2020/08/21 by Avishek Rudra, M. M. Hirschmann, Rudra, Avishek +1
Earth and Planetary Sciences · Physics and Astronomy · #Geological and Geochemical Analysis #High-pressure geophysics and materials #Advanced Condensed Matter Physics
paper · pdf · doi:10.48550/arxiv.2008.09296
Pyroxene is the principal host of Fe3+ in basalt source regions, hosting\n79 and 81% of the Fe3+ in spinel and garnet lherzolite, respectively, with\nopx and cpx hosting 48% and 31%, respectively, of the total Fe3+ in spinel\nperidotite. To better understand partitioning of Fe3+ between pyroxene and\nmelt we conducted experiments at 100 KPa with fO2 controlled by CO-CO2\ngas mixes between \ΔQFM -1.19 to +2.06 in a system containing andesitic\nmelt saturated with opx or cpx only. To produce large (100-150 \μm),\nhomogeneous pyroxenes, we employed a dynamic cooling technique with a\n5-10\degC/h cooling rate, and initial and final dwell temperatures\n5-10\degC and 20-30^\∘C super and sub-liquidus, respectively. Resulting\npyroxene crystals have absolute variation in Al2O3 and TiO2 <0.05 wt.%\nand <0.02 wt.%, respectively. Fe3+/FeT in pyroxenes and quenched glass\nwere measured by XANES. We used a newly developed XANES calibration for cpx and\nopx by only selecting spectra with X-ray vibrating on the optic axial plane at\n50 \± 5^\∘ to the crystallographic c axis. Values of DFe3+ cpx/melt\nincrease from 0.03 to 0.53 as fO2 increases from \ΔQFM -0.44 to 2.06,\nwhile DFe3+ opx/melt remains unchanged at 0.26 between \ΔQFM -1.19\nto +1.37. In comparison to natural peridotitic pyroxenes, Fe3+/FeT in\npyroxenes crystallized in this study are lower at similar fO2, presumably\nowing to lower Al3+ contents. This study shows that the existing\nthermodynamic models implemented in pMELTS and PerpleX over-predict the\nstability of Fe3+ in pyroxenes, causing an anomalous reduced character to\nspinel peridotites at calculated conditions of MORB genesis.\n