THE SOURCES OF ISLAND ARCS AS INDICATED BY Nd AND Sr ISOTOPIC STUDIES

Abstmct: Island arc lavas from New Britain and the Marianas have 143 Nd/144 Nd similar to other oceanic basalts and distinctly different from continental flood basalts and thus appear to be derived from a high Sm/Nd, light-REE-depleted reservoir. Consideration of both Nd and Sr isotopes suggests seawater involvement in the generation of some island arc lavas and thus indicates that they may be derived from altered subducted oceanic crust. Other island arc lavas show no evidence of seawater involvement and may be derived from mantle reservoirs with affinities to the sources of ocean island basalts. Andesite and rhyolite from an Andean volcano reflect assimilation of old continental crust. Nd and Sr in basaltic and ultrapotassic continental rocks indicate that some mafic magmas in continental regions may be derived from old low-Sm/Nd reservoirs or are heavily contaminated with old continental crustal material. Fish debris from the ocean floor provides an estimate of 143 Nd/144 Nd in seawater and indicates that light-REE in the marine environment are derived mainly from contments. Basalts erupted above sea level in oceanic and continental areas are isotopically distinct from those erupted on the ocean floor, suggesting a relationship between parental reservoirs and hydrostatic head. This paper presents Nd and Sr isotopic data on lavas from oceanic island arcs which were obtained to (I) characterize the source of island arc magmas, (2) determine if these magmas are derived from mantle reservoirs which may be associated with either the sources of oceanic basalts or the sources of continental basalts, and (3) relate these observations to the role of island arcs in the growth of continental crust. DePaolo and Wasserburg [ 1976a,b I (referred to as DPW, 1976a,b) showed tliat continental flood basalts are derived from reservoirs which have approximately chondritic Sm/Nd ratios. Mid-ocean ridge (MOR) tholeiitic basalts appear to be derived from ancient reservoirs (>I AE old) which are distinct from those of the continental basalts and characterized by higher Sm/Nd [see also Richard, Shimizu and Allegre, 19761. Basalts from oceanic islands have 14 3 Nd/144 Nd intermediate between MOR basalts and continental flood basalts and may represent a third type of reservoir from which basalts are derived. 0 'Nions, Hamilton and Evensen [ 19771 expanded the data on ocean island basalts and confirmed that they form a coherent group based on 143 Ndfl 44 Nd. DPW [1976a,cl and 0 'Nions et al. [ 1977al also showed that for young volcanic rocks with 8 7 Sr/8 6 Sr less than 0. 705 there is a strong correlation between initial 1 4 3 Ndfl 4 4 Nd and 8 7 SrfS 6 Sr. We have measured samples of basalt, andesite and dacite from two oceanic volcanic arcs to compare their Nd and Sr isotopic compositions with continental and other oceanic volcanic rocks. Nd isotope data provides a means of evaluating the sources of island arc magmas which is not sensitive to seawater contamination which has complicated interpretation of Sr isotope data [cf. Meijer, 19761. We have concentrated primarily on rocks from volcanic arcs built upon oceanic crust to avoid problems of crustal contamination. In particular, the Marianas Arc is far from possible interferences from cqntinental material. New Britain is the only arc segment built upon oceanic crust which contains the entire range of magma compositions from basalt to rhyolite, and has been extensively studied by Johnson [ 197 61 . Samples from a volcano in the Andes of South America, a province which has been associated with the addition of island arc materials to continents, have been included in order to compare possible arc materials in the continental margin environment. A sample of gabbro from a major linear batholith belt, which may have formed in a setting