system. Subsequent hydrothermal processes were characterized by the influx of low-salinity solutions of probable marine origin and by interactions between both fluids. Deep-reaching, advanced argillic alteration formed from high-salinity fluids with 20–30 equiv. wt% NaCl at about 225°C. Near-surface gold precipitation and silification are related to fluids with temperatures of about 175°C and 3–4 equiv. wt% NaCl. Gold was transported as Au(HS) 2 − , and precipitation resulted from boiling with a concomitant decrease in temperature, pressure, and pH and an increase in fO2. All features of the Rodalquilar gold deposit reveal a close relationship to acid-sulfate-type epithermal gold mineralization.

Notes: Abstract In the St. Andreasberg ore district, southwest Harz, at some time between the main and subsequent stages of mineralization sensu Wilke (1952), potassium feldspar (adularia) was formed hydrothermally. Based on isotopic dating (Rb/Sr, 40Ar/39Ar), the formation of this mineral is ascribed to the Early Cretaceous. Thus, part of the St. Andreasberg mineralization is proved to have been formed in the Late Mesozoic. The mineralization process forming feldspar does not coincide with magmatic phases of the area.

Notes: Abstract A new graphical technique has been developed which permits the composition and volumetric properties of mixed CO2-H2O inclusions to be determined using microthermometric measurements. If the values of only two of the four variables 1. density of the CO2 phase derived from the temperature and mode of partial homogenization, 2. temperature and mode of total homogenization, 3. volume fraction of the CO2 phase at 40°C, and 4. molar fraction of CO2 of a mixed CO2-H2O inclusion are known, the values of the other two variables can be taken from a diagram depicting the relationships between all four variables. The diagram can be used to derive the volume fraction of the CO2 phase from measurements of the partial and total homogenization temperature. It can also be employed in the opposite sense for determining the theoretical homogenization temperature of an inclusion which decrepitates before reaching total homogenization, provided a good visual phase volume estimate is available. The graphical technique can also be applied to mixed CO2-H2O inclusions which contain ≤6 wt% NaCl.

Notes: Abstract Drill-hole, geochronologic, and gravity data identify the buried Shishimuta caldera beneath post-caldera lava domes and lacustrine deposits in the center of the Hohi volcanic zone. The caldera is the source of the Yabakei pyroclastic flow, which erupted 1.0 Ma ago with a bulk volume of 110 km3. The caldera is a breccia-filled funnel-shaped depression 8 km wide and 〉 3 km deep with a V-shaped negative Bouguer gravity anomaly up to 36 mgal. Neither ring vents nor resurgence was recognized; instead, post-caldera monogenetic volcanism in an extensional setting dominated the area. The andesitic breccia has a relatively low density and fills the caldera; it possibly formed by fragmentation of disrupted roof rock during the violent Yabakei eruption and related collapse. Fewer normal faults and shallow microearthquakes occur inside the caldera than around it, possibly because rocks beneath the caldera are structurally incoherent. A profile of Shishimuta caldera may be more elongated vertically, and have a more intensely fractured zone, than that of a Valles-type caldera.

Notes: Abstract Peak intensities (magma discharge rate) of 45 Pleistocene and Holocene plinian eruptions have been inferred from lithic dispersal patterns by using a theoretical model of pyroclast fallout from eruption columns. Values range over three orders of magnitude from 1.6 × 106 to 1.1 × 109 kg/s. Magnitudes (total erupted mass) also vary over about three orders of magnitude from 2.0 × 1011 to 6.8 × 1014 kg and include several large ignimbrite-forming events with associated caldera formation. Intensity is found to be positively correlated with the magnitude when total erupted mass (tephra fall, surges and pyroclastic flows) is considered. Initial plinian fall phases with intensities in excess of 2.0 × 108 kg/s typically herald the onset of major pyroclastic flow generation and subsequent caldera collapse. During eruptions of large magnitude, the transition to pyroclastic flows is likely to be the result of high intensity, whereas the generation of pyroclastic flows in small magnitude eruptions may occur more often by reduction of magmatic volatile content or some transient change in magma properties. The correlation between plinian fall intensity and total magnitude suggests that the rate of magma discharge is related to the size of the chamber being tapped. A simple model is presented to account for the variation in intensity by progressive enlargement of conduits and vents and excess pressure at the chamber roof caused by buoyant forces acting on the chamber as it resides in the crust. Both processes are fundamentally linked to the absolute size of the pre-eruption reservoir. The data suggest that sustained eruption column heights (i.e. magma discharge rates) are indicators of eventual eruption magnitude, and perhaps eruptive style, and thus are key parameters to monitor in order to assess the temporal evolution of plinian eruptions.

Notes: Abstract The permeable foam hypothesis is suggested by Eichelberger et al. (1986) to explain a major loss of water from rhyolitic magmas in the volcanic conduit. Evidence for the high-water content of the major portion of the magma is herein examined and rejected. Eichelberger's hypothesis does not take into account the large (∼2 orders of magnitude) viscosity change that would occur in the conduit as a result of water loss. It also requires that the permeable foam collapse and weld to form an obsidian that in thin section displays no evidence of the foam. An alternate hypothesis to explain the existence of small amounts of high water content rhyolite glasses in acid volcanoes is that rhyolite magmas are relatively dry (0.1–0.3% H2O) and that water enters the magma from the environment to produce a water-rich selvage which then is kneaded into the body of the magma.

Notes: Abstract The behaviour of a lava flow is reproduced by a two-dimensional model of a Bingham liquid flowing down a uniform slope. Such a liquid is described by two rheological parameters, yield stress and viscosity, both of which are strongly temperature-dependent. Assuming a flow rate and an initial temperature of the liquid at the eruption vent, the temperature decrease due to heat radiation and the consequent change in the rheological parameters are computed along the flow. Both full thermal mixing and thermal unmixing are considered. The equations of motion are solved analytically in the approximation of a slow downslope change of the flow parameters. Flow height and velocity are obtained as functions of the distance from the eruption vent; the time required for a liquid element to reach a certain distance from the vent is also computed. The gross features of observed lava flows are reproduced by the model which allows us to estimate the sensitivity of flow dynamics to changes in the initial conditions, ground slope and rheological parameters. A pronounced increase in the rate of height increase and velocity decrease is found when the flow enters the Bingham regime. The results confirm the observation according to which lava flows show an initial rapid advance, followed by a marked deceleration, while the final length of a flow is such that the Graetz number is in the order of a few hundreds.

Notes: Abstract More than half of the intensely active East Rift Zone of Kilauea Volcano crops out underwater along the crest of the submarine Puna Ridge. I present multibeam bathmetry of the entire ridge, near-bottom photographic and sonar observations of the plunging crest of its deeper distal half, and seismic profiles across the ridge tip and the adjacent structural moat. Analysis of large-scale relief, small-scale topography, and superficial rock types indicates that the rift zone is actively propagating across the moat but is probably a superficial structure that does not penetrate the underlying oceanic crust, that its tip is covered with large lava flows erupted at high rates and is surrounded with extensive debris flow deposits, and that the axial topography at depths of 2–4 km is dominated by gaping fissures and collapse pits, showing a preponderance of intrusive rather than extrusive events. Some aspects of this central-volcano rift zone, such as its geometry at small lateral offsets, resemble those at interplate rift zones along fast-spreading mid-ocean rises, but the great contrast in lithosphere thickness results in fundamental structural differences.

Notes: Abstract Landsat Thematic Mapper (TM) images acquired in 1984 and 1985 revealed a pronounced thermal anomaly on Lascar volcano, north Chile. Subsequent images showed that the anomaly was persistent but variable and that after a significant eruption on 16 September 1986 it was weaker and divided into several sources. TM studies and other observations of Lascar indicate that the persistent thermal anomaly may be due to high-temperature fumaroles within the summit crater. GOES weather satellite images and field investigations confirm that the 16 September event was a short-lived, Vulcanian-type eruption, which produced an ash column that reached 15 km altitude. The ash cloud can be tracked on GEOS images to about 400 km downwind and covered an area 〉 112 000 km2. Transport occurred in the upper troposphere at speeds up to 180 km/h. Ash fall from the plume was well sorted and moderately fine grained (Md 200 µm). Formation and fallout of ash as aggregates was unimportant in the deposition of the ash layer. Although small, the Lascar 16 September eruption is significant because few historic eruptions have been recorded in the central Andes. Little would have been known of the eruption in the absence of remote-sensed data.

Notes: Abstract Water, F, and Cl contents of melt inclusions in phenocrysts from the 2-ka-old Taupo and Hatepe plinian tephras, and the ∼22-ka-old Okaia tephra from the Taupo volcanic center, New Zealand, were measured by electron and ion microprobe. Major and trace element chemistry of the inclusions is similar to that of bulk rock, supporting our assumption that volatile contents of inclusions are representative of the magma in which the crystals grew. Inclusions in the 2-ka Taupo plinian tephra contain a mean of 4.3 wt% H2O, 450 ppm F, and 1700 ppm Cl; from the Hatepe plinian tephra 4.3 wt% H2O, 430 ppm F, and 1700 ppm Cl; and from the Okaia tephra 5.9 wt% H2O, 470 ppm F, and 2100 ppm Cl. Sulfur was below the detection limit of 200 ppm. The constant H2O, F and Cl from a number of stratigraphic horizons in the tephra deposits suggest that the Taupo and Hatepe plinian tephras (〉8.2 km3 magma volume) were derived from a magma body that did not contain a strong volatile gradient. By inference, there is no pre-eruptive volatile difference between these plinian eruptions and a phrea-toplinian eruption which occurred between the two. Virtually no major element zonation is seen in this eruptive sequence. Although the Okaia tephra was also erupted from the Taupo volcanic center, probably from a similar vent area, its higher volatile contents and distinct composition as compared to the Taupo tephras show that it was derived from a different, and possibly deeper, magma body.

Notes: Abstract A well-defined pahoehoe lava type that is very common medially and distally in Hawaii is characterized by a high concentration and fairly uniform distribution of spherical or near-spherical vesicles. Measurements of vesicle sizes and concentrations have been made on ten of these spongy pahoehoe lava flow-units. The vesicles increase in size toward the middle, accompanied by a moderate increase in lava porosity. The close approach to bilateral symmetry on either side of the horizontal median plane and the common occurrence of a median gas blister shows that no significant upward movement of vesicles occurred, suggesting that the lava possessed a yield strength and was more or less static. Olivine phenocrysts when present are, however, concentrated in the lower half of the same flow units, showing that the lava previously lacked a significant yield strength. The vesicles are regarded as early, inherited from the vent, but the size characteristics of the vesicle population are a late-formed feature. Vesicles grew in static lava mostly by coalescence, and it is postulated that coalescence was promoted by the presence of abundant diktytaxitic voids which punctured the walls of contiguous vesicles. Zones in which the vesicle concentration is lower and the vesicles are larger and strongly deformed interrupt the symmetry of some spongy pahoehoe units, and gas blisters higher than the median plane occur in many examples. These zones are interpreted to result from late-stage shearing, and point to a mechanism by which vesicles may be eliminated from a lava.

Notes: Abstract Narrow-beam maps, selected dredge samplings, and surveys of the Mariana and Volcano Arcs identify 42 submarine volcanos. Observed activity and sample characteristics indicate 22 of these to be active or dormant. Edifices in the Volcano Arc are larger than most of the Mariana Arc edifices, more irregularly shaped with numerous subsidiary cones, and regularly spaced at 50–70 km. Volcanos in the Mariana Arc tend to be simple cones. Sets of individual cones and volcanic ridges are elongate parallel to the trend of the arc or at 110° counterclockwise from that trend, suggesting a strong fault control on the distribution of arc magmas. Volcanos in the Mariana Arc are generally developed west of the frontal arc ridge, on rifted frontal arc crust or new back-arc basin crust. Volcanos in the central Mariana Arc are usually subaerial, large (〉 500 km3), and spaced about 50–70 km apart. Those in the northern and southern Marianas are largely submarine, closer together, and generally less than 500 km3 in volume. There is a shoaling of the arc basement around Iwo Jima, accompanied by the appearance of incompatible-element enriched lavas with alkalic affinities. The larger volcanic edifices must reflect either a higher magma supply rate or a greater age for the larger volcanos. If the magma supply (estimated at 10–20 km3/km of arc per million years at 18° N) has been relatively constant along the Mariana Arc, we can infer a possible evolutionary sequence for arc volcanos from small, irregularly spaced edifices to large (over 1000 km3) edifices spaced at 50–70 km. The volcano distribution and basal depths are consistent with the hypothesis of back-arc propagation into the Volcano Arc.

Notes: Abstract Consideration of published anisotropy of magnetic susceptibility (AMS) studies on welded ignimbrites suggests that AMS fabrics are controlled by groundmass microlites distributed within the existing tuff fabric, the sum result of directional fabrics imposed by primary flow lineation, welding, and (if relevant) rheomorphism. AMS is a more sensitive indicator of fabric elements within welded tuffs than conventional methods, and usually yields primary flow azimuth estimates. Detailed study of a single densely welded tuff sample demonstrates that the overall AMS fabric is insensitive to the relative abundances of fiamme, matrix and lithics within individual drilled cores. AMS determinations on a welded-tuff dyke occurring in a choked vent in the Trans-Pecos Texas volcanic field reveals a consistent fabric with a prolate element imbricated with respect to one wall of the dyke, while total magnetic susceptibility and density exhibit axially symmetric variations across the dyke width. The dyke is interpreted to have formed as a result of agglutination of the erupting mixture on a portion of the conduit wall as it failed and slid into the conduit, followed by residual squeezing between the failed block and in situ wallrock. Irrespective of the precise mechanism, widespread occurrence of both welded-tuff dykes and point-welded, aggregate pumices in pyroclastic deposits may imply that lining of conduit walls by agglutionation during explosive volcanic eruptions is a common process.

Notes: Abstract Irregularly shaped, large and clear (“LAC”) glass inclusions are present in plagioclase phenocrysts in several andesitic lavas erupted from Tolimán volcano, Guatemala. Their morphology is different from densely spaced, fine-grained glass inclusions that form concentric zones in dusty or cellular textured plagioclase phenocrysts. The large size of LAC inclusions make them suitable for microprobe analysis and average bulk compositions are presented for glasses in 30 phenocrysts from eight lava samples. Their compositions are rhyolitic and in disequilibrium, or “out-range” (Anderson 1976) with respect to whole-rock and groundmass glass compositions. LAC inclusions typically occur in large, tabular plagioclase phenocrysts with relatively uniform, sodic compositions (An 40–54). Compositions of feldspar phenocrysts not containing LAC inclusions range from An 41 to An 81. Petrographic and chemical data support a primary origin for LAC glasses, suggest mixing of mafic and silicic magmas, and also constrain a mechanism for magma mixing. Rapid growth of plagioclase and entrapment of LAC glass occurs during mixing in a vapor-rich silicic liquid under low degrees of undercooling. These conditions are possibly produced in a high-level magma body such as that envisioned by Huppert et al. (1982), where replenishment and subsequent crystallization of a hydrous magma induces density instability and mixing with the resident magma.

Notes: Abstract Microgravity measurements and levelling surveys on volcanoes are not always easy to make, but are useful for studying volcanic processes quantitatively. Gravity changes associated with volcanic activity are not always significant. Precision of microgravity measurements depend critically on the procedures adopted, and those applied in the present paper are described. Levelling technique is now orthodox, and some empirical laws relating ground deformation to volcanic activity are deduced from the accumulated data. Gravity changes occur at the same time and places as ground deformations. The relationship between microgravity and height changes are discussed from the standpoint of analyzing the data obtained on volcanoes. The observational results obtained on four volcanoes in Japan are separately analyzed because each volcano exhibits different patterns of gravity changes and deformations. During the 1977–1982 activity of Usu volcano, deformation was accompanied by microgravity changes frequently observed at a particular benchmark at the base of the volcano for about five years. The gravity changes prove to be not a direct effect of magma movements but to be caused by the deformations of ground strata and aquifers around the benchmark. The 1983 eruption of Miyakejima volcano was associated with local gravity changes around the eruptive fissures due to magma intrusion which was approximately modelled. Similarly the 1986 eruption of Ooshima volcano caused gravity changes on the volcano, but these were poorly correlated with elevation changes and their origins were not uniquely interpreted. To detect gravity changes associated with the activity of Sakurajima volcano, an equigravity point was selected at the north of the volcano besides the gravity points on and around the volcano itself. The probable gradual accumulation of magmas beneath the volcano for eight years is substantiated by observed microgravity and elevation changes.

Notes: Abstract Two groups of rhyolites have been recognized at San Vincenzo (Tuscany, Italy). Group A rhyolites are characterized by plagioclase, quartz, biotite, sanidine and cordierite mineral assemblages. They show constant MgO and variable CaO and Na2O contents. Initial87Sr/86Sr ratios in group A samples range between 0.71950 and 0.72535, whereas the Nd isotopic compositions are relatively constant (0.51215–0.51222). Group B rhyolites are characterized by orthopyroxene and clinopyroxene as additional minerals, and show textural, mineralogical and chemical evidence of interaction with more mafic magmas. The Sr and Nd isotopic ratios range between 0.71283–0.71542 and 0.51224–0.51227 respectively. Magmatic inclusions of variable size (1 mm to 10 cm) were found in groups B rhyolites. These inclusions consist mainly of diopsidic clinopyroxene and minor olivine and biotite. They are latitic in composition and represent blobs of hybrid intermediate magmas entrained in the rhyolitic melts. These magmatic inclusions have relatively high Sr contents (996–1529 ppm) and Sr and Nd isotope-ratios of 0.70807–0.70830 and 0.51245–0.51252 respectively.87Sr/87Sr data on minerals separated from both group A and B rhyolites and magmatic inclusions reveal strong isotopic disequilibria due to the presence of both restitic and newly crystallized phases in group A rhyolites and due to interaction of rhyolites with a mantle-de-rived magma in group B rhyolites. Isotopic data on whole rocks and minerals allow us to interpret the group A rhyolites as representative of different degrees of melting of an isotopically fairly homogeneous pelitic source; conversely, group B rhyolites underwent interactions with a mantle-derived magma. The crustal source as inferred from isotopic systematics would be characterized by87Sr/86Sr and143Nd/144Nd ratios close to 0.7194 and 0.51216 respectively. The sub-crustal magma would have Sr isotopic composition close to 0.7077 and a143Nd/144Nd ratio greater than or equal to 0.51252. These isotopic features are different from those reported for the parental magmas postulated for Vulsini and Alban Hills in the nearby Roman Magmatic Province, and are similar to those of the Vesuvius and Ischia magmas.

Notes: Abstract Basal layered deposits of the large-volume Peach Springs Tuff occur beneath the main pyroclastic flow deposit over a minimum lateral distance of 70 km in northwestern Arizona (USA). The basal deposits are interpreted to record initial blasting and pyroclastic surge events at the beginning of the eruption; the pyroclastic surges traveled a minimum of 100 km from the (as yet unknown) source. Changes in bedding structures with increasing flow distance are related to the decreasing sediment load of the surges. Some bed forms in the most proximal part of the study area (Kingman, Arizona) can be interpreted as being shock induced, reflecting a blast origin for the surges. Component analyses support a hydrovolcanic origin for some of the blasting and subsequent pyroclastic surges. The eruption apparently began with magmatic blasts, which were replaced by hydrovolcanic blasts. Hydrovolcanic activity may be partially related to failure of the conduit walls that temporarily plugged the vent. A single large-volume pyroclastic flow immediately followed the blast phase, and no evidence has been observed for a Plinian eruption column. The stratigraphic sequence indicates that powerful hydrovolcanic blasting rapidly widened the vent, thus bypassing a Plinian fallout phase and causing rapid evolution to a collapsing eruption column. Similar processes may occur in other large-volume ignimbrite eruptions, which commonly lack significant Plinian fallout deposits.

Notes: Abstract An exceptionally well-exposed, ancient, intra-arc basin in the Permian Takitimu Group of New Zealand contains 14 km of interbedded primary volcanic and marine volcaniclastic rocks of basaltic to rhyodacitic composition. These are the products of subaerial and submarine arc volcanism and closely associated turbidite sedimentation. The Takitimu oceanic arc/basin setting formed a dynamic closed sedimentary system in which large volumes of volcaniclastic material generated at the arc was rapidly redeposited in marine basins flanking the eruptive centres. Volcanism probably included (1) moderate- to deep-water extrusion of lava and deposition of hyaloclastite, (2) extrusive and explosive eruptions from shallow marine to marginally emergent volcanoes in or on the margin of the basin, and (3) Plinian and phreato-Plinian eruptions from more distant subaerial vents along the arc. Much of the newly erupted material was rapidly transported to the adjacent marine basin by debris flows, slumping and sliding. Hemipelagic sedimentation predominated on the outer margin of the basin, infrequently interrupted by deposition of ash from the most explosive arc volcanism and the arrival of extremely dilute turbidites. Turbidite sedimentation prevailed in the remainder of the basin, producing a thick prograding volcaniclastic apron adjacent to the arc. The volcaniclastic strata closely resemble classic turbidite deposits, and show similar lateral facies variations to submarine fan deposits. Study of such sequences provides insight into poorly understood processes in modern arc-related basins.

Notes: Abstract The vesicularity of juvenile clasts in pyroclastic deposits gives information on the relative timing of vesiculation and fragmentation, and on the role of magmatic volatiles versus external water in driving explosive eruptions. The vesicularity index and range are defined as the arithmetic mean and total spread of vesicularity values, respectively. Clast densities are measured for the 16–32 mm size fraction by water immersion techniques and converted to vesicularities using measured dense-rock equivalent densities. The techniques used are applied to four case studies involving magmas of widely varying viscosities and discharge rates: Kilauea Iki 1959 (basalt), Eifel tuff rings (basanite), Mayor Island cone-forming deposits (peralkaline rhyolite) and Taupo 1800 B.P. (calc-alkaline rhyolite). Previous theoretical studies suggested that a spectrum of clast vesicularities should be seen, depending on the magma viscosity, eruption rate, and the presence and timing of magma: water interaction. The new data are consistent with these predictions. In magmatic “dry” eruptions the vesicularity index lies uniformly in the range 70%–80% regardless of magma viscosity. For high viscosities and eruption rates the vesicularity ranges are narrow (〈 25%), but broaden to between 30% and 50% as the viscosity and eruption rates are lowered and the volatiles and magma can de-couple. In phreatomagmatic “wet” eruptions, widely varying clast vesicularities reflect complex variations in the relative timing of vesiculation and water-induced fragmentation. Magma:water interaction at an early stage greatly reduces the vesicularity indices (〈 40%) and broadens the ranges (as high as 80%), whereas late-stage interaction has only a minor effect on the index and broadens the range to a limited extent. Clast vesicularity represents a useful third parameter in addition to dispersal and fragmentation to characterise pyroclastic deposits.

Notes: Abstract Rothenberg scoria cone Eifel formed by an alternation of three Strombolian and three phreatomagmatic eruptive phases. Eruptions took place from up to six vents on a 600 m-long fissure, building an early tuff ring and then two coalescing scoria cones. Strombolian volcanism dominated volumetrically, as the supply of external water was severely limited. Magma/water interaction only occurred during the opening stages of eruption at any vent, when discharge rates were low and the fragmentation surface was below the water table. The phreatomagmatic deposits consist of relatively well-sorted fall beds and only minor surge deposits. They contain juvenile clasts with a wide range of vesicularity and grain size, implying considerable heterogeneity in the assemblage of material ejected by the phreatomagmatic explosions. the transition from phreatomagmatic to Strombolian eruption at any vent was rapid and irreversible, and Strombolian volcanism persisted even when eruption rates are inferred to have waned at the close of each eruptive phase as, by then, the fragmentation surfaces were high in the growing cones and water was denied access to the magma. The Strombolian deposits are relatively homogenous, consisting of alternating coarser- and finer-grained, well-sorted fall beds erupted during periods of open-vent eruption and partial blockage of the vent respectively. The intervals of Strombolian eruption were always a delicate balance between discharge of freely degassing magma and processes such as ponding of degassed magma in the vent, collapse of the growing cones, and repeated recycling of clasts through the vent. Clear evidence of the instability of the Rothenberg cones is preserved in numerous unconformities within deposits of the inner walls of the cones. The close of Strombolian phases was probably marked by a decreasing supply of magma to the vents accompanied by ponding and stagnation of lava in the craters.

Notes: Abstract The Puu Oo eruption in the middle of Kilauea volcano's east rift zone provides an excellent opportunity to utilize petrologic constraints to interpret rift-zone processes. Emplacement of a dike began 24 hours before the start of the eruption on 3 January 1983. Seismic and geodetic evidence indicates that the dike collided with a magma body in the rift zone. Most of the lava produced during the initial episode of the Puu Oo eruption is of hybrid composition, with petrographic and geochemical evidence of mixing magmas of highly evllved and more mafic compositions. Some olivine and plagioclase grains in the hybrid lavas show reverse zoning. Whole-rock compositional variations are linear even for normally compatible elements like Ni and Cr. Leastsquares mixing calculations yield good residuals for major and trace element analyses for magma mixing. Crystal fractionation calculations yield unsatisfactory residuals. The highly evolved magma is similar in composition to the lava from the 1977 eruption and, at one point, vents for these two eruptions are only 200 m apart. Possibly both the 1977 lava and the highly evolved component of the episode 1 Puu Oo lava were derived from a common body of rift-zone-stored magma. The more mafic mixing component may be represented by the most mafic lava from the January 1983 eruption; it shows no evidence of magma mixing. The dike that was intruded just prior to the start of the Puu Oo eruption may have acted as a hydraulic plunger causing mixing of the two rift-zone-stored magmas.

Notes: Abstract The Christmas Mountains caldera complex developed approximately 42 Ma ago over an elliptical (8×5 km) laccolithic dome that formed during emplacement of the caldera magma body. Rocks of the caldera complex consist of tuffs, lavas, and volcaniclastic deposits, divided into five sequences. Three of the sequences contain major ash-flow tuffs whose eruption led to collapse of four calderas, all 1–1.5 km in diameter, over the dome. The oldest caldera-related rocks are sparsely porphyritic, rhyolitic, air-fall and ash-flow tuffs that record formation and collapse of a Plinian-type eruption column. Eruption of these tuffs induced collapse of a wedge along the western margin of the dome. A second, more abundantly porphyritic tuff led to collapse of a second caldera that partly overlapped the first. The last major eruptions were abundantly porphyritic, peralkaline quartz-trachyte ash-flow tuffs that ponded within two calderas over the crest of the dome. The tuffs are interbedded with coarse breccias that resulted from failure of the caldera walls. The Christmas Mountains caldera complex and two similar structures in Trans-Pecos Texas constitute a newly recognized caldera type, here termed a laccocaldera. They differ from more conventional calderas by having developed over thin laccolithic magma chambers rather than more deep-seated bodies, by their extreme precaldera doming and by their small size. However, they are similar to other calderas in having initial Plinian-type air-fall eruption followed by column collapse and ash-flow generation, multiple cycles of eruption, contemporaneous eruption and collapse, apparent pistonlike subsidence of the calderas, and compositional zoning within the magma chamber. Laccocalderas could occur else-where, particularly in alkalic magma belts in areas of undeformed sedimentary rocks.

Notes: Abstract Historical eruptions have produced lahars and floods by perturbing snow and ice at more than 40 volcanoes worldwide. Most of these volcanoes are located at latitudes higher than 35°; those at lower latitudes reach altitudes generally above 4000 m. Volcanic events can perturb mantles of snow and ice in at least five ways: (1) scouring and melting by flowing pyroclastic debris or blasts of hot gases and pyroclastic debris, (2) surficial melting by lava flows, (3) basal melting of glacial ice or snow by subglacial eruptions or geothermal activity, (4) ejection of water by eruptions through a crater lake, and (5) deposition of tephra fall. Historical records of volcanic eruptions at snow-clad volcanoes show the following: (1) Flowing pyroclastic debris (pyroclastic flows and surges) and blasts of hot gases and pyroclastic debris are the most common volcanic events that generate lahars and floods; (2) Surficial lava flows generally cannot melt snow and ice rapidly enough to form large lahars or floods; (3) Heating the base of a glacier or snowpack by subglacial eruptions or by geothermal activity can induce basal melting that may result in ponding of water and lead to sudden outpourings of water or sediment-rich debris flows; (4) Tephra falls usually alter ablation rates of snow and ice but generally produce little meltwater that results in the formation of lahars and floods; (5) Lahars and floods generated by flowing pyroclastic debris, blasts of hot gases and pyroclastic debris, or basal melting of snow and ice commonly have volumes that exceed 105 m3. The glowing lava (pyroclastic flow) which flowed with force over ravines and ridges...gathered in the basin quickly and then forced downwards. As a result, tremendously wide and deep pathways in the ice and snow were made and produced great streams of water (Wolf 1878).

Notes: Abstract A system of propagating cracks may explain magma transport and the evolution of a volcano. This paper considers only a basaltic magma. The system is controlled by two boundary conditions: the stress field, and the production rate of the magma-filled cracks in the mantle. Numerical solutions of crack propagation for various stress conditions, with a constant production rate high enough to coalesce isolated cracks, were performed, and the results applied to different tectonic conditions. For the hydrostatic stress conditions, most magma-filled cracks beneath a polygenetic volcano become trapped either in the lower crust, because there the density difference between magma and the host rocks (Δρ) becomes suddenly small, compared with that in the mantle, or trapped in the upper crust, because there Δρ is near to zero. Magma traps composed of such cracks may grow into magma reservoirs if the production rate of cracks in the mantle is large. If horizontal stress with a vertical gradient is superimposed on the hydrostatic condition in the crust, that is, tensile stress which increases upward or compressional stress which increases downward, magmafilled cracks, even if the density of magma is higher than that of the crust, may ascend directly without trapping. When the crust undergoes relative tension, magma-filled cracks may become trapped. Then, the lower part of the trap may grow into a magma reservoir, while the upper part may become filled with dikes. When the production rate of cracks is small, an initial magma-filled crack can rise directly to the surface only when the stress with a gradient is superimposed as mentioned above, or when the average density in a crack decreases, owing to, for example, vesiculation of volatile components.

Notes: Abstract Thermal remanent magnetism provides a method of quantitatively determining the emplacement temperature of individual lithic clasts in a volcaniclastic rock. The technique is reviewed and applied to two types of Quaternary pyroclastic deposit on Santorini. Emplacement-temperature estimates for lithic clasts from two co-ignimbrite lithic breccias (Cape Riva and Middle Pumice eruptions) range from 250°C to ≥ 580°C, showing unambiguously that the breccias were emplaced hot. Good precision on temperature estimates (about ±20°C) were obtained from the Cape Riva breccias. Lithics in a Plinian airfall deposit from the Middle Pumice eruption give less precise results because the primary magnetisation has been partly overprinted by chemical (and/or viscous) remanence, and some clasts may have rotated during compaction of the deposit. Temperatures from proximal airfall are consistent with welding of the deposit within 1.5 km from vent. Temperature estimates for lithic clasts further from vent scatter, but a falloff of temperature away from vent can be recognised if an average emplacement temperature for the whole deposit is identified at each location. The study highlights some difficulties in interpreting quantitative temperature estimates for prehistoric pyroclastic deposits.

Notes: Abstract An improved empirical method for the plotting of field data and the calculation of tephra fall volumes is presented. The widely used “area” plots of ln(thickness) against ln(isopach area) are curved, implying an exponential thinning law. Use of ln(thickness)−(area)1/2 diagrams confirm the exponential dependence of many parameters (e.g. thickness, maximum and median clast size) with distance from source, producing linear graphs and allowing volumes to be calculated without undue extrapolation of field data. The agreement between theoretical models of clast dispersion and observation is better than previously thought. Two new quantitative parameters are proposed which describe the rates of thinning of the deposit (b t the thickness half-distance) and the maximum clast size (b c the clast half-distance). Many deposits exhibit different grainsize and thickness thinning rates, with the maximum clast size diminishing 1–3 times slower than the thickness. This implies that the entrained grainsize population influences the morphologic and granulometric patterns of the resulting deposit, in addition to the effects of column height and wind-speed. The grainsize characteristics of a deposit are best described by reference to the half-distance ratio (b c /b t ). A new classification scheme is proposed which plots the half-distance ratio against the thickness half-distance and may be contoured in terms of the column height.

Notes: Abstract Ruapehu composite volcano is a dynamic volcanic-sedimentary system, characterised by high accumulation rates and by rapid lateral and vertical change in facies. Four major cone-building episodes have occurred over 250 Ka, from a variety of summit, flank and satellite vents. Eruptive styles include subplinian, strombolian, phreatomagmatic, vulcanian and dome-related explosive eruptions, and extrusion of lava flows and domes. The volcano can be divided into two parts: a composite cone of volume 110 km3, surrounded by an equally voluminous ring plain. Complementary portions of Ruapehu's history are preserved in cone-forming and ring plain environments. Cone-forming sequences are dominated by sheet- and autobrecciated-lava flows, which seldom reach the ring plain. The ring plain is built predominantly from the products of explosive volcanism, both the distal primary pyroclastic deposits and the reworked material eroded from the cone. Much of the material entering the ring plain is transported by lahars either generated directly by eruptions or triggered by the high intensity rain storms which characterise the region. Ring plain detritus is reworked rapidly by concentrated and hyperconcentrated streams in pulses of rapid aggradation immediately following eruptions and more gradually in the longer intervals between eruptions.

Notes: Abstract A simple system was designed using7LiOH-impregnated filters to collect acidic gases from ambient air and from highly concentrated volcanic plumes or gas streams. These filters were developed for analysis using instrumental neutron activation analysis, but other analytical techniques could be used as well. The sampling system was designed to use a series of 1–3 M7LiOH-impregnated filters to collect high concentrations of acidic gases found in gas plumes of active volcanoes. The filters are quantitative for SO2 and the halide acids when sufficient base is present to neutralize the acidic species. Extremely high concentrations of SO2 may not be collected quantitatively since SO2 is a relatively weak Lewis acid compared to the halide acids. The acidic oxides of Sb, As, and Se were also collected quantitatively. A particle filter preceded the impregnated filters in order to remove particles from the fumes. This system has proven effective under difficult sampling conditions and, since it is portable and light weight, it could be used for many volcanological applications where high levels of acidic gaseous phase species need to be collected.

Notes: Abstract The grain orientations within the matrix of two large-scale welded, two small-scale nonwelded and two nonwelded low-aspect ratio pyroclastic flow deposits are measured to analyze flow behavior. Preferred grain alignments are especially apparent in the middle part of layer 2 of each deposit. Preferred grain alignments do not vary laterally within a 10 m interval. The grain alignments obtained are radial from the source caldera, especially in proximal to medial and plateau-forming facies of pyroclastic flow deposits. Grain alignments are controlled by valley-channel directions for the valley-ponded facies of pyroclastic flow deposits, especially at medial to distal locations. Such local topographic factors strongly affect the data for high-aspect ratio and smallscale deposits, and weakly affect the data for widespread low-aspect ratio pyroclastic flow deposits. This work suggests that grain alignment analysis should be used with care when attempting to determine the location of an unknown source.

Notes: Abstract New field observations with the submersible ALVIN and photographic evidence from a study of the summits of seamounts near the East Pacific Rise show that hyaloclastite deposits occur commonly. Hyaloclastite outcrops were found on six volcanoes at depths from 1240 to 2500 m. These new observations plus laboratory study of new hyaloclastite specimens extend the results of previous studies. Most of the hyaloclastite samples are of hydrovolcanic eruptive origin, but a few show evidence of a predominantly sedimentary origin. Primarily from morphology, we identify several vent areas from which hyaloclastite presumably erupted. The surface appearance of the hyaloclastite deposits varies with distance to these vents, leading us to propose a facies model for deep-sea hyaloclastites on seamount summits. Hyaloclastites of hydromagmatic origin exhibit weak normal grading and bedding-parallel alignment of platy shards. They consist of blocky, sliver and fluidal basalt glass shards and lithics in a matrix that contains pelagic sediment. The shards themselves are remarkably free of even the tiniest crystals and are usually chemically homogeneous. We propose that the shards form mainly by cooling-contraction granulation, but cannot rule out the possibility of limited steam explosivity. Hyaloclastites are closely associated with submarine pahoehoe and we propose that a very rapid eruption rate, promoting clastic-dominated versus flow-dominated eruptive behavior, is the dominant control on hyaloclastite formation. We propose that shard formation occurs during submarine lava fountaining. Gravitational instability of the resulting slurry of shards, sea water and possibly steam causes gravity flow that carries the shards outward from the vent. Further field and modelling studies are needed to test these ideas and more quantitatively constrain the ascent mechanism, eruption dynamics and deposition of deep-sea hyaloclastites.

Notes: Abstract Samples obtained by U.S. Department of Energy research drilling at the 600-year-old Obsidian Dome volcano provide the rare opportunity to examine the transition from volcanic (dome) to plutonic (intrusion) textures in a silicic magma system. Textures in the lavas from Obsidian Dome record multiple periods of crystallization initiated in response to changes in undercooling (ΔT) related to variable degassing in the mag-ma. Phenocr)ysts formed first at low ΔT. A drastic increase in ΔT, related to loss of a vapor phase during initial stages of eruption, caused nucleation of microlites. All of the lavas thus contain phenocrysts and microlites. Extrusion and subsequent devitrification of the dry (0.1 wt% H2O) magma crystallized spherulites and fine-grained rhyolite at high ΔT. A granophyric texture, representing crystallization at a moderate ΔT, formed in the intrusions beneath Obsidian Dome. Textures in the intrusion apparently represent crystallization of hydrous (1–2 wt% H2O) rhyolitic magma at shallow depths.

Notes: Abstract Crystallization paths of basaltic (1763 eruption) and hawaiitic (1865 and 1329 eruptions) scoria from Etna were deduced from mineralogy and melt inclusion chemistry. The volatile behaviour was investigated through the study of melt inclusions trapped in the phenocrysts and those of the whole rocks and the matrix glasses. The results from the 1763 eruption point to the early crystallization of olivine Fo 81.7 from a water-rich alkaline basalt, with high Cl (1750–2000 ppm) and S (2100–2400 ppm) concentrations. The hawaiitic melt inclusions trapped in olivine Fo 74, salite and plagioclase are characterized by a decrease in Cl/K2O and S/K2O ratios. In each investigated system there is good correlation between K2O and P2O5. In the whole rocks, Cl ranges from 980 to 1680 ppm, from basaltic to hawaiitic lavas, whereas S (110–136 ppm) remains low. Cl and S behaviour in the 1763 magma suggests an early degassing stage of Cl and S, with CO2 and a water-rich gaseous phase for a pressure close to 100 MPa, consistent with a permanent outgassing at the summit craters of Etna. During the eruption, the sulphur remaining in the hawaiitic liquid is lost, and the degassing of chlorine is limited. Such a degassing model can be extended to the 1865 and 1329a.d. eruptions.

Notes: Abstract A study of pyroclastic deposits from the 1815 Tambora eruption reveals two distinct phases of activity, i.e., four initial tephra falls followed by generation of pyroclastic flows and the production of major co-ignimbrite ash fall. The first explosive event produced minor ash fall from phreatomagmatic explosions (F-1 layer). The second event was a Plinian eruption (F-2) correlated to the large explosion of 5 April 1815, which produced a column height of 33 km with an eruption rate of 1.1 × 108 kg/s. The third event occurred during the lull in major activity from 5 to 10 April and produced minor ash fall (F-3). The fourth event produced a 43-km-high Plinian eruption column with an eruption rate of 2.8 × 108 kg/s during the climax of activity on 10 April. Although very energetic, the Plinian events were of short duration (2.8 h each) and total erupted volume of the early (F-1 to F-4) fall deposits is only 1.8 km3 (DRE, dense rock equivalent). An abrupt change in style of activity occurred at end of the second Plinian event with onset of pyroclastic flow and surge generation. At least seven pyroclastic flows were generated, which spread over most of the volcano and Sanggar peninsula and entered the ocean. The volume of pyroclastic flow deposits on land is 2.6 km3 DRE. Coastal exposures show that pyroclastic flows entering the sea became highly fines depleted, resulting in mass loss of about 32%, in addition to 8% glass elutriation, as indicated by component fractionation. The subaqueous pyroclastic flows have thus lost about 40% of mass compared to the original erupted mixture. Pyroclastic flows and surges from this phase of the eruption are stratigraphically equivalent to a major ash fall deposit (F-5) present beyond the flow and surge zone at 40 km from the source and in distal areas. The F-5 fall deposit forms a larger proportion of the total tephra fall with increasing distance from source and represents about 80% of the total at a distance of 90 km and 92% of the total tephra fall from the 1815 eruption. The field relations indicate that the 20-km3 (DRE) F-5 deposit is a co-ignimbrite ash fall, generated largely during entrance of pyroclastic flows into the ocean. Based on the observed 40% fines depletion and component fractionation from the flows, the large volume of the F-5 co-ignimbrite ash requires eruption of 50 km3 (DRE, 1.4 × 1014 kg) pyroclastic flows.