Calibration of the relationship between oxygen fugacity and the oxidation state of iron in anorthitic plagioclase

Abstract

Plagioclase, one of the major minerals comprising igneous rocks in the Earth's crust and those of other rocky planetary bodies, incorporates minor quantities of iron in its structure. While ferric iron is known to preferentially partition into plagioclase over its ferrous counterpart, there have been few efforts to characterise its oxidation state directly. To do so, we collected X-ray Absorption Near-Edge Structure (XANES) spectra at the Fe K-edge for a series of quenched anorthite-melt pairs synthesised in the CaO-MgO-Al₂O₃-SiO₂-FeO* (CMAS+Fe) system over a range of oxygen fugacities from two log₁₀-units below the iron-wüstite (IW) buffer; ΔIW-2, to air (ΔIW + 12) at 1140 °C and 1 bar, with an additional experiment up to ΔIW + 16.7 at 1200 °C and 1 GPa. We show that the range of area mean-weighted centroid energies and integrated intensities of the pre-edge feature in glass and anorthite overlap, permitting Fe³⁺/∑Fe ratios for both phases to be determined using existing calibrations for glasses. The equilibrium constant for the reaction FeO + 1/4O₂ = FeO_1.5 in plagioclase determined by XANES is found to be logK^plg = 1.60 ± 0.09 with a stoichiometric coefficient for oxygen, n = 0.25 ± 0.01, indistinguishable from the ideal value, independent of plagioclase orientation. This permits the fO₂ under which anorthite crystallised to be determined from its Fe³⁺/Fe²⁺ ratio alone, by the equation $f_{o_2}$ = (K^plg/[Fe³⁺/Fe²⁺]_plg)^(−1/n). Fits to partition coefficients between coexisting melt and anorthite as a function of $f_{o_2}$ yield $D_{\mathrm{plg}/\mathrm{melt}}^{{\mathrm{Fe}}^{2+}}$ (0.055 ± 0.005) and $D_{\mathrm{plg}/\mathrm{melt}}^{{\mathrm{Fe}}^{3+}}$ (0.43 ± 0.04), which, together with logK^melt (0.78 ± 0.03), lead to a second oxybarometer that can be applied to plagioclase-melt pairs, $f_{o_2}$ = ([Fe³⁺/Fe²⁺]_plg/([$D_{\mathrm{plg}/\mathrm{melt}}^{{\mathrm{Fe}}^{3+}}$/$D_{\mathrm{plg}/\mathrm{melt}}^{{\mathrm{Fe}}^{2+}}$]K^melt))^1/n. For a typical precision of ±0.02 on Fe³⁺/∑Fe in anorthite, both calibrations yield an $f_{o_2}$ indistinguishable to one another within uncertainty, which is ±0.40 and ± 0.25 log units at 1140 °C and 1 bar for the plagioclase-only- and plagioclase-melt oxybarometer, respectively. Use of the first oxybarometer opens the possibility of modelling oxygen fugacity in magmatic systems where the melt composition is unknown (e.g. anorthositic cumulates).