Effect of Burial Depth on the Clumped Isotope Thermometer: An Example from the Green River and Washakie Basins (WY)

Abstract:

The mass-47 clumped isotope thermometer allows one to elucidate the temperature of carbonate formation and avoids speculation about the composition of precipitating water and/or oxygen composition of another mineral phase, as is required for conventional stable isotope thermometry. Recent studies have tried to reconstruct surface paleotemperatures to resolve paleoclimate or paleoelevation using this technique. Mass-47 measured from carbonates preserved in the geologic record; however, are potentially affected by burial and/or diagenetic alteration. These effects are poorly understood. Here we investigate the effect of burial on the ∆-47 composition of lacustrine carbonates. We collected samples, on the surface and recovered from drill cores, from two regions in the greater Green River Basin (Wyoming), the Washakie Basin and the Pinedale Anticline. Both basins were filled with thick, continuous Cretaceous – Eocene lacustrine strata. From these two sections, lacustrine carbonates were sampled over a paleodepth range of 1–6 km.

The ∆-47 compositions of the carbonates have been measured over the first two kilometers of depth and the clumped temperatures compared with peak temperatures estimated from vitrinite reflectance, fluid inclusion thermometry and basin thermal models. Although slightly warmer than MAT estimates from Eocene paleosol geochemistry (~24°C), clumped isotope temperatures are consistently ~40°C across this depth range, similar to clumped-isotope paleotemperature estimates for the Eocene Bighorn Basin. Peak temperatures of the carbonate samples reached 120°C, while modern downhole temperatures of the deepest samples are 50°C.

These results suggest that the ∆-47 thermometer preserves a record of surficial temperatures during Eocene time and that the ∆-47 thermometry is not apparently affected by burial to temperatures of at least 120°C. Effects of burial to greater depths (~6 km) and higher temperatures (~200°C) is in progress to evaluate the robustness of the ∆-47 thermometer at preserving surficial temperatures under these conditions.