ABSTRACT

Whole rock subsurface samples from the Cherry Canyon Formation, Delaware Basin, were used to document the presence of a contaminant gas affecting the isotopic composition of acidified carbonate. The whole rock material, when reacted in purified phosphoric acid using the McCrea (1950) method, produces a contaminant mass-46 gas (nitrogen dioxide, NO₂). This mass-46 gas contributes to the ¹⁸O/¹⁶O ratio (mass 46/44), artificially enriching the ¹⁸O value and masking the true isotope signature of the CO₂ derived from the carbonate minerals. Acidified whole rock samples containing low carbonate (< 5%) tend to yield more positive ¹⁸O (PDB) values, some greater than +30. Conversely, samples with higher carbonate (> 5%) have ¹⁸O values ranging from +5 to -8, hence diluting the effects of the contaminant gas on the mass 46/44 ratio. After purifying the CO₂ produced by the McCrea (1950) method with a reduction furnace, all of the ¹⁸O values fall in the range of 0 to - 12 regardless of carbonate content. Th ¹⁶C (PDB) values remain unchanged after reduction furnace purification, indicating no effect on mass 45/44 ratio.

The inferred source of the contaminant NO₂ gas is from inorganic ammonium adsorbed to interlayer sites of alumina silicates such as illite and mica. This source is supported by the detection of ammonium and clays in the whole rock material and the lack of correlation between percent organic carbon and ¹⁸O values influenced by NO₂. To eliminate the effect of the contaminant gas on ¹⁸O values, we describe a simple procedure that removes the contaminant NO₂ gas, yielding the true isotopic signal from the carbonate minerals in whole rock material.