Carbon biogeochemistry of the Betsiboka estuary (north-western Madagascar)

Madagascar’s largest estuary (Betsiboka) was sampled along the salinity gradient during the dry season to document the distribution and sources of particulate and dissolved organic carbon (POC, DOC) as well as dissolved inorganic carbon (DIC). The Betsiboka was characterized by a relatively high suspended matter load, and in line with this, low DOC/POC ratios (0.4–2.5). The partial pressure of CO₂ (pCO₂) was generally above atmospheric equilibrium (270–1530 ppm), but relatively low in comparison to other tropical and subtropical estuaries, resulting in low average CO₂ emission to the atmosphere (9.1 ± 14.2 mmol m⁻² d⁻¹). Despite the fact that C4 vegetation is reported to cover >80% of the catchment area, stable isotope data on DOC and POC suggest that C4 derived material comprises only 30% of both pools in the freshwater zone, increasing to 60–70% and 50–60%, respectively, in the oligohaline zone due to additional lateral inputs. Sediments from intertidal mangroves in the estuary showed low organic carbon concentrations (<1%) and δ¹³C values (average −19.8‰) consistent with important inputs of riverine imported C4 material. This contribution was reflected in δ¹³C signatures of bacterial phospholipid derived fatty acids (i + a15:0), suggesting the potential importance of terrestrial organic matter sources for mineralization and secondary production in coastal ecosystems.     

A re-examination of the role of hydrogen in Al−Si interdiffusion in feldspars

Recent experimental studies have shown that the rates of Al–Si order-disorder and interdiffusion in alkali feldspars at high pressures under dry conditions increase dramatically in the approximate pressure range 7–14 kb, depending on temperature and feldspar composition (Goldsmith 1987, 1988). Enhancement of Al–Si interdiffusion rates is ascribed to the involvement of hydrogen, but the species of hydrogen involved is undetermined.A simple kinetic analysis of the data of Goldsmith (1987) on disordering of dry albite at 800°–950° C and 6–24 kb in the solid media press is consistent with the NaCl pressure cell acting as a proton donor by enhancing dissociation of water in the pressure medium, generating a high in the experimental environment. The rate constant for disordering of albite is found to increase linearly with the estimated experimental and with the density of aqueous salt solution, implicating H⁺ as the rate-enhancing species.Further experimental studies confirm the importance of . At 16 kb and 850° C, dry albite in sealed Pt capsules in a NaCl cell containing tantalum powder (which reduces H₂O to H₂) remains highly ordered over the same time that complete disordering would occur in the absence of Ta. H₂ cannot therefore be the rate-enhancing species. At 1 kb and 850° C, the extent of Al–Si disorder in albite in direct contact with various NaCl–H₂O solutions increases from partially disordered for pure H₂O to completely disordered for saturated aqueous NaCl solution, giving strong support to the proton model. SIMS scanning ion imaging of albite run products demonstrates conclusively that solution-reprecipitation is not responsible for enhanced disordering rates.Results of disordering experiments in the solid media apparatus cannot be duplicated in Ar gas media internally-heated pressure vessels, even with the same experimental configuration around the albite-bearing capsules, due to the different proton-buffering capacities of the solid and gas media apparatus.     

Cation distribution in pentlandites (Fe,Ni)9S8: Dependence on pressure and temperature and kinetics of the cation exchange reaction

This paper describes the distribution of Fe and Ni between the octahedral and tetrahedral sites in pentlandite (Fe,Ni)₉S₈. The dependence of the distribution on pressure and temperature and the activation energy of the cation exchange reaction were determined through annealing experiments. Synthetic crystals were annealed at 433–723 K and pressures up to 4 GPa, and natural crystals were annealed at 423, 448 and 473 K in evacuated silica capillary tubes for various durations. The cation distributions in the synthetic crystals were determined with an X-ray powder method employing the anomalous dispersion effect of CuK. and FeK radiations, while those of natural crystals were calculated from the cell dimensions. The values of U, S and V for the Fe/Ni exchange reaction are –6818 J mol^–1, 20.52 J K^–1 mol^–1, and 6.99 × 10^–6 m³ mol^–1, respectively. The dependence of the Fe/Ni distribution on pressure (Pa) and temperature (Kelvin) was determined as lnK = 2.47+8.20 × 10² T ^–1+8.41 x 10^–7 T ^–1 P, where K = (Fe/Ni)_octahedral /(Fe/Ni)_tetrahedral. The activation energy of the cation exchange reaction was 185 kJ mol^–1.     

P - V - T equation of state of stishovite to the mantle transition zone conditions

In-situ synchrotron X-ray diffraction experiments were conducted using the SPEED-1500 multi-anvil press of SPring-8 on stishovite SiO₂ and pressure-volume-temperature data were collected at up to 22.5 GPa and 1,073 K, which corresponds to the pressure conditions of the base of the mantle transition zone. The analysis of room-temperature data yielded V₀=46.56(1) ų, K_{T 0}=296(5) GPa and K _T =4.2(4), and these properties were consistent with the subsequent thermal equation of state (EOS) analyses. A fit of the present data to high-temperature Birch-Murnaghan EOS yielded (K_T /T) _P =–0.046(5) GPa K^–1 and = a + bT with values of a =1.26(11)×10^–5 K^–1 and b =1.29(17)×10^–8 K^–2. A fit to the thermal pressure EOS gives ₀=1.62(9)×10^–5 K^–1, ( K _T / T) _V =–0.027(4) GPa K^–1 and (²P /T ²) _V =27(5)×10^–7 GPa K^–2. The lattice dynamical approach by Mie-Grüneisen-Debye EOS yielded ₀=1.33(6), q =6.1(8) and ₀=1160(120) K. The strong volume dependence of the thermal pressure of stishovite was revealed by the analysis of present data, which was not detectable by the previous high-temperature data at lower pressures, and this yields ( K _T / T) _V 0 and q 1. The analyses for the fictive volume for a and c axes show that relative stiffness of c axis to a axis is similar both on compression and thermal expansion. Present EOS enables the accurate estimate of density of SiO₂ in the deep mantle conditions.     

Anisotropic compression of edingtonite and thomsonite to 6 GPa at room temperature

Polycrystalline samples of natural edingtonite (New Brunswick, Canada) and thomsonite (Oregon, USA) were studied up to 6 GPa using monochromatic synchrotron X-ray powder diffraction and a diamond-anvil cell with a methanol:ethanol:water mixture as a penetrating pressure-transmitting fluid. Unlike natrolite, previously studied under the same conditions, edingtonite and thomsonite do not show any apparent pressure-induced hydration (PIH) or phase transitions. All these fibrous zeolites are characterized by their anisotropic compressibilities, with the linear compressibilities of the fibrous chains (c-axis) being as small as one third of those perpendicular to the chains (a-, b-axes); for edingtonite, ₀ ^a =0.0050(3) GPa^–1, ₀ ^b =0.0054(2) GPa^–1, ₀ ^c =0.0034(1) GPa^–1; for thomsonite, ₀ ^a = 0.0080(2) GPa^–1, ₀ ^b =0.0084(2) GPa^–1, ₀ ^c =0.0032(1) GPa^–1. The pressure–volume data were fitted to a second-order Birch–Murnaghan equation of state using a fixed pressure derivative of 4. As a result of the 0000-type connectivity of the chains, the bulk modulus of edingtonite is found to be about 40% larger than that of thomsonite; K^EDI ₀=73(3) GPa, K^THO ₀=52(1) GPa. Distance least-squares refinements were used to model the expected framework, following the observed linear compression behaviors. The chain-bridging T–O–T angle is proposed to be correlated with the different compressibilities across the chains in each framework type.     

MgSiO3 ilmenite: Heat capacity,thermal expansivity,and enthalpy of transformation

A new determination, using high temperature drop-solution calorimetry, of the enthalpy of transformation of MgSiO₃ pyroxene to ilmenite gives H ₂₉₈ = 59.03 ±4.26 kJ/mol. The heat capacity of the ilmenite and orthopyroxene phases has been measured by differential scanning calorimetry at 170–700 K; C_p of MgSiO₃ ilmenite is 4–10 percent less than that of MgSiO₃ pyroxene throughout the range studied. The heat capacity differences are consistent with lattice vibrational models proposed by McMillan and Ross (1987) and suggest an entropy change of -18 ± 3 J-K^-1 ·mol^-1, approximately independent of temperature, for the pyroxene-ilmenite transition. The unit cell parameters of MgSiO₃ ilmenite were measured at 298–876 K and yield an average volume thermal expansion coefficient of 2.44 × 10^-5 K^-1. The thermochemical data are used to calculate phase relations involving pyroxene, -Mg₂SiO₄ plus stishovite, Mg₂SiO₄ spinel plus stishovite, and ilmenite in good agreement with the results of high pressure studies.     

Tridymite: Effect of hydrostatic pressure to 6 kbar on temperatures of two rapidly reversible transitions

Trajectories of two reversible phase transitions in a low-Na synthetic tridymite have been determined to 6 kbar by differential thermal analysis (DTA) in hydrostatic apparatus using Ar or CO₂. The temperature of the lower transition increases from 111 ° C at 1 bar linearly with pressure with slope 1₅ deg kbar^–1. Pressure raises the temperature of the upper transition from 157 ±2 ° or 159 ° C (independently determined) at 1 bar wit a slope of 53 deg kbar^–1, up to 0.7 kbar; for the data above that pressure, the initial slope is 64 deg kbar^–1. Above 2–1/2 kbar, the variation is linear with slope 70 deg kbar^–1. No evidence for other transitions was found at any of the apparent changes of slope. Hystereses for both transitions decreased at high pressures compared to 1-bar. Preferred values for the transition enthalpies, together with these slopes and the Clausius-Clapeyron equation, yield estimates for the volume changes at the transitions of 0.01 (lower) and 0.15 to 0.25 (upper) cm³ gfw^–1. These calculated volume changes are not consonant with many of the high temperature volumetric data on tridymites of varying origins.     

Equation of state,structural behaviour and phase diagram of synthetic MgFe<Subscript>2</Subscript>O<Subscript>4</Subscript>, as a function of pressure and temperature

The behaviour of synthetic Mg-ferrite (MgFe₂O₄) has been investigated at high pressure (in situ high-pressure synchrotron radiation powder diffraction at ESRF) and at high temperature (in situ high-temperature X-ray powder diffraction) conditions. The elastic properties determined by the third-order Birch–Murnaghan equation of state result in K₀=181.5(± 1.3) GPa, K=6.32(± 0.14) and K= –0.0638 GPa^–1. The symmetry-independent coordinate of oxygen does not show significant sensitivity to pressure, and the structure shrinking is mainly attributable to the shortening of the cell edge (homogeneous strain). The lattice parameter thermal expansion is described by _a0+_a1*(T–298)+_a2/(T–298)², where _a0=9.1(1) 10^–6 K^–1, _a1=4.9(2) 10^–9 K^–2 and _a2= 5.1(5) 10^–2 K. The high-temperature cation-ordering reaction which MgFe-spinel undergoes has been interpreted by the ONeill model, whose parameters are = 22.2(± 1.8) kJ mol^–1 and =–17.6(± 1.2) kJ mol^–1. The elastic and thermal properties measured have then been used to model the phase diagram of MgFe₂O₄, which shows that the high-pressure transition from spinel to orthorombic CaMn₂O₄-like structure at T < 1700 K is preceded by a decomposition into MgO and Fe₂O₃.     

Atmospheric C variations derived from tree rings during the early Younger Dryas

Atmospheric radiocarbon variations over the Younger Dryas interval, from 13,000 to 11,600 cal yr BP, are of immense scientific interest because they reveal crucial information about the linkages between climate, ocean circulation and the carbon cycle. However, no direct and reliable atmospheric ¹⁴C records based on tree rings for the entire Younger Dryas have been available. In this paper, we present (1) high-precision ¹⁴C measurements on the extension of absolute tree-ring chronology from 12,400 to 12,560 cal yr BP and (2) high-precision, high-resolution atmospheric ¹⁴C record derived from a 617-yr-long tree-ring chronology of Huon pine from Tasmania, Australia, spanning the early Younger Dryas. The new tree-ring ¹⁴C records bridge the current gap in European tree-ring radiocarbon chronologies during the early Younger Dryas, linking the floating Lateglacial Pine record to the absolute tree-ring timescale. A continuous and reliable atmospheric ¹⁴C record for the past 14,000 cal yr BP including the Younger Dryas is now available. The new records indicate that the abrupt rise in atmospheric Δ¹⁴C associated with the Younger Dryas onset occurs at 12,760 cal yr BP, 240 yrs later than that recorded in Cariaco varves, with a smaller magnitude of 40‰ followed by several centennial Δ¹⁴C variations of 20–25‰. Comparing the tree-ring Δ¹⁴C to marine-derived Δ¹⁴C and modelled Δ¹⁴C based on ice-core ¹⁰Be fluxes, we conclude that changes in ocean circulation were mainly responsible for the Younger Dryas onset, while a combination of changes in ocean circulation and ¹⁴C production rate were responsible for atmospheric Δ¹⁴C variations for the remainder of the Younger Dryas.     

Subseabed disposal of radioactive waste: effects of consolidational fluid flow

Subseabed disposal of radioactive waste applies a multiple-barrier concept with the sediment being the most important barrier for preventing a release of nuclides into the biosphere. While many investigations have been carried out to analyze the risk potential in this type of disposal, the effects of sediment consolidation and associated fluid flow have not fully been taken into consideration. Here, possible effects of consolidational fluid flow in the penetrator disposal option and possible consequences to the transport of nuclides in the sediment are analyzed. Results of numerical experiments demonstrate that consolidation contributes to the transport of radioactive nuclides released from containers buried in the sediment and to the release of nuclides at the sediment-water interface. Both depend on geological conditions and to a large extent on possible alterations of hydraulic conductivity i of the sediment in the vicinity of the entry path of a penetrator.Symbols c concentration ml m^–3 - c _a concentration of adsorbed solute mg kg^–1 (relative to dry weight of sorbing substance) - c _in solute concentration of source q mg m^–3 - c ₀ initial concentration mg m^–3 - ID dispersion tensorm ²s^–1 - ID ^* diffusion tensor m²s^–1 - D coefficient of dispersion m²s^–1 - d ₀ coefficient of molecular diffusion m²s^–1 - d coefficient of effective diffusion m²s^–1 - g gravity m²s^–1 - h piezometric pressure m - k hydraulic conductivity m²s^–1 - m mass kg - p pressure Pa - q source/sink m³s^–1 - S ₀ specific surface m²m^–3 - t time s - v velocity m s^–1 - x, z cartesian coordinates m - compressibiliy of sediment m²N^–1 - _L longitudinal dispersivity m - effective porosity (decimal fraction) - density kg m^–3 - _s density of sediment kg m^–3 - _w density of water kg m^–3 - decay constant per s - kinematic viscosity m²s^–1     

Genetic environment of germanium-bearing gold-silver vein ores from the Wolyu mine,Republic of Korea

The Wolyu mine is one of the largest vein-type gold-silver-bearing epithermal systems in the Youngdong district and is the first gold-silver deposit in Korea found to contain significant germanium, in the form of argyrodite (Ag₈GeS₆). Mineralized veins (78.9 ± 1.2 Ma) crosscutting Late Cretaceous hostrock tuff and quartz porphyry (81.5 ± 1.8 Ma) consist of three stages of quartz and carbonates, the first of which contains pyrite, basemetal sulfides and Au-Ag-minerals. Stage I Au-Ag-Ge-mineralized veins show a systematic variation of mineral assemblage with time: (1) quartz + pyrite; (2) quartz + pyrite + sphalerite + electrum + argentite; (3) carbonate + quartz + sphalerite + electrum + argentite; (4) carbonate + native silver + argentite + Ag-sulfosalts + argyrodite + sphalerite. Calculated values of temperature and sulfur activity are: assemblage (1), 360-280°C and 10^–7-10^–10; (2), 280-210°C and 10^–10-10^–14; (3), 210-180°C and 10^–14-10^–16; (4), 180-155°C and 10^–17-10^–18. These data, the frequent association of gold with sulfides, and the abundance of pyrite in alteration zones indicate that decreasing sulfur activity and cooling were important in triggering gold deposition. Hydrogen and oxygen isotope compositions of ore fluids display a systematic variation with increasing time. Within the main Ag-Au-Ge mineralization, D and ¹⁸O values decrease with the transition from quartz to carbonate deposition (from -78 and –2.8% to –90 and –8.7%., respectively), indicating increasing involvement (mixing) of less evolved meteoric water which resulted in progressive cooling and dilution of ore fluids in the shallow ( 370–600 m) Wolyu epithermal system.     

Single crystal infrared spectra in the range of OH fundamentals of paragenetic garnet,omphacite and kyanite in an eklogitic mantle xenolith

Summary Single crystals of paragenetic garnet (gt), omphacite (cpx) and kyanite (ky) were isolated from an eclogite xenolith from the Zagadochnaya kimberlite, Yakutia (grospydite Z13) and studied by polarized FTIR-microspectrometry in the OH valence vibrational region and by microprobe analyses. The coexisting minerals are homogeneous with respect to major and minor elements and have compositions near gross₄₉pyr₂₆alm₂₀uvar₄ (gt), jad₄₅dio₄₇hed₆kos₂ (cpx) and ky_>97. Single crystal spectra show one _OH-band for gt at 3630 cm^–1 (halfwidth ca. 100 cm^–1) which is very likely caused by vibrations of tetrahedral (OH)₄-^4– clusters replacing SiO₄ ^4– tetrahedra. Cpx shows one strong, but weakly pleochroic band at 3464 cm^–1 (halfwidth ca. 160 cm^–1) and a weak satellite band centered at 3620 cm^–1) with a distinct pleochroism. Ky OH spectra exhibit two sets of weak sharp pleochroic bands, a triplet, characteristic for high pressure ky, at 3439, 3410, 3387 cm^–1) and a doublet at 3279, 3264 cm^–1) (halfwidths ca. 10 cm^–1) From integral and linear absorbances in the unpolarized spectra defect-hydroxyl contents in the three coexisting minerals were estimated and found near O.OX wt% H₂O. The distribution scheme of hydrogen in the paragenetic minerals was evaluated to be c^ky < c^cpx < c^gt.

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Einkristall Infrarotspektren im Bereich der OH Grundschwingungen einer Granat-Omphacit-Kyanit Paragenese in einem eklogitischen Mantelxenolith

Zusammenfassung Aus einer Granat (Gt)-Omphacit (Cpx)-Kyanit (Ky) Paragenese eines Eklogit Xenoliths aus dem Zagadochnaya Kimberlit, Jakutien (Grospydit Z13), wurden Einkristalle isoliert und mit polarisierter FTIR-Mikrospektrometrie im OH Valenzschwingungsbereich und mit der Elektronenstrahl-Mikrosonde untersucht. Die koexistierenden Minerale sind hinsichtlich ihrer Haupt- und Nebenelemente homogen und haben Zusammen setzungen von etwa Gross⁴⁹Pyr²⁶Alm²⁰Uvar₄ (Gt), Jad₄₅Dio₄₇Hed₆Kos₂ (Cpx) and Ky_>97 Einkristallspektren von Gt zeigen eine _OH-Bande bei 3630 cm^–1) (Halbwerts breite ca. 100 cm^–1) die wahrscheinlich von Schwingungen tetraedrischer (OH)₄ ^4– Gruppen, die SiO₄ ^4– Tetraeder ersetzen, herrührt. Cpx zeigt eine starke, aber schwach pleochroitische Bande bei 3464 cm^–1 (Halbwertsbreite ca. 160 cm^–1) and eine schwache, deutlich pleochroitische Satellitenbande bei 3620 cm^–1) Ky OH Spektren zeigen zwei Gruppen von schwachen, scharfen pleochroitischen Banden, ein fur Hochdruck Ky charakteristisches Bandentriplett bei 3439, 3410, 3387 cm^–1) and ein Bandendublett bei 3279, 3264 cm^–1) (Halbwertsbreiten ca. 10 cm^–1) Aus den integralen and linearen Extinktionen der nicht-polarisierten Spektren wurde der Defekt-Hydroxyl Gehalt der drei koexistierenden Minerale abgeleitet and mit O.OX Gew% H₂O festgelegt. Das Verteilungsschema des Wasserstoffs kann in der Mineralparagenese mit c^Ky < c^Cpx c^Gt angegeben werden.

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Dedicated to Prof. Dr. J. Zemann on the occasion of his 70th birthday

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With 3 Figures     

The pleochroism of a gem-quality enstatite in the region of the OH stretching frequency,with a stereochemical interpretation

Summary The pleochroic behaviour of a gem-quality enstatite from Tanzania was investigated in the region =2500 to 4000 cm^–1. Two sharp absorption bands at =3410 cm^–1 and =3510 cm^–1 are interpreted to be caused by OH stretching vibrations. As their absorption coefficients are considerably larger parallel to [001] (=direction of the silicate chains) than perpendicular to this direction, the OH dipoles have to be oriented approximately parallel to [001]. On this basis, a stereochemical interpretation of the incorporation of the OH groups into the structure is given.

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Der Pleochroismus eines Enstatits von Edelsteinqualität im Gebiet der OH-Streckfrequenz und seine kristallchemische Interpretation

Zusammenfassung Das pleochroitische Verhalten eines Enstatits von Edelsteinqualität aus Tansania wurde im Bereich =2500 bis 4000 cm^–1 untersucht. Von zwei scharfen Banden bei =3410 cm^–1 und 3510 cm^–1 wird angenommen, daß sie durch OH-Streckschwingungen herrühren. Da ihre Absorptionskoeffizienten parallel zu [001] (=Richtung der Silikatketten) beträchtlich größer sind als senkrecht dazu, müssen die OH-Dipole ungefähr parallel zu [001] liegen. Auf dieser Basis wird der Einbau der OH-Gruppen in die Struktur diskutiert.

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With 3 Figures     

Glassy orthopyroxene granodiorites of the Pannonian Basin: tracers of ultra-high-temperature deep-crustal anatexis triggered by Tertiary basaltic volcanism

Glassy orthopyroxene granodiorite-tonalite (named pincinite after type locality) was described from basaltic lapilli tuffs of the Pliocene maar near Pinciná village in the Slovakian part of the Pannonian Basin. Two pincinite types exhibit a qualitatively similar mineral composition (quartz, An_20–55 plagioclase, intergranular silicic glass with orthopyroxene and ilmenite, ±K-feldspar), but strongly different redox potential and formation PT conditions. Peraluminous pincinite is reduced (6–7% of total iron as Fe³⁺ in corundum-normative intergranular dacitic glass) and contains ilmenite with 8–10 mol% Fe₂O₃ and orthopyroxene dominated by ferrosilite. High-density (up to 0.85 g/cm³) primary CO₂ inclusions with minor H₂, CH₄, H₂S, CO and N₂ (<2 mol% total) are present in Qtz and Plg. Equilibrium PT conditions inferred from the intergranular Opx–Ilm–Glass assemblage and fluid density correspond to 1,170±50°C, 5.6±0.4 kbar, respectively. Metaluminous pincinite is more oxidised (25–27% of total iron as Fe³⁺ in diopside-normative intergranular glass of rhyolite–trachyte–dacite composition) and contains Fe₂O₃-rich ilmenite (17–29 mol%) associated with enstatite. Fluid inclusions are composed of CO₂–H₂O mixtures with up to 38 mol% H₂O. Raman spectroscopy revealed H₂S along with dominant CO₂ in the carbonic phase. Equilibrium PT parameters for the intergranular Opx–Ilm–Glass assemblage correspond to 740±15°C, 2.8±0.1 kbar, respectively. Reducing gas species (<2 mol% total) in the CO₂-inclusions of the peraluminous pincinite resulted from hydrogen diffusion due to fH₂ gradient imposed during decrease of redox potential from the log fO₂ values near QFM during Qtz + Plg growth, to QFM-2 incidental to the superimposed Opx + Ilm assemblage in the intergranular melt. The decrease in oxygen fugacity was recorded also in the metaluminous pincinite, where log fO₂ values changed from ~QFM + 2.6 to QFM + 0.4, but hydrogen diffusion did not occur. Absence of OH-bearing minerals, major and trace element abundances (e.g. REE 300–320, Nb 55–57, Th 4–31, Zr 240–300 ppm, FeO_tot/MgO up to 11), and Sr–O isotope ratios in the pincinites are diagnostic of high-temperature anorogenic magmas originated by dehydration melting of biotite in quartz-feldspathoid crust (⁸⁷Sr/⁸⁶Sr>0.705–0.706, ¹⁸O>9 V-SMOW) around alkali basalt reservoir in depths between 17 and 20 km, and around late stage derivatives of the basalt fractionation, intruding the crust up to depths of 10–11 km. Low water activity in the pincinite parental melt was caused by CO₂-flux from the Tertiary basaltic reservoirs and intrusions. The anatexis leads to generation of a melt-depleted granulitic crust beneath the Pannonian Basin, and the pincinites are interpreted as equivalents of igneous charnockites and enderbites quenched at temperatures above solidus and unaffected by sub-solidus re-equilibration and metamorphic overprint.     

Relationships among Gibbs free energies and enthalpies of formation of phosphates,oxides and aqueous ions

Two parameters _GO^2– and _HO^2– are defined as the differences between respectively the Gibbs free energies and the enthalpies of formation of an oxide and its corresponding aqueous cation. The Gibbs free energies and enthalpies of formation of phosphates from their consituent oxides are shown to be linear functions of respectively _GO^2– and _HO^2– of their constituent cations.     

Energy associated with dislocations: A calorimetric study using synthetic quartz

High temperature oxide melt solution calorimetry was used to study the energy associated with dislocations in quartz by comparing undeformed and deformed single crystals of synthetic quartz. Samples were deformed at 698 K, 1000–1500 MPa at a strain rate of 10^–5 sec^–1. Two sets of calorimetric measurements were made: (i) using a Pt capsule as a container for powdered sample, and (ii) using pellets made from sample powder without any container. For the first set of measurements, the undeformed sample with a dislocation density of enthalpy is sum of heat content H ₉₇₃-H ₂₉₅ and enthalpy of solution in molten lead borate at 973 K of 39.22 ± 1.00 kJ mol^–1, while the sample deformed in the dislocation creep regime with a dislocation density of 6 × 10¹⁰ to 1 × 10¹¹ cm^–2 gave an enthalpy of 38.59 ± 0.78 kJ mol^–1. For the second set of measurements the measured enthalpy of the undeformed sample was 38.87 ± 0.31 kJ mol^–1, and that of a deformed sample with a dislocation density of 3 × 10¹⁰ to 1 × 10¹¹ cm^–2 was 38.24 ± 0.58 kJ mol^–1.The present study and previous theoretical calculations and estimates are consistent and suggest that the energy associated with dislocations in quartz is 0.6 ± 0.6 kJ mol^–1 for a dislocation density of 10¹¹ cm^–2; a precise value is difficult to determine because of the overlapping errors. These results indicate that for geologically realistic dislocation densities, the maximum excess energy due to dislocations would be 0.5 kJ mol^–1 for most minerals; the exact value would depend on the Burgers vector as well as the shear modulus.     

Mg–Fe interdiffusion rates in wadsleyite and the diffusivity jump at the 410-km discontinuity

Mg–Fe interdiffusion rates have been measured in wadsleyite aggregates at 16.0–17.0 GPa and 1230–1530 °C by the diffusion couple method. Oxygen fugacity was controlled using the NNO buffer, and water contents of wadsleyite were measured by infrared spectroscopy. Measured asymmetric diffusion profiles, analyzed using the Boltzmann–Matano equation, indicate that the diffusion rate increases with increasing iron concentration and decreasing grain size. In the case of wadsleyite containing 50–90 weight ppm H₂O, the Mg–Fe interdiffusion coefficients at compositions of Mg/(Mg + Fe)=0.95 in the coarse-grained region (about 60 m) and 0.90 in the fine-grained region (about 6 m) were determined to be a D_Xmg = 0.95 (m² s^–1)=1.24 × 10^–9 exp[–172 (kJ mol^–1)/RT] and D_Xmg = 0.90 (m² s^–1)=1.77 × 10^–9 exp[–143 (kJ mol^–1)/RT], respectively. Grain-boundary diffusion rates were estimated to be about 4 orders of magnitude faster than the volume diffusion rate. Grain-boundary diffusion dominates when the grain size is less than a few tens of microns. Results for the nominally dry diffusion couple in the present study are roughly consistent with previous studies, taking into account differences in pressure and grain size, although water contents of samples were not clear in previous studies. We observed that the diffusivity is enhanced by about 1 order of magnitude in wadsleyite containing 300–2100 wt. ppm H₂O at 1230 °C, which is almost identical to the enhancement associated with a 300 °C increase in temperature. It is still not conclusive that a jump in diffusivity exists between olivine and wadsleyite because water contents of olivine in previous diffusion studies and effects of water on the olivine diffusivity are uncertain.     

Synthesis and upper stability of paragonite

The mineral paragonite, NaAl₂[AlSi₃O₁₀ (OH)]₂, has been synthesized on its own composition starting from a variety of different materials. Indexed powder data and refined cell parameters are given for both the 1M and 2M₁ polymorphs obtained. The upper stability limit of paragonite is marked by its breakdown to albite + corundum + vapour. The univariant equilibria pertaining to this reaction have been established by reversing the reaction at six different pressures, the equilibrium curve running through the following intervals: 1 kb: 530°–550° C 2 kb: 555°–575° C 3 kb: 580°–600° C 5kb: 625°–640° C 6 kb: 620°–650° C 7 kb: 650°–670° C.Comparison with the upper stability limit of muscovite (Velde, 1966) shows that paragonite has a notably lower thermal stability thus explaining the field observation that paragonite is absent in many higher grade metamorphic rocks in which muscovite is still stable.The enthalpy and entropy of the paragonite breakdown reaction have been estimated. Since intermediate albites of varying structural states are in equilibrium with paragonite, corundum and H₂O along the univariant equilibrium curve, two sets of data pertaining to the entropy of paragonite (S ₂₉₈ ⁰ ) as well as the enthalpy ( H _f,298 ⁰ ) and Gibbs free energy ( G _f,298 ⁰ ) of its formation were computed, assuming (1) high albite and (2) low albite as the equilibrium phase. The values are: (1) (2) S ₂₉₈ ⁰ 67.8±3.9 cal deg^–1 gfw^–1 63.7±3.9 cal deg^–1 gfw^–1 H _f,298 ⁰ –1417.9±2.7 kcal gfw^–1 –1420.2±2.6 kcal gfw^–1 G _f,298 ⁰ –1327.4±4.0 kcal gfw^–1 –1328.5±4.0 kcal gfw^–1.Adapted from a part of the author's Habilitationsschrift accepted by the Ruhr University, Bochum (Chatterjee, 1968).     

The role of hydrogen bonding in the thermal expansion and dehydration of brushite,di-calcium phosphate dihydrate

The unit-cell and atomic parameters of perdeuterated brushite have been extracted from Rietveld analysis of neutron powder diffraction data within the temperature range 4.2 to 470 K. The thermal expansion of brushite is anisotropic, with the largest expansion along the b axis due principally to the effect of the O(1)···D(4) and O(3)···D(2) hydrogen bonds. Expansion along the c axis, influenced by the Ow1···D(5) interwater hydrogen bond, is also large. The high temperature limits for the expansion coefficients for the unit-cell edges a, b and c are 9.7(5) × 10^–6, 3.82(9) × 10^–5 and 5.54(5) × 10^–5 K^–1, respectively, and for the cell volume it is 9.7(1) × 10^–5 K^–1. The angle displays oscillatory variation, and empirical data analysis results in = 1.28(3) × 10^–6sin(0.0105 T) K^–1, within this temperature range. The evolution of the thermal expansion tensor of brushite has been calculated between 50 T 400 K. At 300 K the magnitudes of the principal axes are ₁₁ = 50(6) × 10^{–6 K–1}, ₂₂ = 26.7(7) × 10^–6 K^–1 and ₃₃ = 7.0(5) × 10^–6 K^–1. The intermediate axis, ₂₂, is parallel to b, and using IRE convention for the tensor orthonormal basis, the axes ₁₁ and ₃₃ have directions equal to (–0.228, 0, –0.974) and (–0.974, 0, 0.228) respectively. Under the conditions of these experiments, the onset of dehydration occurred at temperatures above 400 K. Bond valence analysis combined with assessments of the thermal evolution of the bonding within brushite suggests that dehydration is precipitated through instabilities in the chemical environment of the second water molecule.     

Paragenesis and conditions of formation of the Moschellandsberg mercury deposit,SW Germany

The Moschellandsberg mercury deposit (SW Germany) is hosted in a Permo-Carboniferous volcanic caldera and has formed at shallow depths (100–300 m) and over a wide temperature range from about 200°C to less than 65°C. The deposit shows vertical zonation and the mineral paragenesis is unusually complex and contains significant amounts of Sb, As, Cu, and Ag. Fahlores are Hg-and Fe-rich Sb (Bi) and As members displaying a mixing gap. Vesicular ore textures and limited fluid inclusion evidence prove the existence of an immiscible gas phase during ore deposition. The fluid composition can be constrained to pH4, H₂S10^–5 molal, CO₂10^–1 molal, and f_H210^–2–10^–3 bar by comparing the observed mineral associations with stable phase assemblages calculated from thermodynamic data. Calculated metal solubilities indicate that Hg, Ag, and Sb can be transported very efficiently in such a fluid, but that the solubility of gold is very low.