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

Tracing the earliest stages of hydrothermal alteration on the CM chondrite parent body

“…As LAP 04514 contains the same abundance of Fe‐serpentine as CM chondrites of the same petrologic subtype, this is consistent with the Fe‐serpentine in LAP 04514 being partially dehydrated and is additional evidence of thermal metamorphism in this meteorite. The mass loss for LAP 04514 between 300 and 450 °C is higher than that observed for CM chondrites heated to Stage II (300–500 °C) (King et al., 2021; Nakamura, 2005), while, as described above, we still detect Fe‐serpentine peaks in its XRD pattern. We therefore estimate that the peak metamorphic temperature for LAP 04514 was ~300–400 °C, although CM chondrites heated to the upper end of that range (e.g., 400 °C) for short periods of time (e.g., 24 h) can also show coherent diffraction from Fe‐serpentine (Lindgren et al., 2020).…”

“…Some CM chondrites experienced short‐lived, posthydration thermal metamorphism at temperatures up to ~1000 °C. Mineralogical features of heated CM chondrites include dehydrated phyllosilicates (~300–500 °C), and at higher temperatures (>500 °C) recrystallized Fe‐Ni sulfides, olivine, and metal (King et al., 2021; Nakamura, 2005; Tonui et al., 2014). It has been suggested that similar materials may be present on the surface of asteroid Ryugu (e.g., Kitazato et al., 2019).…”

“…XRD is sensitive to the collapse of phyllosilicate structures that occurs during dehydration. XRD patterns of naturally and artificially heated CM chondrites lack coherent phyllosilicate diffraction peaks after heating at temperatures > ~300–400 °C (King et al., 2021; Lindgren et al., 2020; Nakamura, 2005; Tonui et al., 2014). For LAP 04796, LAP 04565, and LAP 02333, we observe typical diffraction peaks from Fe‐ and Mg‐rich serpentines, indicating that these meteorites were not heated to > ~300–400 °C after aqueous alteration had ceased (Fig.…”

“…Figure 6 shows that unlike for heated CM chondrites, the water contents of LAP 04796, LAP 04565, and LAP 02333 are not depleted relative to their phyllosilicate abundances, providing further evidence that they did not witness temperatures high enough for volatile loss. However, there are a few meteorites, for example, QUE 93005, that have been identified as heated CMs using other techniques but still show coherent phyllosilicate diffraction, possibly because the metamorphism preceded the main period of aqueous alteration (King et al., 2021; Lee, Cohen & King, 2019; Quirico et al., 2018).…”

A Spectral Investigation of Aqueously and Thermally Altered CM, CM‐An, and CY Chondrites Under Simulated Asteroid Conditions for Comparison With OSIRIS‐REx and Hayabusa2 Observations

“…This scale has four stages describing peak metamorphic temperature: stage I (<250°C), stage II (250–500°C), stage III (500–750°C), and stage IV (>750°C). Of the 12 samples, seven are classified as CM chondrites that experienced no heating to severe metamorphism; the CM2s Alan Hills (ALH) 83100, ALH 83102, and Lonewulf Nunataks (LON) 94101 are all aqueously altered and have not experienced thermal alteration; the CM2s Elephant Moraine (EET) 96029 and Yamato (Y) 793321 experienced aqueous alteration followed by moderate thermal alteration (temperatures of 300–500°C, stage II (Lee et al., 2016; Nakamura, 2006)); and Pecora Escarpment (PCA) 02010 and PCA 02012, which experienced aqueous alteration followed by severe heating at temperatures of >750°C (stage IV [Alexander et al., 2013; Hanna et al., 2020; King et al., 2021; Mahan et al., 2018]). We also investigated the CM‐anomalous (CM‐an) chondrite Wisconsin Range (WIS) 91600, which has experienced aqueous alteration followed by thermal alteration to ∼300–500°C (stage II) but shows a high magnetite abundance (∼7 vol%) and affinities with CI chondrites (Howard et al., 2015; Tonui et al., 2014).…”