Magma genesis and mantle flow at a subducting slab edge: the South Sandwich arc-basin system
The intra-oceanic South Sandwich subduction system is distinctive in having a narrow slab with slab edges at its northern and southern ends. We present new geochemical data to investigate magma genesis beneath the parts of the arc and back-are segments that lie close to the two slab edges: Kemp and Nelson seamounts at the southern edge of the South Sandwich arc, and segments E1 and E2 in the south, plus segments E9 and E10 in the north, of the East Scotia Sea. In the arc, Kemp and Nelson seamounts exhibit enhanced subduction fluxes compared to the remainder of the arc. The southernmost (Nelson) has the isotope (low Nd and high Sr isotope ratios) and elemental (ultra-high Th and Ba and high Hf/Nd ratios) characteristics of a sediment melt, or supercritical aqueous fluid, component. The more northerly (Kemp) has the same characteristics as the remainder of the arc (high Nd and slightly raised Sr isotope ratios, high Nd/Hf ratios, high Ba/Th ratios), indicative of a fluid component derived mainly from subducted crust, but has a greater mass fraction of that component than the rest of the arc. In the back-are basin, the slab-edge segments are generally fed by more fertile mantle (E-MOR-B in all but E I) than the segments in the centre of the basin (N-MORB). At the edges, segments furthest from the trench (E2, E9) have small subduction components while those nearer to the trench (E1, E10) have larger subduction components and slightly more depleted mantle. We argue that several processes were important at the slab edges: roll-back of the slab, forcing sideways flow of relatively enriched mantle into the mantle wedge; convergence of the arc with the back-are spreading centre, imparting a greater subduction component into the back-arc lavas; and anomalous heating of the subducting slab, increasing subduction fluxes and the contribution of sediment melts to the subduction component.