Alkali-enriched rocks from the Central Eastern Pacific Ocean

Abstract

Rock samples collected from the Mathematician seamount chain (MSC) and from the flank of the East Pacific Rise (EPR) between 9° and 16°N include mugearite, trachyte and tholeiite. Mugearite occurs at a depth of 3500 m on the western flank of the East Pacific Rise, and is not associated with elevated volcanic features. Mineralogical and major-element chemistry suggest that the trachytes from the MSC are possibly formed as a result of various degrees of differentiation due to crystal-liquid fractionation of an alkali-enriched melt. The alkali elements and the Sr-isotopic data indicate a complex genetic history involving a two-stage (reflecting time differences) depletion model for the source material of the MSC. The trachytic rocks and the mugearite found in the central eastern Pacific Ocean resemble alkali island basalt series and are probably derived from similar alkalienriched or relatively undepleted mantle source layers, while the source of EPR tholeiite appears to lie in the depleted or alkali-poor mantle layers. In general, the geochemical variations of oceanic rocks suggest the possible presence of a chemically zoned mantle with systematically arranged alkali-enriched and alkali-poor layers, beneath the central eastern Pacific Ocean.