Basalts from the deep ocean floor

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Abstract
Summary: Basalts dredged from the floor of the deep ocean show general tholeiitic affinities. Some samples are rich in aluminium and of very similar composition to the Warner high-alumina basalt from California. Both olivine tholeiite and high-alumina basalt have been found in the form of glass in dredgings from the Mid-Atlantic Ridge, indicating that liquid magmas of both compositions have been erupted on to the sea bed in this area. In explanation of this association a tentative hypothesis of fractional melting of hydrated upper mantle material is proposed. It is suggested that under the mid-ocean ridges the ‘basaltic fraction’ of the mantle is locally in the form of an amphibole. Fractional incongruent melting of this amphibole appears to be a possible explanation of the range of composition encountered in dredged glass samples.

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