PETROLOGY OF THE POLYMICT EUCRITE PETERSBURG

The polymict eucrite Petersburg contains eucritic and diogenitic mater ials which span a broad range of compositions. The present study focus es on four eucritic clasts for which bulk compositions and mineralogic al data were acquired; the broad compositional range of these clasts d ocuments the diversity of the parent regolith. Clast A is REE-rich (si milar to 26 x chondrite) and magnesian (Mg# = 48.7); composition of th is clast can best be explained by partial melting of a source region d istinctly different from that of other noncumulate eucrites. Compositi on of eucritic clast B is similar to compositions of cumulate eucrites in REE and Sr abundances, but it has a much lower Mg# than Moore Coun ty. Clasts C and D are similar in texture, mineralogy, and composition to main group eucrites. The presence of anomalous clast A in the same polymict breccia as typical eucrite clasts C and D emphasizes the var iability of source materials from which eucritic melts were derived on the HED parent body(ies). After final accretion, this breccia apparen tly was exposed to a minor episode of thermal metamorphism causing sim ilar to 30 mu m wide alteration rims around larger pyroxene fragments and causing alteration of pyroxenes at the edges of eucritic clasts. T his metamorphism completely altered the Mg# of smaller matrix pyroxene s, less than 40-60 mu m in size, to values closer to the bulk Mg# of t he meteorite.