Experimental determination of ferric iron partitioning between pyroxene and melt at 100KPa

Pyroxene is the principal host of Fe$^{3+}$ in basalt source regions, hosting 79 and 81% of the Fe$^{3+}$ in spinel and garnet lherzolite, respectively, with opx and cpx hosting 48% and 31%, respectively, of the total Fe$^{3+}$ in spinel peridotite. To better understand partitioning of Fe$^{3+}$ between pyroxene and melt we conducted experiments at 100 KPa with f$_{O2}$ controlled by CO-CO$_2$ gas mixes between $\Delta$QFM -1.19 to +2.06 in a system containing andesitic melt saturated with opx or cpx only. To produce large (100-150 $\mu$m), homogeneous pyroxenes, we employed a dynamic cooling technique with a 5-10$\deg$C/h cooling rate, and initial and final dwell temperatures 5-10$\deg$C and 20-30$^\circ$C super and sub-liquidus, respectively. Resulting pyroxene crystals have absolute variation in Al$_2$O$_3$ and TiO$_2$ <0.05 wt.% and <0.02 wt.%, respectively. Fe$^{3+}$/Fe$^T$ in pyroxenes and quenched glass were measured by XANES. We used a newly developed XANES calibration for cpx and opx by only selecting spectra with X-ray vibrating on the optic axial plane at $50 \pm 5^\circ$ to the crystallographic c axis. Values of DFe$^{3+}$ cpx/melt increase from 0.03 to 0.53 as fO2 increases from $\Delta$QFM -0.44 to 2.06, while DFe$^{3+}$ opx/melt remains unchanged at 0.26 between $\Delta$QFM -1.19 to +1.37. In comparison to natural peridotitic pyroxenes, Fe$^{3+}$/FeT in pyroxenes crystallized in this study are lower at similar f$_{O2}$, presumably owing to lower Al$^{3+}$ contents. This study shows that the existing thermodynamic models implemented in pMELTS and Perple_X over-predict the stability of Fe$^{3+}$ in pyroxenes, causing an anomalous reduced character to spinel peridotites at calculated conditions of MORB genesis.