THE HIMU CONNECTION IN ENRICHED MID-OCEAN RIDGE BASALTS FROM EXPLORER RIDGE, NORTHEAST PACIFIC OCEAN

The radiogenic isotope characteristics of mid-ocean ridge basalts (MORB) yield information on mantle geochemical heterogeneity. Some ridge segments include MORB that are enriched in incompatible elements (E-MORB) that have distinct isotopic compositions compared to normal N-MORB but are far from a mantle plume, showing that the upper mantle also includes enriched components. Southern Explorer Ridge (SER), at the north end of the Juan de Fuca ridge system, includes abundant E-MORB based on trace element studies, for which scant isotopic data exist. Here we present new geochemical and Pb-Sr-Nd-Hf isotopic data from glass and whole-rock samples from SER to identify the composition of the Explorer enriched mantle component(s). The SER basalts range from N- to E-MORB, with a corresponding increase in K/Na, K/Ti, Ta/La and La/Sm. E-MORB are most common in the neovolcanic zone of the shallowest, central section of the SER (Magic Mountain hydrothermal site) as well as just off-axis, thus E-MORB are able to pool beneath the ridge, fractionate, and erupt largely without mixing with N-MORB magmas that are also erupted nearby. La/Sm and ⁸⁷Sr/⁸⁶Sr roughly decrease from north to south along the SER. Lavas with the greatest light-REE enrichment have the highest ⁸⁷Sr/⁸⁶Sr and Pb isotope ratios and amongst the lowest ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf, indicating that the E-MORB source has a time-integrated history of incompatible element enrichment. While Sr and Nd isotopic ratios differ slightly between N- and E-MORB, Hf isotope ratios are very low and Pb isotope ratios are very high compared to most MORB. Low Hf and radiogenic Pb isotopic compositions are characteristic of HIMU-type ocean island basalts. E-MORB from the northern Juan de Fuca ridge (Cousens et al., 1995; Gill et al., pers.comm.) and SER share these geochemical characteristics, such that MORB from Endeavour Segment north to SER define a HIMU mantle province. North of Explorer Ridge, alkaline lavas related to the Pratt-Welker hotspot (Bowie Seamount, Tuzo Wilson Volcanic Field) are also HIMU-like but follow subtly different isotopic trends compared to SER/northern Juan de Fuca E-MORB. At SER, the HIMU component is likely not derived from an underlying plume, but is dispersed in the shallow mantle beneath a rapidly migrating, reorganizing spreading centre.