Aubrite Pesyanoe: Clues to composition and evolution of the enstatite achondrite parent body

Abstract

The Pesyanoe aubrite is an essentially polymict regolith breccia comprised by fragments of different highly magnesian pyroxenitic lithologies: albite; anorthoclase and labradorite-bearing pyroxenites; diopside and magnesian augite pyroxenites; roedderite-and forsterite-bearing pyroxenites; and impact glasses; porphyritic and melt matrix breccia fragments; FeO-rich chondritic inclusions; and exotic oxidized clasts. The parent magma of Pesyanoe probably was carbon saturated, as suggested by pyroxenite fragments containing igneous-textured carbon phases, possibly graphite. The composition of feldspar and trapped melt inclusions in enstatite indicates occurrence of at least three metaluminous melt sources with different (K + Na)/Al and K/(K + Na) atomic ratios on the Pesyanoe parent body and has records of K and Na loss from the melt, possibly due to evaporation from the parent body surface. The roedderite-and forsterite-bearing rocks probably crystallized from a peralkaline melt. We propose that peralkaline melt could be formed from a metaluminous melt(-s) due to gravitational segregation of djerfisherite-bearing metal-sulfide liquid in the lower horizon of the magma chamber and following oxidation of the magma. This should lead to enrichment of silicate melt in K 2 O and Na 2 O and increasing of (K + Na)/Al > 1, allowing forsterite and roedderite to crystallize. Rocks enriched in K and containing rare Kbearing minerals were found among both magmatic and melt rocks. This may imply an insignificant role of regolith transport in the process of the breccia's formation and, therefore, an origin of all of the breccia components from a local region of the Pesyanoe parent body, probably from a single complex igneous massif.

The effects of highly reduced magmatism revealed through aubrites

“…Moreover, Lorenz et al. (2020) observed voids of 500 µm diameter within enstatite in Pesyanoe and interpreted this to indicate that some of the Pesyanoe rocks crystallized under low lithostatic pressure. It is possible that Mount Egerton could also have crystallized under low lithostatic pressure, which would be at odds with it being from the core–mantle boundary of its parent body, unless it is sourced from its own, smaller, parent body rather than the core–mantle boundary of the aubrite parent body.…”