Helium isotopes in back-arc basins

Whereas the majority of the Manus Basin samples (particularly from the Manus Spreading Centre and the Extensional Transform Zone) are characterized by He/ He ratios greater than those of MORB, this is not the case for lavas from the Lau Basin or the North Fiji Basin. Indeed, a significant number of samples from both basins have 

MORB-like He/ He values (see references in Table 4). Specifically, lavas with He/ He values of8 l are erupted in the central Lau Basin as well as along all spreading centers of the North Fiji Basin. Therefore, in addition to a contribution from a deep mantle plume, the other principal source of helium in active back-arc basins is the same as that supplying mid-ocean ridges. This conclusion reinforces that derived from the majority of island arc studies—namely, that the mantle is the predominant source of helium in subduction zone environments with little or no contribution from the subducted slab. 

The mantle wedge is depleted in helium so that it cannot mask/dilute radiogenic helium derived from the subducted slab (subducted sediments or underlying oceanic basement). 

The mantle wedge is enriched in U- and Th-series isotopes derived from the subducted slab so that the wedge itself is characterized by He/ He ratios MORB. 

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