RETRACTED: Experimental determination of ferric iron partitioning between pyroxene and melt at 100 kPa

Abstract

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. In spinel peridotite, orthopyroxene (opx) and clinopyroxene (cpx) host 48% and 31%, respectively, of the total Fe 3+ . Yet the relationship between mantle mineralogy, pyroxene chemistry, and the oxygen fugacity ( f O2 ) recorded by mantle-derived basalts remains unclear. 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 ∆QFM −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 °C/h cooling rate, and initial and final dwell temperatures 5–10 °C and 20–30 °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 Fe K -edge XANES. We used a newly developed XANES calibration for cpx and opx by selecting spectra with X-rays vibrating on the optic axial plane at 45 ± 5° to the crystallographic c axis. Values of D Fe3+ cpx/melt increase from 0.03 to 0.53 as f O2 increases from ∆QFM −0.44 to +2.06, while D Fe3+ opx/melt remains unchanged at 0.26 between ∆QFM −1.19 to +1.37. In comparison to natural peridotitic pyroxenes, Fe 3+ /Fe T in pyroxenes crystallized in this study are lower at similar f O2 , presumably owing to lower Al 3+ contents. Comparison to thermodynamic models implemented in pMELTS and Perple_X suggest that these over-predict the stability of Fe 3+ in pyroxenes, causing these models to underpredict the f O2 of spinel peridotites under conditions of basalt genesis.