Helium-3 and deuterium in back-arc basalts: Lau Basin and the Mariana Trough

Abstract

Samples of fresh basalt glass from the Mariana Trough and the Lau Basin were analyzed for their isotopic composition of water and helium in order to identify the sources of the volatiles in back-arc basin basalts. In the Mariana Trough basalts, the concentration (0.64–2.1 wt.%) and D/H ratio ( δD = −46 to −32‰) of the water provide important evidence for a water-rich component from the subducting lithosphere. Extrapolation to infinite water content gives an end-member D/H ratio of δD = −25‰. 3He/ 4He ratios are, in general, similar to MORB values ( 8 R A ) and indicate that Mariana Trough basalts represent a mixture of a MORB-type mantle and an alkali and water-rich component from the descending slab. In contrast, the Lau Basin produces both hydrous (1.3 wt.% H 2O) and relatively anhydrous (0.12–0.35 wt.%) basalts. The D/H ratio ( δD = −43‰) in the hydrous Lau Basin basalt resembles those of Mariana Trough lavas. The low water contents (0.12–0.35 wt.%) and MORB-like D/H ratios ( δD = −63 to −70‰) in three of five Lau Basin samples show that the water-rich component, observed in all Mariana Trough lavas, is absent in some Lau Basin lavas and not essential for the production of back-arc basalts. The high 3He/ 4He ratio of Rochambeau Bank in the northern Lau Basin ( 11 R A ), confirms an earlier analysis by Lupton and Craig and provides evidence for an enriched mantle source region beneath the Lau Basin which is perhaps related to the high- 3He Samoan “hotspot”.