Geochemical Constraints for Mechanisms of Planetary Differentiation and Volatile Depletion
The evolution of the terrestrial planets involved a range of complex processes, including accretion, core formation, post-core formation accretion, mantle differentiation and volatile depletion. The earliest processes of accretion and core formation have largely been overprinted on Earth and Mars, but can be investigated using geochemical measurements of extraterrestrial materials. Highly siderophile elements (HSE; Os, Ir, Ru, Rh, Pt, Pd, Re, Au) preferentially partition into metal phases and are therefore powerful tracers for examining mechanisms of core formation on partially differentiated bodies, and differentiation and post-core formation late accretion on fully differentiated planetesimals. Chapter 2 examines the partially-melted, primitive acapulcoite and lodranite meteorites for insight into metal segregation and metal-sulfide partitioning. This work examines the effects of sulfur on HSE partitioning during the earliest stages of core formation, and allows identification of samples that may have derived near the nascent core of the acapulcoite-lodranite parent body. Chapter 3 consists of detailed measurements of the HSE and Os isotopes in eucrite meteorites, revealing new insight into metal-silicate and differentiation of the Asteroid-4 Vesta. These data are used to identify pristine eucrite samples that may represent the first natural examples of metal-silicate signatures of primary planetary differentiation in the Solar System. The fourth chapter transitions to volatile depletion, and investigates the potential for volatile loss using a model of magma ocean differentiation and constraints from zinc abundance and isotope data in lunar samples. The models constructed demonstrate that the lunar zinc signature, which reflects wholesale volatile depletion from the Moon, can be explained by surface volatile depletion and subsequent homogenization in a magma ocean. The continuum from core formation, to metal-silicate differentiation and late accretion, to volatile loss provides an overview of planet formation through detailed measurements and analysis of these fundamental mechanisms that occurred during terrestrial planet evolution in the Solar System.