Major and trace element geochemistry of basalts from the Explorer area, Northeast Pacific Ocean
Creators: name: Cousens, Brian Lloyd
Publisher: name: University of British Columbia
Year: 2010

description: Fifty fragments of young, fresh basalts from the Explorer Ridge, Paul Revere Ridge (Fracture Zone), Dellwood Knolls, and the J. Tuzo Wilson Knolls have been analysed for 12 major and minor elements, as well as 11 trace elements, by X-ray fluorescence spectrometry. Rare earth element concentrations in 25 of the samples have been determined by instrumental neutron activation, and Sr⁸⁷/Sr⁸⁶ ratios have been obtained for 11 of the basalts. The Explorer Ridge basalts have major element compositions similar to most mid-ocean ridge basalts (MORB), and can be classified as ferrobasalts, similar to those of the southern Juan de Fuca Ridge. The incompatible minor and trace elements K, Ti, Rb, Zr, and Nb are weakly to strongly enriched in the Explorer samples, with respect to MORB, part of which is the result of crystal fractionation. The observed trace element and light rare earth element (LREE) enrichment of many of the samples, particularly those from Explorer Deep, suggest that a weak hotspot may exist beneath the Explorer Deep. The adjacent ridge segments, Explorer Rift and the Southern Explorer Ridge, are erupting basalts both enriched and depleted in incompatible elements, which could be an indicator of a chemically heterogenous mantle source, or may be the result of intermittent injection of enriched magmas from the postulated hotspot beneath Explorer Deep into areas producing normal MORB. The enriched basalts do not have significantly different Sr⁸⁷/Sr⁸⁶ ratios from the depleted basalts. All the samples fall within the range of values typical for Juan de Fuca and Gorda Ridge basalts, and East Pacific Rise tholeiites in general. Thus, although the source areas for the 2 basalt types may differ chemically, they are similar radiogenically, unlike-other hypothetically plume-influenced areas such as the Mid-Atlantic Ridge at 45°N and the FAMOUS area. The basalts from the northwest and southeast Dellwood Knolls appear to be related by crystal fractionation, based on major element analysis. However, the very different REE patterns and Sr⁸⁷/Sr⁸⁶ ratios exhibited by the two knolls suggest that they have different mantle sources, one typically depleted (northwest knoll) and one chemically and radiogenically enriched (southeast knoll). In terms of their major and trace element chemistry, the J. Tuzo Wilson Knolls basalts are typical of late-stage volcanism on ocean islands associated with mantle plumes. The hawaiites strongly resemble alkali basalts dredged from several seamounts in the Pratt-Welker Chain, which are co-latitudinal with the J. Tuzo Wilson Knolls on a small circle about the Pacific-Hotspot pole of rotation. Geochronological evidence questions the hypothesis that the mantle plume responsible for Pratt-Welker volcanism is also the source for the J. Tuzo Wilson basalts. The existence of a second mantle plume, 300 km southeast of the first, would explain minor chemical and physiographical differences between the Knolls and the other Pratt-Welker seamounts, as well as the evidence for two phases of volcanism on the southeastern seamounts of the chain. A second plume also explains the coeval volcanism of Bowie Seamount and the J. Tuzo Wilson Knolls. Recent geophysical evidence suggests that the J. Tuzo Wilson Knolls are also part of the Explorer-Dellwood spreading system. Although the JTW basalts are plume-type basalts chemically, the situation appears to be somewhat analagous to other ridge segments where plumes are coincident with the ridge itself.