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Isotopic and chemical heterogeneity of the Hawaiian mantle plume : evaluating mantle geodynamics and characterization of the Loa geochemical trend

University of British Columbia

Oceanic island basalts provide the exceptional opportunity to study deep mantle geochemical reservoirs, mantle geodynamics and, for long-lived systems, the time evolution of their mantle sources. The Hawaiian-Emperor chain represents the geologic record of the long-lived (>81 Ma) and deeply sourced Hawaiian mantle plume. The geochemical record of the entire chain is now complete with analysis of Pb-Hf-Nd-Sr isotopes and elemental compositions of the Northwest Hawaiian Ridge (NWHR), which consists of ~51 volcanoes spanning ~42 Ma between the bend in the chain and the Hawaiian Islands. This segment of the chain previously represented a significant data gap where Hawaiian plume geochemistry changed markedly: only Kea compositions have been observed on Emperor Seamounts (>50 Ma), whereas the Hawaiian Islands (<6 Ma) present both Kea and Loa compositions. Statistical analysis of the new isotopic compositions of NWHR shield-stage basalts confirms the ephemeral presence of the Loa composition. The Hawaiian plume sampled only Kea-type material from the deep Pacific mantle during formation of the Emperor Seamounts and most of the oldest NWHR. Plume movement up the gently sloping edge of the large low-shear velocity province (LLSVP) resulted in entrainment of greater amounts of LLSVP-enriched material over time, which explains why the Hawaiian mantle plume strengthens. Geochemical investigation of NWHR postshield and rejuvenated basalts shows that the rejuvenated component is homogeneous and different from both that of postshield basalts and the depleted component sampled by the oldest Emperor Seamounts. The oldest Emperor Seamounts record interaction between the Hawaiian plume and a mid-ocean ridge. The high Hf isotope ratios of rejuvenated basalts require a source imprinted by ancient partial melting. The robustness of Pb isotope analyses is assessed by investigation of a potential contaminant, ferromanganese crusts, precipitated hydrogenetically on NWHR lavas. Lithium isotopes of Hawaiian Island basalts distinguish between Loa trend heterogeneities; the Loa trend represents singular components sampled for finite time periods. These conclusions demonstrate how integrated geochemical and geophysical studies of oceanic island basalts can further resolve mantle heterogeneity and challenge us to rethink models of how mantle plumes sample the lower mantle.

Text

2018-04-30

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/63408

Geological Sciences

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/