Oxygen fugacity and Fe3+ partitioning during partial melting of peridotite
Abstract
The prevailing oxygen fugacity (fO2) of the convecting upper mantle has geochemical and geodynamic consequences for phase stability, volatile speciation, and onset of melting. Fe3+/ΣFe ratios of MORB record small variations in fO2 (< 1 log unit) at or just below QFM [1,2]. Inferring solid mantle fO2 from MORB Fe3+/ΣFe ratios, however, requires constraints on the Fe3+/ΣFe ratios of melts and solids during partial melting. We present results of experiments covering Earth-relevant fO2 conditions aimed at elucidating effects of variable fO2 on partial melting of peridotite [3].
All 1-atm experiments [3] produced basaltic andesite glass saturated in olivine, opx, and Cr-rich spinel. As fO2 increases from QFM-1.87 to QFM+2.23, Fe3+/ΣFe ratios in glasses and spinels increase, but other compositional parameters in silicate phases change only modestly. Trivalent cations in spinel change appreciably as Fe3+/ΣFe ratios in spinel increase from 0.115 to 0.575, with much of the added Fe3+ replacing Cr at fO2 < QFM and both Cr and Al at fO2 > QFM in a manner not captured by available models [4-6]. As Fe3+ in spinel increases, so does the spinel/melt partition coefficient of Fe3+. Modal abundances of spinel and opx increase with fO2, modal olivine decreases, and experimental melt fraction (F) remains approximately constant [3]. At ridges, Fe3+ behaves incompatibly during fractional crystallization, but does not simply act as an incompatible element during partial melting [7]. Near constant F in our experiments suggests that as Fe3+ increases in a peridotite saturated system, it does not promote melting, but instead drives reactions between solid phases in the residue. As melting proceeds, increasing olivine Mg# and Fe3+ compatibility in spinel may allow peridotites to maintain relatively oxidizing fO2 even as incompatible Fe3+ is lost from the bulk residue [3]. New piston cylinder experiments and those from a recent study [8] allow us to extend these observations to higher pressures and a greater range of spinel Cr#. [1] Zhang et al. 2018 [2] Berry et al., 2018 [3] Davis and Cottrell 2018 [4] Ghiorso and Sack 1995 [5] Ariskin and Nikolaev 1996 [6] Poustovetov and Roeder 2001 [7] Cottrell and Kelley, 2011 [8] Sorbadère et al. 2018- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMDI43C0041D
- Keywords:
-
- 3914 Electrical properties;
- MINERAL PHYSICSDE: 3619 Magma genesis and partial melting;
- MINERALOGY AND PETROLOGYDE: 5724 Interiors;
- PLANETARY SCIENCES: FLUID PLANETSDE: 8147 Planetary interiors;
- TECTONOPHYSICS