The effect of manganese on olivine-quartz-orthopyroxene stability |
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| Authors: | Steven R Bohlen Arthur L Boettcher Wayne A Dollase Eric J Essene |
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| Institution: | 1. Department of Geology and Mineralogy, University of Michigan, Ann Arbor, MI 48104 U.S.A.;2. Institute of Geophysics and Planetary Physics and Department of Earth and Space Sciences, University of California at Los Angeles, Los Angeles, CA 90024 U.S.A.;3. Department of Earth and Space Sciences, University of California at Los Angeles, Los Angeles, CA 90024 U.S.A. |
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| Abstract: | The effect of manganese on the stability of ferrosilite relative to fayalite + quartz has been experimentally determined to assess its importance to orthopyroxene barometry. Reaction reversals in a piston-cylinder apparatus were obtained to within 0.1-kbar intervals indicating instability of Fs95Rh5 below 10.3, 10.9, 11.4, 12.2, 12.9, 13.7 kbar and Fs90Rh10 below 9.8, 10.4, 10.9, 11.6, 12.4 and 13.2 kbar at 750, 800, 850, 900, 950 and 1000°C, respectively. Each mole % MnSiO3 extends the pyroxene stability by approximately 0.12 kbar relative to FeSiO3. Electron microprobe analyses of run products indicate a small preference of Mn for pyroxene over olivine withKDMn-Feopx-oliv = 1.2?1.5, similar to values observed for natural pairs. Mössbauer spectra are consistent with a random distribution of Mn between the M1 and M2 sites in the orthopyroxene. These experimental data allow downward revision of pressure estimates based on the orthopyroxene barometer in areas where Mn is a significant component in orthopyroxene. |
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