Silicon isotopic compositions of chondrule silicates in carbonaceous chondrites and the formation of primordial solids in the accretion disk

Johan Villeneuve, Yves Marrocchi, Emmanuel Jacquet

Earth and Planetary Science Letters
Volume 542, 15 July 2020, 116318



• Chondrules are characterized by large, in situ mass-dependent silicon isotopic variations.
• These variations are inherited from chondrules precursors and kinetic effects during their formation.
• These variations imply that chondrules formed in open-system in interaction with a SiO-rich gas.”

“We determined the silicon isotopic compositions of silicates (olivine and low-Ca pyroxene) in type I and type II chondrules of the carbonaceous chondrites Allende, Kaba, NWA (Northwest Africa) 5958, and MIL (Miller Range) 07342. Type I chondrule silicates show large, mass-dependent Si isotopic fractionations, with Si values ranging from −7‰ to +2.6‰, whereas the Si values of type II chondrule silicates are close to zero and vary by less than 2‰. When present, Mg-rich relict olivine grains in type II chondrules show larger Si variations than their FeO-rich counterparts. In type I chondrules, low-Ca pyroxenes yield systematically lighter Si values than Mg-rich olivines. Our results show that type I chondrules are complex objects whose Si isotopic compositions derived from their precursors and SiO-rich gas-melt interactions. This corroborates that type I chondrules are nebular products that formed under open-system conditions. Our data also suggest that at least some type II chondrules derived from their type I counterparts. Overall, this demonstrates that recycling was common during the evolution of the protoplanetary disk.”