The Nilpena ureilite, an unusual polymict breccia: implications for origin

Abstract

Nilpena (173 g), a new ureilite find from the Parachilna area of South Australia, is an unusual polymict breccia containing polymineralic aggregates, mineral fragments and achondritic and chondritic lithic enclaves in a dark, C-rich matrix. The polymineralic aggregates consist of equigranular-textured olivine Fa 20 and pigeonite En 75Wo 9FS 16, and exhibit evidence of shock in the form of undulose extinction and kink-banding. Monomineralic fragments consist of olivine Fa 19–24 (with highly forsteritic rims up to Fa 3) and pigeonite, and appear to be derived by brecciation of the polymineralic aggregates. The enclave material consists of lithic granular olivine fragments, porphyritic enstatite fragments (either enstatite chondrite or aubrite), olivine-clinobronzite fragments resembling an H3 chondrite, and eucritelike lithic fragments composed of plagioclase An 98, salitic clinopyroxene Wo 48.5En 31.4Fs 20.1 and olivine Fa 49–53. The matrix contains kamacite (generally rich in P), schreibersite and troilite. The texture of Nilpena suggests formation by disruption of a olivine-pigeonite granular aggregate while the presence of the diverse chondritic and achondritic enclave material suggests an origin as a surface or near-surface breccia. Like other ureilites Nilpena is strongly differentiated with respect to cosmic abundances but is significantly enriched in Ba and LREE. A lack of correlation of lithophile elements with Fe (Fe + Mg) ratio among ureilites suggests that the differentiation was not caused by varying degrees of partial melting of a homogeneous source. A cumulate origin therefore seems more plausible.