Experimental evidence for interaction of water vapor and platinum crucibles at high temperatures : Implications for volatiles from igneous rocks and minerals

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 (900-1300 °C) for 5-10 min and then recovered. The experiments show that on average 20% 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‰. Only 20-50% of the lost water was recoverable by re-heating the crucible at 1300 °C. 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 μmol) of water remains trapped in the crucible and that partial 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. Fourteen basaltic samples were also re-analyzed without using a metal crucible and show isotopic compositions of hydrogen enriched by 10-20‰ in deuterium relative to previous studies. The isotopic composition of the upper mantle is closer to −60‰, rather than −80‰ as postulated earlier.