Investigations of trace-element and isotopic variability in mid-ocean ridge basalts

This dissertation consists of two geochemical investigations of mid-ocean ridge basalt (MORB) petrogenesis based on constraints derived from trace element and isotopic chemical data from two separate areas along the global mid-ocean ridge system.; Based on combined isotope and trace element geochemistry, investigations of lavas from the Chile Ridge reveal a petrogenetic history involving both recent and ancient recycling of oceanic lithosphere into the source of MORB. Basalts from the seven southernmost segments of the Chile Ridge (numbered S1–S7 stepping away from the trench) display large variations in Sr, Nd, Pb, and He isotope and trace element compositions. The uniformly MORB-like 3He/4He and modest variations in Pb, Sr, and Nd isotopes of segment S1 (nearest the trench) suggest recent (<20 Ma) introduction of a contaminant into its source. In contrast to segment 1, segment 3 lavas display a dramatic southward increase in enrichment, extending to highly radiogenic Pb and Sr isotopic compositions and the lowest 3He/ 4He yet measured in MORB. The segment S3 variations are most readily explained by ancient (∼2 Ga) recycling of terrigenous sediment and altered crust. The segment S4 isotopic variations are consistent with contamination by anciently recycled pelagic sediment and altered crust. Trace element variations in segment S5 lavas suggest an origin possibly related to contamination with altered oceanic crust alone.; Chapter 3 discusses the recent discovery of convergent-margin geochemical signatures along the SCR spreading center and their implications for the interpretation of ophiolite provenance. The location of the SCR near a ridge-trench-trench (RTT) triple junction suggests that the convergent-margin signatures may relate to processes associated with spreading center subduction. The possibility that ophiolite obduction may also occur near RTT triple junctions where a spreading center is being subducted implies that MORB with supra-subduction zone (SSZ) signatures may have been preferentially emplaced as ophiolite complexes in the past. The discovery of MORB with convergent margin-geochemical signatures near ophiolitic material with similar chemical systematics, points to a modern example of a tectonic setting in which ocean crust with anomalous convergent margin-like compositions is formed and may be emplaced as an ophiolite.; The second study of four basalts from the axis of the slow-spreading Mid-Atlantic Ridge presents the first radiometric ages from geologically-controlled lavas from a slow-spreading ridge. The most recently erupted lavas on the youngest-looking volcanic edifices in the well-studied MARK Area (Mid-Atlantic Ridge south of the Kane Fracture Zone) are 10–20 kyr old. (Abstract shortened by UMI.)...