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Improved “Clumped-Isotope” Measurements: Pressure Baseline Correction

Bo He, Gerard Olack, and Albert Colman
The Dept. of the Geophysical Sciences, The University of Chicago

“Clumped-isotope” measurements (tracking enrichment of 16O13C18O, reported as Δ47, on CO2 derived from carbonates) are becoming central to a wide range of geochemical investigations, including paleoclimate, paleoaltimetry, and thermal history reconstruction. Previously published analytical method is limited by: (1) the need to perform frequent calibrations because of instrumental drifts; and (2) the large sample sizes (typically 5-10 mg of carbonates) required to maintain stable ion beam. As a result, repeated calibrations require a lot of instrument time, and the precision of Δ47 measurements is also compromised. We have determined that drift in instrument calibration results from a shift in the baseline signal (“pressure baseline”, PBL) on sensitive collectors that occurs when gas is admitted to the ion source. In addition, the same root cause has hampered the advancement of Δ47 measurements on small samples. We developed a method for determining the “pressure baseline” and integrating a correction to ion beam intensity measurements during analysis. We then tested this approach for both bellows and micro-volume modes of sample introduction. Heated gas and equilibrated gas lines (Δ47 vs. δ47) established the effectiveness of this correction. PBL-corrected calibrations in bellows and micro-volume modes are nearly identical and stable through time (months of stability, comparing to previously reported weeks of stability), allowing more instrument time to be used for analysis. Introduction of the PBL correction, a revision to the absolute reference frame approach to determining Δ47 values, dramatically improves the external precision of Δ47 measurements to near instrumental analytical uncertainty.

AAPG Search and Discovery Article #90182©2013 AAPG/SEG Student Expo, Houston, Texas, September 16-17, 2013