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Isotopic Tracers for CO 2 Produced During A Planetary Impact Into Limestone Target Rocks

Terrestrial meteorite impacts have been directly linked with multiple mass extinction events throughout Earth’s history. Among these impacts, those that land in sedimentary target rocks are thought to generate large quantities of CO 2 via decarbonation. This injection of CO 2 into the atmosphere has the potential to alter the climate and threaten terrestrial habitability. The magnitude of this change depends upon the net amount of CO 2 released, which is controlled by how much CO 2 is produced by the impact, how much CO 2 is removed by back-reactions after the impact, and other post-impact CO 2 sinks. To interrogate the behavior of CO 2 release into the impact atmosphere, we present carbon (δ 13 C), oxygen (δ 18 O), and clumped (Δ 47 ) isotope results from carbonate clasts preserved within the impact breccia of the Steen River Impact Structure (SRIS) in Alberta, Canada. These clasts exhibit a ~65‰ range in δ 13 C and a ~5‰ variation in δ 18 O. However, while δ 13 C and δ 18 O are positively correlated, Δ 47 unexpectedly has a negative relationship with δ 13 C and δ 18 O. Based on prior assumptions, there would either be (1) no relationship with carbonate Δ 47 and the bulk ratios because Δ 47 would be reset at extreme impact temperatures, or (2) Δ 47 would have a positive correlation with the bulk ratios, reflecting gradual Δ 47 ‘resetting.’ To reconcile the SRIS result with these expectations, we conducted a series of in vacuo heating experiments at temperatures above calcite decomposition. As predicted by Rayleigh fractionation, these heating experiments generated depletions in δ 13 C and δ 18 O that increased with reaction time. Mimicking the SRIS results, these experiments also produced concomitant increases in Δ 47 . Using an adaptation of a mechanistic model for decomposition fractionation (Hayles & Killingsworth, 2022) we hypothesize that these exotic isotope trends are caused by Rayleigh fractionation and a Δ 47 kinetic isotope effect that results from the disproportionation of O to CaO and CO 2 during thermal decomposition. This high Δ 47 CO 2 subsequently exchanges with the residual CaCO 3 . This work highlights a potential pathway for identifying and quantifying CO 2 generation by impacts and builds on the relatively limited literature characterizing the behavior of carbonate clumped isotopes at very high geologic temperatures.

Multi-isotope systems↗
NASA NTRS2012

Measuring Isotope Ratios Across the Solar System

Stable isotope ratios in C, H, N, O and S are powerful indicators of a wide variety of planetary geophysical processes that can identify origin, transport, temperature history, radiation exposure, atmospheric escape, environmental habitability and biology [1]. For the Allan Hills 84001 meteorite, for example, the (sup 1)(sup 3)C/(sup 1)(sup 2)C ratio identifies it as a Mars (SNC) meteorite; the ??K/??Ar ratio tells us the last time the rock cooled to solid, namely 4 Gya; isotope ratios in (sup 3)He, (sup 2)(sup 1)Ne and (sup 3)?Ar show it was in space (cosmic ray exposure) for 10-20 million years; (sup 1)?C dating that it sat in Antarctica for 13,000 years before discovery; and clumped isotope analysis of (sup 1)?O(sup 1)(sup 3)C(sup 1)?O in its carbonate that it was formed at 18+/-4 ?C in a near-surface aqueous environment [2]. Solar System Formation

planetary geophysical process↗
NASA NTRS2006

Carbon-rich Chondritic Clast PV1 from the Plainview H-chondrite Regolith Formation from H3 Chondrite Material by Possible Cometary Impact

Chondritic clast PV1 from the Plainview H-chondrite regolith breccia is a subrounded, 5-mm diameter unequilibrated chondritic fragment that contains 13 wt% C occurring mainly within irregularly shaped 30-400-micron-size opaque patches. The clast formed from H3 chondrite material as indicated by the mean apparent chondrule diameter (310 micron vs. approximately 300 micron in H3 chondrites), the mean Mg-normalized refractory lithophile abundance ratio (1.00 +/- 0.09 XH), the previously determined 0-isotopic composition (Delta O-17 = 0.66% vs. 0.68 +/- 0.04%0 in H3 chondrites and 0.73 +/- 0.09% in H4-6 chondrites), the heterogeneous olivine compositions in grain cores (with a minimum range of Fal-19), and the presence of glass in some chondrules. Although the clast lacks the fine-grained, ferroan silicate matrix material present in type 3 ordinary chondrites, PV1 contains objects that appear to be recrystallized clumps of matrix material. Similarly, the apparent dearth of radial pyroxene and cryptocrystalline chondrules in PV1 is accounted for by the presence of some recrystallized fragments of these chondrule textural types. All of the chondrules in PV1 are interfused indicating that temperatures must have briefly reached approximately 1100C (the approximate solidus temperature of H-chondrite silicate). The most likely source of this heating was by an impact. Some metal was lost during impact heating as indicated by the moderately low abundance of metallic Fe-Ni in PV1 (approximately 14 wt%) compared to that in mean H chondrites (approximately 18 wt%). The carbon enrichment of the clast may have resulted from a second impact event, one involving a cometary projectile, possibly a Jupiter-family comet. As the clast cooled, it experienced hydrothermal alteration at low water/rock ratios as evidenced by the thick rims of ferroan olivine around low-FeO olivine cores. The C-rich chondritic clast was later incorporated into the H-chondrite parent-body regolith and extensively fractured and faulted.

Rubin, Alan E.↗
NASA NTRS1997

Observational Evidence for Mixing and Dust Condensation in Core-Collapse Supernovae

Recent findings of isotopic anomalies of Ca-44 (the decay product of Ti-44) and the enhanced ratio of Si-28/Si-30 in SiC grains X, TiC subgrains, and graphite dust grains within primitive meteorites provides strong evidence that these presolar grains came from core-collapse supernovae. The chemical composition of the presolar grains requires macroscopic mixing of newly nucleo-synthesized elements from explosive silicon burning at the innermost zone of the ejects to higher velocities where C exists and where C/O > 1 in either the outer edge of the oxygen zone or in the He-C zone. To date, the only core-collapse supernova observed to form dust is the brightest supernova of the past four centuries, SN1987A in the Large Magellanic Cloud. Observations of SN1987A confirm large scale macroscopic mixing occurs in the explosions of massive stars. Rayleigh-Taylor instabilities macroscopically mix most of the ejects into regions which are still chemically homogeneous and which cool with different time scales. Only small clumps in the ejects are microscopically mixed. Observations show that dust condensed in the ejects of SN1987A after approx.500 days in the Fe-rich gas. Neither silicates nor SiC grains were seen in the dust emission spectrum of SN1987A. SN1987A, the Rosetta Stone of core-collapse supernovae, shows that while the mixing required to explain presolar grains occurs, the rapid cooling of the Fe zone and the sustained high temperatures of the O-Si, O-C, and He-C zones favor the formation of iron-rich rather than oxygen- or carbon-rich grains.

Wooden, Diane↗