Accretion, metamorphism, and brecciation of ordinary chondrites: Evidence from petrologic studies of meteorites from Roosevelt County, New Mexico

Abstract

We have made petrologic studies of a suite of 29 newly discovered ordinary chondrites from Roosevelt County, New Mexico, including an H3 chondrite that is rich in solar wind gases and contains clasts of melt rock and graphite‐magnetite‐rich material, and an L5 chondrite with a unique igneous inclusion. Our data and published work show that mean CaO concentrations of low‐Ca pyroxene can be used to distinguish petrologic types in H, L, and LL groups. Mean CaO concentrations in olivine show significant variations among equilibrated chondrites, but these are not correlated with petrologic type; varied cooling rates could be responsible. The degree of heterogeneity of FeO concentrations in olivines of types 4–6 is also not correlated with degree of metamorphism; it may be controlled more by postmetamorphic brecciation and mixing of material than by metamorphism. Mean FeO concentrations of silicates show average increases of 3–5% from type 4 to type 6 in each group. This increase and the accompanying increase in bulk concentrations of siderophile elements appear to predate parent body formation. Correlation of nebula‐controlled bulk chemistry and planetary metamorphic effects could only have been produced under special conditions of accretion. One possible origin is runaway growth, which allowed parent bodies to form by accretion kilometer‐sized planetesimals to larger bodies, accompanied by differential movement of chemically distinct large and small bodies to produce radially zoned parent bodies that were subsequently metamorphosed by internal heating. Alternatively, ordinary chondrites may have been metamorphosed in a variety of planetesimals, each of which had a uniform chemical composition and metamorphic temperature. Correlations between composition and temperature may have arisen if both were correlated with another parameter, such as heliocentric distance.