HR: 13:45h
AN: V33F-01 INVITED     [Abstracts]
TI: Experimental studies bearing on the role of brines in deep-crustal metamorphism
AU: * Newton, R C
EM: rcnewton@ucla.edu
AF: Dept. of Earth & 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 & Space Sciences, University of California, Los Angeles, CA 90095-1567 United States
AB: Action of an aqueous fluid in granulite facies metamorphism can explain such commonly observed features as alkali-exchanged feldspars, garnet corrosion textures, synmetamorphic quartz veins, apparent mobility of Ca, Fe, Mg, and REE, depletion of Rb and Th, and, in some granulites, anomalously high oxidation state. Such a fluid, though geochemically effective, must have low H$_{2}$O activity to suppress hydrous minerals and prevent wholesale melting. CO$_{2}$-rich fluids have often been invoked as granulite-forming fluids, but have very low solubility for most common minerals. Concentrated chloride solutions satisfy the low H$_{2}$O activity requirement. (Na, K)Cl brines at 600-900 $\deg$C undergo pressure-induced ionization near 5 kbar, which results in a sudden decrease in H$_{2}$O activity (Aranovich and Newton, CMP 127, 261, 1997). A 30 mol % solution of (K$_{.2}$Na$_{.8}$)Cl at 8-10 kbar and 800 $\deg$C would have a(H$_{2}$O) near 0.5. Aranovich and Newton (Am. Min. 83, 193, 1998) used measured a(H$_{2}$O) of KCl solutions in equilibrium with the model granulite assemblage phlogopite-quartz-enstatite-K-feldspar to show that the threshold a(H$_{2}$O) in medium- to high-pressure granulite facies metamorphism is 0.4-0.5, much higher than previously thought. Thus, alkali chloride solutions of moderate concentration, well short of salt saturation, have suitably low a(H$_{2}$O). Recent studies in this laboratory and elsewhere reveal contrasting solubility behavior of common minerals in NaCl solutions at high T and P. Enhancement factors of molality (mol solute/kg H2O) relative to salt-free H$_{2}$O at 10 kbar, 800 $\deg$C and X(H$_{2}$O)=0.3 are 0.4 for quartz, 0.34 for albite (9 kbar: Shmulovich et al, CMP 141, 95, 2001), 16 for calcite, 120 for anhydrite, 15 for wollastonite, 34 for forsterite and 110 for fluorapatite. These data show that carbonate, sulfate, phosphate, the divalent cations, and rare earth elements are very soluble in high P-T alkali chloride solutions, and that the aluminosilicate constituents retain moderate solubility. A complex brine at high P and T could be effective in deep-crustal metasomatism, even at low fluid/rock ratios. The large solubility enhancement of anhydrite may be significant in that a CaSO$_{4}$-rich brine would necessarily be highly oxidized, with f(O$_{2}$) midway between NNO and HM in the presence of granulite facies assemblages. This could explain the high f(O$_{2}$) shown by some terranes (Wilson Lake, Labrador; Shevaroy Hills, S. India). Recent measurements (Webster et al, GCA 63, 729, 1999) have shown that Cl is very soluble in H$_{2}$O-undersaturated basalt magmas at elevated P and high T, and strongly pressure-dependent, so that Cl could cause early out-gassing during ascent of basalts, liberating concentrated brines. Thus, underplating of basalts could not only supply heat for metamorphism, but also geochemically active fluids compatible with granulite facies assemblages and only limited anatexis of country rocks.
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3660 Metamorphic petrology
DE: 3665 Mineral occurrences and deposits
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting