DISTRIBUTIONS AND ABUNDANCES OF MAJOR AND MINOR ELEMENTS IN BRECCIATED DIOGENITE MIL 11201

Diogenites are part of the HED suite of meteorites originating from asteroid 4-Vesta. Sample MIL 11201 was collected in Antarctica in 2011 and obtained from the Meteorite Working Group (MWG). MIL 11201 was initially classified as a diogenite with a compositional range for pyroxene (PYX) of Fs_30-48 and Wo_1-4. Diogenites are classified by abundance of olivine (OL): orthopyroxenite (0% OL), harzburgite (10-90% OL), and dunite (>90% OL), and also grouped into brecciated and unbrecciated varieties. We analyzed a thin section of MIL 11201 through optical microscopy, EPMA, BSE, and x-ray mapping. Based on our analyses, MIL 11201 is a brecciated orthopyroxenite. There is a wide range in PYX compositions, with currently measured Mg# from 37-58; the highest Mg# PYX measured by the MWG has not yet been found in our thin section but we have located much more Fe-rich PYX and substantial exsolution of Ca-rich CPX from OPX, with Wo_1-46.

Previous diogenite x-ray maps have demonstrated that minor-element heterogeneity can be measured within samples. We collected EPMA x-ray maps of Mg, Cr, Al, and Ti. As in sample MET 01084, many annealed cracks in MIL 11201 show elevated Cr; but in contrast with MET 01084, only some cracks exhibit an exsolved spinel phase. The annealed cracks indicate multiple episodes of brecciation and heating occurred in this sample. EPMA data indicates a negative correlation between Ti and both Cr and Al in the brecciated PYX fragments. Based on the inverse relationship between the minor elements, MIL 11201 represents a brecciated PYX from a single magmatic source, that crystallized PYX with a range of compositions. An area of exsolved OPX/CPX exhibits diffusive exchange of major elements and sharp boundaries in minor elements; further analysis is needed to determine if these PYX originate from the same magmatic source. The most Fe-rich PYX exsolved into the CPX/OPX pair, and the original minor element relationships have been preserved through metamorphism.