Melting of Carbonated MORB-like Eclogite and Genesis of Ocean Island Basalts

William Marsh Rice University, Houston TX

Investigators

Abstract

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5) Chemistry of oceanic island basalts (OIBs) reflects the complexity of compositions and processes in the Earth's mantle. Isotope and trace element geochemistry have established that parent lithologies of OIBs in the Earth's mantle are heterogeneous. However, the case of major element heterogeneity in OIBs is not completely understood as the full spectrum of major element compositions of OIBs have so far not been produced by laboratory experiments. Especially, the major element compositions of OIBs that carry isotopic signatures of recycled crust cannot be reconciled with existing melting experiments on subducted oceanic crust. This proposal is to test whether melting of subducted oceanic crust, in the presence of CO2-rich fluids or carbonates, produces magmas (partial melts of the Earth's mantle) with major element compositions similar to alkalic OIBs. The aim is to constrain the major and trace element compositions of silicate partial melts derived from carbonated basaltic eclogite, and the compositional evolution of such melts through reaction with mantle peridotite. The experiments will be performed at a pressure range of 2 to 4 GPa. In particular, the experiments will be performed to address the following fundamental questions: (1) What are the compositions of silicate partial melts of silica-excess eclogite in the presence of CO2 and can carbonated crust act as a root source of alkalic OIBs? (2) How do eclogite+CO2-derived melt compositions change owing to interaction with shallow mantle, depleted harzburgite? (3) What is the contribution of recycled carbonated crust in the origin of intraplate basalts? (4) What are the plausible conditions (depths/temperatures/source compositions) of generation of alkali OIBs? In this work, laboratory experiments will be conducted to simulate high pressure, high temperature conditions relevant for magma generation processes beneath oceanic islands. Analysis of the partially molten quenched rocks will be carried out using well established techniques of optical and electron microscopy, electron probe microanalysis, and mass spectrometry. The obtained results will further the understanding of genesis of basalts (the dominant rock that makes up the ocean floor) and will add to link melting of heterogeneous mantle rocks and deep carbon cycle. The results will be integrated with other sister-disciplines in the broad field of solid Earth science that seek to capture the complex processes in the Earth's mantle in general and in the source regions of basalts in particular. The work will support a newly established experimental petrology laboratory and a starting faculty member at Rice University. The research will also involve education and training of graduate and undergraduate researchers.