HR: 0800h
AN: V31A-1407    [Abstracts]
TI: Experimental constraints on H$_{2}$O activity in high-pressure metamorphic brines
AU: * Tropper, P
EM: Peter.Tropper@uibk.ac.at
AF: Dept. of Earth & Space Sciences, University of California, Los Angeles, CA 90095-1567 United States
AU: * Tropper, P
EM: Peter.Tropper@uibk.ac.at
AF: Inst. of Mineralogy and Petrography, University of Innsbruck, Innrain 52, Innsbruck, A-6020 Austria
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: Subduction of crustal materials is accompanied by metamorphic reactions liberating fluids. Fluid inclusions in eclogite minerals range from dilute solutions to chloride-rich brines; however, the effect of salinity variations on the stability of hydrous phases in subduction zones is poorly understood. To address this problem, we carried out reversed piston-cylinder experiments on the equilibrium (1) paragonite = jadeite + kyanite + H$_{2}$O at 700$\deg$C, 1.5-2.5 GPa, in the presence of H$_{2}$O-NaCl fluids. The experiments were conducted using fluids with nominal starting compositions: X(H$_{2}$O)=1.0, 0.90, 0.75 and 0.62. At X(H$_{2}$O) = 1.0, the equilibrium lies between 2.25 and 2.30 GPa. Lowering X(H$_{2}$O) decreases the pressure of paragonite breakdown to 2.10 - 2.20 GPa at X(H$_{2}$O) = 0.90 and 1.85-1.90 GPa at X(H$_{2}$O) = 0.75. The experiments at X(H$_{2}$O) = 0.62 yielded albite + corundum at $\leq$1.60 GPa, and jadeite + kyanite at $\geq$1.70 GPa. The shift in the equilibrium pressure constrains a-X relations in the sytem H$_{2}$O-NaCl and indicates that a(H$_{2}$O) varies as nearly the square of its mole fraction. The results are consistent with the extrapolated non-ideal activity model of Aranovich and Newton (1996, CMP, 125, 200). Our results permit use of appropriate paragonite-bearing or -absent assemblages to quantify a(H$_{2}$O) in high-P metamorphic environments, such as the Austroalpine units in the Western Alps. For example, jadeite and kyanite in a metapelite from Val Savenca in the Sesia Lanzo Zone formed during the Eo-Alpine high-P metamorphic event at 1.7-2.0 GPa, 550-650$\deg$C. The absence of paragonite requires a fluid with low a(H$_{2}$O) of 0.3-0.6, which could be due to the presence of saline brines. Petrologic investigations of Sesia Lanzo eclogites from Val Ianca show that paragonite occurs as inclusions in garnet cores but gives way to omphacite + kyanite toward rims, suggesting a decrease in a(H$_{2}$O) from $\sim$1.0 to $<$0.81 (Tropper and Essene, 2002, SMPM, 82, 487-514). During the subsequent hydration in the late eclogite-facies at slightly lower P, a(H$_{2}$O) increased to 0.84-0.98, stabilizing the assemblage paragonite + omphacite. The new results can also be used to compare a(H$_{2}$O) from phase-equilibrium studies with fluid-inclusion constraints. Paragonite-bearing eclogites from the Mt. Emilius unit in the Western Alps interacted with fluid with a(H$_{2}$O)=0.62-0.72, based on fluid-inclusion data from Scambelluri et al. (1998, Lithos, 43, 151) during Eo-Alpine high-P metamorphism of 1.1-1.3 GPa, 450-550$\deg$C (Dal Piaz et al., 1983, Am. J. Sci., 283A, 438). Calculation of reaction (1) with reduced a(H$_{2}$O) shows no incompatibilities with the observed phase assemblage. This study shows that paragonite + cpx + kyanite-bearing rocks have great potential for constraining a(H$_{2}$O).
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