Boron and Lithium Isotope Determinations in Minerals from Subduction-Related Rocks by LA-MC-ICPMS

Abstract:

Lithium (Li) and boron (B) are incompatible light elements that preferentially partition into the fluid phase, whether melt or aqueous liquid, and thus are useful for tracking fluid-related processes in rocks. Currently, most of the Li isotopic data available on subduction-related rocks are whole-rock analysis; and the few B isotopic analyses on subduction material have been carried out on whole-rock or in-situ in accessory phase (tourmaline).

The new method presented here couples an ESI New Wave UP-193-FX ArF* (193 nm) excimer laser ablation microscope with a Neptune Plus (Thermo Scientific) MC-ICP-MS. In situ B and Li isotopic analyses were carried out on mica and pyroxenes from jadeitites and albite mica-rocks from a subduction-related mélange. These results have been compared with SIMS analyses for B on the same mineral samples and with MC-ICP-MS analyses for Li on whole-rock or a mineral separate from the same sample.

The results show that for B concentrations above 15 µg/g, the data obtained by LA-MC-ICPMS and by SIMS are similar within error, for both mica and pyroxene. The significant improvement of LA-MC-ICPMS compared to SIMS is the duration of measurements: one analysis takes 3 minutes by LA-MC-ICPMS, and 32 minutes by SIMS. Such a method therefore permits multiplying the number of isotopic analyses, providing a better interpretation of the studied samples, with a lateral resolution and an error on each measurement similar to those obtained with SIMS. The results show that for Li concentrations above 10µg/g, the data obtained by LA-MC-ICPMS and by MC-ICP-MS are also similar within error, for both mica and pyroxene. The significant improvement of LA-MC-ICPMS compared to MC-ICP-MS is better spatial resolution. Indeed, many of the mineral phases encountered in subduction-related rocks display a strong chemical zoning, and the analyses carried out by wet chemistry and MC-ICP-MS average the isotopic values of the different zones. This newly developed method permits in-situ analysis of B and Li isotopes in a short time in subduction rock-forming minerals, with an error equal or lower than methods used previously.