Petrology, mineralogy, and geochemistry of the olivine diogenite NWA 4255: new insights into the magmatic evolution of asteroid 4 Vesta

Abstract

This research work is based on the petrographic, mineralogical and geochemical study of meteorites discovered in the Sahara of southwest Algeria. These meteorites are of the HEDDO type (howardite-eucrite-diogenite-olivine diogenite-diogenite) which belong to the group of achondrites from the 4Vesta asteroid. They are considered a very valuable source for understanding the formation of this asteroid. In this thesis we group our meteorite samples into three main groups: olivine diogenites and diogenites (NWA 4255 and NWA 4272), eucrites (AW 200, Tata, Soueliki and Said) and howardites (GW 25-1 and NWA 2251). These meteorites are analyzed on separate minerals and total rock. The meteorite NWA 4255 is formed by two lithologies (harzburgitic and orthopyroxenitic) and classified as olivine diogenite. Harzburgitic and orthopyroxenitic lithologies are formed by equilibrium crystallization (EC) and fractional crystallization (FC) respectively. These lithologies come from the deepest areas of the crust, or rather at the interface with the mantle of the 4Vesta asteroid. Our oxygen isotopic data show that our samples have 17O values close to the average value of -0.241 ± 0.018 ‰ (2σ). In addition, petrological observations, our geochemical data and the results of modelling work allow us to perfectly explain the petrological and geochemical evolution of the magmatic ocean on Vesta. We use our data to model the major processes related to formation and lithological variation observed in our meteorites. The purpose of this modelling work is to examine the differentiation and crystallization of the magmatic ocean on Vesta. Thus, to see how these meteorite samples could be placed in the global crystallization sequence. In particular, we focus on the formation of olivine diogenites, diogenites and eucrites in general. Olivine diogenites (NWA 4255) crystallize first in equilibrium from a magmatic liquid resulting from early crystallization and after separation of the harzburgitic mantle, followed by peritectic reactions between olivines, pyroxenes and the magmatic liquid. Then, the diogenites (NWA 4272) crystallize by fractional crystallization from the liquid that remains from the crystallization of the olivine diogenites. Cumulative eucrites (eucrite Said) crystallize from the liquid that remains from the crystallization of diogenites via fractional crystallization. The last remaining liquids continue to crystallize to give the non-cumulative eucrites (AW 200, Soueliki and Tata) by fractional crystallization.