LITHIUM ISOTOPE MEASUREMENTS IN SPODUMENE, LITHIOPHILITE, TOURMALINE, PETALITE, AND BERYL BY LA-MC-ICP-MS

Lithium is a highly fluid-mobile element with two isotopes that are useful in studying how hydrous fluids transfer components from a subducting slab into the mantle wedge. Most of the Li isotopic data available from subduction-related rocks are from whole-rock analyses, and only few in-situ data were acquired by SIMS. A new in-situ method to obtain isotopes analyses faster than by SIMS, coupling an excimer laser-ablation microscope with a MC-ICP-MS has been developed. Previous measurements by this method have shown that tourmaline does not yield comparable results, when compared to SIMS data. Lithium content and isotope measurements were conducted on 13 Li-bearing minerals from the mineral collections of the American Museum of Natural History and Harvard Museum of Natural History. Lithium isotopes were measured by LA-MC-ICP-MS in both low and medium resolution (LR and MR) to test possible interferences on ⁶Li or ⁷Li, and Li contents were acquired by LA-ICP-MS. Samples include tourmaline (e.g., elbaite, schorl), phosphate (lithiophilite), pyroxene (spodumene), Cs-Li beryl, and petalite.

The Li contents range from ~ 20,000 ppm in lithiophilite to ~ 200 ppm in schorl, which ensures measurements over a wide range of Li concentrations. The δ⁷Li values of most of the minerals measured in this study using an ESI New Wave UP-193-FX ArF* (193 nm) LA microscope coupled to a Neptune Plus (Thermo Scientific) MC-ICP-MS display significant offsets between LR and MR analyses and also between different sessions of analyses. For example, elbaite H-98144 ([Li] ~ 2000 ppm) yielded a δ⁷Li of +27.3 ± 0.9 ‰, +32.1 ± 2.3 ‰, and +29.7 ± 8.6 ‰ in LR, each representing the average of 3 points from 3 different sessions of analyses (Jul. 2015, Nov. 2014, and Dec. 2012). The same elbaite measured in MR yielded δ⁷Li of +20.7 ± 0.3 ‰ (n = 3; Jul. 2015), and +13.0 ± 5.8 ‰ (n = 11; Mar. 2014). The δ⁷Li of H-98144 measured by SIMS was +7.3 ± 0.6 ‰. Similar offsets have been observed for schorl H-112566 ([Li] ~ 200 ppm) and the lithiophilite H-134825 ([Li] ~ 20,000 ppm). The careful observation of Li peaks in both LR and MR does not show any interference that could explain such discrepancies. These data will be compared with measurements of these samples by MC-ICP-MS. However, until the issue is resolved, the use of LA-MC-ICP-MS to measure Li isotopes must be discontinued.