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Mineral Physics Institute
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| Kerry Da Silva The Incorporation Mechanism of Hydrogen in Forsterite
Olivine is the most common mineral in the Earth's upper mantle, and forsterite (Mg 2 SiO 4 ), one of the endmembers of the olivine solid solution series, constitutes about ninety percent of mantle olivine. Forsterite is a nominally anhydrous mineral (NAM), but trace amounts of hydrogen occur within its crystalline structure. Hydrogen enters forsterite by attaching to one or more of its oxygen atoms, and is primarily present as a hydroxyl (OH - ) group. Mantle NAMs such as forsterite could contain a large water reservoir, which may well be the source of water delivered to the surface through volcanism, and then back down to the deep mantle through the subduction of oceanic plates (plate tectonics). Water effects mantle minerals' mechanical strength, electrical conductivity, plasticity, melting temperature, and phase transformation rate. Therefore, the presence of hydrogen in forsterite effects overall geophysical properties of the mantle. Since natural mantle minerals such as olivine contain impurities that could contaminate research results, we will be using synthetic forsterite in our experiments. First we will cook a sample of forsterite in the DIA press with the presence of water at 5 GPa and 1200° C, in order to simulate the conditions of the upper mantle (~ 150 km deep). We will then analyze the hydrogen concentration in the forsterite sample and the incorporation modes using Fourier Transform Infrared Spectroscopy (FTIR). Incorporation of hydrogen into forsterite is dependent upon pressure, temperature, silica activity, and perhaps oxygen fugacity. Through our research, we hope to shed light on the mechanisms with which hydrogen is incorporated into mantle olivine. The results will have significant implications on the water capacity in mantle NAMs. |
Modified June 20, 2007
