Rare earth and trace element geochemistry of a fragment of jurassic seafloor,Point Sal,California |
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| Authors: | Martin Menzies Douglas Blanchard Joyce Brannon Randy Korotev |
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| Institution: | Department of Geology, University of California, Davis, CA 95616, U.S.A.;NASA Johnson Space Center, Houston, TX 77058, U.S.A.;Lockheed Electronics Corp., Houston, TX 77058, U.S.A.;Department of Chemistry, University of Wisconsin, Madison, WI53706, U.S.A. |
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| Abstract: | A suite of rocks from the Point Sal ophiolite, California, were analyzed for rare earth elements (REE), Sc, Co, Na2O, Cr, Zn and FeO. The lavas all have either flat or slightly light REE (LREE) depleted profiles relative to chondrites. The lavas contain smectite or greenschist facies mineralogy and some have radiogenically enriched ratios. This is interpreted as evidence of basaltseawater interaction (Hopsonet al., 1975; Davis and Lass, 1975). The smectite and zeolite bearing lavas that have been exposed to seawater for prolonged periods have anomalous Ce abundances. At higher grades of metamorphism, the lavas show no marked changes in light REE. The plutonic igneous rocks vary from early cumulus dunite to late stage, noncumulus diorite. All the plutonic rocks are light REE depleted with total REE abundance varying by a factor of 100 × between the dunites and diorites. Analyses of clinopyroxene and hornblende separates indicate that these two minerals strongly influence the REE characteristics of the early cumulates and late stage fractionates, respectively.In general, REE contents are: hornblende > clinopyroxene > plagioclase > orthopyroxene > olivine. Estimates of the REE compositions of parental lavas were obtained by calculating the REE contents of liquids in equilibrium with early cumulate clinopyroxenes. This reveals that the parent to the stratiform sequence was more depleted in light REE than the parent to the lava pile. |
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