Experimental determination of ferric iron partitioning between pyroxene and melt at 100KPa
Abstract
Pyroxene is the principal host of Fe3+ in basalt source regions, hosting 79 and 81% of the Fe3+ in spinel and garnet lherzolite, respectively, with opx and cpx hosting 48% and 31%, respectively, of the total Fe3+ in spinel peridotite. To better understand partitioning of Fe3+ between pyroxene and melt we conducted experiments at 100 KPa with fO2 controlled by CO-CO2 gas mixes between -1.19 to +2.06 in a system containing andesitic melt saturated with opx or cpx only. To produce large (100-150 μm), homogeneous pyroxenes, we employed a dynamic cooling technique with a 5-10/h cooling rate, and initial and final dwell temperatures 5-10 and 20-30 super and sub-liquidus, respectively. Resulting pyroxene crystals have absolute variation in Al2O3 and TiO2 <0.05 wt.% and <0.02 wt.%, respectively. Fe3+/FeT 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 5 to the crystallographic c axis. Values of DFe3+ cpx/melt increase from 0.03 to 0.53 as fO2 increases from -0.44 to 2.06, while DFe3+ opx/melt remains unchanged at 0.26 between -1.19 to +1.37. In comparison to natural peridotitic pyroxenes, Fe3+/FeT in pyroxenes crystallized in this study are lower at similar fO2, presumably owing to lower Al3+ contents. This study shows that the existing thermodynamic models implemented in pMELTS and PerpleX over-predict the stability of Fe3+ in pyroxenes, causing an anomalous reduced character to spinel peridotites at calculated conditions of MORB genesis.