The phenocryst compositions were determined by electron microprobe for the 34 igneous units. Table T3 provides average compositions for plagioclase, clinopyroxene, olivine, and chromian spinel. The full data set is presented in "Appendix B" of the "Explanatory Notes" chapter.
Plagioclase phenocrysts in the upper series lavas show large compositional ranges with average anorthite contents between An58 and An86. Clinopyroxene coexisting with plagioclase has average Mg numbers between 0.70 and 0.81, although there is a poor correlation between the coexisting plagioclase and clinopyroxene compositions. Fresh olivine phenocrysts were found in only five samples and range in composition from Fo61 to Fo69. The groundmass olivine in Unit I-1 of Hole SEG61B is the most iron-rich olivine analyzed (Fo54).
Phenocrysts of the lower series olivine phyric lavas typically reach more primitive compositions than the upper series lavas. Plagioclase varies between An76 and An89 and clinopyroxene between Mg number 0.77 and 0.85. Fresh olivine was found in only two samples with composition ranging from Fo68 to Fo75. Spinel has Cr/(Cr + Al + Fe3+) ratios between 0.42 and 0.59, with the most Cr-rich spinels occurring as inclusions in olivine.