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A garnet-hornblende Fe-Mg exchange geothermometer has been calibrated against the garnet-clinopyroxene geothermometer of Ellis & Green (1979) using data on coexisting garnet + hornblende + clinopyroxene in amphibolite and granulite facies metamorphic assemblages. Data for the Fe-Mg exchange reaction between garnet and hornblende have been fitted to the equation.where K, is the Fe-Mg distribution coefficient, using a robust regression approach, giving a thermometer of the form:with very satisfactory agreement between garnet-hornblende and garnet-clinopyroxene temperatures. The thermometer is applicable below about 850 "C to rocks with Mn-poor garnet and common hornblende of widely varying chemistry metamorphosed at low sol.Application of the garnet-hornblende geothermometer to Dalradian garnet amphibolites gives temperatures in good agreement with those predicted by pelite petrogenetic grids, ranging from 520°C for the lower garnet zone to 565-610°C for the staurolite to kyanite zones. These results suggest that systematic errors introduced by closure temperature problems in the application of the garnet4nopyroxene geothermometer to the 'calibration' data set are not serious. Application to 'eclogitic' garnet amphibolites suggests that garnet and hornblende seldom attain Fe-Mg exchange equilibrium in these rocks.Quartzo-feldspathic and mafic schists of the Pelona Schist on Sierra Pelona, Southern California, were metamorphosed under high Present address:

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The martian meteorite ALH84001 contains small, disk-shaped concretions of carbonate with concentric chemical and mineralogical zonation. Oxygen isotope compositions of these concretions, measured by ion microprobe, range from delta18O = +9.5 to +20.5 per thousand. Most of the core of one concretion is homogeneous (16.7 +/- 1.2 per thousand) and over 5 per thousand higher in delta18O than a second concretion. Orthopyroxene that hosts the secondary carbonates is isotopically homogeneous (delta18O = 4.6 +/- 1.2 per thousand). Secondary SiO2 has delta18O = 20.4 per thousand. Carbon isotope ratios measured from the core of one concretion average delta13C = 46 +/- 8 per thousand, consistent with formation on Mars. The isotopic variations and mineral compositions offer no evidence for high temperature (>650 degrees C) carbonate precipitation and suggest non-equilibrium processes at low temperatures (< approximately 300 degrees C).

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The solubility of quartz has been measured in a wide range of salt solutions at 800°C and 0.5 GPa, and in NaCl, CaCl 2 and CsCl solutions and H 2 O-CO 2 fluids at six additional P-T conditions ranging from 400°C at 0.1 GPa to 800°C at 0.9 GPa. The experiments cover a wide range of compositions along each binary. At P-T conditions where the density of pure water is low (0.43 g cm )3 ), addition of most salts produces an enhancement of quartz solubility at low to moderate salt concentrations (salt-in effect), although quartz solubility falls with further decrease in X H 2 O. At higher fluid densities (0.7 g cm )3 and greater), the salt-in effect is generally absent, although this depends on both the cation present and the actual P-T conditions. The salt-in effect is most readily produced by chloride salts of large monovalent cations, while CaCl 2 only produced a salt-in effect at the most extreme conditions of high-T and low-P investigated (800°C at 0.2 GPa). Under most crustal conditions, the addition of common salts to aqueous fluids results in a lowering of quartz solubility relative to that in pure water (salt-out effect). Comparing quartz solubility in different fluids by calculating X H 2 O on the basis that all salts are fully associated under all conditions yields higher quartz solubility in solutions of monovalent salts than in solutions of divalent salts, absolute values are also influenced by cation radius.Quartz solubility measurements have been fitted to a Setchenow-type equation, modified to take account of the separate effects of both the lowering of X H 2 O and the specific effects of different salts, which are treated as arising through distinct patterns of non-ideal behaviour, rather than the explicit formation of additional silica complexes with salt components. Quartz solubility in H 2 O-CO 2 fluids can be treated as ideal, if the solvation number of aqueous silica is taken as 3.5. For this system the solubility (molality) of quartz in the binary fluid, S is related to its solubility in pure water at the same P-T conditions, S o , by: log S ¼ log S o þ 3:5 log X H2O :Quartz solubility in binary salt systems (H 2 O-RCl n ) can be fitted to the relationship:where salt concentration mRCl n is expressed as molality and the exponent b has a value of 1 except under conditions where salting-in is observed at low salt concentrations, in which case it is <1. Under most crustal conditions, the solubility of quartz in NaCl solutions is given to a good approximation by:We propose that quartz solubility in multicomponent fluids can be estimated from an extended expression, calculating X H 2 O based on the total fluid composition (including dissolved gasses), and adding terms for each major salt present. Our experimental results on H 2 O-NaCl-CO 2 fluids are satisfactorily predicted on this basis. An important implication of the results presented here is that there are circumstances where the migration of a fluid from one quartz-bearing host into another, if it is accompanied by re-equilibration through catio...