  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.
