Volcanism in the Sumisu Rift, I. Major element, volatile, and stable isotope geochemistry
A bimodal volcanic suite with K Ar 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. Volatile concentrations distinguish Sumisu Rift basalts from island arc basalts and MORB. H 2 O 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 2 O 3 depletion and delays the initiation of Fe 2 O 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 2 O 3 (tot) and TiO 2 enrichment. Thus, major element trends are more similar to those of MORB than arcs. H 2 O, CO 2 and S are undersaturated relative to pure phase solubility curves, indicating exsolution of an H 2 O-rich mixed gas phase. HighH 2 O/S, highδD, and low (MORB-like)δ 34 S ratios are considered primary and distinctive of the back-arc basin setting.