ABSTRACT

Enrichments in Sr, Ba, and Pb relative to the light rare-earth elements in island-arc basalts have been taken as evidence for the involvement of subducted, upper-crust-derived sediments, or altered oceanic crust, or sea-water, in the upper-mantle source regions of primary island-arc magmas. However, the Sr enrichments are unlikely to have been caused by addition of these ⁸⁷Sr-rich materials, because there is no positive correlation between the magnitude of the Sr anomalies and ⁸⁷Sr/⁸⁶Sr values in most island-arc rock suites. On the contrary, these parameters in some arcs are negatively correlated. Furthermore, Sr, Ba, and Pb anomalies are found in non-island-arc rocks, such as those from continental-flood lava sequences and other areas where subduction is unlikely to have taken place; and Sr and Nd isotopic data for both non-arc and island-arc rocks plot in similar parts of ε_Nd-versus-ε_Sr diagrams. These relationships, as well as the absence of marked Th and U enrichments in island-arc basalts, are generally consistent with primitive island-arc magmas having been contaminated by ⁸⁷Rb-poor (low ⁸⁷Sr/⁸⁶Sr) lower crust. Contamination by crust in the island-arcs themselves may be a more important process than addition of subducted material to the mantle wedge above downgoing slabs.