Geochemistry of Late Cenozoic basalts from the Crary Mountains: characterization of mantle sources in Marie Byrd Land, Antarctica

Late Cenozoic (9.34 to 0.04 Ma) alkaline basalts from the Crary Mountains, eastern Marie Byrd Land, Antarctica have low Sr-87/Sr-86 (0.70267-0.70283), moderately high Nd-143/Nd-144 (0.51186-0.51391) and high Pb-206/Pb-204 (19.9-20.9) ratios. The Crary Mountains basalts together with Hobbs Coast basalts from western Marie Byrd Land have the strongest HIMU signature (Pb-206/Pb-204 > 20.6) in the West Antarctic Rift System (WARS). There is no evidence of crustal contamination and their trace-element patterns are similar to HIMU basalts from oceanic islands. Higher concentrations of some large-ion lithophile (LIL) elements, particularly K, relative to oceanic HIMU, suggest melting of an enriched mantle. Models for small degrees of partial melting between 2% and 3% of a mantle peridotite containing 2.5% amphibole are consistent with trace-element variations in the basalts. The Crary data confirm the presence of a moderately depleted, lower-mu (mu = U-238/Pb-204) mantle component with a Pb-306/Pb-204 signature between 19.3 and 19.8 in the source regions of West Antarctic basalts. The lower-mu component, defined by the focus of West Antarctic data arrays on 2-D and 3-D isotope plots, is prevalent as a mixing end-member throughout the WARS, whereas the HIMU component, defined by Pb-206/Pb-204 ratios greater than 20.5, is present at only a few localities. The positive correlation of many highly incompatible trace-elements with Pb-206/Pb-204 ratios indicate that smaller-degree melts preferentially sample the HIMU component, whereas larger degree melts sample the lower-mu component. We also suggest, based on variations in elements moderately incompatible to highly compatible in the mantle phases phlogopite and amphibole, that the lower-mu component is more hydrous than the HIMU source. The lower-mu and HIMU components are also found in alkaline basalts from Tasmania and New Zealand which were adjacent to the Antarctica coast prior to the mid-Cretaceous fragmentation of the proto-Pacific margin of Gondwanaland. We propose that before continental breakup, a HIMU-type mantle plume was trapped and stored beneath a much more extensive pre-existing metasomatised layer within the Gondwanaland lithosphere. Source regions for all known extreme HIMU basalts in the continental borderlands of the southwest Pacific would be encompassed by a modest plume head, 600-800 km in diameter, emplaced around 100 Ma. We suggest that an earlier metasomatic event, possibly related to the Jurassic Bouvet-plume, enriched the originally depleted upper mantle in highly incompatible elements. A fertile lithosphere formed by plume-driven melts and fluids during the Jurassic would allow for the widespread development of the present-day isotopic signature of the lower-mu source. (C) 2000 Elsevier Science B.V. All rights reserved.