An Anomalous Eucrite, Dhofar 007, and a Possible Genetic Relationship with Mesosiderites

Introduction: An achondrite, Dhofar 007 (~27 kg) was found in the Oman desert and was initially classified as a cumulate eucrite [1]. However, Yamaguchi et al. [2] showed that a coarse-grained clast (CG clast) in Dhofar 007 has a high abundance of metallic phases and the high bulk Ni and Co contents, and suggested a possible relationship with mesosiderites. We performed an additional petrologic and geochemical study to better understand the differentiation, shock, and thermal history of Dhofar 007. We compared Dhofar 007 to HED meteorites and mesosiderites to fill the gap in our knowledge of the geologic history of these meteorite groups. Analytical techniques: We made polished thick and thin sections (PTSs) of several portions of Dhofar 007. A portion of the CG clast (0.882 g) was powdered for INAA, PGA, and ICP-MS. We visually inspected the portion for the chemical analyses to insure that it did not contain any material from the breccia matrix. PTSs were examined by optical microscope, SEM, and EPMA. The modes of various phases were determined by X-ray mapping of EPMA. Texture and mineral chemistry: Dhofar 007 is a breccia, mainly composed of coarse-grained clasts set in a brecciated matrix. Although most of the matrix materials are similar to the CG-clast, we found xeno-lithic components such as an impact melt clast (1.9 x 2.0 mm), a Mg-rich orthopyroxene fragment (1.5 x 2.5 mm), and a recrystallized plagioclase. This suggests that Dhofar 007 is a polymict breccia. The boundaries between clast and brecciated portion is in many cases gradational. Clasts and brecciated matrix are transect-ed by glassy impact melt veins (10-30 µm thick), indicating that Dhofar 007 experienced at least two shock events after crystallization. The PTSs of the CG clast show a granular texture composed of pyroxene (~51 vol%) and plagioclase (~47 vol%) with minor amounts of silica minerals (0.7 vol%), chromite (0.3 vol%), troilite (0.6 vol%), kama-cite (0.02 vol%), taenite (0.04 vol%), Ca-phosphate (0.06 vol%) and weathering products often associated with metallic phases. Pyroxenes in the CG clast are pigeonites that have closely spaced, very fine augite exsolution lamellae (<<1 µm wide). Chemical compositions of pyroxenes are scattered on a single tie line from Wo 2.9 En 53.3-Wo 16.7 En 48.2 because thin augite lamellae are unresolvable by electron beam. Orthopy-roxene grain is chemically homogeneous (Wo 1.3 En 83.0).