HR: 1340h
AN: V13A-1452    [Abstracts]
TI: Modeling Open System Metamorphic Devolatilization of Subducted Metabasalts
AU: * Gorman, P
EM: pgorman@geosc.psu.edu
AF: Pennsylvania State University, Department of Geosciences, University Park, PA 16802 United States
AU: Kerrick, D M
EM: kerrick@geosc.psu.edu
AF: Pennsylvania State University, Department of Geosciences, University Park, PA 16802 United States
AU: Connolly, J A
EM: james.connolly@erdw.ethz.ch
AF: Swiss Federal Institute of Technology, Earth Sciences Department, Sonnegstrasse,Zurich, 8092 Switzerland
AB: Fluids derived from the devolatilization of a subducting slab play a critical role in the melting of the mantle wedge and consequent arc volcanism. In this study we present results from thermodynamic modeling of subduction zone metamorphic devolatilization of metabasalts for pressures up to 6 GPa using an approach which considers fluid fractionation from source lithologies and infiltration from subjacent lithologies. This open system approach offers a more realistic model of subduction zone devolatilization than closed system models using similar computational techniques. Along high temperature geotherms, decarbonation is complete in the case where the maximum H$_{2}$O flux likely to be achieved in subduction settings is assumed. In the case of a dry mantle and lower crust, decarbonation along high temperature geotherms is limited to $\sim$20 wt.% of original carbonate content. Along low and intermediate temperature geotherms, decarbonation is insignificant except in the case of maximal H$_{2}$O infiltration where carbonate loss reaches a maximum of $\sim$20%. Relative to closed system behavior, our open system modeling leads to more extensive decarbonation along high and intermediate temperature geotherms, whereas decarbonation along low temperature geotherms is not appreciably different. Because our model assumes pervasive compaction-driven fluid flow, the effect of fluid infiltration is maximized and therefore it is possible that the results presented here overestimate the extent of infiltration induced decarbonation in subducting crust. Subsequently, fluid propagation models which account for channelized flow could diminish the magnitude of carbonate loss compared to what is predicted in this study. The extent of metabasalt dehydration for open system behavior is not significantly different when compared to closed system devolatilization for all geotherms considered in this study. Because metabasalts undergo appreciable dehydration under all but the coldest geotherms, this lithology provides a fertile H$_{2}$O source for infiltration driven decarbonation of overlying marine sediments as well as fluid induced partial melting in the mantle wedge. Due to limited decarbonation along low and intermediate temperature geotherms and limited dehydration along low temperature geotherms, oceanic metabasalts represent an effective lithology for recycling of volatiles into the deep mantle.
DE: 8450 Planetary volcanism (5480)
DE: 8105 Continental margins and sedimentary basins
DE: 5480 Volcanism (8450)
DE: 3660 Metamorphic petrology
DE: 0330 Geochemical cycles
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting