Geochemical mapping of the Mariana arc‐basin system: Implications for the nature and distribution of subduction components

Abstract

A new ICP‐MS database for glasses from the Mariana Trough, together with published and new ICP‐MS data from the Mariana arc, provides the basis for geochemical mapping of the Mariana arc‐basin system. The geochemical maps presented here are based on the graphic representation of spatial variations in geochemical proxies for the principal mantle and subduction components. The focus is on three elements with high and similar partition coefficients but different behavior in subduction systems, namely, Ba, Th, and Nb. Two elements with different partition coefficients, Ta and Yb, are used as normalizing factors. Ratio maps (Ta/Yb, Nb/Ta, Th/Ta, Ba/Ta, Ba/Th) provide the simplest petrogenetic insights, subduction zone addition maps based on deviations from a MORB array provide more quantitative insights, and component maps represent an attempt to isolate the different subduction components. The maps shown here indicate the presence of a variably depleted asthenosphere and three added components: a Nb‐Th‐Ba component, a Th‐Ba deep‐subduction component, and a Ba‐only shallow‐subduction component. The asthenosphere entering the system is enriched relative to N‐MORB and appears to be focused at three sites within the Mariana Trough. The Nb‐Th‐Ba component is present mainly in the north of the arc (the Northern Seamount province and northern Central Island Province), the northern edge of the Mariana Trough, and two locations within the Southern Seamount Province. It has a distinctively high Nb/Ta ratio and a moderate enrichment in Th and Ba relative to Nb. Its composition and distribution indicate that it may not be part of the present subduction system but instead originates in mantle lithosphere previously enriched above the subduction zone by addition of small‐degree, subduction‐modified mantle melts. The Th‐Ba component is present throughout the arc and, in minor amounts, in parts of the back‐arc basin. The Ba‐only component is mainly present in the central part of the arc and at the edges of the back‐arc basin. Overall, the geochemical maps provide a new perspective on the geochemical processes that accompany the evolution of an arc basin system from prerifting lithospheric enrichment, through arc‐rifting to arc volcanism and back‐arc spreading.