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Abyssal tholeiites from the Oceanographer Fracture Zone

I. Petrology and fractionation

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Abstract

Fresh basalts from the Oceanographer Fracture Zone are petrographically and chemically similar to typical abyssal tholeiites, but are somewhat enriched in large ion lithophile elements, with consistent differences among separate dredge hauls. Olivine compositions are in equilibrium with host basalt for reasonable K D values, but some plagioclases are anomalously calcic (e.g., a rock highly differentiated in Mg/Mg+Fe bears some plagioclase of An86). Ti/Al in clinopyroxene is approximately constant at 1/6, but Ti+Al abundance decreases in the sequence basalt groundmass cpx, basalt phenocryst+microphenocryst cpx, gabbro cpx (samples from adjacent dredges), an effect that may be related to decrease in cooling rate. Least-squares calculations indicate that 45% of magma with the composition of the more primitive sample must be removed as plagioclase, clinopyroxene and olivine (in the ratio 4.4∶2.7∶1, respectively) to obtain the composition of the more differentiated sample. Both samples have excess plagioclase on the liquidus, however, which should not be the case for the differentiated composition if it is produced by fractionation of all three phases. The excess of plagioclase and the anomalous plagioclase compositions indicate that the samples cannot be related solely by fractional crystallization. Additional processes such as magma mixing or plagioclase accumulation (or both) must have effected these differences.

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Authors and Affiliations

  1. Department of Earth Sciences, Okayama University, Tsushima, 700, Okayama, Japan

    Tsugio Shibata

  2. Department of Geological Sciences, State University of New York at Albany, 12222, Albany, New York, USA

    Stephen E. DeLong

  3. Hoffman Laboratory, Harvard University, 02138, Cambridge, Massachusetts, USA

    David Walker

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  1. Tsugio Shibata
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  2. Stephen E. DeLong
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  3. David Walker
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Shibata, T., DeLong, S.E. & Walker, D. Abyssal tholeiites from the Oceanographer Fracture Zone. Contr. Mineral. and Petrol. 70, 89–102 (1979). https://doi.org/10.1007/BF00371875

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