| Subduction of mid-ocean ridge spreading centers is believed to have occurred frequently throughout tectonic history. Presently, there are two sites where spreading centers are undergoing subduction: the Southern Chile Ridge and the Woodlark Basin. This study examines the geochemistry of young lavas recovered from these two systems to investigate the possible effects of subduction on the mantle melting and source processes of mid-ocean ridge magmatic systems.; Major element glass data for normal and enriched mid-ocean ridge basalt samples collected from four ridge segments of the southern Chile Ridge between the Chiloe Fracture Zone and the Chile Margin Triple Junction indicate that the subridge mantle is heterogeneous at small spatial scales. The normal mid-ocean ridge basalts (N-MORB) have experienced variable extents of low pressure fractionation, but were generated by relatively uniform extents (F) and initial pressures (P{dollar}sb{lcub}rm o{rcub}){dollar} of melting of a heterogeneous depleted source. The enriched mid-ocean ridge basalts (E-MORB) were double-backtracked to 8 wt % MgO and a K/Ti ratio of 0.1 in order to assess melting conditions. Although the magnitudes of F and P{dollar}sb{lcub}rm o{rcub}{dollar} are model-dependent, the N-MORB and E-MORB were probably generated under similar melting conditions. These observations indicate that the spreading rate and mantle temperature, rather than ridge subduction, exert the primary control on the southern Chile Ridge melting process.; These lavas were also analyzed for volatile components (Cl, F, S, H{dollar}sb2{dollar}O, CO{dollar}sb2{dollar}) and oxygen isotopes to investigate the contribution of materials recycled at subduction zones to the petrogenesis of these lavas. Low Cl/K ratios were found in most of the N- and E-MORB, suggesting that pre-eruptive assimilation of seawater-altered crustal materials was minimal. Oxygen isotope ratios fall in a narrow range close to typical MORB ratios, consistent with a small volume of recycled material. Distinct volatile signatures are found in the E-MORB. These signatures reflect a combination of processes: metasomatism by Cl-rich aqueous fluids or hydrous siliceous melts, melting in the presence of fluorapatite, or amphibole and/or phlogopite, and shallow recycling of slab-derived materials. |
