Experimental determination of calcite solubility in H2O-NaCl solutions at deep crust/ upper mantle pressures and temperatures: Implications for metasomatic processes in shear zones
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Robert C. Newton
Abstract
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 × 10-4 T + X2NaCl exp(-3.071 + 4.749 × 10-6T2)] (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for the enigmatic carbonated shear zones, such as the Attur Valley of southern India, where crustal rocks have been replaced by up to 20% by calcite and ankerite with mantle-like stable-isotope signatures.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.
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
- Atomic-scale observations of franckeite surface morphology
- Surface control vs. diffusion control during calcite dissolution: Dependence of step-edge velocity upon solution pH
- First-principles calculation of the infrared spectrum of lizardite
- Hard-mode infrared spectroscopy of perovskites across the CaTiO3-SrTiO3 solid solution
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- Electron paramagnetic resonance spectroscopic study of synthetic fluorapatite: Part III. Structural characterization of sub-ppm-level Gd and Mn in minerals at W-band frequency
- Experimental determination of the equilibria: rutile + magnesite = geikielite + CO2 and zircon + 2 magnesite = baddeleyite + forsterite + 2 CO2
- Symplectites derived from metastable phases in martian basaltic meteorites
- Copper speciation in vapor-phase fluid inclusions from the Mole Granite, Australia
- Trace-element partitioning between vacancy-rich eclogitic clinopyroxene and silicate melt
- Fluorite particles inducing butterfly aggregates of incipient microperthite in alkali feldspar from a syenite, the Patagonian Andes, southern Chile
- Thermal stability and spectroscopic studies of zemkorite: A carbonate from the Venkatampalle kimberlite of southern India
- Temperatures from triple-junction angles in sulfides
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- High-pressure behavior of bikitaite: An integrated theoretical and experimental approach
- Raman spectroscopic study of H2O in bikitaite: “One-dimensional ice”
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- Letters. Equation of state of Al-bearing stishovite to 40 GPa at 300 K
- Order parameter behavior at the structural phase transition in cummingtonite from Mössbauer spectroscopy
- Deciphering Ni sequestration in soil ferromanganese nodules by combining X-ray fluorescence, absorption, and diffraction at micrometer scales of resolution
- Use of the spindle stage for orientation of single crystals for microXAS: Isotropy and anisotropy in Fe-XANES spectra
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