O, S, Sr, and Pb isotope variations in volcanic rocks from the Northern Mariana Islands: implications for crustal recycling in intra-oceanic arcs |
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| Authors: | Jon D Woodhead Russell S Harmon Donald G Fraser |
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| Abstract: | Quaternary lavas from the Northern Mariana Islands have respective O- and S-isotope ranges ofδ18O = +5.7 to +6.6 (‰ SMOW) andδ34S = +2.0to+20.7 (‰ CDT). Chemically evolved andesites and dacites with meanδ18O = +6.3 ± 0.2 are slightly enriched in18O with respect to unfractionated basalts of<53%SiO2 with meanδ18O = +6.0 ± 0.1. This18O enrichment can be explained in terms of differentiation of parental mafic magmas havingδ18O values between +5.7 to +6.2‰ through closed-system crystal fractionation because the lavas from all nine islands of the arc define a coherentδ18OSiO2 trend. The S-isotope composition of oxidized magmas is not modified extensively through the degassing of SO2; therefore, the meanδ34S value of ca. +11‰ for the Mariana lavas is considered to be representative of their source region.The enrichment of18O and34S in Mariana Arc parental magmas relative to ocean floor basalts withδ18O ca. + 5.7‰ andδ34S = ca.0.3‰ is attributed to the recycling of18O- and34S-rich crustal components (sediment withδ18O = ca. + 25‰ and seawater sulfate withδ34S = ca. +20‰ into the upper mantle source region for these arc magmas. This interpretation is consistent with enrichments of radiogenic Sr and Pb in the same lavas relative to ocean-floor basalts erupted either side of the arc, which are presumed to share a common upper mantle source. This enrichment is considered to reflect the mixing of two components, one having a typical upper mantle composition and the other having a more radiogenic character similar to that of western Pacific pelagic sediments. |
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