Partial Melting on FeO-Rich Asteroids: Insights to the First Stage of Planetary Differentiation

Abstract

The melting of planetesimals was a widespread geologic phenomenon taking place in the early inner solar system. Petrologic and geochemical evidence shows that this melting frequently resulted in full differentiation of planetary bodies into a core, mantle, and crust. The extent of this early planetary melting is evidenced in the breadth of achondrite meteorites. In the achondrite meteorite group, there exist meteorites that experienced low degrees of melting, such that the parent body underwent partial melting and did not fully differentiate. These meteorites, called the primitive achondrites, are a window to the first stage of melting in the early solar system. The primitive achondrites with FeO-poor silicate compositions have been well-studied, but little is known about the formation conditions and history of the FeO-rich primitive achondrites, which includes the brachinites and several ungrouped meteorites.The brachinites are olivine-dominated meteorites with a recrystallized texture that show evidence of partial melting and melt removal on their parent body. The ungrouped primitive achondrites are also olivine-dominated meteorites with a recrystallized texture, but they exhibit a larger range in mineralogy with most being essentially chondritic and containing relict chondrules. In this dissertation, I present a study of the petrology, geochemistry and formation conditions of the FeO-rich primitive achondrites. I analyze the petrology and bulk composition of the meteorites, and I conduct thermodynamic modelling of the mineral assemblages to determine oxidation conditions during their formation. Finally, I attempt to simulate the formation of the brachinite meteorites through 1-atmosphere, gas-mixing partial melting experiments of an FeO-rich chondritic meteorite.These meteorites represent a continuum of partial melting, akin to that seen in the acapulcoite-lodranite clan of primitive achondrites. Mineral compositions and oxygen fugacity formation conditions indicate that the brachinites could have formed from a parent body much like the R chondrites. Gas-mixing, partial melting experiments of a R4 chondrite LaPaz Ice Field 03639 at 1250 °C and an oxygen fugacity of IW-1 create the mineralogy and mineral compositions of the brachinites. The experiments also confirm that the brachinites formed by the partial melting of an FeO-rich chondritic source and not as igneous cumulates.