An experimental investigation of Fe-sulfides as an indicator of heating conditions in CM and CI carbonaceous chondritesOPEN ACCESS
C.S. Harrison, A.J. King, R.H. Jones
Geochimica et Cosmochimica Acta, In Press, Journal Pre-proof, Available online 8 April 2026
“We conducted a series of experiments to better understand how the mineralogy and composition of Fe-sulfide grains respond to heating under conditions comparable to those on the CM (“Mighei-like”) and CM/CI-like (“Ivuna-like”) chondrite parent bodies. Understanding these conditions provides insights into the nature of the heating event that dehydrated some CM and CM/CI-like carbonaceous chondrites, a process that is fundamentally important for understanding the evolution and distribution of volatiles in the Solar System. We experimentally heated pieces of CM and CI chondrites to peak temperatures ranging from 500 °C to 900 °C, followed by controlled cooling over durations on the order of hours to days. We characterized coarse (>10 μm) Fe-sulfide grains in unheated and experimentally heated samples using Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS) analysis to constrain changes in Fe-sulfide mineralogy and chemistry with heating. The experiments replicate several heating effects observed in dehydrated chondrites. We have developed schematic time–temperature-transformation (T-T-T) diagrams, based on the presence of pentlandite and/or metal, that can be used as tools to interpret the thermal history of naturally heated hydrated meteorites. The atomic Fe/(Fe + Ni) ratio of pentlandite increases with increasing peak temperatures in the experimentally heated samples, offering a useful diagnostic tool for determining the relative peak temperatures in naturally heated chondrite samples. Over the short heating durations considered in the experiments we typically find patches and blebs of pentlandite associated with grain boundaries or defects in pyrrhotite grains, but we did not replicate pentlandite exsolution textures within the pyrrhotite crystal lattice, such as blades and lamellae. This indicates that longer duration heating and / or slower cooling (over a period > 4 days) is necessary to form the fine-scale exsolution textures observed in naturally heated chondrites. Finally, by experimentally heating a naturally heated Stage II (300 – 500 °C) CM sample, we find that the cumulative effects of two heating events are not significantly different from a single heating event in terms of Fe-sulfide mineralogy and texture. This means that it is difficult to interpret the number of heating events experienced by individual heated CM and CM/CI-like chondrites, from sulfides alone.”
