2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 76-10
Presentation Time: 4:45 PM

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


COUSENS, Brian, Ottawa-Carleton Geoscience Centre, Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada, WEIS, Dominique, Earth, Ocean and Atmospheric Sciences, University of British Columbia, Pacific Centre for Isotopic and Geochemical Research, 2020-2207 Main Mall, Vancouver, BC V6T 1Z4, Canada, CONSTANTIN, Marc, Département de géologie et de génie géologique, Université Laval, 1065 ave de la Médecine, Québec, QC G1V 0A6, Canada and SCOTT, Steve, Department of Earth Sciences, University of Toronto, 22 Russell St., Toronto, ON M5S 3B1, Canada, brian.cousens@carleton.ca

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.