Potential sources for arc magmas studies to distinguish and put limits on inputs include subducted midocean ridge basalt and from each of these sources in individual arcs. sediment, subarc mantle peridotire, and overlying Nevertheless, there is no consensus among lower 143Nd/144Nd and higher A1203/CaO , than lavas 1980], there is no positive correlation between from volcanoes 38•-41•S, which are sited on thinner crust. Paper number 4B5354. 0148-0227 / 86 / 004 B-5354 $ 05. O0 the magnitude of Sr, Ba and Pb abundance anomalies and 87Sr/86Sr or 207pb/204pb ratios in arc lavas. The absence of such correlation between Sr abundance anomalies and 87Sr/86Sr ratios led Arculus and Johnson [1981] to conclude that these geochemical characteristics do not result from "...upper-crust-derived sediment, subducted seawater, or seawater-altered basaltic layers .... " Recent models for arc magma genesis explain decoupling of isotope and abundance ratios, as well as the similar Sr and Nd isotope ratios in arc basalts and oceanic island basalts, by deriving arc basalts from OIB-source mantle in which a process, unique to subduction zones, creates high Ba/La and Sr/Nd ratios. Possible processes include partial melting of OIB mantle sources that contain a refractory REE and HFSE-retaining mineral stabilized only under subduction zone conditions (Fe-Ti oxides [Saunders et al., 1980] and perovskite [Morris and Hart, 1983]), and preferential leaching of Pb, U, Th, and alkali and alkaline earth elements from incompatible element enriched veins in the mantle by water released from the subducted crust [Arculus and 5963 5964 Hickey etal.: Multiple Sources for Basaltic Arc Rocks •,.,.-A RC BASALT 2O o •: MORB 2-incorporation of subducted sediment into the sources of arc magmas. Incorporation of a continent-derived component, i.e., pelagic sediments, in the source of arc lavas is consistent with decoupling between isotopic and trace element abundance ratios. Consider that Pb, U, Th, and alkali and alkaline earth elements are selectively removed from subducted crust either by partial melts that leave a residual light REE and HFSE-retaining mineral or by preferential incorporation into a volatilerich phase. For example, in the model of DePaolo 1983; Kay, 1984]; therefore incorporation of pelagi•4clay results in higher 87Sr/86Sr and pelagic sediments can also account for the lower 3Nd/144Nd. In contrast, an increase in enrichment of Cs in arc rocks. Based on these the proportion of metalliferous sediment •0[ considerations, we conclude, like Sun [1980], not result in high 207pb/204pb and 208pb/ Pb Cohen and O'Nions [1982], and White and Patchett because metalliferous sediments can have low [1984], that there is compelling evidence for the values for these ratios (Figure 2a).
