Fig. 7.

Photomicrographs of microstructures in the picrodolerite, with pairs of images photographed with plane polarised light and under crossed polars, showing the progressive change in olivine morphology. a, b Sample SC533, from 22.92 m stratigraphic height. Note the wide range of grain sizes and that the olivine grains commonly form loose clusters. Augite forms a single oikocryst in these two images. Scale bar is 200 µm long. c, d Sample S81/8, from 30.51 m stratigraphic height. In comparison with a and b, this sample contains very few apparently isolated grains, suggesting that most olivine forms loose clusters. Scale bar is 200 µm long. e, f Sample S81/24, from 56.57 m stratigraphic height. All olivine is found in clusters that are well-sintered with large areas of olivine–olivine grain boundary, no interstitial material and 120° triple junctions. Scale bar is 1 mm long. g, h S81/27, from 65.87 m stratigraphic height. Olivine is coarse-grained and forms well-sintered clusters with no interstitial material and 120° triple junctions. Note the prominent compositional zoning in the grains labelled 1 and 7, in comparison with grains 2, 3 and 4, which are enclosed by an augite oikocryst. Scale bar is 1 mm long