HR: 0800h
AN: T41B-1186    [Abstracts]
TI: Water Partition Coefficients Between Nominally Anhydrous Minerals and Basaltic Melts: Implication on Mantle Melting
AU: * Aubaud, C P
EM: aubau001@umn.edu
AF: University of Minnesota, Dept. Geology and Geophysics, 310 Pillsbury Drive SE, Minneapolis, MN 55455 United States
AU: Hauri, E H
EM: hauri@dtm.ciw.edu
AF: Carnegie Institute of Washington, DTM, 5241 Broad Branch Rd. NW, Washington, DC 20015 United States
AU: Hirschmann, M M
EM: Marc.M.Hirschmann-1@UMN.EDU
AF: University of Minnesota, Dept. Geology and Geophysics, 310 Pillsbury Drive SE, Minneapolis, MN 55455 United States
AB: Partitioning of water between peridotite minerals and basaltic magma has a significant influence on the initiation of melting in the mantle and on the rheological structure of the lithosphere. To investigate mineral/melt and mineral/mineral partitioning of H$_{2}$O applicable to the mantle, we have conducted multiple saturation experiments consisting of hydrous basalt$\pm$ol$\pm$opx$\pm$cpx in a piston-cylinder apparatus at pressures of 1--2 GPa, temperatures of 1230--1380\deg C and bulk initial water contents of 3.3 to 6.3 wt.%. We measured H$_{2}$O in melts and minerals (ol, opx, cpx) by SIMS using methods described by [1]. Resulting liquids have 3.1-6.4 wt.% H$_{2}$O and average mineral/melt partition coefficients as follows: D$^{ol-melt}$=0.0017$\pm$0.0005 (n=9), D$^{opx-melt}$=0.019$\pm$0.004 (n=8), and D$^{cpx-melt}$=0.023$\pm$0.005 (n=2). Mineral/mineral partition coefficients are D$^{ol-opx}$=0.11$\pm$0.01 (n=4), D$^{ol-cpx}$=0.08$\pm$0.01 (n=2) and D$^{cpx-opx}$=1.4$\pm$0.3 (n=1). Observed partition coefficients are reproducible between experiments and systematic variations with pressure, temperature or concentration of H$_{2}$O are not apparent, possibly because of the relatively small range of pressures and compositions examined. The D$^{pyroxene-melt}$ increases with the Al$_{2}$O$_{3}$ content of the pyroxene due to enhanced solubility of water in Al-bearing pyroxenes. For a peridotite consisting of 58% ol, 30% opx, 10% cpx, and 2% spinel (assumed nominally anhydrous) the calculated bulk sol-liq D is 0.009$\pm$0.002 confirming that water is highly incompatible in mantle minerals. Compared to conventional trace elements, water has a behavior similar to Ce, in accordance with studies on natural basaltic glasses (e.g., [2]). If this bulk D is applicable to the deeper parts of the MORB melting regime, then following [3], we can estimate the effect of H$_{2}$O on peridotite partial melting: for mantle water concentrations of 50--200 ppm, the near-solidus melt would contain 0.6-2.3 wt.% water. Using the data of [4] for $\Delta$H$_{fusion}$, the freezing point depression is 20--60\deg C, which corresponds to initiation of melting beneath along ridge geotherms 5--20 km deeper than the anhydrous solidus, somewhat less than previous estimates ($\sim$50 km, [5]). For concentrations of 500--1000 ppm H$_{2}$O along plume geotherms, melting will begin 60--130 km deeper than the dry solidus. References [1] Koga et al. (2003) G3 4,1--20. [2] Michael (1995) EPSL 131, 301--320. [3] Hirschmann et al. (1999) J. Petrol.40, 831--851. [4] Kojitani and Akaogi (1997) EPSL 153, 209--222. [5] Hirth and Kohlstedt (1996) EPSL 144, 93--108.
DE: 1025 Composition of the mantle
DE: 1030 Geochemical cycles (0330)
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 1065 Trace elements (3670)
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting