Cooling history of lunar Mg-suite gabbronorite 76255, troctolite 76535 and Stillwater pyroxenite SC-936: The record in exsolution and ordering in pyroxenes

We have determined cooling rates of orthopyroxene crystals from two Mg-suite lunar samples (gabbronorite 76255 and troctolite 76535) and one terrestrial sample (orthopyroxenite SC-936 from the Stillwater Complex), on the basis of their Fe–Mg ordering states. In addition, a cooling rate of 76255 was determined by modeling the formation of exsolution lamellae in pyroxenes. The M1–M2 site occupancies of the orthopyroxene crystals were determined by single crystal X-ray diffraction and the rate constant for the ordering reaction was used along with calibrations of the equilibrium intracrystalline fractionation of Fe and Mg as a function of temperature to calculate cooling rates. The closure temperatures (T_C) of cation ordering are 525 °C for 76255, 500 °C for 76535 and 350 °C for SC-936 corresponding to cooling rates of 4 × 10⁻² °C/year at the closure temperature for the lunar samples and 10⁻⁶ °C/year for the Stillwater sample. A cooling rate for 76255, determined by simulating the exsolution process, is 1.7 × 10⁻² °C/year at a closure temperature for exsolution of 700 °C. The Fe–Mg ordering cooling rate determined for 76535 reflects a complex thermal history superimposed on the initial plutonic provenance established for this sample [McCallum, I.S., Schwartz, J.M., 2001. Lunar Mg suite: thermobarometry and petrogenesis of parental magmas. J. Geophys. Res. 106, 27969–27983]. The preservation of a crystallization age of 4.51 Ga and a metamorphic age of 4.25 Ga for 76535 is consistent with a model in which excavation of this sample from the lower lunar crust took place while the sample was at a temperature above the closure temperatures for the Sm–Nd, U–Pb and Ar–Ar isotopic systems. Temperatures in excess of the isotopic closure temperatures (i.e., >600 °C) in the lower lunar crust were maintained by heat diffusing from concentrations of U- and Th-rich KREEP material at the base of the crust. On the other hand, 76255 formed at a much shallower depth in the lunar crust (2 km) and was well below its isotopic closure temperatures at the time of excavation, most likely during the Serenitatis basin-forming impact event. Both lunar samples were reheated during transport to the surface and deposition in hot ejecta blankets. The reheating was short lived but apparently sufficient to redistribute Fe and Mg in M sites in orthopyroxenes. For the lunar samples, the cooling rates based on Fe–Mg ordering represent final stage cooling within an ejecta blanket.     

Solubility of silica polymorphs in electrolyte solutions, I. Activity coefficient of aqueous silica from 25° to 250°C, Pitzer''s parameterisation

Within the framework of Pitzer's specific interaction model, interaction parameters for aqueous silica in concentrated electrolyte solutions have been derived from Marshall and co-authors amorphous silica solubility measurements. The values, at 25°C, of the Pitzer interaction parameter (λ_SiO2(aq)−i) determined in this study are the following: 0.092 (i = Na⁺), 0.032 (K⁺), 0.165 (Li⁺), 0.292 (Ca²⁺, Mg²⁺), −0.139 (SO₄²⁻), and −0.009 (NO₃⁻). A set of polynomial equations has been derived which can be used to calculate λ_SiO2(aq)−i for these ions at any temperature up to 250°C. A linear relationship between the aqueous silica-ion interaction parameters (λ_SiO2(aq)−i) and the surface electrostatic field (Z_i/r_e,i) of ions was obtained. This empirical equation can be used to estimate, in first approximation, λ_SiO2(aq)−i if no measurements are available. From this parameterisation, the calculated activity coefficient of aqueous silica is 2.52 at 25°C and 1.45 at 250°C in 5 m NaCl solution. At lower concentrations, e.g. 2 m NaCl, the activity coefficient of silica is 1.45 at 25°C and 1.2 at 250°C. Hence, in practice, it is necessary to take into account the activity coefficient of aqueous silica (λ_SiO2(aq)≠1) in hydrothermal solutions and basinal brines where the ionic strength exceeds 1. A comparison of measured [Marshall, W.L., Chen, C.-T.A., 1982. Amorphous silica solubilities, V. Prediction of solubility behaviour in aqueous mixed electrolyte solutions to 300°C. Geochim. Cosmochim. Acta 46, 289–291.] and computed amorphous silica solubility, using this parameterisation, shows a good agreement. Because the effect of individual ions on silicate and silica polymorph solubilities are additive, the present study has permitted to derive Pitzer interaction parameters that allow a precise computation of γ_SiO2(aq) in the Na---K---Ca---Mg---Cl---SO₄---HCO₃---SiO₂---H₂O system, over a large range of salt concentrations and up to temperatures of 250°C.     

Low temperature Phanerozoic history of the Northern Yilgarn Craton, Western Australia

The Phanerozoic cooling history of the Western Australian Shield has been investigated using apatite fission track (AFT) thermochronology. AFT ages from the northern part of the Archaean Yilgarn Craton, Western Australia, primarily range between 200 and 280 Ma, with mean confined horizontal track lengths varying between 11.5 and 14.3 μm. Time–temperature modelling of the AFT data together with geological information suggest the onset of a regional cooling episode in the Late Carboniferous/Early Permian, which continued into Late Jurassic/Early Cretaceous time. Present-day heat flow measurements on the Western Australian Shield fall in the range of 40–50 mW m⁻². If the present day geothermal gradient of  18 ± 2 °C km⁻¹ is representative of average Phanerozoic gradients, then this implies a minimum of  50 °C of Late Palaeozoic to Mesozoic cooling. Assuming that cooling resulted from denudation, the data suggest the removal of at least 3 km of rock section from the northern Yilgarn Craton over this interval. The Perth Basin, located west of the Yilgarn Craton, contains up to 15 km of mostly Permian to Lower Cretaceous clastic sediment. However, published U–Pb data of detrital zircons from Permian and Lower Triassic basin strata show relatively few or no grains of Archaean age. This suggests that the recorded cooling can probably be attributed to the removal of a sedimentary cover rather than by denudation of material from the underlying craton itself. The onset of cooling is linked to tectonism related to either the waning stages of the Alice Springs Orogeny or to the early stages of Gondwana breakup.     

P–T–t paths and post-metamorphic exhumation of the Day Nui Con Voi shear zone in Vietnam

The Day Nui Con Voi belt in Vietnam is the southeasternmost part of the Red River shear zone in Asia. It is a narrow high-grade metamorphic core complex consisting of garnet–sillimanite–biotite gneisses, mylonite bands, amphibolite layers and migmatites. Geothermobarometric study of the complex revealed that the peak metamorphism took place under amphibolite-facies conditions of 690₋₆₀⁺³⁰°C and 0.65±0.15 GPa and the subsequent mylonitization occurred under greenschist-facies conditions of 480°C and under 0.3 GPa. Fifteen synkinematic hornblende and biotite separates from gneisses, amphibolites and mylonites were dated with the K/Ar method. Hornblende separates from the Day Nui Con Voi give K–Ar ages of 26.4–28.5 Ma, and the biotite separates do give 24.5–24.7 Ma. Combination of thermobarometric and geochronological data yields the cooling history of 500°C at 28 Ma and 300°C at 24 Ma with a cooling rate of 70–110°C Ma⁻¹, and 23 km post-metamorphic exhumation of the core complex. The first 16 km exhumation from the peak of metamorphism (at probably 31 Ma) to 28 Ma was triggered by the left-lateral strike-slip displacement of the Red River shear zone.     

Brine substitute liquids for soil freezing at very low temperatures

The strength of a frozen soil increases with decreasing temperature. Furthermore, the speed it takes to form a frozen wall increases on lowering the temperature of the freezing liquid.

With the traditional freezing systems using brine it is difficult to work with temperatures below −30°C. To go lower than this limit, it is necessary to substitute the brine by using freezing liquids that maintain good hydraulic and thermal characteristics at much lower temperatures.

Different organic liquids have been tested and good results have been obtained with some aromatic hydrocarbon mixes from the terpene family.

As a result of the research, for practical purposes a by-product ofa distilling citrus fruit skins has been selected. This liquid solidifies at −100°C approx. and maintains a low viscosity rate below −30°C.

The present paper describes the thermal and hydraulic properties of this product as a function of the temperature concerned and compares them to the same properties of classic brines of CaCl₂.     

Gas pressures and redox reactions in geothermal fluids in Iceland

The gas and redox chemistry of 100–300 °C geothermal fluids in Iceland has been studied as a function of fluid temperature and fluid composition. The partial pressures of CO₂ in dilute (mCl<500 ppm) and saline (mCl>500 ppm) geothermal fluids above 200 °C are controlled by the mineral buffer clinozoisite+prehnite+calcite+quartz. Two buffers are considered to control the H₂S and H₂ partial pressures above 200 °C depending on fluid salinity, epidote+prehnite+pyrite+pyrrhotite for dilute fluids and pyrite+prehnite+quartz+magnetite+anhydrite+clinozoisite+quartz for saline fluids. Below 200 °C, the partial pressures of CO₂, H₂S and H₂ also seem to be buffered but other minerals must be involved. Zeolites are expected to replace prehnite and epidote. Redox potential calculated on the assumption of equilibrium for the H⁺/H₂ redox couple decreases in dilute geothermal fluids with increasing temperature from about −0.5 V at 100 °C to −0.8 V at 300 °C, whereas saline geothermal fluids at 250 °C display a redox potential of about −0.45 V. A systematic discrepancy between redox couples of about 0.05–0.09 V is observed in the redox potential for the dilute geothermal fluids, whereas redox potentials agree within 0.02–0.04 V for saline geothermal waters. The discrepancies in the calculated redox potential for dilute geothermal fluids are thought to be due to a general lack of equilibrium between CH₄, CO₂ and H₂ and between H₂S, SO₄ and H₂. It is, accordingly, concluded that an overall equilibrium among redox species has not been reached for dilute geothermal fluids whereas it appears to be more closely approached for the saline geothermal fluids. The latter conclusion is based on limited database and should be treated with care. Since the various redox components are not in an overall equilibrium in geothermal fluids in Iceland these fluids cannot be characterised by a unique hydrogen fugacity, oxygen fugacity or redox potential at a given temperature and pressure.     

Correlations between metamorphic events and Rb-Sr ages in metasediments and eclogite from Western Tasmania

The Strathgordon area of low metamorphic grade (450±50°C, 4±1 Kb) and the eclogitebearing Lyell Highway-Collingwood River area, which is of higher grade (670±20°C, 11±1 Kb), have been studied by the Rb-Sr technique.

Three deformational events, D₁ to D₃ correlate in time with two important P,T events, here termed D₂₋₂ and D₃, D₁₋₂ reflects the continuous prograde P,T environment up to the metamorphic maximum of the two areas (D₂), while D₂ is a superimposed deformational event occuring at lower P,T conditions. According to stratigraphical constraints D₃ must have occured before or about 550 m.y. ago.

It is possible from rather complex Rb-Sr data to indicate ages for the D_1–2 and D₃ events. Two-point total rock+phengite (mica) ages together with total rock isochrons using closely spaced samples show that the main metamorphic event D_1–22 occurrred at about 800 m.y. in both areas. Similar data suggest that the superimposed event D₃ occurred between 550 and 630 m.y. ago, probably near the lower age limit. This deformation (crenulation cleavage) was responsible for the local resetting of total rock and mineral systems.

Consistent with later Palaeozoic events, the Cambrian-early Ordovician Jukesian Movement and the Devonian Tabberabberan Orogeny, which affected some Rb-Sr mineral systems (phengite, amphibole and chlorite), have ages of 500±20 m.y. and 385-350 m.y. respectively.

Due to dispersion of the eclogite total rock data points along secondary isochrons it is difficult to estimate an original age. The age of the original minerals must however have been set by the 800 m.y. D_1–2 event, as the P,T history of the eclogite is the same as that of the surrounding schists.     

Oxygen and carbon isotope composition of cold-water Berriedale Limestone (Lower Permian), Tasmania, Australia

The Berriedale Limestone formed at about 80°S paleolatitude and contains many glacial dropstones. It formed during a period of major Gondwana deglaciation.

The Berriedale Limestone contains mostly bryozoans, brachiopods and bivalves, with some intraclasts and rare pellets. The faunal diversity is low and the fauna are similar to the modern cold-water foramol faunal assemblage. Micrite, microspar and spar occur as equant to well developed rhombs of calcite. The coarse spar cements are bored and are ruptured by dropstones, indicating submarine origin of low-Mg calcite at water-temperatures of around 3°C. The mixing zone cementation was preceded by erosion of early formed crystals. The eroded crystals occur as inclusions in mixing zone cements.

The fauna are characterized by heavy δ¹³C and light δ¹⁸O. The whole-rock field of δ¹⁸O-δ¹³C falls at the edge of “Normal Marine Limestone” and deviates to lighter δ¹⁸O values (down to −16.7‰ PDB). Lightest δ¹⁸O values ( −22‰ PDB) of fresh-water sparry calcite cement are similar to those in the Early Permian continental tillites, suggesting that the Permian sea was diluted by isotopically light melt waters. Micrite δ¹⁸O values (−9.2 to −12.6‰ PDB) are within the range of whole-rock values. The δ¹⁸O values of calcite in shales are lighter than limestone values.

The δ¹⁸O values of the fauna give an unrealistic range of sea-water temperatures because the fauna have equilibrated with variable amounts of melt waters. However, calculated original δ¹⁸O values of the fauna indicate temperatures < 4°C. The heaviest δ¹⁸O of fauna gives cold temperatures of 9°C (with δ_w −2.8‰) and −3°C (with δ_w −6‰). The lightest values of sparry calcite cements (−22‰ PDB) indicate that the limestone reacted with cold melt waters.

The δ¹⁸O of Permian sea is estimated to be about +1.2‰ and was diluted by melt waters as light as −27‰ SMOW.     

Fluid inclusions in Scourian granulites from the Lewisian complex of NW Scotland: evidence for CO2-rich fluid in Late Archaean high-grade metamorphism

In this paper the first fluid-inclusion data are presented from Late Archaean Scourian granulites of the Lewisian complex of mainland northwest Scotland. Pure CO₂ or CO₂-dominated fluid inclusions are moderately abundant in pristine granulites. These inclusions show homogenization temperatures ranging from − 54 to + 10 °C with a very prominent histogram peak at − 16 to − 32 °C. Isochores corresponding to this main histogram peak agree with P-T estimates for granulite-facies recrystallization during the Badcallian (750–800 °C, 7–8 kbar) as well as with Inverian P-T conditions (550–600 °C, 5 kbar). The maximum densities encountered could correspond to fluids trapped during an early, higher P-T phase of the Badcallian metamorphism (900–1000 °C, 11–12 kbar). Homogenization temperatures substantially higher than the main histogram peak may represent Laxfordian reworking (≤ 500 °C, < 4 kbar). In the pristine granulites, aqueous fluid inclusions are of very subordinate importance and occur only along late secondary healed fractures. In rocks which have been retrograded to amphibolite facies from Inverian and/or Laxfordian shear zones, CO₂ inclusions are conspicuously absent; only secondary aqueous inclusions are present, presumably related to post-granulite hydration processes. These data illustrate the importance of CO₂-rich fluids for the petrogenesis of Late Archaean granulites, and demonstrate that early fluid inclusions may survive subsequent metamorphic processes as long as no new fluid is introduced into the system.     

The continental tectosphere and Earth's long-wavelength gravity field

To estimate the average density contrast associated with the continental tectosphere, we separately project the degree 2–36 non-hydrostatic geoid and free-air gravity anomalies onto several tectonic regionalizations. Because both the regionalizations and the geoid have distinctly red spectra, we do not use conventional statistical analysis, which is based on the assumption of white spectra. Rather, we utilize a Monte Carlo approach that incorporates the spectral properties of these fields. These simulations reveal that the undulations of Earth's geoid correlate with surface tectonics no better than they would were it randomly oriented with respect to the surface. However, our simulations indicate that free-air gravity anomalies correlate with surface tectonics better than almost 98% of our trials in which the free-air gravity anomalies were randomly oriented with respect to Earth's surface. The average geoid anomaly and free-air gravity anomaly over platforms and shields are significant at slightly better than the one-standard-deviation level: −11±8 m and −4±3 mgal, respectively. After removing from the geoid estimated contributions associated with (1) a simple model of the continental crust and oceanic lithosphere, (2) the lower mantle, (3) subducted slabs, and (4) remnant glacial isostatic disequilibrium, we estimate a platform and shield signal of −8±4 m. We conclude that there is little contribution of platforms and shields to the gravity field, consistent with their keels having small density contrasts. Using this estimate of the platform and shield signal, and previous estimates of upper-mantle shear-wave travel-time perturbations, we find that the average value of ∂lnρ/∂lnν_s within the 140–440 km depth range is 0.04±0.02. A continental tectosphere with an isopycnic (equal-density) structure (∂lnρ/∂lnν_s=0) enforced by compositional variations is consistent with this result at the 2.0σ level. Without compositional buoyancy, the continental tectosphere would have an average ∂lnρ/∂lnν_s≈0.25, exceeding our estimate by 10σ.     

A hydrothermal investigation of the system FeO, Fe2O3, TiO2

Phase relations in the system FeO---Fe₂O₃---TiO₂, at temperatures ranging between 300°C and 700°C, have been investigated experimentally with special refference to the reaction Fe₃O₄ + TiO₂ = Fe₂O₃ + FeTiO₃. Pressure was varied between 500 and 2000 bars but its effect was negligible. Magnetite and rutile are the stable assemblage at temperatures above 550 dgC, and hematite and ilmenite are stable for lower temperatures. The equilibrium oxygen fugacity is estimated to be 10^−17.5 bars at equilibrium temperature. It is suggested that intermediate hematite-ilmenite solid solutions are inhomogeneous, consisting of ‘domains’ of hematite and ilmenite. The ‘domains’ are too small to be resolved by X-ray diffraction techniques. The top of the solvus curve in the hematite-ilmenite solution corresponds to a temperature of 660°C. Regular solution theory is not applicable to the solid solution.     

Oxidative dissolution of pyritic sludge from the Aznalcóllar mine (SW Spain)

As a result of the collapse of a mine tailing dam, a large extension of the Guadiamar valley was covered with a layer of pyritic sludge. Despite the removal of most of the sludge, a small amount remained in the soil, constituting a potential risk of water contamination. The kinetics of the sludge oxidation was studied by means of laboratory flow-through experiments at different pH and oxygen pressures. The sludge is composed mainly of pyrite (76%), together with quartz, gypsum, clays, and sulphides of zinc, copper, and lead. Trace elements, such as arsenic and cadmium, also constitute a potential source of pollution. The sludge is fine grained (median of 12 μm) and exhibits a large surface (BET area of 1.4±0.2 m² g⁻¹).

The dissolution rate law of sludge obtained is r=10^{−6.1(±0.3)} [O₂(aq)]^0.41(±0.04) a_H+^0.09(±0.06) g_sludge m⁻² s⁻¹ (22 °C, pH=2.5–4.7). The dissolution rate law of pyrite obtained is r=10^{−7.8(±0.3)} [O₂(aq)]^0.50(±0.04) a_H+^0.10(±0.08) mol m⁻² s⁻¹ (22 °C, pH=2.5–4.7). Under the same experimental conditions, sphalerite dissolved faster than pyrite but chalcopyrite dissolves at a rate similar to that of pyrite. No clear dependence on pH or oxygen pressure was observed. Only galena dissolution seemed to be promoted by proton activity. Arsenic and antimony were released consistently with sulphate, except at low pH conditions under which they were released faster, suggesting that additional sources other than pyrite such as arsenopyrite could be present in the sludge. Cobalt dissolved congruently with pyrite, but Tl and Cd seemed to be related to galena and sphalerite, respectively.

A mechanism for pyrite dissolution where the rate-limiting step is the surface oxidation of sulphide to sulphate after the adsorption of O₂ onto pyrite surface is proposed.     

Design of tunnel support systems using ground freezing

One of the more promising techniques in soft ground tunneling through urbanized areas is the use of artificially frozen ground for temporary tunnel support. This paper describes the general design considerations involved in the ground freezing method. Various factors are discussed which influence the selection of the freezing temperature, the thickness of the frozen zones and the spacing of the freeze pipes. The time required to achieve freezing is discussed in addition to the amount and rate of frost heave caused by the freezing.

To illustrate the applicability of the freezing method, various considerations in the design of an 8-ft. diameter tunnel in upstate New York, a 75-ft. diameter tunnel in Georgia, and a 12 1/2-ft. diameter tunnel in Washington, D.C. are discussed. All three of the tunnels were to pass immediately beneath mainline railroad tracks. A laboratory testing program was implemented to determine the effects of the repetitive train loads on the zone of frozen soil around the tunnel perimeter. Stress-controlled repeated load triaxial tests were performed on both undisturbed and remolded samples frozen from temperatures of −7°C for the New York tunnel to −10°C for the Atlanta and Washington tunnels. Static testing consisted of both quick triaxial tests and creep tests on frozen samples of the various soil types.

It was found that there was little difference between the cumulative strain response from repeated load tests and static tests for the low frequencies investigated (one-quarter to one-half cycles per minute). Hyperbolic stress—strain functions were developed to simulate the stress—strain relationship for various cumulative loading times.

The stresses and strains in the frozen soil tunnel configuration were computed by the finite element method, using both linear and hyperbolic stress—strain functions. Tangent modulus values were varied to reflect the decreasing modulus with increasing loading time. The analyses indicated that zones of frozen soil of approximately 3 ft. thick were required for both the New York and Washington tunnels. However, high tensile stresses were calculated for the Atlanta tunnel, precluding the use of the freezing method.     

A review and analysis of silicate mineral dissolution experiments in natural silicate melts

We present a database and a graphical analysis of published experimental results for dissolution rates of olivine, quartz plagioclase, clinopyroxene, orthopyroxene, spinel, and garnet in basaltic and andesitic melts covering a range of experimental temperatures (1100–1500°C) and pressures (10⁵ Pa-3.0 GPa). The published datasets of Donaldson (1985, 1990) and Brearly and Scarfe (1986) are the most complete. Experimental dissolution rates from all datasets are recalculated and normalized to a constant oxygen basis to allow for direct comparison of dissolution rates between different minerals. Dissolution rates (ν) range from 5·10⁻¹⁰ oxygen equivalent moles (o.e.m.) cm⁻² s⁻¹ for olivine in a basaltic melt to 1.3·10⁻⁵ o.e.m. cm⁻² s⁻¹ for garnet in a basaltic melt. Values of ln ν are Arthenian for the experiments examined and activation energies range from 118 to 1800 kJ/o.e.m. for quartz and clinopyroxene, respectively.

The relationship between calculated A/RT for the dissolution reactions, where A is the thermodynamic potential affinity, and values of ν is linear for olivine, plagioclase, and quartz. We interpret this as strong evidence in support of using calculated A as a predictor of ν for, at least, superliquidus melt conditions.     

Gum rosin (colophony): A suitable material for thermomechanical modelling of the lithosphere

We investigate the use of a ductile material with temperature-sensitive viscosity for thermomechanical modelling of the lithosphere. First, we consider the scaling of mechanical and thermal properties. For a normal field of gravity, the balance of stresses and body forces sets the stress scale, in proportion to the linear dimensions and the densities. The equation of thermal conduction sets the time scale. The activation enthalpy for creep sets the temperature scale; but the thermal expansivity provides an additional constraint on this temperature scale.

Gum rosin appears to be a suitable material for lithospheric modelling. We have measured its flow properties, at various temperatures, in a specially designed rotary viscometer with unusually low machine friction. The rosin is almost Newtonian. Strain rate depends upon stress to the power n, where 1.0 <n < 1.14. The viscosity varies over 5 orders of magnitude, from about 10² Pa s at 80°C, to about 10⁷ Pa s at 40°C. The activation enthalphy is thus about 250 kJ/mol. Measured with a needle probe, the thermal conductivity is 0.113 ± 0.001 W m⁻¹K⁻¹; the thermal diffusivity, (6±3) ×10⁻⁷ m² s⁻¹. Calculated from X-ray profiles, the thermal expansivity is about 3 × 10⁻⁴ K⁻¹. These thermal and mechanical properties make gum rosin suitable for thermomechanical models, where linear dimensions scale down by a factor of 10⁶; time, by 10¹¹; viscosity, by 10¹⁷; and temperature change, by 10¹.     

Marcasite precipitation from hydrothermal solutions

Pyrite and marcasite were precipitated by both slow addition of aqueous Fe²⁺ and SiO₃²⁻ to an H₂S solution and by mixing aqueous Fe²⁺ and Na₂S₄ solutions at 75°C. H₂S₂ or HS₂⁻ and H₂S₄ or HS₄⁻ were formed in the S₂O₃²⁻ and Na₂S₄ experiments, respectively. Marcasite formed at pH < pK₁ of the polysulfide species present (for H₂S₂, pK₁ = 5.0; for H₂S₄, pK₁ = 3.8 at 25°C). Marcasite forms when the neutral sulfane is the dominant polysulfide, whereas pyrite forms when mono-or divalent polysulfides are dominant. In natural solutions where H₂S₂ and HS₂ are likely to be the dominant polysulfides, marcasite will form only below pH 5 at all temperatures.

The pH-dependent precipitation of pyrite and marcasite may be caused by electrostatic interactions between polysulfide species and pyrite or marcasite growth surfaces: the protonated ends of H₂S₂ and HS₂ are repelled from pyrite growth sites but not from marcasite growth sites. The negative ions HS₂ and S₂²⁻ are strongly attracted to the positive pyrite growth sites. Masking of 1π_g^* electrons in the S₂ group by the protons makes HS₂ and H₂S₂ isoelectronic with AsS²⁻ and As₂²⁻, respectively ( et al., 1981). Thus, the loellingitederivative structure (marcasite) results when both ends of the polysulfide are protonated.

Marcasite occurs abundantly only for conditions below pH 5 and where H₂S₂ was formed near the site of deposition by either partial oxidation of aqueous H₂S by O₂ or by the reaction of higher oxidation state sulfur species that are reactive with H₂S at the conditions of formation e.g., S₂O₃²⁻ but not SO₄²⁻. The temperature of formation of natural marcasite may be as high as 240°C ( and , 1985), but preservation on a multimillion-year scale seems to require post-depositional temperatures of below about 160°C ( , 1973; and , 1985).     

Abiogenic Fischer–Tropsch synthesis of hydrocarbons in alkaline igneous rocks; fluid inclusion, textural and isotopic evidence from the Lovozero complex, N.W. Russia

A detailed fluid inclusion study has been carried out on the hydrocarbon-bearing fluids found in the peralkaline complex, Lovozero. Petrographic, microthermometric, laser Raman and bulk gas data are presented and discussed in context with previously published data from Lovozero and similar hydrocarbon-bearing alkaline complexes in order to further understand the processes which have generated these hydrocarbons. CH₄-dominated inclusions have been identified in all Lovozero samples. They occur predominantly as secondary inclusions trapped along cleavage planes and healed fractures together with rare H₂O-dominant inclusions. They are consistently observed in close association with either arfvedsonite crystals, partially replaced by aegirine, aegirine crystals or areas of zeolitization. The majority of inclusions consist of a low-density fluid with CH₄ homogenisation temperatures between −25 and −120 °C. Those in near-surface hand specimens contain CH₄+H₂ (up to 40 mol%)±higher hydrocarbons. However, inclusions in borehole samples contain CH₄+higher hydrocarbons±H₂ indicating that, at depth, higher hydrocarbons are more likely to form. Estimated entrapment temperatures and pressures for these inclusions are 350 °C and 0.2–0.7 kbar. A population of high-density, liquid, CH₄-dominant inclusions have also been recorded, mainly in the borehole samples, homogenising between −78 and −99 °C. These consist of pure CH₄, trapped between 1.2 and 2.1 kbar and may represent an early CH₄-bearing fluid overprinted by the low-density population. The microthermometric and laser Raman data are in agreement with bulk gas data, which have recorded significant concentrations of H₂ and higher hydrocarbons up to C₆H₁₂ in these samples. These data, combined with published isotopic data for the gases CH₄, C₂H₆, H₂, He and Ar indicate that these hydrocarbons have an abiogenic, crustal origin and were generated during postmagmatic, low temperature, alteration reactions of the mineral assemblage. This would suggest that these data favour a model for formation of hydrocarbons through Fischer–Tropsch type reactions involving an early CO₂-rich fluid and H₂ derived from alteration reactions. This is in contrast to the late-magmatic model suggested for the formation of hydrocarbons in the similar peralkaline intrusion, Ilímaussaq, at temperatures between 400 and 500 °C.     

A mechanism of shape-transformation of quartz inclusions in albite of regional metamorphic rocks

The process of shape-transformation of quartz inclusions from polyhedral to spherical grains in albite single crystals during metamorphism is mainly controlled by the grain boundary diffusion of oxygen along the quartz/albite interface to reduce the interfacial free energy. The rate of the process, which is represented by the growth rate of the curvature of the edge surface of the grain, depends significantly on temperature and on the grain size of the quartz inclusion. The relations between temperature, T, the time, t_r, and the critical radius, R_c, which is equal to the radius of maximum spherical grains, are given by log R_c = −0.11E_b/RT + 0.25log t_r + C, in which E_b is the activation energy of the grain boundary diffusion of oxygen along the quartz/albite interface and C is a material constant.

The mean critical radius of spherical quartz inclusions in albite is 5 μm for the upper chlorite zone and garnet zone, 10 μm for the lower biotite zone, and 20 μm for the upper biotite zone in the Sambagawa metamorphic terrain. The mean values of the critical radii of spherical quartz inclusions in oligoclase of the Ryoke metamorphic rocks is about 5 μm for the chlorite zone and about 10–20 μm for the sillimanite zone.

Assuming temperatures of about 350°C for the upper chlorite and garnet zones, 400°C for the lower biotite zone, 550°C for the upper biotite zone, and 700°C for the sillimanite zone, the activation energy for the grain boundary diffusion of oxygen along the quartz/plagioclase interfase is estimated to be about 30 kcal/mol.     

D–H evidence for the timing of kaolinization in Northeast Bavaria, Germany

The δD_SMOW values of sedimentary kaolins from the western border of the Bohemian Massif, northeast Bavaria, that did not suffer a deep burial (less than 1000 m) nor a hydrothermal overprint, change systematically from Late Triassic (−50‰) to Mid-Jurassic and Late Cretaceous (−56‰ to −66‰) to Upper Oligocene–Mid-Miocene (−77‰ to −90‰). All analyzed clays are far from hydrogen isotope equilibrium with present-day meteoric waters. Combined oxygen and hydrogen isotope data of selected samples indicate low temperatures of formation (<30°C) and no evidence for preferential D/H exchange with younger waters. The hydrogen isotopic evolution of kaolins is interpreted as reflecting a systematic isotopic change of paleo-meteoric waters in that region. This can be related mainly to the northward drift of stable Europe after the break-up of Gondwana. Increasing continentality, surface uplift and global cooling are additional factors responsible for decreasing δD_SMOW values since the Mid-Cretaceous.

Kaolinite hydrogen isotope ratios of two large residual economic deposits (Tirschenreuth: δD_SMOW=−80‰ to −76‰; Hirschau–Schaittenbach: δD_SMOW=−70‰ to −63‰) can be used in combination with additional geological evidence to constrain the timing of weathering in these areas. A late Early Cretaceous kaolinization age is suggested for the Early Triassic sandstone-hosted deposits near Hirschau–Schnaittenbach, whereas a Late Oligocene to Mid-Miocene age is indicated for the Carboniferous granite-hosted Tirschenreuth deposits.     

Equilibria in the system Au–Ag–S–fluid: computed and experimental data

We present results of computations on the interaction of solid-phase electrum–argentite–pyrite (weight ratios 210⁻⁵/ 210⁻³/1 and 210⁻⁵/410⁻²/1) association with Cl-containing aqueous moderately acid solutions (0.5m NaCl, pH = 3.08) at 300 °C and 500 bars. These data are a physicochemical basis for predicting the geochemical behavior of Au and Ag during the hydrothermal-metasomatic transformation of Au-Ag-pyrite. We also propose a technique of study of this process based on the phase equilibria of the subsystem Au–Ag–S with the aqueous solution at different liquid/solid (l/s) ratios, with the use of new graphic diagrams. The relationship of the composition of the solid-phase association with l/s ratio in real boundary conditions (Au = 17 ppm, mAu/mAg = 10^–3.57–10^–2.28) is shown. The maximum l/s values for complete leaching of gold and silver (l/smax = 200–800) are estimated. It has been established that argentite is the first to dissolve when mAu/mAg(s) > mAu/mAg(sol), and electrum, when mAu/mAg(s) < mAu/mAg(sol).

The experimental results showed that at 300 °C, the conversion of electrum (N_Au = 300‰) nonequilibrated with pyrite into an Au-richer form (N_Au = 730‰) and argentite follows an intricate kinetic scheme. Using the Pilling-Bedwords kinetic equation for processing data yielded the process rate constant K = 2.8(±0.5)10⁻⁵ g²cm⁻⁴day⁻¹. With this equation, the time of the complete conversion of 200 μm thick flat gold grains is 604 days. These data evidence a significant role of kinetic factors in hydrothermal-metasomatic processes involving native gold, which requires combination of thermodynamic and kinetic approaches on the construction of geologo-genetic models for hydrothermal sulfide formation.