NASA NTRS1993
Ureilites contain a LREE-enriched component whose origin and identity are unknown, but which may have been introduced into 4.55 Ga olivine + pyroxene assemblages at various times in the period 4.55 - 3.74 Ga. This component is volumetrically minor, inhomogeneously distributed, and can be removed with concentrated HNO3, which suggests that it may be contained in minor, interstitial phases not previously recognized in ureilites. There is evidence, however, that not all of this component is leachable. Whole rock samples of Kenna, Novo Urei, and ALHA77257 form a Nd-143/Nd-144-Sm-147/Nd-144 line with a slope corresponding to an age of 3.74 Ga. Although this line could be a mixing line, with one end-member being the LREE-enriched component and the other being a 4.55 Ga olivine + pyroxene assemblage, the observation that a pyroxene separate from Kenna plots on the line suggests that the LREE-enriched component at least partially equilibrated with the olivine and pyroxene and that the line is an isochron. We performed a leaching experiment on Kenna, using various acids and concentrations, in an attempt to isolate the LREE-enriched component and differentiate between possible hosts for it, and to determine whether all of it is leachable. Our results suggest that most of the LREE-enriched component is surface-sited, rather than contained in a discrete mineral. However, some of it is not readily leachable because it equilibrated with Kenna olivine + pyroxene at 3.79 plus or minus 0.05 Ga.