Solubility of Anhydrite, CaSO4, in NaCl–H2O Solutions at High Pressures and Temperatures: Applications to Fluid–Rock Interaction

Anhydrite solubility in H2O–NaCl solutions was measured at 6–14 kbar, 600–800 C and NaCl mole fractions (XNaCl) of 0–0 3 in piston–cylinder apparatus. Solubilities were determined by weight changes of natural anhydrite in perforated Pt envelopes confined with fluid in larger Pt capsules. In initially pure H2O at 10 kbar and 800 C, CaSO4 concentration is low (0 03molal), though much larger than at the same temperature and 1 kbar. Hematite-buffered experiments showed slightly lower solubilities than unbuffered runs. CaSO4 solubility increases enormously with NaCl activity: at 800 C and 10 kbar and XNaCl of 0 3, CaSO4 molality is 200 times higher than with pure H2O. Whereas CaSO4 solubility in pure H2O decreases with rising T at low T and P, the high-P results show that anhydrite solubility increases with T at constant P at all XNaCl investigated. The effects of salinity and temperature are so great at 10 kbar that critical mixing between sulfate-rich hydrosaline melts and aqueous salt solutions is probable at 900 C at XNaCl 0 3. Recent experimental evidence that volatile-laden magmas crystallizing in the deep crust may evolve concentrated salt solutions could, in light of the present work, have important implications regarding such diverse processes as Mount Pinatubo-type S-rich volcanism, high-fO2 regional metamorphism, and emplacement of porphyry Cu–Mo ore bodies, where anhydrite–hematite alteration and fluid inclusions reveal the action of very oxidized saline solutions rich in sulfur.