Effect of a Small Amount of Melt on Diffusion and Deformation in Earth's Mantle

University Of Minnesota-Twin Cities, Minneapolis MN

Investigators

Abstract

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). (a) Non-technical explanation of the project's broader significance and importance: Small amounts of melt that form deep beneath Earth?s surface rise slowly to feed volcanoes such as those observed in Iceland and Hawaii. As this melt moves upward through the Earth, it weakens the rocks through which it travels. The decrease in strength associated with the onset of melting significantly influences the extent of convection in partially molten regions, such as beneath mid-ocean ridges and in subduction zones, and thus the nature of plate tectonics and the style of volcanic eruptions along the boundaries between surface plates. The experiments described in this proposal are designed to develop a fundamental understanding of the role of a small amount of melt on the strength of mantle rocks which can be applied to important geological processes. The results of these experiments will provide the framework necessary for understanding the rate of transport of melt from depth to Earth?s surface. As melt finds its way to the surface, it transports heat and thus geothermal energy. As melt erupts at the surface, it generates earthquakes and thus causes destruction. Therefore, important long-range goals of our research are to provide the knowledge necessary to understand the rate of heat delivered to localities such as Iceland and the nature of earthquakes that occur in volcanically active regions of the Earth. (b) Technical description of the project: The goal of this research is to develop a fundamental understanding of the physical-chemical mechanisms and geophysical-geochemical implications of a small amount of melt (&#8818;1 vol %) on the strength of mantle rocks. We are motivated by two recent results, one theoretical and one experimental: (1) A new model of creep of partially molten rocks predicts an increase in strength of a factor of ~10 if melt content is reduced from 1 to 0 vol %. (2) A recent set of experiments suggest that high-purity, melt-free aggregates of olivine are a factor of ~100 stronger than anticipated based on flow laws determined from laboratory experiments on impure samples with <1 vol % melt. Therefore, our experiments are designed to determine the influence of a small amount of melt, &#8818;1 vol %, versus the influence of grain boundary impurities on the strength of dunite rocks. An important question thus arises: ?What is the role of grain boundary chemistry versus the role of a small amount of triple junction melt on diffusion and deformation?? The answer to this question will profoundly influence extrapolation and application of results obtained from laboratory experiments to geodynamical and geochemical processes occurring in Earth?s mantle. Grain boundary creep will be investigated for a series of mantle samples (i) starting with high-purity, melt-free olivine, then (ii) progressing to olivine doped with basaltic composition impurities at low enough concentrations to avoid forming a melt phase, and finally (iii) moving to olivine doped with basalt at high enough concentrations to form a small amount of melt. Experiments will be carried out under well-controlled thermal-chemical-mechanical conditions at high P and T. Results will be analyzed using the recently developed constitutive relation for flow of melt-free and partially molten rocks. Microstructural and microchemical characterization of melt distribution and grain boundary chemistry will be performed using high-resolution scanning and transmission electron microscopy imaging and analytical techniques. Our results will impact understanding of chemical, mechanical, electrical and seismological properties of Earth?s upper mantle.