Early Meteoritic Records of Asteroidal Melt and Impact Environments

The characterization of meteorites formed in early melt and impact environments helps deepen our understanding of the processes involved in the formation and modification of terrestrial bodies in the solar system. The main objective of this thesis is to interpret and describe a range of igneous and metamorphic environments on asteroidal bodies, through the chemical, microstructural and isotopic analysis of meteorites, and to place these in the context of evolving rocky bodies in the early protoplanetary disk. Study of the meteorite Northwest Africa (NWA) 869 has led to the novel discovery of a eucrite impactor clast in chondritic regolith material. Secondary heating from additional impact(s) formed a basaltic melt, in a rim surrounding the clast, which has bulk characteristics similar to some planetary achondrites. This finding reveals an alternate pathway to achondrite formation and illustrates the mixing of chemical reservoirs in the solar system. Phosphate thermochronology measurements carried out in situ on NWA 7680 and 6962 revealed that they have remained below 350-550 °C since the time of the early protoplanetary disk (4578 ± 17 Myr ago for NWA 7680). Both meteorites have compositions and textures consistent with formation through short-lived differentiation processes on a primitive CR chondrite-like parent body, preserve evidence of some of the earliest known asteroidal melts, and contain Cr and O isotopic evidence for rapid (within several million years) establishment of chemical reservoirs within the protoplanetary disk. Finally, a textural analysis of graphite and diamond disposition in the ureilites NWA 11950 and 11951 was conducted to test recent planetary models invoking diamond as a relic of static metamorphism within an early planet-sized body. The results of the textural study are instead consistent with a paragenesis by shock waves passing from lower (carbon) to higher (silicates) shock impedance materials. This thesis has established the nature and timescale of some of the earliest formation and modification processes on rocky bodies in our solar system, providing physical evidence with which to improve our models of planetary evolution