Experimental Evidence for Fast Lithium Diffusion and Isotope Fractionation in Water-bearing Rhyolitic Melts at Magmatic Conditions

Abstract:

The aim of this work is to extend the existing database of experimentally-determined lithium diffusion coefficients to more natural cases of water-bearing melts at the pressure-temperature range of the upper crust. In particular, we are investigating Li intra-melt and melt-vapor diffusion and Li isotope fractionation, which have the potential to record short-lived magmatic processes (seconds to hours) in the shallow crust, especially during decompression-induced magma degassing.

Hydrated intra-melt Li diffusion-couple experiments on Los Posos rhyolite glass [1] were performed in a piston cylinder at 300 MPa and 1050 °C. The polished interfaces between the diffusion couples were marked by addition of Pt powder for post-run detection. Secondary ion mass spectrometry analyses indicate that lithium diffuses extremely fast in the presence of water. Re-equilibration of a hydrated ~2.5 mm long diffusion-couple experiment was observed during the heating period from room temperature to the final temperature of 1050 °C at a rate of ~32 °C/min. Fractionation of ~40‰ δ⁷Li was also detected in this zero-time experiment. The 0.5h and 3h runs show progressively higher degrees of re-equilibration, while the isotope fractionation becomes imperceptible. Li contamination was observed in some experiments when flakes filed off Pt tubing were used to mark the diffusion couple boundary, while the use of high purity Pt powder produced better results and allowed easier detection of the diffusion-couple boundary. The preliminary lithium isotope fractionation results (δ⁷Li vs. distance) support findings from [2] that ⁶Li diffuses substantially faster than ⁷Li. Further experimental sets are in progress, including lower run temperatures (e.g. 900 °C), faster heating procedure (~100 °C/min), shorter run durations and the extension to mafic systems.

[1] Stanton (1990) Ph.D. thesis, Arizona State Univ., [2] Richter et al. (2003) GCA 67, 3905-3923.