Iron partitioning between basaltic melts and clinopyroxene as a function of oxygen fugacity

Abstract

Oxygen fugacity (fO₂) is an important parameter because it influences the crystallization sequences of magmas, as well as the composition of the resultant minerals. Traditionally fO₂ has been estimated using coexisting Fe-Ti oxide compositions or Fe3+/Fe2+ ratios in silicate melts, for which the relationship between fO₂ and Fe3+ has been calibrated. Problems arise in systems where oxides and/or glasses (melts) are absent, or when oxides are suspected to have been re-equilibrated. Therefore, it is important to develop alternate ways to look at fO₂ in volcanic rocks. In this work, we describe a set of experiments to establish the relationship between the Fe3+ content of clinopyroxene and a basaltic melt as a function of oxygen fugacity. The starting bulk compositions were chosen to be representative of martian parental melts, with pyroxene (either augite or pigeonite) on their liquidi, in order for the results to be applicable to the martian meteorite magma source regions. Experimental conditions ranged from fO₂ = IW to ~IW+8.6 at 1165 °C for pigeonite/melt pairs and at 1235 °C for melts alone, and fO₂ = IW-1 to IW+4 at ~1190 °C for augite/melt pairs. Results show that Fe3+ varies from 0-65% of the total Fe over this range of experimental conditions, and mass D (wt%Fe3+Pyx/wt%Fe3+ melt) ranged from 0.75-1.44 and from 0.00-0.77, for pigeonite and augite respectively. XANES and Mössbauer analytical methods give similar results within known error bars of the two techniques, with the best agreement in compositions with >10% Fe3+. Our data show that only at high values of fO₂ (>IW + 3.5, or ≈QFM) can Fe3+ distribution in pyroxene be used to estimate crystallization oxygen fugacity.