III. Weathered And Metamorphosed Basalts. Altered basalts from Swallow Bank, an abyssal hill in the NE Atlantic, and from a nearby seamount

Abstract
Three hundred and fifty-seven specimens dredged from Swallow Bank in 1958 are vesicular hypocrystalline porphyritic basalts. Chemical, thin section and X-ray studies were made. The least altered specimens gave an analysis close to the tholeiitic ocean floor basalt type. The glassy selvage of pillows altered sequentially to palagonite, fibro-palagonite and montmorillonite. Within the flows the mesostasis and pyroxene phenocrysts have been replaced by chlorophaeite, fibro-chlorophaeite and obscure chlorites, the calcic cores of plagioclase phenocrysts by a mesh of orthoclase plates and their sodic rims by (?) montmorillonite. Similar, but more extensive, mineralogical changes have affected the lavas from the seamount. Chemically, at Swallow Bank, the alteration of the flow interior involved increases in K 2 O (from 1.0 to 3.5g/100g), in H 2 O + (from 1.2 to 5.2 %) and in oxidation ratio, Fe 2 O 3 :Fe20 3 + FeO (from 71 to 98%) with concomitant loss in CaO (from 11.1 to 2.3 %) and MgO (from 4.7 to 1.8%). In these rocks radioactivity increased, seismic velocity decreased and intensity of magnetization remained substantially unchanged. Similar altered rocks could be widespread in layer 2 beneath the ocean.

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