| The sizes, temperatures, and geochemical structures of the Hawaiian mantle plume UC Berkeley Summary The Hawaii Scientific Drilling Project recovered 2.85 km of core from the Mauna Kea volcano. The lavas show systematic stratigraphic variation of isotopic ratios. These variations can be mapped into radial variations in the mantle plume magma source. Interpretations must incorporate models for the relationships between the plume thermal structure, upwelling velocity structure, and melting processes. Models of Ribe and Christensen (1999) and Hauri et al. (1994) are used. Due to the ca. 80 km thickness of the lithosphere under Hawaii, only the hottest, central portion of the plume melts, and hence the lava sequence records only the structure of the plume core region. At the average depth of melting (120 km), the melting region is roughly circular and 100 km wide, whereas the thermal anomaly that represents the plume is about 400 km wide. At deep levels, before the plume starts to spread beneath the lithosphere, the thermal anomaly may be 250-300 km wide. Isotopic data on He, Sr, Nd, Pb, Hf, and Os (among other elements) have been collected and are mostly published (Eisele et al., 2003; Blichert-Toft et al., 2003; Kurz et al., 2004; Bryce et al., 2004). Within the plume melting region there are strong gradients in 3He and 208Pb, weaker gradients in Hf and Nd isotopes, and very weak gradients in Os, Sr, and 206Pb isotopes. The 3He anomaly has a width of only about 25 km, 10% of the thermal radius of the plume, which suggests that Hawaiian 3He is coming from the base of the mantle (the core or a dense layer mantling the core), and that most of the lower mantle has R/Ra ≈ 6-8. Hawaiian Sr, Nd, Os, Hf isotopes show much weaker stratification in the lower mantle, suggesting that these isotopic signatures have a different origin than that of helium. The tentative conslusion is that most of the lower mantle under Hawaii is not enriched in 3He/4He; but instead is similar to the upper mantle. Only the base of the lower mantle is enriched in 3He/4He. The distinct isotopic signatures of the other elements are more broadly representative of the lower mantle. References [1] Ribe N, U Christensen, EPSL 171,
517 (1999) |