Mo isotope evidence for evaporation loss of volatiles in CKs, and implications for external heating sources of the CK metamorphism

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Yingnan Zhang, Liping Qin

Earth and Planetary Science Letters
Volume 641, 1 September 2024, 118807

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“Highlights

  • Heavy Mo isotopes enriched in CK chondrites, and the enrichment generally decreases with Mo content.
  • Evaporation loss of oxidative Mo during thermal metamorphism cause the Mo isotopic fractionation.
  • The heat source of the thermal metamorphism of CKs could be solar radiation.
  • The oxidation of CKs and subsequent thermal metamorphism may occur after the disintegration of the CK parent body.”

“Thermal metamorphism for asteroids influences their structures and chemical compositions. As the only CCs with the petrologic type of 3-6, their metamorphis history and heat sources are unclear. The isotopic composition of molybdenum (Mo) was used as an indicator of oxidation state to investigate the oxidation and thermal metamorphic history of CK chondrites. CK chondrites are characterized by positively fractionated Mo isotopic composition relative to other chondritic groups, and the degree of enrichment in heavy Mo isotopes in CKs generally decreases with Mo content. Combined with numerical simulations for hexavalent Mo evaporation at elevated temperatures and thermodynamic calculations of the valence transformation of Mo, the Mo isotopic characteristic of CKs is proven in concordance with oxidative Mo lost during thermal metamorphism. This evaporation loss highlights the importance of the parent body process on the MVE depletion. Our results also require oxidation of CKs and subsequent thermal metamorphism to have occurred after the disintegration of the CK parent body. A viable heat source in this scenario could be solar radiation, and the oxidization may perform by the water accretion in orbits.
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