Deuterium abundance in hydrous minerals using pyrolysis-facilitated continuous-flow gas-chromatography isotope ratio mass spectrometry: a new analytical method
Michael R. Sheehan (’05)
(research completed while affiliated with the School of Earth and Space
Exploration, Arizona State University, Tempe, Arizona)
A new analytical method is proposed for measuring the deuterium to hydrogen ratio (D/H) of non-stoichiometric water in hydrous minerals via pyrolysis-facilitated continuous-flow gas-chromatography isotope ratio mass spectrometry (P-CF-GCMS). Prior publications have reported a poorly understood nonlinear dependence of D/H on the amount of hydrogen liberated from the mineral sample. Methods of correction for this nonlinearity have been ad hoc and patently ineffective. This sample-size effect has been variously attributed to kinetic isotope fractionation within the mass spectrometer or peripheral instruments, ion source linearity issues, an unstable H incorrect H +
3
-factor calculations.
+
3
-factor, and
It is here determined that this sample-size effect is primarily an artifact of the calculations employed by the computer program, ISODAT, which is used in these types of systems to both monitor the functions of the mass spectrometer and to compile data. Secondary causes of the sample size effect include persistent, background interference and chromatographic separation of the isotopologues of molecular hydrogen. A new method of H +
3
-factor determination and D/H calculation is proposed which eliminates the need for portions of the ISODAT software. The analytical methods described in prior publications are evaluated in light of these findings.
Using this new analytical method, D/H is measured in non-stoichiometric water extracted from chert belonging to the Cretaceous Edwards Group, Texas, and the Precambrian Kromberg Formation, South Africa. Of principle interest in this study is the maximum average surface temperature, as well as the nature of the hydrological conditions, during chert formation. Data from Cretaceous chert are consistent with previously published data and interpretations. Data from
Precambrian chert are consistent with maximum average surface temperatures approaching 65°C. In addition, D/H is measured in non-stoichiometric water in silicified basalt from the Precambrian Hooggenoeg Complex, South Africa. The basalt data suggest that D/H of the Archean ocean was comparable to the modern value.