The Effects of Carbonate Decomposition on Clumped Isotopes from Heavily Altered Limestone Clasts in the Steen River Impact Structure Breccia.

Abstract:

The Steen River impact structure (SRIS) is a ~25 km diameter crater located in northwestern Alberta, Canada, recently dated to 141 ± 4 Ma. The SRIS-forming impact event excavated Devonian calcareous shales, carbonates, evaporites and Precambrian granitic basement rock, and thus crater-fill deposits accessible from drill cores are breccias dominated by the Devonian sedimentary cover in the upper part of the cores and by crystalline basement in the lower core. The behavior of carbonates in impact settings is poorly known despite their prevalence (~1/3 of terrestrial impact targets have a carbonate component) and probable contribution to post-impact carbon cycle perturbations. Here, we focus on the stable isotope geochemistry of cm-scale limestone clasts from core samples of the SRIS impact breccia. Carbon and oxygen isotope ratios (δ¹³C and δ¹⁸O) are variable by up to 10’s of ‰ within a single clast, with both ratios decreasing substantially from the center to the rim. All breccia carbonate clasts are several ‰ lower in both isotope ratios than the Devonian carbonate target rock. Intraclast carbonate clumped isotope ratios (Δ₄₇) have the opposite trend. Values at the center tend to be lower, equivalent to Δ₄₇ temperatures of ~250 °C, than values measured at the rims. A hypothesis that reconciles these values is isotopic distillation associated with carbonate decomposition during extreme impact heating, which enriches the clast rims in ¹²C and ¹⁶O, but apparently leaves them depleted in ¹²C-¹⁶O bonds (and therefore enriched in ¹³C-¹⁸O). Such δ¹³C and δ¹⁸O trends are known from degassing in metamorphic systems, but this clumped isotope behavior is novel. To verify this finding, we conducted calcite decomposition experiments in vacuo that produce δ¹³C and δ¹⁸O depletions predicted by open-system Rayleigh fractionation and also result in large Δ₄₇ enrichments. These results suggest rapid heating of the carbonate clasts and subsequent cooling on the order of 10³-10⁴ years based on the clast center Δ₄₇, and provide direct evidence of target rock carbon release. Carbonate clumped isotopes may therefore complement existing thermochronology of impact crater cooling and provide new constraints on the amount of carbon released to Earth’s ancient atmosphere by impacts.