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Diffusive fractionation of Li isotopes in wet, highly silicic melts

M.E. Holycross1,2,

1Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180, USA
2National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA

E.B. Watson1,

1Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180, USA

F.M. Richter3,

3Department of the Geophysical Sciences, University of Chicago, Chicago, IL 60637, USA

J. Villeneuve4

4CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-les-Nancy F-54501, France

Affiliations  |  Corresponding Author  |  Cite as  |  Funding information

Holycross, M.E., Watson, E.B., Richter, F.M., Villeneuve, J. (2018) Diffusive fractionation of Li isotopes in wet, highly silicic melts. Geochem. Persp. Let. 6, 39–42.

National Science Foundation.

Geochemical Perspectives Letters v6  |  doi: 10.7185/geochemlet.1807
Received 16 November 2017  |  Accepted 09 February 2018  |  Published 5 March 2018
Copyright © The Authors

Published by the European Association of Geochemistry
under Creative Commons License CC BY-NC-ND 4.0



Table 1 Experiment conditions for diffusion couple runs and measured 7Li diffusion coefficients.
experimentT (°C)t (s)D7Li (m2/s)2σ SE
LiDiff28754927.40E-10~7.4 E-11
LiDiff38502616.50E-10~6.5 E-11
LiDiff4*8102665.74E-107.53E-11
LiDiff57902055.25E-105.02E-11
LiDiff6*8301336.28E-101.14E-10

*SIMS analysis

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Figure 1 Arrhenius plot showing temperature dependence of Li diffusion in silicate materials. Lithium diffusion in wet rhyolite melt is significantly faster than in dry obsidian glass due to the decrease in viscosity from glass to liquid and from the addition of 6 wt. % dissolved H2O to the melt.
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Figure 2 δ7Li profiles produced from SIMS analyses. 6Li diffuses into the “low” Li glass (x > 0) faster than 7Li, fractionating the isotopes in the melt. If the Li isotopes were diffusing at the same speed in rhyolitic melt (β = 0), a smoothly varying isotope gradient would be present near x = 0. When β ≠ 0, calculated profiles show a shallow or reversed slope near the diffusion couple interface at x = 0, as is seen in the data in both panels. The kinetic fractionation of Li isotopes in hydrous rhyolitic melt is best fit by an average β = 0.228.
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Figure 3 The reported β factors for isotopes of an element i vary with its Si-normalised diffusivity, Di/DSi. Data point in red is for rhyolitic melt containing 6 wt. % H2O at 810 ºC. Silicon is an extremely slow diffuser in rhyolite while Li is extremely fast. This suggests diffusion of Li may be decoupled from the melt network and exhibit a greater mass discrimination. Figure after Watkins et al. (2017)

Watkins, J.M., DePaolo, D.J., Watson, E.B. (2017) Kinetic fractionation of non-traditional stable isotopes by diffusion and crystal growth reactions. Reviews in Mineralogy and Geochemistry 82, 85-125.

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