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

Abstract

Strongly deformed greenschist facies carbonate rocks in western Arizona exhibit large 18 O and 13 C depletions relative to unmetamorphosed correlative rocks in northeastern Arizona. Unmetamorphosed Permian Kaibab Formation carbonates on the Colorado Plateau yield δ 13 C values of +1‰ to +5‰ and δ 18 O values of +28‰ to +37‰. In contrast, greenschist facies Kaibab Formation in the Little Harquahala and Granite Wash Mountains yields δ 13 C values of −6‰ to +4‰ and δ 18 O values of +9‰ to +24‰. The magnitude of the observed 13 C and 18 O depletions cannot be explained by decarbonation reactions and requires extensive exchange with low δ 13 C and δ 18 O fluids. The 13 C depletions may reflect isotopic exchange with lower δ 13 C carbonate in the underlying Paleozoic section and exchange with CO 2 derived from oxidation of methane via reaction with hematite or sulfate in interbedded rocks. These data demonstrate that large 13 C and 18 O depletions can occur in strongly deformed terranes during very low-grade metamorphism. Depletions in 13 C and 18 O observed in some high-grade metamorphic terranes may be inherited from much lower metamorphic grades.