Pyroxenes in the Patwar mesosiderite

Major and minor elements were determined with an electron microprobe in pyroxene from Patwar, one of the least metamorphosed mesosiderites. All but two orthopyroxene crystal fragments fall in the range Wo(2.3)En(73) to Wo(3.9)En(59), while pigeonite from a basalt clast clusters around Wo(13)En(38). Absolute and relative amounts of Ti, Cr, and Al are distinct from those in pyroxenes in irons-with-silicate inclusions. Zoning within orthopyroxene grains suggests crystallization from an igneous melt in the presence of plagioclase and chromite, followed at some later time by brecciation and subsequent minor thermal metamorphism. Compositional data support petrographic evidence that the silicates in Patwar were derived from a heterogeneous source.