Biotite as a recorder of an exsolved Li-rich volatile phase in upper-crustal silicic magma reservoirs

The magmatic-hydrothermal transition is key in controlling the fate of many economically important elements due to the change in partitioning when melt and magmatic fluid coexist. Despite its increasing economic importance, the behavior of lithium (Li) in such environments remains poorly known. We illustrate how compositionally unusual biotites from the rhyolitic Bishop Tuff (California, USA) and Kos Plateau Tuff (Greece) may contain a magmatic volatile phase trapped between layers of biotite crystals. Despite originating in pristine deposits and showing the expected X-ray diffraction spectra, these biotites return low (<95 wt%) analytical totals via electron microprobe (EMP) consistent with the presence of considerable amounts of light elements (non-measurable by EMP). Lithium contents and isotope ratios in these biotites are remarkable, with abundances reaching >2300 ppm, exceptionally light Li isotopic compositions (δ7Li as low as –27.6‰), and large isotopic fractionation between biotite and corresponding bulk samples (Δ7Libt–bulk as low as –36.5‰). Other mineral phases, groundmass glass, and melt inclusions from the same units do not support an extremely Li-rich melt prior to eruption. Biotites from phonolitic systems (Tenerife [Canary Islands] and Campi Flegrei [Italy]) do not show such extreme compositional differences, with biotite and melt showing roughly equivalent Li contents, underscored by significantly reduced Δ7Libt–bulk to a maximum of –10.9‰. We ascribe the difference in behavior to the near-liquidus appearance of biotite in alkaline magmatic suites, before widespread exsolution of a magmatic volatile phase in the magma reservoir, while in rhyolitic suites, biotite crystallizes at low temperature, trapping the coexisting exsolved fluid phase in the reservoir.