Experimental heating of CI chondrite: Empirical constraints on the evolution of micrometeorite O-isotopes during atmospheric entry

Extraterrestrial dust exhibits a wide range of textural, chemical and oxygen isotopic compositions due to the heterogeneity of their precursors and modification during atmospheric entry. Experimental heating provides an opportunity to investigate the relationship between thermal processing and micrometeorite composition for a known precursor material. We conducted experiments to simulate the atmospheric entry of micrometeorites (MMs) using controlled, short-duration (10–50 s) flash heating (400–1600 °C) of CI chondrite chips (<1500 µm) in atmospheric air (1 bar, 21% O₂) combined with microanalysis (textures, chemical and isotopic compositions) of the experimental products. The heated chips closely resemble natural samples, with materials similar to unmelted MMs, partially melted (scoriaceous) MMs and fully melted cosmic spherules produced. We reproduced several key features such as dehydration cracks, magnetite rims, volatile gas release, vesicle formation and coalescence, melting and quench cooling. Our parameter space allows for discriminating peak temperature and heating duration effects. Peak temperature is the first-order control on MM mineralogy, while heating duration controls vesicle coalescence and homogenization. When compared against previous heating experiments, our data demonstrates that CI chondrite dust is more thermally resistant, relative to CM chondrite dust, by approximately +200 °C. The 207 measurement of O-isotopes allows, for the first time, petrographic effects (such as volatile degassing and melting) to be correlated against bulk O-isotope evolution. Our results demonstrate findings applicable to CI chondrites and potentially to all fine-grained hydrated carbonaceous chondrite dust grains: (1) O-isotope variations arising during sub-solidus heating are dominated by the release of water from phyllosilicates, forcing the residual MM composition towards its anhydrous precursor composition. (2) Oxygen isotope compositions undergo the most significant changes at supra-solidus temperatures. As previously demonstrated and now empirically confirmed, most of these changes are driven by a mass-dependent fractionation effect caused by evaporation, which shifts residual rock compositions toward heavier values. Mixing with atmospheric air alters compositions toward the terrestrial fractionation line. Notably, these two processes do not begin simultaneously. Our data indicate that at 1200 °C, isotopic evolution is dominated by evaporative mass loss. However, at higher temperatures (1400–1600 °C), both pronounced evaporation and mixing with atmospheric oxygen become active, resulting in a more complex isotopic signature. (3) The total change in Δ¹⁷O during heating up to 1600 °C is < 3‰ and in most scenarios < 2‰.