Implications of Pt Crucibles-H2O Vapour Interaction on Past DeltaD Measurements in Silicate Glasses and Minerals

Clog, M. , Cartigny, P. and Aubaud, C.P. (2010) Implications of Pt Crucibles-H2O Vapour Interaction on Past DeltaD Measurements in Silicate Glasses and Minerals. AGU Fall Meeting, San Francisco, CA USA, 13-17 Dec 2010.

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The extraction of water from igneous rocks and minerals is classically achieved by induction heating of a platinum alloy crucible where the sample has been deposited. Here, we show that chemical interaction between water and Pt-10%Rh crucibles occurs at high temperature. Known amounts of water were reacted with a Pt crucible held at high temperatures (900-1300oC) for 5 to 10 minutes and then recovered. The experiments show that in average 18% of the water was lost to the crucible during the reaction, and that the isotopic composition of the remaining water was shifted by up to 25‰ . From 20 to 50% of the lost water was recoverable by re-heating the crucible at 1300oC. Repeated experiments using the same standard water on the crucible showed a decrease of the isotopic shift to only 2‰ . This is compatible with a memory effect of the Pt-10%Rh crucible. We propose that a large amount (at least several tens of {μ }mol) of water remains trapped in or at the surface of the crucible and that isotopic exchange between trapped and introduced water affects subsequent isotopic composition of injected water. We conclude that the use of Pt alloys, as crucibles or foils, to extract water from rocks or minerals should be avoided. The interaction highlighted in this study shed light on previously inconsistent observations made on several mantle-derived samples. This effect could potentially explain the very low δ D ( ∼-110‰ ) measured by Bell and Ihinger (2000) in anhydrous mantle minerals with low water concentrations. Moreover, 14 basaltic glasses previously measured using Pt or Pt alloys crucibles were re-analysed using externally heated silica tubes, yielding δ D heavier by about 15‰ and suggesting a δ D for the source of N-MORB closer to ∼-60‰ rather than -80‰ .

Item Type:Conference or Workshop Item
Glasgow Author(s) Enlighten ID:Clog, Dr Matthieu
Authors: Clog, M., Cartigny, P., and Aubaud, C.P.
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre

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