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Fluid-mediated calcium metasomatism is often associated with strong silica mobility and the presence of chlorides in solution. To help quantify mass transfer at lower crustal and upper mantle conditions, we measured quartz solubility in H2O-CaCl2 solutions at 0.6–1.4 GPa, 600–900 °C, and salt concentrations to 50 mol%. Solubility was determined by weight loss of single-crystals using hydrothermal piston-cylinder methods. All experiments were conducted at salinity lower than salt saturation. Quartz solubility declines exponentially with added CaCl2 at all conditions investigated, with no evidence for complexing between silica and Ca. The decline in solubility is similar to that in H2O-CO2 but substantially greater than that in H2O-NaCl at the same pressure and temperature. At each temperature, quartz solubility at low salinity (XCaCl2 < 0.1) depends strongly on pressure, whereas at higher XCaCl2 it is nearly pressure independent. This behavior is consistent with a transition from an aqueous solvent to a molten salt near XCaCl2 ~0.1. The solubility data were used to develop a thermodynamic model of H2O-CaCl2 fluids. Assuming ideal molten-salt behavior and utilizing previous models for polymerization of hydrous silica, we derived values for the activity of H2O (aH2O), and for the CaCl2 dissociation factor (α), which may vary from 0 (fully associated) to 2 (fully dissociated). The model accurately reproduces our data along with those of previous work and implies that, at conditions of this study, CaCl2 is largely associated (<0.2) at H2O density <0.85 g/cm3. Dissociation rises isothermally with increasing density, reaching ~1.4 at 600 °C, 1.4 GPa. The variation in silica molality with aH2O in H2O-CaCl2 is nearly identical to that in H2O-CO2 solutions at 800 °C and 1.0 GPa, consistent with the absence of Ca-silicate complexing. The results suggest that the ionization state of the salt solution is an important determinant of aH2O, and that H2O-CaCl2 fluids exhibit nearly ideal molecular mixing over a wider range of conditions than implied by previous modeling. The new data help interpret natural examples of large-scale Ca-metasomatism in a wide range of lower crustal and upper mantle settings. [ABSTRACT FROM AUTHOR]

The standard Gibbs energy of formation of zircon was constrained by measuring the solubility of silica in [H.sub.2]O in equilibrium with zircon and baddeleyite at 800 [degrees]C, 12 kbar, by a sensitive weight- change method. Dissolution occurs incongruently according to the reaction: ZrSi[O.sub.4] = Zr[O.sub.2] + Si[O.sub.2(t)] zircon baddeleyite where Si[O.sub.2(t)] is total dissolved silica. Blank runs demonstrated that the effects of Zr[O.sub.2] solubility and/or capsule-Pt transfer were near the weighing detection limit, so weight losses or gains could be ascribed quantitatively to Si[O.sub.2] solubility. Precise Si[O.sub.2(t)] concentrations were ensured by use of three types of starting material, by approaching equilibrium from zircon-undersaturation and oversaturation, and by demonstrating time-independence of the measurements. The results yielded a Si[O.sub.2] concentration of 0.069 [+ or -] 0.002 (1 se) moles per kg [H.sub.2]O ([m.sub.s]), or a mole fraction ([X.sub.s]) of 1.23 x [10.sup.-3] [+ or -] 3.3 x [10.sup.-5]. Two runs on zircon solubility in NaCl-[H.sub.2]O solutions at 800 [degrees]C and 10 kbar showed silica solubility to decrease by nearly 1% per tool% NaCl. The standard molar Gibbs free energy of formation of zircon from the oxides at a constant P and T is given by: [DELTA][G.sup.o.sub.f,ox,zr] = RTln [[gamma].sup.ZB.sub.s] [X.sup.ZB.sub.s]/[[gamma].sup.Q.sub.s][X.sup.Q.sub.s] where ZB and Q refer, respectively, to equilibrium with zircon-baddeleyite and quartz, [[gamma].sub.s] is the activity coefficient of total silica, and the relationship between [[gamma].sub.s] and [X.sub.s] accounts for aqueous silica activity. Our results yield [DELTA][G.sup.o.sub.f,oxzr] = -18.5 [+ or -] 0.7 kJ/mol at 800 [degrees]C, 12 kbar (95% confidence), or a standard apparent Gibbs free energy of formation from the elements of -1918.3 [+ or -] 0.7 kJ/mol at 25 [degrees]C, l bar. Our value is consistent with determinations based on phase equilibrium studies, within reported error limits, but is more precise than most previous values. However, it is less negative than high-temperature determinations by calorimetry and electrochemistry. Our results indicate that solubility measurements at high T and P may be a superior method of free energy determination of other refractory silicate minerals. Keywords: Experimental petrology, phase equilibria, thermodynamics, zircon DOI: 10.2138/am.2010.3213

Quartz solubility in [H.sub.2]O-C[O.sub.2] fluids was measured at 800[degrees]C and 10 kbar. Mixed fluids were generated from hydrous oxalic acid, silver oxalate, silver carbonate, and liquid [H.sub.2]O; solubility was determined by weight changes of the quartz crystals. Stringent blank tests and weighing procedures were used to establish the C[O.sub.2] and [H.sub.2]O contents of experimental fluids. Using experimentally constrained models of [H.sub.2]O activity and mixing of silica monomers and dimers, a logarithmic plot of monomer activity vs. [H.sub.2]O activity yields a linear data array of constant slope n insignificantly different from 4 (n = 4.024, [R.sup.2] = 0.997), where n is the total monomer hydration number. Moreover, all high-quality quartz solubility data in [H.sub.2]O-C[O.sub.2] fluids at lower temperature and pressure fall on the same line when Si[O.sub.2] and [H.sub.2]O activities are calculated with this formulation. The same analysis for the dimer yields slope n of 7 (n = 7.049, [R.sup.2] = 0.996). Our results show that neutral silica monomers and dimers have fixed stoichiometry of Si[(OH).sub.4]*2[H.sub.2]O and [Si.sub.2]O[(OH).sub.6]*4[H.sub.2]O (i.e., two solvating [H.sub.2]O molecules per Si) in [H.sub.2]O-C[O.sub.2] fluids for a large range of crustal metasomatic processes. This provides a simple basis for modeling [H.sub.2]O-C[O.sub.2]-Si[O.sub.2] fluids over a range of pressure, temperature, C[O.sub.2], and Si[O.sub.2] activity. Keywords: Quartz solubility, crustal fluids, experimental petrology, thermodynamics

The solubility of andradite garnet was determined at 800 [degrees] C and 10 kbar in a solution of 10 mol% NaCl and 90 mol% [H.sub.2]O. Experiments were syntheses carried out with high-purity namral wollastonite, reagent hematite or natural specular hematite containing ~1 wt% Ti[O.sub.2], and fluid. All experiments were performed with [Mn.sub.2][O.sub.3]-[Mn.sub.3][O.sub.4] or [Fe.sub.2][O.sub.3]-[Fe.sub.3][O.sub.4] oxygen buffers in a piston-cylinder apparatus with NaCl pressure media for 1-3 days. Andradite saturation was determined by the presence or absence of gamet in quenched charges. Andradite dissolves incongruently to hematite and fluid with CaSi[O.sub.3] molality ([m.sub.cs]) of 0.0838 [+ or -] 0.0015 for the reagent hematite and both buffers. Slightly higher [m.sub.cs] of 0.0895 [+ or -] 0.0005 for the natural hematite and Mn-oxide [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] buffer is interpreted as due to incomplete equilibration and/or ~9 mol% Ti in the run-product andradite. Dissolved Fe molality could be determined only approximately, but must be at least ten times lower than [m.sub.cs]. Quenched fluids were very basic (pH 11-12). The solubility of [Fe.sub.2][O.sub.3] in andradite-saturated [H.sub.2]O-NaCl fluids is lower than that of [Al.sub.2][O.sub.3] at grossular saturation at the same pressure (P), temperature (T), and fluid composition. The results permit a test of a model of CaSi[O.sub.3] dissolution in NaCl solutions to three dominant aqueous species: Ca[Cl.sup.+], O[H.sup.-], and [H.sub.3]NaSi[O.sub.4]. Combination of the CaSi[O.sub.3] molality at andradite saturation with wollastonite solubility at the same conditions (0.1253 [+ or -] 0.0047 molal; Newton and Manning 2006) leads to a Gibbs free energy change of the reaction 3 wollastonite + hematite = andradite at 800 [degrees] C and 10 kbar of -32.21 [+ or -] 2.45 kJ. The good agreement between this value and that derived from previous studies supports the dissolution model of CaSi[O.sub.3] in NaCl solutions. The low solubility of the [Fe.sub.2][O.sub.3] component of andradite contrasts with the high solubility of magnetite and of Fe in pelitic and granitic mineral assemblages measured in acidic chloride solutions by previous workers at lower P and T. The results imply that [Fe.sub.2][O.sub.3] is conserved during metasomatic processes affecting calc-silicates at high metamorphic grades. Keywords: Experimental petrology, thermodynamics, fluid, crystal growth, metasomatism

The activity coefficient of aqueous silica was determined at 800 [degrees]C and 12 kbar by measuring the concentration of total dissolved aqueous silica (Si[O.sub.2(t)]) in [H.sub.2]O in equilibrium with four buffering mineral assemblages: quartz (Q), kyanite--corundum (KC), enstatite-forsterite (EF), and forsterite-rutile-geikielite (FRG). Si[O.sub.2(t)] concentrations were determined by reversed equilibration of synthetic minerals with [H.sub.2]O using a weight gain/weight loss technique, yielding molalities of Si[O.sub.2(t)] ([m.sub.s]) of Q, 1.413(1); KC, 0.984(43); EF, 0.276(9); FRG, 0.079(1) (1[sigma] errors). The solubility data can be fitted well according to the homogeneous equilibrium 2 Si[O.sub.2] monomers (m) = 1 [Si.sub.2][O.sub.4] dimer (d) assuming ideal mixing, such that the equilibrium constant ([K.sub.md]) is [K.sub.md] = [X.sub.d]/[X.sub.m.sup.2], where X represents the mole fraction of Si[O.sub.2](t) occurring as the subscripted species. This formulation is valid irrespective of the hydration states of the silica species because of the small silica concentration at these conditions ([less than or equal to] 2.5 mol%). For a standard state of unit activity of the hypothetical pure monomer solution at the P and T of interest, the monomer-dimer model leads to [[gamma].sub.s] = [X.sub.m]/[X.sub.s] and [K.sub.md] = (1 - [[gamma].sub.s])/2[[gamma].sup.2.sub.s][X.sub.s] where [[gamma].sub.s] and [X.sub.s] are the activity coefficient and mole fraction of total silica. Our data yield [K.sub.md] = [155.sup.+25.sub.-39] at 800 [degrees]C and 12 kbar, in excellent agreement with results from in situ Raman spectroscopy of a quartz-saturated solution at the same P-T. In the system Si[O.sub.2]-[H.sub.2]O, a solution in equilibrium with quartz at 800 [degrees]C, 12 kbar, contains 2.5 mol% silica, of which 70% occurs in dimers, and [[gamma].sub.s] is small (0.30). Even at the low concentration of the FRG buffer (0.1 tool%), the activity coefficient is only 0.75 and the dissolved silica is substantially polymerized (25%).

The solubility of calcite in NaCl-[H.sub.2]O solutions was measured at 600-900 [degrees]C, 10 kbar, at NaCl concentrations ranging from dilute to near halite saturation, and at 6-14 kbar, 700[degrees]C, in 30 mol% NaCl solutions. Solubility was determined from the weight loss of cleavage rhombs of a pure natural calcite after experiments of 1/2 to 6 days in a piston-cylinder apparatus with NaCl-graphite furnaces. CaC[O.sub.3] molality ([m.sub.CaC[O.sub.3]]) increases greatly with NaCl mole fraction ([X.sub.NaCl]): at 800[degrees]C and 10 kbar, [m.sub.CaC[O.sub.3]] increases from ~0.1 in pure [H.sub.2]O to near 4.0 at halite saturation ([X.sub.NaCl] ~ 0.6). There is also a large temperature effect at 10 kbar, with [m.sub.CaC[O.sub.3]] increasing from 0.25 at 600[degrees]C to 3.0 at 900[degrees]C at [X.sub.NaCl] = 0.3. There is only a 20% increase with increasing pressure between 6 and 14 kbar at 700[degrees]C and [X.sub.NaCl] = 0.3. Melting to a carbonate-rich liquid was inferred at 900[degrees]C, 10 kbar, from [X.sub.NaCl] of 0 to 0.2. The composition, temperature, and pressure dependence of [m.sub.CaC[O.sub.3]] are described by: [m.sub.CaC[O.sub.3]] = [-0.051 + 1.65 x [10.sup.4] T + [X.sup.2.sub.NaCl]exp(-3.071 + 4.749 x [10.sup.-6][T.sup.2])] (0.76 + 0.024P) with T in Kelvins and P in kbar. The predicted increase of calcite solubility with salinity and temperature is so great that critical mixing of NaCl-rich hydrous carbonate liquid and CaC[O.sub.3]-rich saline solution is probable at 10 kbar near 1000[degrees]C and [X.sub.NaCl] ~ 0.4. The experimental results suggest a genetic mechanism for the enigmatic carbonated shear zones, such as the Attur Valley of southern India, where crustal rocks have been replaced by up to 20% by calcite and ankerite with mantle-like stable-isotope signatures. The high CaC[O.sub.3] carrying capacity of concentrated alkali-chloride solutions, together with the drastic decrease in solubility between 1000 and 700[degrees]C, plausibly account for large-scale emplacement of mantle-derived carbonate from concentrated chloride-carbonate solutions (or hydrosaline magmas) formed as immiscible fluids in the late stages of alkalic magmatism. Such solutions may also mobilize sulfate and phosphate minerals, which would have important consequences for redistribution of incompatible and heat-producing elements in the crust.

The P-T conditions of both equilibria were determined precisely by reversal experiments in a piston-cylinder apparatus. On the basis of 8 experiments, brackets for the rutile-magnesite-geikielite equilibrium are 7.0-7.1 kbar at 800 [degrees]C, 8.6-8.7 kbar at 850[degrees]C, and 10.5-10.7 kbar at 900[degrees]C. On the basis of 9 experiments, brackets for the zircon-magnesite-baddeleyite-forsterite equilibrium are 7.1-7.7 kbar at 800[degrees]C, 9.2-9.4 kbar at 850[degrees]C, and 10.7-10.9 kbar at 900[degrees]C. Considering experimental uncertainties in P ([+ or -]300 bars) and T ([+ or -]3[degrees]C), equilibrium curves calculated from both the Berman and the Holland and Powell databases pass through all brackets. Molar Gibbs free energy of formation from the elements at 1 bar and 298 K for geikielite and zircon, derived from the experiments and consistent with the Berman database, are -1481.94 [+ or -] 0.67 kJ and -1917.54 [+ or -] 1.25 kJ, respectively. Corresponding values consistent with the Holland and Powell database are -1479.30 [+ or -] 0.74 kJ and -1918.47 [+ or -] 1.49 kJ. Application of the two equilibria indicate that: (1) the mole fraction of C[O.sub.2] in fluid was 0.54-1.00 when geikielite and baddeleyite formed during contact metamorphism of siliceous dolomites in the Ballachulish aureole, Scotland; (2) the activity of C[O.sub.2] could have been as low as 2 [10.sup.-5] during ultra-high pressure metamorphism of magnesite-bearing ecologites; and (3) the activity of C[O.sub.2] was

The P-T equilibrium curve of the reaction of magnesite + enstatite = forsterite and C[O.sub.2] was determined in the ranges 6-25 kbar and 700-1100 [degrees] C by 53 reversed experiments carried out in 1.91 cm diameter piston-cylinder apparatus with NaCl pressure medium. Silver oxalate was used as a C[O.sub.2] source and charges were buffered at hematite-magnetite oxygen fugacity. Reaction progress was monitored by weight changes upon puncturing of C[O.sub.2]-inflated quenched charge capsules and was confirmed by comparing X-ray diffractograms of charges made before and after the experiments. The data satisfy the relation: [P.sub.EQ] = -3.215 - 4.784 x [10.sup.-3] T + 2.542 x [10.sup.-5] [T.sup.2] with P in kbar and Tin [degrees] C. A P-T curve calculated with the 1994 version 2.3 THERMOCALC program agrees well with our results except in the highest pressure range. Our data, together with three existing tight experimental brackets on the decarbonation reaction of magnesite to periclase and C[O.sub.2], give [Mathematical Expression Omitted] (298 K) (forsterite) = -61.20 [+ or -] 0.87 kJ/mol from the oxides. This value is in agreement with the value adopted by Berman (1988). An analysis of various proposed equations of state for C[O.sub.2] shows that only those of Mader and Berman (1991) and Frost and Wood (1997) meet the constraints imposed by this study near 1000 [degrees] C and 10-20 kbar. The modified Redlich-Kwong (MRK)-based equations of state overestimate C[O.sub.2] fugacity in this pressure range.

The equilibrium P-T curve of the reaction (1) Mg[CO.sub.3] + Si[O.sub.2] = MgSi[O.sub.3] + [CO.sub.2]/ magnesitequartzenstatite was determined by reversed reactions in well-calibrated, solid-pressure medium (NaCl) piston-cylinder apparatus. External hematite + magnetite + [H.sub.2]O [f.sub.o2] buffers were used in all experiments. A reversed point at 715 [degrees]C of the reaction (2) Mg[CO.sub.3] = MgO + [CO.sub.2]/ magnesitepericlase was obtained in an internally heated Ar pressure vessel, using a coarse-grained synthetic periclase to inhibit quenching back reaction, along with synthetic magnesite and silver oxalate. At 715 [degrees]C our pressure for Reaction 1 is 10.4 [+ or -] 0.2 kbar, and for Reaction 2 it is 590 [+ or -] 30 bars. From these two reactions we derive [Mathematical Expression Omitted] = 33.41 [+ or -] 0.83 kJ for enstatite from the oxides. Using an extrapolated heat-capacity equation for magnesite, we obtain [Mathematical Expression Omitted] = -116.68 Kj (From the oxides) and -1111.68 kJ from the elements for magnesite, somewhat less negative than the values of either the Holland and Powell (1990) or the Berman (1988) data sets. A calculated 2 kbar temperature for the magnesite + quartz reaction is 492 [degrees]C, about 25 [degrees]C lower than the data set predictions but in agreement with a prediction based on natural occurrences of magnesite, quartz, and enstatite in metacarbonates from the Alps by Trommsdorff and Connolly (1990).

Meionite structure constitutes AlO(sub 4)(super 5-) and SiO(sub 4)(super 4-) tetrahydra rings, joined laterally from oval-shaped cages containing Ca2+ and CO(sub 3)(super 2-)ions. Meionite structure exhibits appreciable rotational disorder for both Al/Si and CO3, arising out of tetrahedral occupancy. Meionite is stable compared to anorthite and calcite phases at temperature above 875 degrees Celsius, irrespective of pressure changes.

Researchers reversed the metastable equilibrium kyanite + corundum + quartz with a H2O flux at 800 degrees centigrade and 7.75 plus or minus 0.25 kbar in the piston-cylinder instrument with a NaCl pressure medium, and at 700 degrees centigrade and 5.17 plus or minus 0.32 kbar in an internally heated pressure container. Results indicate that the addition of Fe to the kyanite + corundum + quartz assemblage, buffered with magnetite-hematite fo2, failed to noticeably alter the equilibrium. The coexisting corundum and quartz discovered in some high-grade rocks are metastable when compared to sillimanite.