Thermally metamorphosed CM chondritic clasts reveal deep interior conditions: Insights from thermometry

We studied four coarse-grained crystalline clasts within CM carbonaceous chondrites to constrain their thermal history and origin. These clasts have near-identical mineralogies, dominated by olivine (Fa37 or Fa40), with lesser quantities of high-Ca pyroxene (diopside: Fs10-11 Wo46-48), plagioclase (andesine: An36-40), Cr-rich spinel, Fe-Ni sulphides and nepheline. Combined SEM-EDS-EBSD and EMPA analyses yield olivine-spinel closure temperatures of ∼690-750 °C. Available O–Cr–Ti isotope data from one clast support a CM affinity (Kerraouch et al. 2025 [LPSC]), while their equilibrated mineral assemblages and textures indicate petrologic type 6 (Kerraouch et al. 2019 [Geochemistry]). Textural evidence suggests that Fe sulphides underwent melting, small-scale (<1 mm) migration and coalescence into larger interstitial masses, implying peak temperatures between 988 °C (the Fe-FeS eutectic) and 1050 °C (the approximate silicate solidus of chondritic material). Retention of Fe sulphides requires closed-system heating, consistent with pressure suppressing sulphur volatilisation and stabilising sulphides before melting. All clasts show minimal post-metamorphic shock deformation. Together, these findings indicate that either the CM parent body was sufficiently large to sustain prolonged high-temperature heating at depth, or at least two CM-like parent bodies accreted from similar starting materials at different times, accounting for their matching compositions but contrasting thermal histories.