Quantifying F and Cl concentrations in granitic melts from apatite inclusions in zircon

Apatite inclusions hosted by zircon offer a means to probe the magmatic history of granitic rocks and better constrain the volatile budgets of crystallising granitic melts. Building on recently developed F–Cl–OH partitioning models for apatite and coexisting melt, we outline an approach for estimating the melt concentrations of F and Cl from the composition of apatite inclusions in zircon, constrained by Ti-in-zircon crystallisation temperatures. The melts in equilibrium with apatite inclusions in zircon for the ‘I-type’ Jindabyne, Why Worry and Cobargo granitic suites of the Lachlan Orogen (eastern Australia), have Cl concentrations of 20–2880 ppm and F concentrations of 65–575 ppm. Variations in melt Cl and F concentrations between the granitic suites is attributed to differences in source compositions, specifically the relative contribution of F-rich turbiditic sediments and Cl-rich juvenile arc magmas. Within individual granitic suites, the calculated melt F and Cl concentrations decrease with magmatic differentiation and falling melt temperatures, and this appears to reflect the partitioning of Cl and F into biotite and hornblende, and into exsolving aqueous fluids. This study demonstrates that apatite-melt exchange coefficients for F, Cl and OH can be applied to apatite inclusions in zircon to quantify the F and Cl content of the melt, without additional context from the host rock samples.