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

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 δ¹³ values of +1‰ to +5‰ and δ¹⁸O values of+28‰ to +37‰. In contrast, greenschist facies Kaibab Formation in the Little Harquahala and Granite Wash Mountains yields δ¹³C values of-6‰ to +4‰ and δ¹⁸O values of +9‰ to +24‰ The magnitude of the observed ¹³C and ¹⁸O depletions cannot he explained by decarbonation reactions and requires extensive exchange with low δ¹³C and δ¹⁸O fluids. The ¹³C depletions may reflect isotopic exchange with lower δ¹³C 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.