Redox-sensitive partitioning of vanadium and other heterovalent elements between apatite and biotite in high silica magmas

Apatite and biotite, ubiquitous minerals in a multitude of rock types, host a variety of trace elements, including those whose valence state, and hence ionic radius and charge, can vary over the oxygen fugacity (fO2) of natural magmatic systems. In this study, we determine trace element (X) partition coefficients in five suites of natural, mostly silicic, metaluminous to peraluminous rocks that formed under reduced to oxidized conditions (FMQ-3.6 to +6.2; FMQ = fayalite-magnetite-quartz buffer). The analyzed suites include Fish Canyon tuff (FCT, USA), Mascota minette (MM, Mexico), Picotani intrusive suite (MAC, Peru), Tuk-Tuk tuff (TT, Indonesia), and Umiakovik pluton (UP, Canada). In three of the suites (MAC, TT, FCT), representing a narrower fO2 range (FMQ-3.6 to 2.9), trace element apatite/glass and biotite/glass partition coefficients (DXap/gl =CXapatite/CXglass and DXbt/gl = CXbiotite/CXglass, respectively) were also determined. Elements considered include alkalis, alkaline earths, rare earth elements, transition metals, metalloids, and high field strength elements, of which V, As, Eu, Mo, Sn, and W are expected to be present in species of more than one valence state (heterovalent) over the fO2 range considered.
Results reveal changes in partitioning behavior of the heterovalent elements over the sample fO2 range. With increasing fO2 from FMQ-3.6 to FMQ+2.9, values of DVbt/gl decrease from ~580 to ~50 whereas DVap/gl increases from ~0.5 to ~5. Arsenic becomes more compatible in apatite with DAsap/gl increasing from ~ 0.04 to ~1.6. Values of Dap/gl for Eu increase from ~20 to ~90. Molybdenum Dbt/gl decreases from ~2 to ~0.1. Tin becomes more compatible with both minerals as fO2 increases, with DSnbt/gl increasing from ~0.1 to ~1.0 and DSnap/gl from ~0.05 to ~0.3. Although the uncertainty on DWap/gl is too large to derive a meaningful fO2 relation, values for DWbt/gl decrease from ~0.13 to ~0.02 with increasing fO2. As a result of DVap/gl and DVbt/gl exhibiting opposite relations with fO2, values of apatite/biotite partitioning (DVap/bt) systematically increase from ~0.001 to ~0.5 with an increase in fO2.