Biotite/Melt Trace-Element, Lithium, and F-OH Partitioning in Silicate Magmas

Abstract Biotite is a key hydrous silicate mineral in evolved magmatic systems, but its control on the behaviour of minor- and trace-elements, in particular Li, Nb, F and the REE is not well understood. Here, we quantify that control in sodic (per)alkaline ${\text{H}_{2}\text{O}}$-saturated magmas with variable F-content through crystallisation experiments at 650–800${}^\circ $C and 200 MPa total pressure, at log $f$O$_{2}$ $\approx $ FMQ +1. Biotite-glass pairs from tephriphonolite to phonolite fall deposits from Tenerife, Canary Islands, and a broad compilation from literature, complement our experimental data set. The new biotite-melt $Kd_{\text {F/OH}}$ exchange coefficients are 2.9–47.0, typically 6.5–12.5, with minima for Al-rich, Mg-poor biotite. Nernst partition coefficients ($D$) for lithium are 0.24–32.8 with minima in F-poor biotite formed at high temperatures from peraluminous silicate melt. $D$ values for the large alkali metal ions Na–Cs define Onuma parabolae consistent with their incorporation on the biotite ${{}^\text{XII}\text{A}}$-site. Niobium partition coefficients are 0.1–1.2, and highest in Ti-rich biotite. The REE and actinides have $D$ values less than 0.01 as their ionic radii fall between the size of the ${{}^\text{XII}\text{A}}$-site and ${{}^\text{VI}\text{M}}$-sites of biotite. Our data, alongside a literature compilation, constrain empirical models that: (1) describe the exchange of F and OH between the silicate melt and the biotite W-site; (2) predict the partitioning of 1+ cations Li–Cs between silicate melt and the biotite A- and M-sites; (3) predict ${D_\text{Nb}}$ values. The models use the major-element composition of biotite and silicate melt, pressure and temperature as input. Models are calibrated for use over a wide range of pressure, temperature and bulk composition ($P$-$T$-$X$) and can be used to interrogate biotite from natural systems to determine the composition of their source melt, or to forward model the trace-element evolution of mafic to evolved peralkaline syenite or peraluminous granite systems at upper-mantle to crustal pressures.