TWO UNUSUAL TYPE B REFRACTORY INCLUSIONS IN THE NINGQIANG CARBONACEOUS CHONDRITE: - EVIDENCE FOR RELICTS, XENOLITHS AND MULTI-HEATING

Abstract

Two Type B refractory inclusions, consisting mainly of melilite, fassaite and spinel +/- anorthite, were found in the anomalous Ningqiang carbonaceous chondrite. The composition of melilite varies from #9k4-15 near the diopside rims to #9k80-90 in the centers of these inclusions. In addition, melilite exhibits intergrowths with fassaite and/or anorthite in the centers of the inclusions. These observations suggest that both inclusions were once molten. The Na2O content of melilite is positively correlated with the #9k content for #9k<70, but the correlation becomes negative for more #9kermanitic grains. These are the most #9kermanitic compositions reported in Type B refractory inclusions, and they could be related to a secondary heating of the inclusions.Beyond crystallization from melts, the Ningqiang Type B refractory inclusions contain possible relict fassaite fragments. These fragments are embedded in gehlenitic melilite and have corroded outlines surrounded by highly TiO2-enriched fringes, as distinguished from the fassaites intergrown with melilite in the centers of the inclusions. In inclusion NQJ331, most grains of anorthite occur as irregular coarse-grained fragments, distinct from those intergrown with melilite. Toward these anorthite fragments, melilite shows a steep decrease in #9k content. We propose that these anorthite fragments are xenoliths and were probably injected into the host while the latter was crystallizing. Palisades occur only in NQJ331, and are probably relicts too. Distinctly low V2O3 concentrations of the spinels from the palisade bodies and the presence of palisade bodies consisting of one or more corroded crystals of fassaite +/- anorthite are new lines of the evidence for a relict origin of palisades.The other Type B refractory inclusion, NQJ354, contains a spherule consisting of a grossite core and a spinel mantle enclosing laths of hibonite. The modal abundance and mineral chemistry of the spinel-hibonite spherule are distinctly different from the host inclusion. We propose that the spinel-hibonite spherule crystallized from a SiO2-poor melt at a very high temperature and was injected into the Type B host before the latter was completely solidified. This grossite-bearing spinel-hibonite spherule may be related to the spinel-hibonite spherules in CM2 chondrites.