IGNEOUS INCLUSIONS FROM ORDINARY CHONDRITES - HIGH-TEMPERATURE CUMULATES AND A SHOCK MELT

We report microprobe, instrumental neutron activation analysis, and ra diochemical neutron activation analysis data for three large igneous i nclusions in the Yamato (Y-)75097, Y-793241, and Y-794046 ordinary cho ndrites. The inclusions in the first two chondrites are troctolitic cu mulates that have undergone appreciable reactions with their hosts eit her during emplacement and/or cooling. Olivine-spinel Fe-Mg exchange p airs in these two inclusions record equilibration temperatures of abou t 710 degrees C, and these temperatures are similar to those exhibited by mineral pairs in the Y-75097 and Y-793241 hosts. The inclusion in Y-794046 is texturally unique, consisting of fine-grained, randomly di stributed olivines, coarse (similar to 2 mm) fascicular pyroxene laths , and angular pockets of maskelynite/plagioclase feldspar. The phase c ompositions are readily interpreted as having resulted from extremely rapid, essentially isochemical cooling to temperatures <1000 degrees C of a melt with an initial temperature > 1670 degrees C. We suggest th at this igneous inclusion formed in situ by shock.