Nature of the H chondrite parent body regolith: Evidence from the Dimmitt breccia

Abstract

The Dimmitt H chondrite regolith breccia consists of (in vol.%) 40% H4 and H5 chondrite clasts, 3% impact melt rock clasts, 0.5% shocked H chondrite clasts, 1.5% exotic clasts (including carbonaceous and LL5 chondrites), and 55% gas‐rich matrix. The LL5 clast is the best documented example of an ordinary chondrite in a host of a different compositional group. The matrix contains unequilibrated material, which differs from typical H3 material in having little (0.2 vol.%) fine‐grained opaque silicate matrix, and having 20% of the olivines with compositions in the range Fa21–24. About 10–15% of this unequilibrated material is probably derived from graphite‐magnetite‐rich chondrites and 2% from H3.0–3.5 chondrites. The absence of H3 clasts suggests that most of the unequilibrated material was derived from unconsolidated type 3 components. Many exotic clasts may have been derived from planetesimals that accreted to the H chondrite parent body prior to regolith development. One slowly cooled melt rock clast formed beneath a 500‐m‐thick melt breccia pile on the floor of a large impact crater and was later excavated by additional impacts, incorporated into the regolith and consolidated with other components to form the Dimmitt breccia.