Geokolloquium: Montag 28.11.2022

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The ‘tales’ of lithium in quartz: A look into felsic shallow magma reservoirs

  Vortragender: Alejandro Cortés Calderón

ETH Zürich

The inventory of lithium (Li) in felsic igneous rocks has proven to be highly valuable to unravel ore genesis of Li deposits and to identify fast processes taking place in evolved magma reservoirs prior to and during eruptions. The mechanisms that control Li distribution in felsic magmas are, however, not fully understood. Although most studies suggest that Li is an incompatible element along common liquid lines of descent (LLD), there is still a shortfall of data concerning partition coefficients in the rhyolite spectrum of compositions. Despite Li in natural rhyolites seems to be a ‘mobile’ element during magma degassing, such Li mobility to fluids remains elusive in experimental studies. In this seminar, I will present the results of decompression and equilibrium experiments at conditions of shallow felsic water-saturated reservoirs with a special focus on Li distribution in quartz. Experiments were performed using externally heated pressure vessels (EHPV) with a cold seal design and contained haplogranitic synthetic glass chunks, quartz cores and fluids. The experimental setup allowed fluid trapping at equilibrium conditions by thermal shock of quartz cores. Regarding decompression experiments, the vessels used allowed instant quenching of experimental charges to avoid Li mobility after decompression. Data from such experiments stress that the importance of studying quartz goes beyond being a thermometer of magma reservoirs or a host of melt inclusions. Fluid inclusions trapped in quartz during equilibrium experiments induce a shift in Li isotopic composition of the bulk quartz and shows that Li can be mobilised preferentially into hydrosaline liquid-like fluids. Furthermore, partitioning between quartz and melt from experiments agrees with distribution coefficients derived from ‘cold’ and ‘wet’ rhyolites but differ by an order of magnitude lower than those from ‘hot’ and ‘dry’ counterparts, which is linked to the different water activities in both systems. Such dependency on H2O is also observed in decompression experiments, whereby Li inventory along quartz cores displays arrested ‘tales’ due to H-out followed by Li-in diffusion.