Stable Isotope Analyses of Carbonate Complicated by Nitrogen Oxide Contamination: A Delaware Basin Example

Abstract

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 (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.