Justin A. Hayles | Search NASA

NASA NTRS2021

Constraints on Triple Oxygen Isotope Kinetics

Isotopic disequilibrium is not as well constrained as equilibrium, hindering interpretation of isotopic variations. Kinetic isotope effects, a subset of disequilibrium, are common in nature and have been assumed to be distinct from equilibrium and mass independent isotope effects based on underdeveloped criteria. Using basic physical principles, we provide needed mechanistic constraints on mass-dependent kinetic isotope effects for the triple oxygen isotope system. We find some kinetic isotope effects yield large isotopic variations, exceeding equilibrium, which could be mistaken for mass independent relationships. Meanwhile, other kinetic isotope effects are found to have triple oxygen isotope relationships that could be mistaken for equilibrium isotope effects. Comparison against prior case studies of thermal decomposition of calcite (CaCO3) and brucite (Mg(OH)2) further tests our results. Although oxygen is the focus here, our approach applies to any system with more than two isotopes.

Isotope Geochemistry↗

NASA NTRS2021

3. Climbing to the Top of Mount Fuji: Uniting Theory and Observations of Oxygen Triple Isotope Systematics

The near-simultaneous discovery of both minor isotopes of oxygen in 1929 was a watershed moment for modern science, to say nothing of its impacts on isotope geochemistry. At the time, oxygen was the international standard for atomic weight, as it had been for over twenty-five years. However, chemists and physicists had grown fond of different definitions: physicists used the weight of the 16O atom, while chemists used half the weight of atmospheric oxygen (O2) to define the precise weight of 16 atomic mass units. While they usually avoided direct conflicts, these contrasting definitions found an unexpected impasse in the near-infrared absorption spectrum of atmospheric oxygen: it contained a series of faint lines that could not be explained by any known atmospheric constituents.

Laurence Y. Yeung↗

NASA NTRSDate not supplied

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↗