CompuServe Messages

Hi Doug & Joesph, I have a meteorite book that offers more on the Ureilites origin and may complicate our theories even greater <G>. From the Meteorites and the Early Solar System : Ureilites. They are carbonaceous olivine- pyroxene achondrites whose igneous history is complex (Berkley and Jones 1982; Goodrich et al. 1987). A two stage model for the formation of the ureilites has been proposed (Goodrich et al. 1987): chondrite material underwent a large degree of partial melting (10 – 25%) leaving olivine-pigeonite- plagioclase(?) residues. Later melting ( <10% ) of mafic cumulates from the earlier magmas produced the ureilites parent magmas with high Ca/Al ratios and negative Eu anomalies. Carbon becomes reducing at low pressures and, when the C- bearing ureilite magmas were intruded into the crust, they equilibrated at varying depths and were reduced to diffrent extents, losing diffrent amounts of Fe metal ranging from 20% to 27%. The cumulus urelite minerals then crystallized from the reduced magmas. That sugests the urelites did not experience a core formation event. The urelites were finally intruded by less than 1% of an interstitial liquid, unrelated to the original magmas from which the cumulates formed. Goodrich et al. (1987) suggests that a large parent body ( > 470 km diameter ) is required to explain the ureilites. ——-> Get this next bit! However new O isotope data show that ureilites are igneous but not related to each other by differentiation of a common source, which means they cannot have been formed by large-scale igneous activity on a single parent body (Clayton et al. 1987). The model of Takeda involving partial melting of carbonaceous material therefore deserves serious consideration. I thought that to be fasinating. Sorry for the long message. –AL