CSEDI: Whole-Mantle Convection and the Transition-Zone Water Filter

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A major question about the Earth's interior is why the mantle appearsboth well-mixed and unmixed at the same time. Geochemical analyses implythat the two main types of mantle convective upwellings (which createmid-ocean ridge basalts MORB and ocean-island basalts OIB, respectively)have distinct chemical signatures and thus come from different isolatedlayers in the mantle. However, seismological studies suggest thatcold downwellings (called subducting slabs) sink across the entiremantle and would therefore stir the mantle and destroy any layering.Thus there appears to be completely contradictory pieces of evidenceof how the mantle convects. However, it has been recently suggestedthat melting at 410 km depth in the mantle can act as a filter thatsequesters incompatible elements into the deep (sub-410km) mantle, butallows whole-mantle circulation of major components, thereby satisfyingboth geochemical and geophysical constraints. As most of the ambientupwelling mantle rises out of the high-water-solubility transitionzone into the low-solubility upper mantle above 410km, it becomeswater-supersaturated and undergoes dehydration melting that filtersout incompatible elements. The filtered solid phase supplies dry,depleted materials to the MORB source region. The filter, however,is suppressed for hotter, faster mantle plumes which therefore deliverenriched OIBs. This "water-filter" model raises many new questions: (1)What is the predicted structure of the putative melt layer at the 410km,and how can this be tested seismologically? (2) What is the influenceof the filter mechanism on mantle convection and chemical circulation?(3) What is the nature of hydrous melt at deep upper-mantle conditions?Answering these and a host of other important new questions demands aninterdisciplinary approach and therefore involves a team comprised ofa geodynamicist, an experimental mineral physicist, a seismologist anda geochemist.Broader impact:This project involves the education of two graduate students in a highlyinterdisciplinary project entailing geodynamics, mineral physics,seismology and geochemistry. The water-filter model also raises newquestions and fields of research that will drive interaction betweendisciplines and provide focus and hypothesis-testing for the largerobservational and experimental programs such as EarthScope, OMD (OceanMantle Dynamics) and COMPRES.