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

1982; Taylor & Francis; Volume: 25; Issue: 1 Linguagem: Inglês

10.1080/00288306.1982.10422505

1175-8791

R. V. Fodor, G. R. Bauer, Klaus Keil,

Crystal Structures and Properties

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.