Earth and Planetary Science Letters, 83 (1987) 39-52 39 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands [31 O, S, Sr, and Pb isotope variations in volcanic rocks from the Northern Mariana Islands: implications for crustal recycling in intra-oceanic arcs Jon D. Woodhead 1, Russell S. Harmon 2 and Donald G. Fraser 1 1 Department of Earth Sciences, University of Oxford, Oxford OXI 3PR England (U. K ) 2 Department of Geological Sciences, Southern Methodist University, Dallas, TX 75275 (U.S.A.) Received March 6 1986; revised version accepted February 13 1987 Quaternary lavas from the Northern Mariana Islands have respective O- and S-isotope ranges of 8180 = + 5.7 to +6.6 (oh, SMOW) and 8~S = +2.0 to +20.7 (%0 CDT). Chemically evolved andesites and dacites with mean 8180= +6.3_+0.2 are slightly enriched in 1SO with respect to unfractionated basalts of < 53% SiO2 with mean 81gO = + 6.0 _+0.1. This lSO enrichment can be explained in terms of differentiation of parental mafic magmas having 8180 values between +5.7 to +6.2c~ through closed-system crystal fractionation because the lavas from all nine islands of the arc define a coherent 81SO-SIO2 trend. The S-isotope composition of oxidized magmas is not modified extensively through the degassing of SO2; therefore, the mean ~34S value of ca. + 11700 for the Mariana lavas is considered to be representative of their source region. The enrichment of 180 and 34S in Manana Arc parental magmas relative to ocean floor basalts with 8180 ca. + 5.7% and 834S = ca.0.3%o is attributed to the recycling of 1sO- and 34S-rich crustal components (sediment with 81~O = ca. + 25~, and seawater sulfate with 834S = ca.+ 207oo 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. I. lnU'oduction An understanding of the various processes con- tributing to magma genesis in island arcs is im- portant because the main mass transfer between crust and mantle occurs in convergent plate margin settings. For example, quantifying the extent to which material of crustal derivation is recycled into the upper mantle, either via sediment subduc- tion or within the hydrothermally altered oceanic lithosphere of the descending slab is central to current attempts to interpret the material balances that have determined the Earth's chemical evolu- tion and isotopic history. There can be little doubt that the bulk mass contribution to arc magmas comes from the upper mantle, but trace element and radiogenic isotope studies imply that compo- nents derived from the subducting slab can mod- ify the composition of the mantle source region for arc magmas [1-4] and may be involved in the initiation of partial melting [5]. Recent t°Be [6] 0012-821x/877503.50 o 1987 Elsevier Science Publishers B.V. and Sr-, Nd-, and Pb-isotope [7-12] studies sug- gest that calc-alkaline lavas in some arcs contain a component of recycled sediment, whereas others do not. The Mariana Island Arc in the western Pacific is an ideal location in which to look for the chemical and isotopic signatures of recycled sedi- ment because geophysical studies have shown that the arc is sited entirely on oceanic crust [13], thereby eliminating the possibility of contamina- tion by overlying continental crust during magma ascent. An early Sr- and Pb-isotope study by Meijer [14] and a more recent O- and Sr-isotope study by Ito and Stern [15], both based on small numbers of samples from the Marina Arc, did not provide any firm evidence for sediment subduc- tion. The latter work concluded that magmagen- esis in the Mariana Arc occurred in a mantle source region that was indistinguishable from that of oceanic island basalts and that very little, if any, sediment was involved in magma generation.