Subduction zone geochemical characteristics in ocean ridge basalts from the southern Chile Ridge: Implications of modern ridge subduction systems for the Archean

Abstract

The southern portion of the Chile Ridge is one of few sites where active subduction of a spreading center and its consequences for ridge axis magmatism can be investigated. New major element, trace element, and isotopic data for lavas recovered from the ridge axis between 43 °S and 46 °20′S of the southern Chile Ridge have revealed a suite of mid-ocean ridge basalts which possess typical major element variations, but diverse and sometimes unusual trace element characteristics. For several Chile Ridge lavas, key trace element ratios, such as Rb Cs , Ce Pb , Nb U , La Ta , Hf Th and Nb La , extend well outside the fields for normal MORB or ocean island basalts and have values more commonly associated with arc volcanics and continental crust. This hybrid mixture between MORB-like major elements and arc-like trace element signatures has only previously been seen in back-arc basins, and is considered to primarily reflect contamination of a depleted MORB source mantle with slab-derived components. Along the southern Chile Ridge, contamination with slab components is occurring in advance of the subduction zone, possibly as a result of slab break-up or shearing in conjunction with subduction of young, buoyant lithosphere, and subsequent entrainment of these slab components into the sub-ridge mantle. Interestingly, many Archean greenstone basalts share the unusual hybrid MORB-arc geochemical characteristics found along the southern Chile Ridge. On the basis of theoretical modeling, it has been suggested that the mantle was hotter, plate motions were more rapid and ridge-trench interactions were more frequent during the Archean. Although use of geochemical signatures to discriminate tectonic setting must be approached with caution, the observed geochemical affinity of modern lavas from the southern Chile Ridge and some Archean greenstone lavas lends support to the idea that ridge subduction may have been an important mechanism in the formation of Archean greenstone basalts.