Formation of solar nebula reservoirs by mixing chondritic components
Résumé
We determined proportions of Type I (reduced) and Type II (oxidized) chondrules in ordinary chondrites (OC) and found linear relationships between chondrule abundances and chondrite bulk chemical and oxygen isotopic compositions. Similar relationships exist between bulk oxygen isotopic compositions of carbonaceous chondrites and modal abundances of their chondritic components (matrix, Type I and Type II chondrules, refractory calcium–aluminium-rich inclusions and amoeboid olivine aggregates). These correlations can be used to predict the bulk oxygen isotopic composition of chondrites based on their petrology. We can also define model isotopic compositions associated with each petrologic component, which are not their current actual isotopic compositions due to alteration or mixing. These compositions for refractory inclusions and chondrules plot close to a slope 1 line, consistent with refractory inclusions (RI) forming from an early 16O-rich gas, the evolution of the gas to more 16O-poor compositions, possibly involving photodissociation and subsequent ice transport, followed by chondrule formation. Our results open a new understanding of the oxygen 3-isotope space and explain the unique position of OC as well as the differences between H, L and LL chondrites. They indicate that major chemical and isotopic variations between chondritic reservoirs were established after chondrule and CAI formation. They may have some bearing on the formation of planetary reservoirs: the Δ17O calculated for type I chondrules is appropriate for terrestrial planet progenitors, consistent with their chemical similarity to Earth mantle.