Triple Oxygen Isotope Paleoaltimetry of the Kettle Metamorphic Core Complex (WA, USA)

The application of the triple oxygen isotope system (¹⁶O-¹⁷O-¹⁸O) provides a new tool for stable isotope paleoaltimetry. Here, we use triple oxygen isotope (Δ’¹⁷O) geochemistry to determine the past elevation of the Eocene Kettle Metamorphic Core Complex (MCC) (Washington, USA). We analyze quartz-muscovite pairs from mylonitic quartzites from the MCC-bounding shear zone. δ¹⁸O values range from 4.8 to 11.3‰ (muscovite) and 6.8 to 14.6‰ (quartz) and Δ’¹⁷O values (λ_ref = 0.528) range from -0.054 to -0.077‰ (muscovite) and -0.052 to -0.071‰ (quartz). The calculated quartz-mica oxygen isotope equilibrium temperature averages 390°C ± 90°C, which in line with observed quartz microstructures. Compared to existing muscovite hydrogen isotope data (δD = -101 to -138‰), both approaches, δD-δ¹⁸O and δ’¹⁸O-Δ’¹⁷O, indicate oxygen and hydrogen isotopic exchange between syntectonically formed minerals and a meteoric-derived fluid within the Kettle shear zone. We find that the shear zone minerals are in isotopic equilibrium with a fluid having a δ¹⁸O_water value of ~-14‰, which likely reflects high-elevation inland precipitation. Combined with a low-elevation δ¹⁸O_water estimate (-6 to -8 ‰) from the Eocene near-coastal Chumstick Basin (WA, USA), the δ¹⁸O_water estimate translates into a paleoelevation of 3-4 km for the Eocene Kettle MCC. This is consistent with δD-based elevation estimates of 4.2 km and underscores the robustness and complementary nature of the two different isotopic approaches.