O, S, Sr, and Pb isotope variations in volcanic rocks from the Northern Mariana Islands: implications for crustal recycling in intra-oceanic arcs

Abstract

Quaternary lavas from the Northern Mariana Islands have respective O- and S-isotope ranges ofδ 18O = +5.7 to +6.6 (‰ SMOW) andδ 34S = +2.0to+20.7 (‰ CDT). Chemically evolved andesites and dacites with meanδ 18O = +6.3 ± 0.2 are slightly enriched in 18O with respect to unfractionated basalts of<53%SiO 2 with meanδ 18O = +6.0 ± 0.1. This 18O enrichment can be explained in terms of differentiation of parental mafic magmas havingδ 18O values between +5.7 to +6.2‰ through closed-system crystal fractionation because the lavas from all nine islands of the arc define a coherentδ 18O SiO 2 trend. The S-isotope composition of oxidized magmas is not modified extensively through the degassing of SO 2; therefore, the meanδ 34S value of ca. +11‰ for the Mariana lavas is considered to be representative of their source region. The enrichment of 18O and 34S in Mariana Arc parental magmas relative to ocean floor basalts withδ 18O ca. + 5.7‰ andδ 34S = ca.0.3‰ is attributed to the recycling of 18O- and 34S-rich crustal components (sediment withδ 18O = ca. + 25‰ and seawater sulfate withδ 34S = 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.