THE SHAPE OF OIKOCRYSTS IN SERIAL SECTION IN THE KEWEENAWAN GREENSTONE FLOOD-BASALT FLOW

The 500-m-thick Keweenawan Greenstone flood-basalt flow, Keweenaw Peninsula Michigan, has a striking ophitic texture with augite oikocrysts up to 5 cm in diameter enclosing 0.1 mm chadocrysts of plagioclase and olivine. According to the MELTS program, augite would not have started to crystallize in this basalt until it was 40% solid. If the oikocrysts grew unimpeded in the residual liquid, they would be expected to be spherical. However, if the crystal mush was undergoing compaction during solidification, the growth of an oikocryst could have been impeded if it were brought into close proximity with other oikocrysts. Their shapes might therefore have become elliptical. Oikocryst boundaries were digitized in scanned images of six oriented serial sections through a 1.2-cm-thick block of basalt from 10 m above the base of the flow where typical oikocryst diameter is approximately .5 cm. Using NIH Image, an ellipse was fitted to each oikocryst and these were then normalized and averaged to obtain a mean ellipse for the entire sample. The result shows that the mean shape of the oikocrysts is nearly spherical, with the major and minor axes differing by only 2%. They are slightly flattened in a near horizontal plane, which is consistent with compaction. Such a small amount of compaction may result from the relatively late stage at which augite crystallizes and the proximity of the sample to the base of the flow.