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

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 (Formula presented) -saturated magmas with variable F-content through crystallisation experiments at 650–800(Formula presented) C and 200 MPa total pressure, at log (Formula presented) O(Formula presented) (Formula presented) 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 (Formula presented) exchange coefficients are 2.9–47.0, typically 6.5–12.5, with minima for Al-rich, Mg-poor biotite. Nernst partition coefficients ((Formula presented)) for lithium are 0.24–32.8 with minima in F-poor biotite formed at high temperatures from peraluminous silicate melt. (Formula presented) values for the large alkali metal ions Na–Cs define Onuma parabolae consistent with their incorporation on the biotite (Formula presented) -site. Niobium partition coefficients are 0.1–1.2, and highest in Ti-rich biotite. The REE and actinides have (Formula presented) values less than 0.01 as their ionic radii fall between the size of the (Formula presented) -site and (Formula presented) -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 (Formula presented) 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 ((Formula presented) -(Formula presented) -(Formula presented)) 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.