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The effect of chrysotile nanotubes on the serpentine-fluid Li-isotopic fractionation

Abstract : We determined the lithium isotope fractionation between synthetic Li-bearing serpentine phases lizardite, chrysotile, antigorite, and aqueous fluid in the P, T range 0.2-4.0 GPa, 200-500°C. For experiments in the systems lizardite-fluid and antigorite-fluid, 7Li preferentially partitioned into the fluid and Δ7Li values followed the T-dependent fractionation of Li-bearing mica-fluid (Wunder et al. 2007). By contrast, for chrysotile-fluid experiments, 7Li weakly partitioned into chrysotile. This contrasting behavior might be due to different Li environments in the three serpentine varieties: in lizardite and antigorite lithium is sixfold coordinated, whereas in chrysotile lithium is incorporated in two ways, octahedrally and as Li-bearing water cluster filling the nanotube cores. Low-temperature IR spectroscopic measurements of chrysotile showed significant amounts of water, whose freezing point was suppressed due to the Li contents and the confined geometry of the fluid within the tubes. The small inverse Li-isotopic fractionation for chrysotile-fluid results from intra-crystalline Li isotope fractionation of octahedral Li[6] with preference to 6Li and lithium within the channels (Li[Ch]) of chrysotile, favoring 7Li. The nanotubes of chrysotile possibly serve as important carrier of Li and perhaps also of other fluid-mobile elements in serpentinized oceanic crust. This might explain higher Li abundances for low- T chrysotile-bearing serpentinites relative to high- T serpentinites. Isotopically heavy Li-bearing fluids of chrysotile nanotubes could be released at relatively shallow depths during subduction, prior to complete chrysotile reactions to form antigorite. During further subduction, fluids produced during breakdown of serpentine phases will be depleted in 7Li. This behavior might explain some of the Li-isotopic heterogeneities observed for serpentinized peridotites.
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Submitted on : Wednesday, December 22, 2010 - 6:31:18 PM
Last modification on : Friday, September 25, 2020 - 3:13:59 AM

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Bernd Wunder, Fabien Deschamps, Anke Watenphul, Stéphane Guillot, Anette Meixner, et al.. The effect of chrysotile nanotubes on the serpentine-fluid Li-isotopic fractionation. Contributions to Mineralogy and Petrology, Springer Verlag, 2010, 159, pp.781-790. ⟨10.1007/S00410-009-0454-X⟩. ⟨insu-00549855⟩

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