Two unusual Type B refractory inclusions in the Ningqiang Carbonaceous Chondrite: Evidence for relicts, xenoliths and multi-heating

Two Type B refractory inclusions, consisting mainly of melilite, fassaite a nd spinel +/- anorthite, were found in the anomalous Ningqiang carbonaceous chondrite. The composition of melilite varies from Angstromk(4-15) near th e diopside rims to Angstromk(80-90) 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 inclusi ons were once molten. The Na2O content of melilite is positively correlated with the Angstromk content for Angstromk(<70), but the correlation becomes negative for more <Angstrom>kermanitic grains. These are the most Angstrom kermanitic 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 inclusio ns contain possible relict fassaite fragments. These fragments are embedded in gehlenitic melilite and have corroded outlines surrounded by highly TiO 2-enriched fringes, as distinguished from the fassaites intergrown with mel ilite 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 Angstromk content. We propose that these anorthite fra gments are xenoliths and were probably injected into the host while the lat ter was crystallizing. Palisades occur only in NQJ331, and are probably rel icts too. Distinctly low V2O3 concentrations of the spinels from the palisa de bodies and the presence of palisade bodies consisting of one or more cor roded 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 consisti ng of a grossite core and a spinel mantle enclosing laths of hibonite. The modal abundance and mineral chemistry of the spinel-hibonite spherule are d istinctly different from the host inclusion. We propose that the spinel-hib onite spherule crystallized from a SiO2-poor melt at a very high temperatur e and was injected into the Type B host before the latter was completely so lidified. This grossite-bearing spinel-hibonite spherule may be related to the spinel-hibonite spherules in CM2 chondrites. Copyright (C) 2000 Elsevie r Science Ltd.