Resolving mantle components in oceanic lavas from segment E2 of the East Scotia back-arc ridge, South Sandwich Islands

The East Scotia Ridge, situated in the South Atlantic, is the back-arc spreading centre to the intra-oceanic South Sandwich arc. Samples from the ridge show a wide diversity in erupted magma compositions. Segment E2, in the northern part of the ridge, has an axial topographic high, which contrasts with the rift-like topography common to most of the ridge. Lava compositions in the segment have been modelled by mixing of magmas derived from normal mid-ocean ridge basalt (N-MORB)-like mantle, a mantle plume component similar in composition to that sampled by Bouvet Island and mantle modified by addition of components from the subducting slab. The ‘Bouvet’-like plume signature has higher 87Sr/86Sr, 206Pb/204Pb, Nb/Yb, and lower 143Nd/144Nd and 4He/3He, than the local upper mantle. It can be traced geochemically from the Bouvet Island hot spot to segment E2, via the South American-Antarctic Ridge, which connects the Bouvet triple junction to the South Sandwich subduction system. Four samples dredged from segment E2 have 4He/3He ratios of 85 000–90 200 (8.5–8.0 R/RA, where) R/RA is the 4He/3He ratio normalized to air) and three wax core samples taken from the segment axis have values of 104 300, 101 560 and 176 620 (6.9, 7.1 and 4.1 R/RA). These latter data are similar to values from the South American-Antarctic Ridge which have no discernable plume input. Whilst the dredge samples have a measurably lower 4He/3He ratio than the South American-Antarctic Ridge and samples from the segment axis, these He isotope data contrast with a dominant plume signature recorded by other petrogenetic tracers. This is interpreted to be due to re-melting of an entrained plume component, with an inherent low He concentration, incorporated into the E2 mantle. Helium depletion from the plume component can be seen to be a consequence of mantle processing and does not imply shallow-level degassing prior to entrainment within the upper-mantle-melting zone. As a consequence, He is characterized in the back-arc by values more similar to the upper mantle, whereas lithophile tracers are more influenced by the plume component.

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