A unique high Mn/Fe microgabbro in the Parnallee (LL3) ordinary chondrite: nebular mixture or planetary differentiate from a previously unrecognized planetary body?

Abstract

The study of planetary materials in chondritic meteorites constrains the compositional diversity of materials in different nebular environments and provides information on the degree of differentiation of early planetary bodies. We studied a unique microgabbro fragment from the Parnallee (LL3) unequilibrated ordinary chondrite. The fragment, which was originally identified by its ophitic to sub-ophitic texture, exhibits features characteristic of lunar and terrestrial tholeiitic basalts—extreme compositional zoning in clinopyroxene (Wo_(10)En_(65)Fs_(25) to Wo_(15)En_2Fs_(83)), a multiply saturated major element composition similar to mid-ocean ridge basalt with 3.1 wt% Na_2O and 0.15 wt% K_2O, and uniformly enriched rare earth elements (c.10 × C1). A high bulkMnO/FeO ratio (0.064) distinguishes the microgabbro from other basaltic rocks and suggests the precursor material formed or reached equilibrium in a reducing environment. However, the absence of Fe metal and the extreme enrichment of FeO (up to 40 wt%), in late crystallizing pyroxferroite, requires the last crystallization event to have occurred in a relatively oxidizing environment. We suggest the microgabbro formed by partial melting in a planetary body after removal of metallic Fe. Examination of possible planetary source materials, such as alkali-rich eucritic material, a volatile-depleted C1 carbonaceous chondrite or H-group chondrite, shows that multiple-stage fractionation is required to produce a melt with the FeO/MgO ratio of the microgabbro from these materials. The increasing number of planetary igneous fragments observed in unequilibrated ordinary chondrites (UOC) suggests the picture of UOC as primitive assemblages of unprocessed material is overly simplistic.