Conolly, J.

Hort, Matthias

Phipps Morgan, Jason

Rüpke, Lars

English

OceanRep

Geophysical Research Abstracts, Vol. 6, 05554, 2004SRef-ID: 1607-7962/gra/EGU04-A-05554c© European Geosciences Union 2004IMPLICATIONS OF SUBDUCTIONREHYDRATION FOR EARTH’S DEEP WATERCYCLEL. Rüpke (1), J. Phipps Morgan (1), M. Hort (2), J. Connolly (3)(1) SFB 574, Kiel University, Germany, (lruepke@geomar.de), (2) Hamburg University,Germany, (3) ETH Zurich, SwitzerlandThe Earth’s global geochemical water cycle is controlled by two major competingprocesses: water outgassing at mid-ocean ridges and hotspots and the potential re-injection of surface water into the mantle at subduction zones. The substantial un-certainty in this water cycle is the amount of water that is recycled at subductionzones. It is now generally accepted that some of the more enriched mantle compo-nents (HIMU&EM) have formed by the recycling of sediments and crustal material.One way to determine the amount of water that is recycled back into the mantle is,therefore, to measure the volatile contents of these recycling associated mantle com-ponents as sampled in OIB and MORB. Dixon et al. (2002) has recently taken this ap-proach and finds that slabs most likely efficiently dehydrate during subduction and thatthe most primitive (FOZO) mantle components are significantly wetter than the recy-cling associated EM or HIMU mantle components and the even drier depleted mantlesource that melts to form MORB. Dixon et al.’s conclusions on slab-dehydration arein striking agreement with our recent conclusions from numerical modelling of slabdehydration beneath modern and paleo-subduction zones when coupled to a param-eterised whole mantle convection model. Based on these numerical experiments wepresent here a mantle evolution scenario in which the oceans have formed by wateroutgassing from the mantle but in which the Earth’s surface and deep water cycleare still in close contact through mantle rehydration by plate subduction. In this sce-nario, the present day Earth is characterized by a mantle that has been largely, but notcompletely outgassed (∼93%). Due to slab recycling, it contains about ∼25% of theexosphere’s water content and ∼20% of its initial water content, of which ∼2/3 hasbeen recycled back from the exosphere. These numbers are in close agreement withthe results of Dixon et al. in that we find similar water concentrations for wet primi-tive mantle and much drier recycled slabs. It seems, therefore, completely consistentthat hotspots with a FOZO-component in their source will make wetter basalts thanhotspots whose mantle sources contain a larger fraction of EM and HIMU compo-nents, and also consistent that progressive melting beneath a mid-ocean ridge shouldeventually be able to ’dehydrate’ the mantle, so that the MORB source is even drier.Thus these results are consistent with a plum-pudding whole mantle convection evo-lution scenario in which plate subduction plays an important role in partially filteringthe water content of the recycled sediments and crust that form the sources of the moreenriched mantle plums.

Implications of subduction rehydration for earth's deep water cycle

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