Diffusive fractionation of lithium and boron isotopes in silicate melts: a tool to study degassing in ascending magmas

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We propose to experimentally calibrate fractionation of lithium isotopes (6Li vs. 7Li, 10B vs. 11B) during magma degassing as a function of the decompression rate. This calibration will b the basis of a new eruption speedometer, quantitatively constraining magma ascent rates in volcanic conduits.Magma ascent rate is one of the main parameters controlling volcanic eruption style (effusive vs. explosive) and thus associated volcanic hazards. But previous studies on correlating bubble number densities (BND) to ascent velocities have shown that the homogeneous bubble nucleation and growth strongly depends on the decompression path (i.e. single-step vs. multi-step and vs. continuous decompression) and that especially at low decompression rates the bubble signatures can be overprinted by the interaction of bubble growth and coalescence (e.g. Ostwald ripening). Furthermore, the experimental BND values of continuously decompressed melts are limited to decompression rates larger than 0.02 MPa/s and scatter significantly over several orders of magnitude for homogeneous bubble nucleation. Thus, it is difficult to make straightforward correlations solely on BNDs, and demonstrates the need for methods that better quantify the timescales of degassing during magma upwelling. The proposed calibration of lithium and boron isotope fractionation between melt and gas bubbles will be a powerful tool to complement bubble texture analysis.Lithium is volatile and diffuses quickly in magmas, while boron has similar characteristics but diffuses slower compared to lithium. The stable isotopes of Li and B diffuse at sufficiently different speeds so that fractionation develops in the hours-to-days timescales typical of volcanic eruptions. Therefore, measurements of 6Li vs. 7Li and 10B vs. 11B isotopes in volcanic products have great potential to be used as a speedometer to estimate magma ascent rates associated with a range of eruption styles. Effective development of this speedometer requires quantitative calibration of 6Li/7Li and 10B/11B isotopic fractionation. The proposed work will conduct two sets of experiments carefully designed to perform this calibration in internally heated pressure vessels (IHPV): (1) lithium and boron diffusion-couple experiments between a Li- and/or B-free and an equilibrated Li- and/or B-enriched melt, in order to quantify Li and B isotope diffusivity and (2) decompression-induced degassing experiments, in order to quantify lithium and boron isotope fractionation between melts and exsolved gas phases using an IHPV specially designed and configured with a gas release valve to simulate continuous ascent. Both sets of experiments will be performed mainly on rhyolite melts (corresponding to well-studied Los Posos obsidian, Valles Caldera, New Mexico, USA) and will be later extended to more basic melts.