Effect of grain growth on cation exchange between dunite and fluid: implications for chemical homogenization in the upper mantle |
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Authors: | Tomohiro Ohuchi Michihiko Nakamura Katsuyoshi Michibayashi |
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Institution: | (1) Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan;(2) Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan;(3) Institute of Geosciences, Shizuoka University, Shizuoka 422-8529, Japan |
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Abstract: | The effect of grain growth on the cation exchange between synthesized forsterite aggregates (i.e., dunite) and nickel-rich
aqueous fluid was evaluated experimentally at 1.2 GPa and 1,200°C. The grain boundary (GB) migration caused nickel enrichment
in the area swept by the GBs in a fashion similar to that reported for stable isotope exchange in the quartz aggregates. The
progress of the grain growth resulted in an increase in the average nickel concentration in the dunites of up to ~80 times
that was calculated for a system having stationary GBs. The overall diffusivity of the nickel along the wet GBs and interconnected
fluid networks was found to be 6.5 × 10−19–6.7 × 10−18 m3/s, which is 4–5 orders of magnitude higher than the grain boundary diffusivity in the dry dunite. These results show that
the grain growth rate is a fundamental factor in the evaluation of the time scale of chemical homogenization in the upper
mantle. |
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