HR: 1330h
AN: V22D-0612    [PDF]
TI: Solubility of Anhydrite (CaSO$_{4}$) in NaCl-H$_{2}$O Fluids at High T and P
AU: * Newton, R C
EM: rcnewton@ucla.edu
AF: Dept. of Earth and Space Sciences, University of California, Los Angeles, CA 90095-1567 United States
AU: Manning, C E
EM: manning@ess.ucla.edu
AF: Dept. of Earth and Space Sciences, University of California, Los Angeles, CA 90095-1567 United States
AB: Weight losses of single crystals of a very pure natural anhydrite exposed to NaCl solutions of 0-0.3 mol fraction were measured at 600-800 \deg C and 6-14 kbar. Experimental charges were contained in welded Pt capsules in 1.91 cm-diameter piston-cylinder apparatus with NaCl pressure medium for 5-72 hr. Measurements in initially pure H$_{2}$O were made with HM, NNO, and MnO$_{2}$ buffers, as well as without buffering. At 800 \deg C and 10 kbar, CaSO$_{4}$ molalities are: MnO$_{2}$, 0.014 mol/kg H$_{2}$O; HM, 0.017; NNO, 0.148; and unbuffered, 0.026. Variation in oxygen fugacity thus has a large effect on CaSO$_{4}$ solubility, increasing with H$_{2}$S/SO$_{2}$ in the fluid. Unbuffered (self-buffered) charges gave solubilities much closer to HM than NNO. Melting occurred in the NNO experiment at this P and T. NaCl increases CaSO$_{4}$ solubility enormously, with m(CaSO$_{4}$) reaching 5.4, or 23.5 wt. %, at 800 \deg C, 10 kbar and X(NaCl)=0.3. There is also a very large increase with temperature. Regression of all the data give: log(m-m$_{o}$) = -1.533 + 0.00291T + (1.441 + 0.00016T)logX(NaCl) + 0.0413(P-10) where m$_{o}$ is molality in pure H$_{2}$O, P is in kbar, and T is in Kelvins. The very large carrying capacity for sulfate in even mildly saline fluids at high P and T, together with the high oxygen potential generated when these solutions react with FeO in rocks to yield pyrrhotite, indicates that such fluids should be considered as principal agents in S-rich, highly oxidizing processes such as Pinatubo-type volcanic eruptions, certain deep-crustal granulite facies metamorphism, as in Bamble, Norway and Shevaroy Hills, S. India, and the anhydrite-related, oxidized Au ore deposits like Abitibi, Ontario, and Kalgoorlie, Australia.
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting