Collaborative Research: The Effects of Melt Depletion on Mantle Density and Velocity

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To understand flow in the upper mantle, the density variations that drive this flow must be quantified. Processes that create density variations are changes in temperature, the amount of melting, and the composition of the mantle. A primary cause of compositional variations is melt depletion, which occurs when mantle rock (peridotite) is partially melted. In this case, some elements, such as Fe, preferentially partition into the melt, leaving the unmelted rock (the residuum) depleted in these elements. Melt depletion is of fundamental importance in tectosphere formation, melt focusing at mid-ocean ridges, plume-plate interaction, and melt-driven convection. Melt depletion may also modify seismic velocities, allowing depleted mantle to be imaged. Furthermore, the density effects of melt depletion are increasingly being used in geophysical studies to infer the composition of the mantle. The aim of this project is to quantify the compositional, density, and velocity effects of melt depletion, and resolve the differences between theoretical models and xenolith studies. To do so, the investigators will measure the compositional effects of melt removal on peridotite at pressures between 1-5 GPa, and convert these compositional variations to density and anharmonic velocity variations. This study will enhance discovery and understanding by providing research experience to a new scientist; by encouraging cooperation between the University of Wyoming and the University of California, Davis; and by promoting interdisciplinary research in mantle petrology, mineral physics, and seismology. To encourage broad dissemination of this work, results will be posted on a web site hosted by the University of Wyoming, which will include parameterizations of density and velocity for geodynamical modelers (initial results already are being incorporated into convection models at UCLA), and freely available Matlab scripts for calculating mantle density and velocities as function of composition, temperature, and pressure. Research will also be presented at scientific meetings and departmental seminars.