| Abstract: | Abstract— Y74123 is an olivine-rich, relatively unshocked ureilite and contains more interstitial pigeonitic materials than do ureilites which have been reported previously. Thus, Y74123 is especially suited for detailed study of the interstitial materials. We have studied these materials by optical microscope, electron microprobe, scanning electron microscope, high resolution transmission electron microscope (TEM) and analytical TEM to gain a better understanding of their nature and origin. Y790981, with shock partial melts, has also been examined by the same techniques. Bulk chemical compositions of the interstitial materials in Y74123 are pyroxene-like and have higher CaO and Al2O3 contents than the large pigeonite and olivine core. Interstitial materials at olivine-pigeonite grain boundaries are richer in CaO and Al2O3 than those at olivine-olivine grain boundaries. TEM observations of the interstitial material of Y74123 show that it consists of alternating pigeonite-augite lamellae more than 3.5 μm thick on (001). This texture suggests that the rim material had already crystallized before the parent body breakup. The shock-produced glassy veins in Y790981 cut through the rim materials. These observations are consistent with the idea that the interstitial materials in this ureilite are a mixture of residual liquids of high Ca melts and shock-produced partial melts of olivine and pigeonite. This mixture accumulated along the grain boundaries and some of it is trapped within grains. |
