Lithium isotopes in fluid inclusions as tracers of crustal fluids: An exploratory study

摘要

Lithium isotopes are extensively used to trace fluid migration and fluid-rock interactions in the shallow Earth's crust. Although, Li isotope systematics might give important information about deep crustal fluids, this stable isotope system remains surprisingly under exploited in fluid inclusions. Here, twenty-three samples from a variety of deep environments, from diagenetic, metamorphic to magmatic-hydrothermal conditions (~150 to 600°C), have been investigated in order to provide the first exploratory overview of the range in Li isotopic composition of fluid inclusions. From 1 to 2g of hydrothermal quartz or dolomite, cation analysis (Na, K, Ca, Mg, Sr, Li), Li fraction recovery and Li isotopic analysis of bulk fluid inclusion leachates were carried out by using an innovative approach coupling crush-leach extraction, high performance ion chromatography and MC-ICPMS analysis. Reconstructed Li concentrations and δ7Li values of these fluid inclusions (respectively 12 to 653mg/l and −1.4±0.2‰ to +41.3±0.8‰) are compatible with previously proposed models for the origin of the fluids and fluid-rock interactions. δ7Li values are independent from other tracers (Br/Cl, I/Cl) and parameters such as temperature, salinity, and cation content (Na/Ca, Na/K, Na/Mg, Na/Li, Na/Sr ratios). The results show that, in conjunction with other parameters, the Li isotopic compositions of fluid inclusions are potentially powerful source and process tracers of deep crustal fluids within a wide range of geological environments and related ore-forming systems, for example in from sedimentary basins, seafloor hydrothermal systems and subduction zones.