Experimental and analytical studies of partial melting in planetesimals and the Martian mantle

Abstract

Planetesimals and planetary embryos, the building blocks of planets, started to melt within a few million years of the formation of the solar system. This thesis explores, through experiments and the analysis of meteorites, the magmatic processes that affected those early-formed bodies. Chapter 1 presents low-pressure experiments that simulate the onset of melting of planetesimals made of different chondritic materials (H, LL, CI, CM and CV). H, LL and CI compositions, melted at lower temperature and produced partial melts with higher SiO₂, Al₂O₃ and alkali element concentrations compared to CM and CV compositions. They formed unique trachyandesite achondrites upon crystallization. In Chapter 2, the experiments are compared to primitive achondrites, distinct groups of meteorites that represent the melting residues "left behind" within planetesimals.

Cumulative evidence from trachyandesite achondrites and primitive achondrites suggests that the planetesimals that accreted in the inner solar system were not depleted in alkali elements relative to the composition of the sun's photosphere. Chapter 3 is a detailed study of ureilites, the largest group of primitive achondrites. Twelve ureilites were analyzed to determine the chemical composition and relative proportions of olivine and pyroxene. Those analyses, together with additional experiments, constrain the initial Mg/Si ratio of the ureilite parent body. The experiments are used to develop a new geothermometer, based on the partitioning of Cr between olivine and pyroxene, which demonstrates that ureilites are residues of incremental melting. Chapter 4 is the first of two chapters describing igneous processes on Mars, a planet sometimes referred to as a planetary embryo due to its small size and early accretion age.

It describes a high-pressure experimental study of the partial melting of the primitive Martian mantle and discusses the origin of rocks from the Martian crust. Finally, chapter 5 is a study of Fe-Mg isotopic fraction in the olivine of the "enriched" shergottite Northwest Africa 1068. The composition and crystallization history of the parental melt, which represents a melt extracted from the Martian mantle, are constrained by modeling diffusion and crystal growth simultaneously.