Journal article

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Abstract: 

Rationale

Carbonate clumped isotope thermometry examines the thermodynamic preference of 13C-18O bonds to form within the carbonate crystal lattice. The 13C18O16O isotopologue in analyte CO2 has a natural abundance of 44.4 ppm necessitating stringent purification procedures to remove contaminant molecules that may produce significant isobaric effects within range of the mass 47 isotopologue. Strict purifications of analyte CO2 are thus required as well as reliable contamination indicators.

Methods

CO2 purification was carried out by vacuum cryogenic purification through a static trap packed with Porapak™ Q (PPQ). The correlation between mass excesses on on m/z 47, 48 and 49 in CO2 produced by acid digestion of 12 natural samples was measured by IRMS. CO2 from two contaminated carbonate samples was then purified at PPQ trap temperatures between -25°C and -65°C and measured by IRMS to determine changes in mass excesses on m/z 47, 48 and 49. Finally carbonate standards, Carrara Marble (CM) and ETH3, were purified at PPQ trap temperatures -35°C and -60°C to identify isotopic fractionation associated with lowering trap temperature.

Results

The correlation between mass excesses on m/z 47, 48 and 49 is determined to be sample dependent. Lowering the PPQ trap temperature to -60°C has a 78% success rate in decreasing ∆48offset, a measure of sample contamination, to within an acceptable range (<1.5‰). Lowering the PPQ temperature in purification of CM and ETH3 is associated with decreases in the δ13C and δ18O values as a result of isotopic fractionation. We demonstrate that we can correct for fractionation at a trap temperature of -60°C.

Conclusions

Lowering the temperature of the Porapak Q trap to -60° C results in improved sample cleaning. It is possible to correct for fractionation in δ13C and δ18O values at lower PPQ temperatures using identically prepared standards. This result has important connotations for laboratories using similar sample preparation methods.

Publication date: 
Wednesday, May 10, 2017