LITHIUM ISOTOPES IN THE SYSTEM QZ-MS-FLUID: AN EXPERIMENTAL STUDY

Abstract

The fractionation of lithium isotopes among quartz, muscovite, and a chloride-bearing aqueous fluid has been investigated experimentally at 400°–500°C and 50–100 MPa. Experiments were performed for 15–60 days in cold seal vessels with natural mineral specimens. Lithium was introduced primarily through the fluid, which also contained KCl and HCl. In most runs, the fluid was prepared with the L-SVEC standard (δ7Li = 0) and was 1 M in total chloride with K/Li/H = 100/10/1. In two experiments, a 6Li spike was employed. The experiments demonstrate that quartz and muscovite are susceptible to pronounced, rapid shifts in Li isotopic composition by diffusion through interaction with a Li-bearing fluid, particularly at 500°C. At 500°C, fractionation factors were determined to be Δquartz-aqueous ≅ +8 to + 12‰ and Δmuscovite-aqueous ≅ +18 to + 20‰. An intermineral fractionation factor is given by Δmuscovite-quartz ≈ +9‰. At 400°C, the results suggest Δquartz-aqueous ≈ +4 to + 6‰. The study provides evidence of systematic fractionation in lithium isotopes at the temperatures of some magmatic processes, such as those associated with porphyry-type ore systems and pegmatites.