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

Abstract

Apatite and biotite, ubiquitous minerals in a multitude of natural rocks, 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 partition coefficients (D values) between apatite (ap), biotite (bt), and glass (gl) in five suites of natural, mostly silicic, metaluminous to peraluminous rocks of reduced to oxidized conditions (FMQ-2.8 to +4.3; FMQ = fayalite-magnetite-quartz buffer). 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 heterovalent over the fO2 range considered. Results reveal changes in partitioning of the heterovalent elements over the fO2 range. With increasing fO2 from FMQ-2.8 to FMQ+2.8, values of Dbt/gl for V decrease from ~580 to ~50 whereas Dap/gl increases from ~0.6 to ~5. Arsenic becomes more compatible in apatite with Dap/gl increasing from ~ 0.04 to ~1.6. Europium Dap/gl values 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 Dbt/gl increasing from ~0.1 to ~1.0 and Dap/gl from ~0.05 to ~0.3. Although the uncertainty on Dap/gl for W is too large to derive a meaningful fO2 relation, values for Dbt/gl decrease from ~0.13 to ~0.02 with increasing fO2. As vanadium shows counter variation in individual mineral/glass partitioning, the apatite/biotite partitioning (Dap/bt) sympathetically increases from ~0.003 to ~0.5 with increase in fO2. In addition to shifts in the relative proportions of oxidized and reduced species with fO2, the observed partitioning relations for heterovalent elements could be caused by differences in other parameters that affect the activity of single species, species proportions, temperature, etc. These confounding aspects are assessed by comparison of the homovalent element partitioning data, which are relatively constant for the samples investigated, and with a suggestion that larger D-values involving one suite (Mascota, Mexico) are likely due to the stabilization of higher valence V species in a more alkaline melt composition. Results therefore indicate that the observed variation in heterovalent element partitioning is predominantly the result of fO2 control on element species and corresponding ionic radius. With this interpretation, we present a preliminary vanadium partitioning model that considers changes in the proportion of vanadium species with fO2 and estimated values Dap/bt for vanadium species. Model results are used to estimate the fO2 of the peraluminous South Mountain Batholith (Nova Scotia, Canada) and Palabora carbonatite (South Africa) for which oxybarometry has not been straightforward. Estimated oxygen fugacities are consistent with other redox indications, however, other factors, including melt and biotite composition, need to be considered to enable general application of the model. The partitioning of vanadium between apatite and ferromagnesian minerals has the potential of a novel oxybarometer, applicable to a broad range of igneous rocks, including Fe-Ti poor plutonic systems.