Basalts from Leg 6 of the Deep-Sea Drilling Project

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Abstract
Basalt was successfully cored at Site 54 in the Parace Vela Basin of the Philippine Sea, and at Site 57 on the Caroline Ridge as part of Leg 6 of the Deep Sea Drilling Project. Site 54 basalts are altered, but selected major and trace elements, particularly REE allow their characterization as high-alumina olivine tholeiites, with flat REE patterns, no Eu anomalies, and low dispersed trace-element contents. Basalt from Site 57 is distinctly different, with higher TiO2, P2O5, Fe/Mg ratio, and dispersed trace elements, low Ni, Mg, and a strongly fractionated REE pattern. Derivation by fractionation of transitional basalt is suggested. Varying degrees of alteration of Site 54 basalts has caused depletion in MgO and addition of Rb, Sr, Ba, Ka2O, and Na2O. Mineralogically these changes have resulted in alteration of olivine and pyroxene, but plagioclase and iron-titanium oxides remain unaffected. Microprobe data are presented for olivine, clinopyroxene, plagioclase, Fe-Ti oxides, and chrome-spinel in all three basalts. In 54–4, 54–8 pyroxenes are zoned from salite to ferroaugite, and show erratic decrease in A12O3 and TiO2 with increasing iron content. In 57–2, pyroxenes are more magnesian than those in site 54 basalts, and show an increase in A12O3 and TiO2 with iron-enrichment. The anomalous behaviour of Al2O3 and TiO2 in some clinopyroxenes, and the close textural relationship of pyroxene and magnesian olivine in 57–2 basalt, suggest these minerals are xenocrystic, and were incorporated during magma ascent. Spinels of picotite composition in early-formed, and possibly xenocrystic, olivine, and plagioclase crystallized prior to magma eruption, and may have begun crystallization within the mantle. Site 54 basalts add further confirmation that inter-are basalts closely resemble ocean ridge tholeiites, although there is a paucity of data from inter-are environments. Site 57 basalt is chemically and mineralogically distinct from ocean ridge basalts and may be related to similar volcanics erupted on the Caroline Islands.

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