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Large ¹⁸O and ¹³C depletions in greenschist facies carbonate rocks, western Arizona

¹ Department of Geology, Arizona State University, Tempe, Arizona 85287-1404

Strongly deformed greenschist facies carbonate rocks in western Arizona exhibit large ¹⁸O and ¹³C depletions relative to unmetamorphosed correlative rocks in northeastern Arizona. Unmetamorphosed Permian Kaibab Formation carbonates on the Colorado Plateau yield ¹³C values of +1 to +5 and ¹⁸O values of +28 to +37. In contrast, greenschist facies Kaibab Formation in the Little Harquahala and Granite Wash Mountains yields ¹³C values of –6 to +4 and ¹⁸O values of +9 to +24. The magnitude of the observed ¹³C and ¹⁸O depletions cannot be explained by decarbonation reactions and requires extensive exchange with low ¹³C and ¹⁸O fluids. The ¹³C depletions may reflect isotopic exchange with lower ¹³C carbonate in the underlying Paleozoic section and exchange with CO₂ derived from oxidation of methane via reaction with hematite or sulfate in interbedded rocks. These data demonstrate that large ¹³C and ¹⁸O depletions can occur in strongly deformed terranes during very low-grade metamorphism. Depletions in ¹³C and ¹⁸O observed in some high-grade metamorphic terranes may be inherited from much lower metamorphic grades.

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