Geochemistry and mineralogy of selected Atlantic Ocean basalts

Doctoral Thesis


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University of Cape Town

Bulk rock compositional variations in lavas from four localities in the Atlantic Ocean are evaluated quantitatively in terms of fractional crystallisation and partial melting models. Samples studied are from the Mid-Atlantic Ridge at 36°49 'N (FAMOUS), from the Islas Orcadas fracture zone on the Southwest Indian Ocean Ridge, from the Conrad fracture zone on the America-Antarctica Ridge and from Bouvet Island, situated at the southernmost tip of the Mid- Atlantic Ridge and in the vicinity of both the fracture zones mentioned above. Bulk rock major and trace element variations in selected basalts from the FAMOUS area, in conjunction with a detailed study of the chemistry of phenocryst minerals and associated melt inclusions are used to place constraints on the genetic relationships between the various lava types. The distribution of NiO in olivine and Cr-spinel phenocrysts distinguishes the picritic basalts, plagioclase phyric basalts and plagioclase-pyroxene basalts from the olivine basalts. The NiO content of these phenocrysts in the former three basalt types is low, or a given Mg Mg+Fe²⁺ atomic ratio of the mineral, relative to the phenocrysts in the olivine basalts. Consideration of the Zr/Nb ratio of the lavas similarly distinguishes the olivine basalts from the plagioclase phyric and plagioclase- pyroxene basalts but, in addition, distinguishes the picritic basalts from the former three basalt types.

Bibliography: p. 238-268.