Foreign Materials in Polymict Breccias from Vesta

Previous studies have documented fragments of petrogeneti-cally unrelated materials in HED polymict breccias [e.g., 1,2,3,4,5,6]. Observed clasts range in size from as large as 5 mm in diameter [2,3] to very small (~25 μm diameter) [4]. Most fall into two groups: clasts with textures similar to CM materials and those similar to CR materials [2,3,4]. CM-like clasts predominate among larger fragments, whereas CR-like clasts are more abundant among smaller fragments [4]. TEM analysis of matrix phyl-losilicates in these fragments indicates that they have been heated to variable degrees [2,3]. Oxygen isotope analysis of a large CM-like fragment in EET 87513 combined with textural, mineralogi-cal, and compositional data confirm that it is a fragment of CM material [2]. Based on TEM analysis, CR-like fragments are mineralogically similar to some CR materials [3]. HED polymict breccias are relatively rich in siderophile elements compared to HED monomict meteorites. For example, Ni abundances of polymict breccias commonly range between 100 ppm and 600 ppm [e.g.,1,5,6] compared with values commonly less than 35 ppm for monomict eucrites [6,7] and less than 172 ppm for diogenites [e.g.,1,6]. Analysis of other siderophile elements confirms this enrichment [1,5,6,7]. However, petrographic and SEM analyses of some of these meteorites do not reveal observable clasts [1,3,6]. Oxygen isotopic analysis supports the presence of CM material in some of these breccias (e.g., Bhol-ghati) [8]. Previous authors have interpreted these chondritic fragments in two ways: (i) fossil IDPs [4], and (ii) fragments of meteorites that impacted Vesta during regolith formation and mixing [2,5,6]. The uniformly high abundances of siderophile elements in HED polymict breccias indicate that these foreign materials are pervasively mixed throughout these breccias and, hence, that the mixing process was very efficient [2,6]. These materials represent a continuous range from relatively large fragments to submicro-scopic individual mineral grains [5,6]. Energetic mixing associated with impact of meteorites onto the surface of Vesta seems the most probable mechanism for this efficient mixing [5,6].