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

Yale University, New Haven CT

Investigators

Abstract

A major question about the Earth's interior is why the mantle appears both well-mixed and unmixed at the same time. Geochemical analyses imply that the two main types of mantle convective upwellings (which create mid-ocean ridge basalts MORB and ocean-island basalts OIB, respectively) have distinct chemical signatures and thus come from different isolated layers in the mantle. However, seismological studies suggest that cold downwellings (called subducting slabs) sink across the entire mantle and would therefore stir the mantle and destroy any layering. Thus there appears to be completely contradictory pieces of evidence of how the mantle convects. However, it has been recently suggested that melting at 410 km depth in the mantle can act as a filter that sequesters incompatible elements into the deep (sub-410km) mantle, but allows whole-mantle circulation of major components, thereby satisfying both geochemical and geophysical constraints. As most of the ambient upwelling mantle rises out of the high-water-solubility transition zone into the low-solubility upper mantle above 410km, it becomes water-supersaturated and undergoes dehydration melting that filters out 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 deliver enriched 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 influence of 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 an interdisciplinary approach and therefore involves a team comprised of a geodynamicist, an experimental mineral physicist, a seismologist and a geochemist. Broader impact: This project involves the education of two graduate students in a highly interdisciplinary project entailing geodynamics, mineral physics, seismology and geochemistry. The water-filter model also raises new questions and fields of research that will drive interaction between disciplines and provide focus and hypothesis-testing for the larger observational and experimental programs such as EarthScope, OMD (Ocean Mantle Dynamics) and COMPRES.