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Pyroxene oikocrysts in troctolitic cumulates — evidence for supercooled crystallisation and postcumulus modification

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Abstract

Studies of pyroxene oikocrysts in plagioclase-olivine cumulates crystallised from high alumina olivine tholeiite magmas in layered intrusions inferred to represent periodically replenished magma chambers show that small plagioclases enclosed within the oikocrysts differ significantly from plagioclases in the host rock. Plagioclases within oikocrysts are very elongate with a high surface area, show extensive mutual attachment at high angles resembling skeletal growth, and tend to be randomly orientated. Some are rounded and embayed, are reversely zoned and more sodic by An 5–10 than some of the plagioclase in the host rock. Plagioclase crystal centres tend to be closer spaced in the oikocrysts than in the host rock. In contrast, plagioclases in the host rock are discrete, coarser grained, less elongate, strongly aligned, and locally polygonised.

Plagioclases in the oikocrysts are inferred to be self-nucleated clusters resulting from supercooled crystallisation at up to 25–40° C below equilibrium crystallisation temperatures. The oikocrysts crystallised early and are cumulus, and at least some crystallised in place. They are significant because they probably preserved the early condition of the crystal mush. Comparison of plagioclases within the oikocrysts and in the host rocks allows the textural evolution during solidification to be inferred. Features of supercooled plagioclases in the oikocrysts are generally absent beyond the oikocrysts because they were largely overprinted by postcumulus processes involving grainsize coarsening due to selective resorption, and the development of lamination. Supercooling may thus have been more common than is recognised but the inherent disequilibrium of supercooled products made them susceptible to postcumulus overprint except where preserved in early oikocrysts. Thus, much of the cumulus-looking plagioclase in cumulates may not be truly cumulus, but really the result of substantial postcumulus overprint or ‘magmatic diagenesis’.

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Mathison, C.I. Pyroxene oikocrysts in troctolitic cumulates — evidence for supercooled crystallisation and postcumulus modification. Contr. Mineral. and Petrol. 97, 228–236 (1987). https://doi.org/10.1007/BF00371242

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