Doctor of Philosophy
Royal Ontario Museum
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.
Summary for Lay Audience
Meteorites provide snapshots of our solar system’s formation and evolution through time. Key processes in the transition from the presolar disk to planets include melting and collision. These processes are important because they ultimately led to the formation of the Earth and other planets and the outgassing of volatiles to their surfaces. The main objective of this thesis is to describe some of the earliest melting processes which initiated on asteroids, as represented in meteorites, to obtain a better understanding of how and for what duration rocky bodies experienced melting and differentiation. The results of this research include evidence of rocks that have gone through the differentiation process impacting and mixing with more primitive rocks on the surface of an asteroid. Information about the melt mixing occurring during impact processes is vital to our understanding about how distinct objects accrete, grow and evolve over time. The study of a second set of meteorites has revealed new insights into the diversity and timing of melt processes; including evidence of previously unknown early (> 4.5 billion years ago) melt environments on a primitive asteroid in the outer solar system. Additional work carried out on a third set of meteorites has found evidence of high pressure impact shock-related diamonds in meteorites from a partially differentiated parent body. The shock formation of diamonds in these meteorites challenges the views of early melt processes, as some studies suggest the requirement of a large planet to form the diamonds at depth. The knowledge gained through this thesis stems from customized methodologies and integrating analytical techniques, enabling a deepening of our understanding of early formation and modification processes on rocky bodies in our solar system.
Hyde, Brendt, "Early Meteoritic Records of Asteroidal Melt and Impact Environments" (2020). Electronic Thesis and Dissertation Repository. 6962.