Ultramafic inclusions and megacrysts in olivine nephelinite, Aitutaki Island, Cook Islands

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Abstract Electron microprobe analyses of lherzolite and clinopyroxenite nodules and clinopyroxene megacrysts in 0.70 m.y. old olivine nephelinite of Aitutaki Island, Cook Islands, yielded the following: (1) Olivine, Fo90–92; orthopyroxene, Fs8–10Wo1–3; clinopyroxene, Fs46Wo41–50; and brown spinel, Sp75 in the lherzolite. All have compositions characteristic of lherzolites worldwide. (2) With 1 exception, textures are also characteristically lherzolitic; the excepted nodule is atypically poikiloblastic where orthopyroxene insets are surrounded by olivine. (3) The opx-cpx pairs in lherzolite form 2 groups of equilibrium temperatures, 774–834°C and 1040–1140°C; these correspond to different modal compositions where the higher T nodules have more olivine (pale green overall versus greenish-black for lower T nodules). (4) Orthopyroxene-spinel intergrowths in the lower T lherzolite nodules suggest a history involving inversion from garnet lherzolite during diapiric upwelling, although both modal groups equilibrated within a 25–70 km depth range. (5) Clinopyroxenites and cpx-megacrysts have compositions agreeable with a high pressure cognate origin. Some contain spongy textures, with nepheline in the vuggy areas, that probably developed from disequilibrium with the undersaturated host magma causing partial melting. The composition of 1 clinopyroxene megacryst, however, qualifies as an early phenocryst of the host; it, too, is spongy, containing nepheline and leucite, plus olivine of a composition like that in the host’s groundmass.