Evidence for an O-18-depleted mantle plume from contrasting O-18/O-16 ratios of back-arc lavas from the Manus Basin and Mariana Trough

Back-are basin glasses from the Mariana Trough and Manus Basin display cont rasting oxygen isotope characteristics that require differences in their ma ntle sources. In both basins, the lavas that are most depleted in high fiel d strength elements possess delta(18)O values of around 6.0 parts per thous and, that are elevated with respect to mid-ocean ridge basalt (MORB). This characteristic is consistent with a mantle source that has been infiltrated by fluids released from subducted oceanic lithosphere. The nature of the m ore fertile mantle component differs between the two basins, The lowest del ta(18)O values in the Mariana Trough are similar to MORB and suggest that t he ambient upper mantle interacts with a subduction-modified mantle to prod uce Mariana Trough back-arc basin basalts. Oxygen isotope ratios of basalti c glasses from the Manus Basin display a negative correlation with helium i sotope ratios. The subduction-modified component is associated with He-3/He -4 ratios typical of the upper mantle, Glasses with He-3/He-4 ratios greate r than average MORB, characteristic of a deep mantle plume, have delta(18)O values that are lower than expected for upper mantle melts. This suggests that the Manus Basin plume taps a reservoir that is O-18-depleted relative to the source of MORB. Two mechanisms are identified that might generate th is reservoir. Deep recycling of oceanic crust that has been hydrothermally altered at high temperature may provide large O-18-depleted domains in the deep mantle. Alternatively, we propose that interactions between silicate a nd iron alloy during the segregation of the Earth's core may have the poten tial to generate such reservoirs. Resolution between these mechanisms:requi res further experimental investigation of oxygen partitioning between silic ates and iron alloy. Each of these mechanisms has distinct implications for the origins and dynamics of the Manus Basin plume. (C) 2000 Elsevier Scien ce; B.V, All rights reserved.