OCEAN-RIDGE BASALTS WITH CONVERGENT-MARGIN GEOCHEMICAL AFFINITIES FROM THE CHILE RIDGE

COLLISIONS between active spreading centres and subduction zones have occurred frequently throughout Earth history(1). Of the few sites of ridge subduction active today, the southern Chile Ridge has the simplest tectonic history(2). We report here that basalts recovered from the Chile Ridge adjacent to the Chile Trench have geochemical characteristics unlike those of mid-ocean-ridge basalts sampled elsewhere, and show affinities with are volcanics. The observed chemical variations are consistent with contamination of depleted sub-oceanic mantle by marine sediments and altered oceanic crust, which may have been derived from the adjacent subduction zone. Although some ocean islands show evidence of crustal recycllng(3-5), the Chile Ridge lavas are the first ocean-ridge basalts to exhibit such extreme geochemical characteristics, and may thus offer the opportunity to study the development of one type of mantle heterogeneity as it occurs. The discovery of ocean-ridge basalts with convergent-margin chemical characteristics suggests that caution is warranted in using trace-element systematics to identify the provenance (mid-ocean ridge or supra-subduction zone) of ophiolites(6,7).