Thermal alteration of CM carbonaceous chondrites: mineralogical changes and metamorphic temperatures

The CM carbonaceous chondrite meteorites provide a record of low temperature aqueous reactions in the early solar system. A number of CM chondrites also experienced short-lived, post-hydration thermal metamorphism at temperatures of 200C to over 750C. The exact conditions of thermal metamorphism and the relationship between the unheated and heated CM chondrites are not well constrained but are crucial to understanding the formation and evolution of hydrous asteroids. Here we have used position-sensitive-detector X-ray diffraction (PSD-XRD), thermogravimetric analysis (TGA) and transmission infrared (IR) spectroscopy to characterise the mineralogy and water contents of 14 heated CM and ungrouped carbonaceous chondrites. We show that heated CM chondrites underwent the same degree of aqueous alteration as the unheated CMs, however upon thermal metamorphism their mineralogy initially (300 to 500C) changed from hydrated phyllosilicates to a dehydrated amorphous phyllosilicate phase. At higher temperatures (over 500C) we observe recrystallisation of olivine and Fe-sulphides and the formation of metal. Thermal metamorphism also caused the water contents of heated CM chondrites to decrease from 13 wt percent to 3 wt percent and a subsequent reduction in the intensity of the 3 micron feature in IR spectra. We estimate that the heated CM chondrites have lost 15 to 65 percent of the water they contained at the end of aqueous alteration. If impacts were the main cause of metamorphism, this is consistent with shock pressures of 20 to 50 GPa. However, not all heated CM chondrites retain shock features suggesting that some were instead heated by solar radiation. Evidence from the Hayabusa2 and ORSIRS-REx missions suggest that dehydrated materials may be common on the surfaces of primitive asteroids and our results will support upcoming analysis of samples returned from asteroids Ryugu and Bennu.

💡 Research Summary

The study investigates the thermal metamorphism of CM carbonaceous chondrites, which preserve a record of low‑temperature aqueous alteration in the early Solar System. While many CM meteorites remained unaltered after hydration, a subset experienced brief heating events ranging from ~200 °C to >750 °C. The authors aim to constrain the conditions of this post‑hydration metamorphism and to clarify the relationship between unheated and heated CM specimens, a key issue for interpreting the evolution of hydrous asteroids.

Fourteen meteorite samples—including both heated CM chondrites and ungrouped carbonaceous chondrites—were examined using three complementary techniques: position‑sensitive‑detector X‑ray diffraction (PSD‑XRD), thermogravimetric analysis (TGA), and transmission infrared (IR) spectroscopy. PSD‑XRD provided high‑resolution diffraction patterns that distinguished crystalline phases from amorphous material. TGA quantified water loss as a function of temperature, while IR spectroscopy monitored the intensity of the 3 µm OH absorption band, a proxy for bound water.

Key mineralogical findings are as follows. At temperatures between 300 °C and 500 °C, the hydrated phyllosilicates (primarily microcrinolite and serpentine) lose their long‑range order, giving way to a dehydrated amorphous phyllosilicate phase. This transition is marked by the broadening and weakening of characteristic X‑ray peaks and a concurrent linear decline in the 3 µm band intensity. Above ~500 °C, the system undergoes recrystallisation: olivine (dominantly forsteritic) and Fe‑sulfides (e.g., pyrrhotite, pentlandite) become crystalline again, and metallic Fe⁰ appears, indicating reduction reactions typical of high‑temperature metamorphism.

Water loss is quantified precisely. Unheated CM chondrites contain ~13 wt % water. TGA reveals two distinct dehydration steps: a modest loss (≈5 wt %) between 200 °C and 350 °C, corresponding to structural water in phyllosilicates, and a second, larger loss (≈4–6 wt %) above 500 °C, associated with the breakdown of the amorphous phase and the formation of new crystalline minerals. The final water content of heated samples drops to ≤3 wt %, implying that 15 %–65 % of the original water was expelled, with the proportion strongly dependent on peak temperature (≈20 % loss for 300–400 °C, >60 % loss for >500 °C).

The authors evaluate two possible drivers of the heating. Shock metamorphism can generate temperatures of several hundred degrees under pressures of 20–50 GPa, and some samples display classic shock textures (planar deformation features, melt veins). However, roughly half of the heated specimens lack any shock signatures, suggesting that impact heating alone cannot account for all cases. An alternative mechanism is prolonged solar radiation exposure on asteroid surfaces. Modeling shows that near‑Earth asteroids can sustain surface temperatures around 300 °C for extended periods, sufficient to produce the observed amorphous dehydrated phase without requiring high‑pressure shock.

Comparisons with data from the Hayabusa2 (Ryugu) and OSIRIS‑REx (Bennu) missions reinforce the relevance of the findings. Both missions have identified dehydrated, low‑albedo material on the surfaces of these primitive bodies, consistent with the thermal histories inferred for the heated CM chondrites. Consequently, the mineralogical and water‑loss trends documented here provide a predictive framework for interpreting returned samples from Ryugu and Bennu, especially regarding the presence of amorphous silicates, recrystallised olivine, Fe‑sulfides, and native metal.

In summary, the paper demonstrates that heated CM chondrites experienced the same degree of aqueous alteration as their unheated counterparts, but subsequent thermal metamorphism induced a systematic sequence: (1) dehydration of phyllosilicates into an amorphous phase (300–500 °C), (2) recrystallisation of olivine and Fe‑sulfides plus metal formation (>500 °C), and (3) substantial water loss (15–65 %). The results support a dual‑origin model for metamorphism—impact‑driven shock and solar‑radiation heating—and provide essential context for the analysis of asteroid sample‑return missions.