Abstract

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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 H3+ -factor, and
incorrect H3+ -factor calculations.
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 H3+ -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.
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