An experimental study of the isotopic fractionation and partitioning of lithium among quartz, muscovite, and fluids.

摘要

The isotopic fractionation and elemental partitioning of lithium have been investigated experimentally among quartz, muscovite, and a chloride-bearing aqueous fluid at 400°–500°C and 50–100 MPa. Experiments were performed for 15–60 days with natural mineral specimens in Pt capsules, which were loaded into cold seal vessels and pressurized with argon. Lithium was introduced primarily through the fluid, which also contained KCl and HCl. In most runs, the fluid was prepared with L-SVEC (δ7Li = 0) and was 1M total chloride with K/Li/H = 100/10/1. In two experiments, a 6Li spike was employed. The charges included two other phases, K-feldspar and phlogopite, to permit coupled substitutions by Li + Al and by Li + Mg in quartz and muscovite. In contrast with other isotope systems (e.g., O), Li partitioning experiments require allowance for coupled substitutions because some crystallographic sites only become available to Li through coupled substitutions and because the chemical potentials of components necessary for coupled substitutions must be fixed to define fully the thermodynamic conditions of Li partitioning. Lithium isotopic compositions were determined by thermal ionization mass spectrometry with a triple-filament technique, in which samples were evaporated from side filaments as a phosphate and Li+ ions were measured directly. Analytical reproducibility was ±2.1‰ (2σ). Li concentrations were measured by isotope dilution.; The experiments demonstrated that quartz and muscovite are susceptible to rapid shifts in Li isotopic composition by diffusion through interaction with a Li-bearing fluid, particularly at 500°C. Approach to equilibrium was indicated by lack of variance with run duration or with approach from higher as compared with lower temperature. At 500°C, fractionation factors were determined to be Δ_{quartz-aqueous} ≅ +8 to +12‰ and Δ_{muscovite-aqueous} ≅ +18 to +20‰. An inter-mineral fractionation factor is given by Δ_{muscovite-quartz} ≅ +9‰. At 400°C, a tentative Δ_{quartz-aqueous} was determined to be ∼+4 to +6‰. Partition coefficients were tentatively determined at 500°C. These are D_{aqueous/quartz} = 36 ± 6 and D_{aqueous/muscovite} = 3.9 ± 0.9. The results provide evidence of systematic fractionation in lithium isotopes at temperatures of some magmatic-hydrothermal processes, suggesting applications in geological modeling.