The Petrogenetic Evolution of Lavas from Easter Island and Neighbouring Seamounts, Near-ridge Hotspot Volcanoes in theSE Pacific

Abstract

Major and trace element, mineralogical and petrographical data are presented for submarine and subaerial lavas from Easter Island and two neighbouring seamounts. The samples can be divided into three groups based on their major element composition and incompatible element enrichment, typified by their (La/Sm)N ratios. Tholeiitic samples with (La/Sm)N of ∼1.2 are comparable with lavas constituting three young volcanic fields closer to the spreading axis. The other two lava series from Easter Island and the two seamounts are transitional to slightly alkaline, one having an intermediate enrichment with (La/Sm)N of 1.5–2, the other being even more enriched [(La/Sm)N ∼2.3]. Large phenocrysts and xenocrysts of olivine and plagioclase in the submarine lavas indicate a crustal reservoir with crystallizing magma. Compared with the relatively mafic submarine samples, the more fractionated lavas on Easter Island suggest the presence of another, more shallow magma chamber. The extreme differentiation of rhyolites and trachytes probably occurred as a result of decreasing magma supply during waning phases of volcano activity. Each volcano of the Easter Hotspot probably evolved through at least two stages. A tholeiitic stage appears to form large volcanoes at depths <1000 m and is succeeded by a transitional group of lavas which can be observed on Easter Island. A third stage with the most enriched lavas may form small eruptive centres on the flanks of older volcanoes or as flank cones on the seafloor. The degree of partial melting decreases from the tholeiitic to the most enriched alkaline series. The occurrence of different stages of volcanism at the Easter Hotspot not only resembles the magmatism of other Pacific hotspots such as Hawaii but also the volcanism of non-plume near-ridge seamounts at the East Pacific Rise. We suggest that the development of each magmatic stage depends on the age of the overlying plate, which determines (1) the amount of depleted mid-ocean ridge material mixed into the magma source and (2) the degree of partial melting.