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

Abstract The magmatic-hydrothermal transition is key in controlling the fate of many economically important elements due to change partitioning when melt and magmatic fluid coexist. Despite its increasing economic importance, behavior lithium (Li) such environments remains poorly known. We illustrate how compositionally unusual biotites from rhyolitic Bishop Tuff (California, USA) Kos Plateau (Greece) may contain a volatile phase trapped between layers biotite crystals. originating pristine deposits showing expected X-ray diffraction spectra, these return low (<95 wt%) analytical totals via electron microprobe (EMP) consistent with presence considerable amounts light (non-measurable by EMP). Lithium contents isotope ratios are remarkable, abundances reaching >2300 ppm, exceptionally Li isotopic compositions (?7Li as –27.6‰), large fractionation corresponding bulk samples (?7Libt–bulk –36.5‰). Other mineral phases, groundmass glass, inclusions same units do not support an extremely Li-rich prior eruption. Biotites phonolitic systems (Tenerife [Canary Islands] Campi Flegrei [Italy]) show extreme compositional differences, roughly equivalent contents, underscored significantly reduced ?7Libt–bulk maximum –10.9‰. ascribe difference near-liquidus appearance alkaline suites, before widespread exsolution magma reservoir, while crystallizes at temperature, trapping coexisting exsolved reservoir.