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 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.

Remobilization and eruption of an upper crustal cumulate mush: the Singkut caldera (North Sumatra, Indonesia)

“…Indeed, the water content estimated using plagioclase-melt hygrometry (4.5-4.8 wt%) is very close to or above water saturation in rhyolitic magmas at storage conditions similar to those inferred for the Singkut tuff (4.5 wt% at 150 MPa and 800 • C; Liu et al, 2005). Additionally, the presence of low analytical total biotites only in the Singkut tuff could be the result of crystallization in volatile saturated conditions, leading to the entrapment of a volatile phase in the crystal structure as interlayer fluid inclusions (Ellis et al, 2022). Alternatively, low analytical total could be the result of interaction with meteoric-hydrothermal fluids immediately after eruption.…”

“…3c,d). Most biotites in the Singkut tuff have relatively low analytical totals (<95 wt%; definition of Ellis et al, 2022), and show higher CaO and lower K 2 O, Na 2 O, BaO, and MnO contents compared to those from the lavas (Fig. S5).…”