TRIPLE OXYGEN ISOTOPE PALEOALTIMETRY OF METAMORPHIC CORE COMPLEXES

We use the triple oxygen isotope system (¹⁶O-¹⁷O-¹⁸O) to determine the past elevation of the Eocene Kettle Dome Metamorphic Core Complex in the western North American Cordillera (Washington, USA). Prerequisites for triple oxygen isotope paleoaltimetry are met by vigorous fluid flow and mineral-water interaction within the extensional detachment system bounding the core complex.

We analyze quartz-white mica pairs from the mylonitic quartzites of the Kettle Dome shear zone. δ¹⁸O values range from 4.7 to 11.5‰ (white mica) and 6.8 to 14.5‰ (quartz) and Δ’¹⁷O values (λ_ref = 0.528) range from -0.046 to -0.095‰ (white mica) and -0.044 to -0.083‰ (quartz). The calculated quartz-mica oxygen isotope equilibrium temperatures range from 275°C to 550°C and agree with observed quartz microstructures. Hydrogen isotope data indicate highly D-depleted white mica with δD values between -101 and -138‰. Both isotopic systems, δD and δ’¹⁸O-Δ’¹⁷O, indicate pervasive fluid flow throughout the Kettle Dome detachment and shear zone and oxygen and hydrogen isotopic exchange with meteoric fluids.

Two different approaches (δD-δ¹⁸O and δ’¹⁸O-Δ’¹⁷O) to infer time-integrated water-rock ratios and isotopic equilibrium among the hydrogen and oxygen isotopic systems yield comparable results. Both show that relatively small amounts of water in the detachment system are sufficient to alter/set the hydrogen isotopic composition of white mica. We find no evidence of later (post-recrystallization) hydrogen exchange in the rapidly cooling, extensional MCC setting, potentially due to the neo-crystallization of white mica and/or its rapid cooling.

Our oxygen isotope results show that the white micas equilibrated with meteoric derived fluids with an isotopic composition of δ¹⁸O = -14‰. These results yield paleoelevations of 3-4 km, which is consistent with previous δD-based elevation estimate (4.2 km; Mulch et al. 2007). The agreement in these paleoelevation estimates underscores the robustness and complementary nature of the two different approaches.