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Basalts from Leg 6 of the Deep-Sea Drilling Project |
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| Authors: | RIDLEY W IAN; RHODES J M; REID ARCH M; JAKES P; SHIH C; BASS M N |
| Institution: | Lunar Science Institute Houston, Texas 77058 1 Lockheed Electronics Corporation Houston, Texas 77058 2 Johnson Space Center Houston, Texas 77058 * Present address: Lamont-Doherty Geological Observatory, Palisades, New York 10964. |
| Abstract: | Basalt was successfully cored at Site 54 in the Parace VelaBasin of the Philippine Sea, and at Site 57 on the CarolineRidge as part of Leg 6 of the Deep Sea Drilling Project. Site54 basalts are altered, but selected major and trace elements,particularly REE allow their characterization as high-aluminaolivine tholeiites, with flat REE patterns, no Eu anomalies,and low dispersed trace-element contents. Basalt from Site 57is distinctly different, with higher TiO2, P2O5, Fe/Mg ratio,and dispersed trace elements, low Ni, Mg, and a strongly fractionatedREE pattern. Derivation by fractionation of transitional basaltis suggested. Varying degrees of alteration of Site 54 basalts has causeddepletion in MgO and addition of Rb, Sr, Ba, Ka2O, and Na2O.Mineralogically these changes have resulted in alteration ofolivine and pyroxene, but plagioclase and iron-titanium oxidesremain 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, andshow erratic decrease in A12O3 and TiO2 with increasing ironcontent. In 57–2, pyroxenes are more magnesian than thosein site 54 basalts, and show an increase in A12O3 and TiO2 withiron-enrichment. The anomalous behaviour of Al2O3 and TiO2 insome clinopyroxenes, and the close textural relationship ofpyroxene and magnesian olivine in 57–2 basalt, suggestthese minerals are xenocrystic, and were incorporated duringmagma ascent. Spinels of picotite composition in early-formed,and possibly xenocrystic, olivine, and plagioclase crystallizedprior to magma eruption, and may have begun crystallizationwithin the mantle. Site 54 basalts add further confirmation that inter-are basaltsclosely resemble ocean ridge tholeiites, although there is apaucity of data from inter-are environments. Site 57 basaltis chemically and mineralogically distinct from ocean ridgebasalts and may be related to similar volcanics erupted on theCaroline Islands. |
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