Petrology and geochemistry of lunar dimict breccia 61015

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Abstract
Dimict breccia 61015 consists dominantly of granulated anorthosite and impact melt rock of VHA (very‐high‐alumina)‐basalt composition. The anorthosite is a typical Apollo 16 ferroan anorthosite, except that it is slightly more augite‐rich than most other ferroan anorthosites. The breccia probably formed in the floor of an expanding crater cavity by the following process: Impact melt generated within the cavity was injected into underlying anorthositic bedrock and solidified rapidly, and the anorthosite and melt rock were then fragmented by deformation in the later stages of the impact. All the Apollo 16 dimict breccias (except perhaps 61016) appear to have formed in the same impact. The source crater cannot yet be identified, but it may be an intermediate‐sized (50–150 km diameter) crater within a few hundred kilometers of the Apollo 16 site, in the direction of the Imbrium basin, of either pre‐Nectarian or post‐Nectarian/pre‐Imbrian age. The breccia has been affected by at least one, and perhaps as many as four, recent small impacts (the most recent was probably the South Ray impact). The recent impact(s) produced internal shock‐induced melting of the breccia and injected and coated the rock with externally derived impact melts. The coating glass is heterogeneous. Its average composition is similar to the compositions of other Apollo 16 coating glasses, except that its TiO2 content is lower. The vein glass is unusually mafic and has very low TiO2 for its Al2O3 content. The TiO2 contents of these glasses establish that they cannot be melted soils; their compositions require a mafic, low‐TiO2 protolith component that has not yet been identified among the large rock samples, perhaps a very mafic member of the ferroan‐anorthosite suite.