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Séminaires externes 2017-2018

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Mercredi 6 septembre 2017 *reporté* : Julia Ribeiro (Rice University Houston Tx, USA) The fate of water at subduction zones

Water is efficiently cycled from the Earth’s surface into its deep interior via subduction zones. As the descending plate dehydrates into the asthenospheric mantle, it triggers arc magmatism. The depth at which the slab dehydration and arc magmatism initiate is thought to be dictated by the thermal state of the slab. Following this view, cold slabs are thought to mostly dehydrate beneath the arc front, while warmer slabs should mostly dehydrate beneath the fore-arc. Cold subduction zone lavas are thus predicted to have interacted with greater extent of water-rich fluids released from the downgoing slab, and should thus display higher water content and be elevated in slab-fluid proxies (i.e., high Ba/Th, H2O/Ce, Rb/Th, etc) compared to hot subduction zone lavas. Arc lavas, however, display similar slab-fluid signatures and 4wt% water regardless of the thermal state of the slab, suggesting more complexity to volatile cycling in subduction zones. Additional questions arise from the observations that Mariana fore-arc lavas display the highest slab fluid proxies recorded so far, implying that a dehydration peak may occur well before the slab reaches the arc front. Such paradoxical observations thus imply that the model of water delivery to the mantle wedge by slab dehydration must be more complicated.

Here, we explore the possibility that water delivery to the mantle wedge is not just controlled by release of fluids from the slab beneath the arc front. We evaluate the possibility that water is delivered to the mantle wedge by the serpentinized fore-arc mantle, in addition to direct dehydration of the slab, as originally proposed by Tatsumi (1989) using stability of hydrous mineral phases along the subducting path. To quantify the water outfluxes released at the arc and back-arc basin spreading center, we performed new mass balance calculations by using the maximum water content of olivine-hosted melt inclusions, basaltic glass shards and gas outfluxes released at arc volcanoes. The extent to which the fore-arc mantle can be dragged down to subarc depths is assessed with simple fluid dynamic equations. We find that the downgoing plate does not carry enough water down to sustain the water outfluxes released in the Mariana and Cascadia arcs. We show that dragging down of fore-arc mantle, followed by its dehydration, is a viable mechanism that can compensate for this water imbalance.

publié jeudi 13 septembre 2012