Trace element analysis of ureilite meteorites and implications for their petrogenesis.

Ureilite meteorites are the second largest class of achondrite meteorites. They are perhaps the most enigmatic of all classes of meteorites and the dilemma over how they were formed has perplexed the meteoritic community for over a decade. This research was undertaken to illuminate the details of the petrogenesis of ureilites through the collection of trace element data. The results presented here indicate that many of the petrogenesis hypotheses promoted are no longer tenable and that the available data on ureilites are not sufficient to unravel the mystery of their origin. This dissertation discusses the data collected using two techniques: neutron activation analysis and inductively coupled plasma-mass spectrometry. Specific procedures required to study the ureilites (due to low concentrations of some trace elements) are delineated. Results are presented for the following elements: Ca, Co, Zn, Ga, Cu, Cs, Rb, Sr, Mo, Y, Ba, REE, Hf, W, Re and Ir. The results lead to the conclusion that the ureilites must be considered a mixture of materials--an original ultramafic rock formed by melting of processed source material combined with carbonaceous material added after the ultramafic formation. This conclusion explains the trace element chemical signatures and is supported by the age information obtained from Sm-Nd and Rb-Sr isotopic systems