Micro-Analysis of D/H Ratios in Mantle Minerals by Carrier-Gas Mass Spectrometry

Isotopic analysis of water released by vacuum heating of hydrous minerals and sub-marine basaltic glasses indicates that they vary in δD_(SMOW) between ca. +10 and -115 per mil (e.g., [1]). However, with few exceptions variations within that range are not well correlated with other geochemical and geologic properties and it is debated whether they reflect isotopic heterogeneity in the mantle, fractionations produced during ascent to the surface, or sub-solidus alteration. This ambiguity is at least partly due to the large sample size and slow rate of conventional analyses, which precludes linking isotopic ranges to grain-scale petrographic variables or constructing large data bases (many 10's of samples) in a reasonable period of time. Ion microprobe methods provide one solution to these problems, although they suffer from analytical uncertainties nominally 5 to 10 times worse than conventional measurements and large fractionations that can be a source of systematic error.