Solar nebula evolution and planetary differentiation

With the advance of next generation mass spectrometers, isotope variations of less than 50 ppm can be resolved between the Earth and various bulk meteorites. We present new Ba and Nd isotopic compositions for bulk chondrites and a Calcium Aluminum Inclusion. We show that all meteorites exhibit differences from Earth in 135 Ba/ 136 Ba and 142 Nd/ 144 Nd. By correlating these variations with other known isotopic anomalies a nucleosynthetic origin for these variations can be established. Incomplete mixing of fresh supernova material in the solar nebula is the best explanation for all bulk isotopic variations. Thus the Earth does not have the same isotopic composition for heavy refractory elements as carbonaceous chondrites. This cautions the use of chondritic reference values for examining early differentiation processes on Earth. The p-process Sm isotopes are also heterogeneous in the Solar System and we revise the initial 146 Sm/ 144 Sm ratio of the Solar System showing that uncertainties in this ratio can account for variations seen in 142 Nd/ 144 Nd in the Earth. Isotopes can also be used to study planetary differentiation. We measure the Sm and Nd isotopic composition of a suite of lunar soils to complete an isotopic mass balance model for lunar differentiation. Our results show that the lunar magma ocean was in fact at least 1000 km deep, larger than previous estimates but more consistent with lunar formation models. We establish that when measuring sub 50 ppm variations in isotopic ratios the method of mass fractionation correction becomes crucial. We show that for Ba isotopes a Rayleigh law best corrects for mass fractionation while the common exponential law is adequate for measuring Nd isotopes. The results presented in this thesis have implications for models of Solar System formation as well as thermal aspects of planetary differentiation.