CSEDI: Melt stability and dynamics in the deep Earth

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5) The relatively low shear modulus and high temperature of Earth's ultralow-velocity zones (ULVZ) imply that these thin (5-40 km thickness) regions at the core-mantle boundary (CMB) may be partly molten. In order to maintain the cooling rate necessary to drive a dynamo by convection, Earth's ancient core must have been hotter than at present and the lowermost mantle would therefore have been more extensively melted. It has been hypothesized that the higher density of melt relative to solids in the deepest mantle stabilized this molten layer, forming a dense basal magma ocean (BMO) that appeared early in Earth's history and whose remains are at present the ULVZs. The existence of a BMO has large effects on the dynamics and differentiation of the deep Earth. Incompatible elements would have been sequestered into the melt by fractionation, while the chemical signature of solids crystallized from the BMO would be governed by the phase diagram. This crystallization signature should appear at Earth's surface in volcanic products from deep-seated mantle plumes. Other structures in Earth's lowermost mantle such as "chemical piles" may be formed or modified by BMO crystallization. The presence of a BMO will also enhance the extent of core-mantle chemical interactions relative to a solid lowermost mantle, possibly leading to the formation of a buoyant layer at the top of the core that is enriched in light elements. This project aims to further explore the consequences of a BMO by developing a two-phase dynamics model that will be used to better constrain the dynamical evolution of a BMO, to address several questions raised by the BMO hypothesis, and to begin to test the hypothesis with geochemical data and seismological observations. The effects of a buoyant stratified layer at the top the core produced by reactions between the BMO and core will also be explored using a numerical dynamo model to test whether features of dynamos in such cores are compatible with geomagnetic observations. The project will provide two years of support for a post-doctoral researcher, who will benefit from interdisciplinary training, and involves direct international collaboration among researchers in 4 different countries (United States of America, United Kingdom, France, and Canada). Most will be new collaborations. The two-phase flow models can be applied to a large variety of mush-slurry systems in other disciplines, such as the evolution of crustal magma chambers and volcanic systems. The code will be used in upcoming benchmark exercises, a time consuming but essential aspect of code development and modeling. The code will also be made available to the broader research community for use in other projects.The relatively low shear modulus and high temperature of Earth's ultralow-velocity zones (ULVZ) imply that these thin (5-40 km thickness) regions at the core-mantle boundary (CMB) may be partly molten. In order to maintain the cooling rate necessary to drive a dynamo by convection, Earth's ancient core must have been hotter than at present and the lowermost mantle would therefore have been more extensively melted. It has been hypothesized that the higher density of melt relative to solids in the deepest mantle stabilized this molten layer, forming a dense basal magma ocean (BMO) that appeared early in Earth's history and whose remains are at present the ULVZs.