Experimental determination of calcite solubility in H2O-NaCl solutions at deep crust/ upper mantle pressures and temperatures: Implications for metasomatic processes in shear zones

The solubility of calcite in NaCl-H2O solutions was measured at 600–900 ∞C, 10 kbar, at NaCl concentrations ranging from dilute to near halite saturation, and at 6–14 kbar, 700 ∞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. CaCO3 molality (mCaCO3) increases greatly with NaCl mole fraction (XNaCl): at 800 ∞C and 10 kbar, mCaCO3 increases from ~0.1 in pure H2O to near 4.0 at halite saturation (XNaCl ~ 0.6). There is also a large temperature effect at 10 kbar, with mCaCO3 increasing from 0.25 at 600 ∞C to 3.0 at 900 ∞C at XNaCl = 0.3. There is only a 20% increase with increasing pressure between 6 and 14 kbar at 700 ∞C and XNaCl = 0.3. Melting to a carbonate-rich liquid was inferred at 900 ∞C, 10 kbar, from XNaCl of 0 to 0.2. The composition, temperature, and pressure dependence of mCaCO3 are described by: mCaCO3 = [–0.051 + 1.65 ¥ 10 T + XNaClexp(–3.071 + 4.749 ¥ 10T)] (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 CaCO3-rich saline solution is probable at 10 kbar near 1000 ∞C and XNaCl ~ 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for the enigmatic carbonated shear zones, such as the Attur Valley of southern India, where crustal rocks have been replaced by up to 20% by calcite and ankerite with mantle-like stable-isotope signatures.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 CaCO3 carrying capacity of concentrated alkali-chloride solutions, together with the drastic decrease in solubility between 1000 and 700 ∞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. NEWTON AND MANNING: CALCITE SOLUBILITY IN H2O-NaCl AT HIGH P AND T 1402 Na-Ca metasomatism of associated non-carbonate-bearing lower-grade rocks indicates that highly saline fluids were involved in the metasomatism there (DeJong and Williams 1995). All of the cited carbonate-replacement events occurred at 500– 700 ∞C and several kbar. The Attur Valley and South Tien Shan occurrences are associated with numerous small intrusions of alkali gabbro, syenite, and carbonatite, which, though much smaller in exposed volumes than the carbonate-metasomatized country rocks, enhance the idea of a mantle origin of the metasomatizing fluids. Experimental determination of the aqueous solubility of calcite immediately poses a problem for the origin of largescale carbonate metasomatism. Calcite solubility decreases sharply with increasing temperature along the boiling curve of water (Ellis 1959), and the negative temperature dependence persists at supercritical conditions to 2 kbar and temperatures of 600 ∞C (Fein and Walther 1987). The calcite solubility at these conditions is only 10 mol/kg of H2O. If the temperature trend can be extrapolated for as little as 100–200 ∞C, it can be inferred that calcite is one of the most insoluble of common minerals at high-grade metamorphic conditions. Another factor that must be considered is the low H2O activity that is necessary in deep-crustal metamorphism if aqueous fluids are to be present and compatible with high-grade mineral assemblages. Fein and Walther (1987) explored the effect of lowering H2O activity by addition of CO2 at 400–500 ∞C and 2 kbar. They found an initial enhancement of calcite solubility for CO2 mole fractions up to 0.05, but then a rapid decrease in solubility between 0.05 and 0.15 mole fraction CO2 (XCO2). Fluids compatible with granulite-facies assemblages would have to have XCO2 ≥ 0.6 (Aranovich and Newton 1999). If calcite solubility is governed primarily by the stability of the bicarbonate ion, HCO3, the calculations of Fein and Walther (1987) indicate that calcite solubility in CO2-H2O solutions should remain vanishingly small at elevated P-T conditions. The great carbonate transport and metasomatism at near-granulite-grade P-T conditions, as described by Dahlgren et al. (1993) in the Bamble region, is difficult to explain unless some additional factors operate to counteract the effects of high temperature and low H2O activity. Concentrated NaCl solutions have the property of very low H2O activity at pressures above 4 kbar at elevated temperatures (Aranovich and Newton 1996; Shmulovich and Graham 1996), and hence deserve consideration as feasible metamorphic fluids (Newton et al. 1998). Moreover, primary fluid inclusions in some rocks of deep-seated origin, such as the eclogite veins in the Western Alps described by Philippot and Selverstone (1991), contain extremely concentrated brines with daughter crystals of calcite and dolomite, as well as anhydrite, barite, monazite, and other trace-element-rich minor minerals. These features suggest that concentrated brines may be powerful solvents at elevated P-T conditions for carbonates and other non-silicate oxysalt minerals. NaCl induces a large enhancement of calcite solubility at very low temperatures and pressures, similar to the effect of CO2 at these conditions (Ellis 1963). Fein and Walther (1989) found a significant NaCl enhancement of calcite solubility at supercritical conditions of 600 ∞C and 2 kbar; however, the maximum salinity that they investigated was 0.02 molal. At higher NaCl concentrations, it might be anticipated that calcite solubility should decrease with decreasing H2O activity, as in the CO2-H2O system. The effect of NaCl on calcite solubility at higher P-T conditions, where the solute properties of NaCl change from those of an undissociated neutral complex to a completely ionized solution (Aranovich and Newton 1996), has not been investigated. The present study was designed to measure calcite solubility at elevated P-T conditions in NaCl solutions sufficiently concentrated to have the low H2O activity suitable for deep crustal-upper mantle metamorphic fluids. EXPERIMENTAL METHODS Solubility experiments were carried out by reacting a very pure natural calcite with H2O of variable NaCl content from pure H2O to halite saturation in sealed Pt envelopes in the internally heated piston-cylinder apparatus with NaCl pressure medium and graphite heater sleeves. Pressures were 6 to 14 kbar at 700 ∞C, and temperatures were 600 to 900 ∞C at 10 kbar. The experimental procedures were identical to those of Newton and Manning (2000) except that the calcite crystals in most of the present experiments were encapsulated in a small inner Pt container that was perforated with numerous pinholes to allow access of the salt solution from the outer capsule. This procedure was necessary because the starting calcite cleavage chips usually recrystallized to an aggregate of rounded crystals, which required containment. Pressures are accurate to ±300 bars and automatically controlled temperatures, measured by calibrated matched pairs of W3%Rh-W25%Rh thermocouples, are accurate to ±3 ∞C. Clear cleavage rhombs of calcite from Rodeo, Durango, Mexico, were used. Microprobe analysis showed that the maximum departure from CaCO3 composition was 0.07 wt% FeO (Caciagli 2001). Reagent-grade NaCl and distilled and deionized water completed the starting materials. Ingredients were successively weighed into the outer Pt capsule on a Mettler M3 microbalance with 1s = 0.002 mg. H2O content was checked after an experiment by puncturing and drying the outer capsule at 310 ∞C for 15 minutes (“H2O out” in Table 1). Calcite solubility values were determined by weighing the inner Pt capsules before and after each experiment. Inner Pt capsules retrieved from an experiment were soaked in warm H2O to dissolve away precipitated salt inside the capsule. A series of 5 to 8 ten-minute soakings, followed by dryings at 115 ∞C and reweighings were performed until a capsule came to constant weight. Because of the high calcite solubilities encountered at run conditions, a significant portion of H2O-insoluble quench calcite remained in the inner capsules; therefore, the solubility determined by weight loss of the inner capsule was a minimum value. The Pt capsule was cut open with a razor blade and the calcite charge was transferred quantitatively to a Pt weighing pan. The quenched “fuzz” of skeletal calcite crystals was clearly distinguishable from the rounded undissolved crystals. It was possible to remove most of the fuzz with a needle under a binocular microscope, but it proved impossible to accomplish this cleaning process without the loss of a small amount of granular calcite. The weight of the cleaned residual crystals thus provided an upper limit for calcite solubility. NEWTON AND MANNING:CALCITE SOLUBILITY IN H2O-NaCl AT HIGH P AND T 1403