Todd Anthony Bianco

Geochemical variations at intraplate hot spots caused by variable melting of a veined mantle plume

Three-dimensional geodynamic models of plume-lithosphere interaction were used to explore the causes of spatial patterns of magmatic compositions at intraplate hot spots. The models couple mantle flow, heat transfer, and the melting of multiple components present in the mantle as small blobs or veins with different solidi and composition. Predicted magma compositions evolve from having a strong signature from the deepest-melting component in the early stages of volcanism to a strong signature from the shallowest-melting component in the later stages. This compositional trend arises by progressive melt extraction of the different components and a horizontal displacement of their melting zones due to shear flow associated with plate motion. When three or more components are present, the composition of a volcano evolves along arrays in isotope space that trend toward mixed compositions of the components rather than the components themselves. Models explain the average Pb isotope trends in the Hawaiian Scientific Drilling Program core at Mauna Kea volcano. Observed scatter about the average trends and the distinction between the Kea and Loa subchains are explained by spatial variability in the relative proportions of the components in the mantle. Monte Carlo simulations show that linear Pb isotope arrays are unlikely to originate from nonsystematic, binary mixing if the scale of the magma capture zone is much larger than the scale of isotopic heterogeneity. However, systematic sampling by progressive melt extraction naturally generates such linear arrays if the capture zone is large compared to the scale of heterogeneity.

Widespread Secondary Volcanism Near Northern Hawaiian Islands

Hot spot theory provides a key framework for understanding the motion of the tectonic plates, mantle convection and composition, and magma genesis. The age-progressive volcanism that constructs many chains of islands throughout the worlds ocean basins is essential to hot spot theory. In contrast, secondary volcanism, which follows the main edifice building stage of volcanism in many chains including the Hawaii, Samoa, Canary, Mauritius, and Kerguelen islands, is not predicted by hot spot theory. Hawaiian secondary volcanism occurs hundreds of kilometers away from, and more than 1 million years after, the end of the main shield volcanism, which has generated more than 99% of the volume of the volcanos mass [Macdonald et al., 1983; Ozawa et al., 2005]. Diamond Head, in Honolulu, is the first and classic example of secondary volcanism.