Phenocryst-bulk rock composition relations of abyssal tholeiites and their petrogenetic significance

Tsugio Shibata

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25 Citations (Scopus)


In order to infer equilibrium phase relations of abyssal tholeiites, olivine, plagioclase, augite, and pigeonite tholeiites from the ocean floor are plotted in terms of the CIPW norm proportions in the tetrahedron olivine-plagioclase-diopside-quartz. The phase relations of abyssal tholeiites have a general similarity in form to those of the experimentally studied relevant systems. Experimental studies on natural basalts allow the pressure of crystallization for abyssal tholeiitic magmas to be evaluated approximately. It appears that the pressure at which the phenocryst-stage crystallization of abyssal tholeiites takes place is as high as 2 or 3 kbar, provided that abyssal tholeiitic magmas are 'dry'. Abyssal tholeiites could be derived from liquids that are in equilibrium with Ca-poor pyroxene in the pressure range of about 5-8 kbar. Major element chemistry of abyssal tholeiites is incompatible with the view that these tholeiitic basalts are derived from picritic magma by olivine fractionation.

Original languageEnglish
Pages (from-to)1407-1417
Number of pages11
JournalGeochimica et Cosmochimica Acta
Issue number11
Publication statusPublished - Nov 1976

ASJC Scopus subject areas

  • Geochemistry and Petrology


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