Abstract
A continuous chain of volcanic islands and seamounts extends on the Nazca plate eastward from the Easter and Sala y Gomez islands to the islands of San Felix and San Ambrosio, off the coast of Chile (Easter line), in a latitudinal band about 200 km wide. A similar chain of seamounts and islands probably extends west of Easter Island on the Pacific plate up to Pitcairn Island, which was recently active. The hypothesis that Easter Island marks the site of a fixed mantle hot spot responsible for these chains is tested here. Easter Island consists predominantly of basalts but also contains terms of a differentiation series including trachytes and rhyolites. The Easter Island basalts have higher contents of Fe, Ti, Ba, and Zr and higher (super 87) Sr/ (super 86) Sr and (super 206) Pb/ (super 204) Pb ratios than ocean ridge basalts; they also have light rare earth enriched patterns. Thus they are similar to basalts which elsewhere (i.e., Iceland and the Azores) have been ascribed to mantle plumes. Rocks from Sala y Gomez, San Felix, and San Ambrosio and various seamounts along the Easter line have similar petrochemistry. But consideration of the spreading history of the Nazca plate (with the fossil Galapagos ridge as an active spreading center up to about 10 m.y. ago and spreading occurring subsequently from the east Pacific rise) excludes the possibility that the Easter line chain could have been formed by one mantle hot spot. At least two hot spots had to be active, one of them located at the intersection of the fossil ridge with the Easter line. These hot spots must have moved with respect to each other. Recent (< 2 m.y. old) activity occurred in at least five areas along the Easter line, namely, at its intersection with the east Pacific rise, at Easter Island, at Sala y Gomez, in the San Felix-San Ambrosio area, and at Pitcairn. We conclude that volcanism of the "mantle plume type" occurred intermittently at various sites along the Easter line. Mantle activity along a line parallel to the direction of motion of the Nazca plate could explain the features of the Easter line. This mantle "hot line" may correspond to upwelling limbs or spouts of mantle convective rolls which according to Richter (1973) develop below fast moving plates with their axes parallel to plate motion. A major transverse morphological, seismic, volcanological, and lithological discontinuity existing at the Chile trench and in western South America along the landward extension of the Easter line might be caused either by the extension of the mantle hot line across a plate boundary and beneath South America or, more likely, by subduction of the anomalous lithosphere produced along the Easter line.
