Other
Scientific paper
May 1987
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987e%26psl..83...39w&link_type=abstract
Earth and Planetary Science Letters, Volume 83, Issue 1-4, p. 39-52.
Other
39
Scientific paper
Quaternary lavas from the Northern Mariana Islands have respective O- and S-isotope ranges of δ18O = +5.7 to +6.6 (‰ SMOW) and δ34S = +2.0 to +20.7 (‰ CDT). Chemically evolved andesites and dacites with mean δ18O = +6.3 +/- 0.2 are slightly enriched in 18O with respect to unfractionated basalts of <53% SiO2 with mean δ18O = +6.0 +/- 0.1. This 18O enrichment can be explained in terms of differentiation of parental mafic magmas having δ18O values between +5.7 to +6.2‰ through closed-system crystal fractionation because the lavas from all nine islands of the arc define a coherent δ18O-SiO2 trend. The S-isotope composition of oxidized magmas is not modified extensively through the degassing of SO2; therefore, the mean δ34S 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 with δ18O ca. + 5.7‰ and δ34S = ca.0.3‰ is attributed to the recycling of 18O- and 34S-rich crustal components (sediment with δ18O = ca. + 25‰ and seawater sulfate with δ34S = 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.
Fraser Donald G.
Harmon Russell S.
Woodhead Jon D.
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