P. Fryer

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

[1]xa0A 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.

Volcanism in the Sumisu Rift, I. Major element, volatile, and stable isotope geochemistry

A bimodal volcanic suite with KAr ages of 0.05–1.40 Ma was collected from the Sumisu Rift using alvin. These rocks are contemporaneous with island arc tholeiite lavas of the Izu-Ogasawara arc 20 km to the east, and provide a present day example of volcanism associated with arc rifting and back-arc basin initiation. Major element geochemistry of the basalts is most similar to that of basalts found in other, more mature back-arc basins, which indicates that back-arc basins need not begin their magmatic evolution with lavas bearing strong arc signatures. n nVolatile concentrations distinguish Sumisu Rift basalts from island arc basalts and MORB. H_2O contents, which are at least four times greater than in MORB, suppress plagioclase crystallization. This suppression results in a more mafic fractionating assemblage, which prevents Al_2O_3 depletion and delays the initiation of Fe_2O_3_((tot)) and TiO_2 enrichment. However, unlike arc basalts,Fe^(3+)/ΣFe ratios are only slightly higher than in MORB and are insufficient to cause magnetite saturation early enough to suppress Fe_2O_3_(tot) and TiO_2 enrichment. Thus, major element trends are more similar to those of MORB than arcs. n nH_2O, CO_2 and S are undersaturated relative to pure phase solubility curves, indicating exsolution of an H_2O-rich mixed gas phase. High H_2O/S, high δD, and low (MORB-like) δ^(34)S ratios are considered primary and distinctive of the back-arc basin setting.