Near-fractional Melting of Pyroxenite: Experimental Investigations and Applications to Basalt Petrogenesis

University Of Utah, Salt Lake City UT

Investigators

Abstract

The rocks that make up Earth's crust are subject to a cycle of life - new crust is formed at divergent plate boundaries (spreading centers) via the melting of the mantle (material beneath the crust) and transport of magma to the surface, while old crust sinks back into the mantle at convergent plate boundaries. Specifically, the return of oceanic crust to the mantle creates chemical and mineralogical heterogeneities (mixtures of olivine-rich and pyroxene-rich lithologies) that can lead to complex melting behavior when this material rises again beneath spreading centers and in mantle plumes. The abundance and time scales of crustal recycling are important aspects of the plate tectonic cycle, but difficult to constrain based solely on the petrology and geochemistry of crustal rocks. Quantitative modeling requires knowledge of melting behavior of different rock types. The goal of this research is to obtain new high quality experimental data on the melting of pyroxenite, which represents one the most important rock types that are recycled into the mantle. Experiments will be performed at conditions found in the mantle where temperatures are thousands of degrees hotter and pressures tens of thousands of times greater than found at the Earth's surface. The results obtained will be integrated into forward models of melting of heterogeneous mantle that simulate the dynamic processes operating beneath the world's spreading centers and in mantle plumes. The primary objective of this project is to understand the effect of near-fractional melting of pyroxenites, to better quantify the role of pyroxenite in basalt genesis. The experimental approach will consist of performing incremental melting experiments, with increments of melt fraction less than 5 wt. % at each stage, on three different compositions previously investigated by batch melting experiments. The selected compositions span the likely compositional range of pyroxenite lithologies in the mantle. Melt composition at each temperature increment will be determined with the microdike technique and used to define new melting parameterizations that can be used to model near-fractional melting and integrate them into forward models of melting of heterogeneous mantle. NiO and MnO proxies used to determine the pyroxenite contribution in typical basaltic magmas and their source regions will be recalibrated based on the experimental results. Because melt extraction during generation of MORB and ocean island basalts is likely to occur via near fractional melting rather than batch melting, these new calibrations will allow us to better quantify the proportion of pyroxenite in the mantle source of the most abundant magmas on Earth.