Two Forms Of Secondary Hawaiian Volcanism

Abstract

This thesis addresses causes of two forms of secondary Hawaiian volcanism: rejuvenated onshore eruptions and offshore Hawaiian Arch flows. It is proposed that secondary volcanism is generated as a direct consequence of lithospheric flexural uplift that surrounds new volcanic shields as they grow. This uplift causes decompression of the underlying asthenosphere, which is assumed to be chemically and isotopically heterogeneous, near its solidus, and derived from the Hawaiian mantle plume. Uplift is modeled as the axisymmetric response of an elastic plate to a (volcanic) point load that grows linearly in time. To model flow in the asthenosphere, the rate of flexure of the lithosphere is taken as the upper boundary condition on an isoviscous, incompressible, fluid half-space. The first feature of secondary volcanism this model explains is the observed spatial gap between secondary volcanism and active shields. Best agreement is found with the majority of the observed spatial gaps with a lithosphere of effective elastic thickness Te=25-35 km. Secondly, this work demonstrates that the flexural model can produce observed crustal production if some magma focusing toward individual eruption sites occurs from the mantle over an area two to ten times the eruption area. The third feature this model addresses is that secondary lavas are isotopically distinct from shield lavas. In this model, melting the same two-component mantle forms the secondary and shield lavas, but the components are sampled by melting at rates that differ between the locations as predictable functions of depth. Flexural decompression produces melts that preferentially sample the mantle component that begins melting shallowest and which is associated with Sr and Nd isotope ratios most like those of secondary lavas. Melting in the center of a mantle plume is assumed to generate shield volcanism and is predicted to preferentially sample the component that begins melting deepest which is associated with Sr and Nd isotope ratios more similar to shield lavas. Models therefore successfully predict the observed mean difference in 87Sr/86Sr and 143Nd/144Nd compositions between the secondary and shield lavas. The fourth feature addressed is that secondary lavas are alkalic and shield lavas are dominantly tholeiitic. To explain this difference, the mean extent of partial melting is computed, and it is found that a model plume composed mostly of depleted peridotite (90%) and some pyroxenite (10%) will yield a lower extent of melting for secondary lavas than shield lavas. This particular model assumes lithospheric thicknesses (90-100 km) and plume potential temperatures (mean of 1550 °C) that are consistent with independent studies of the Hawaiian hotspot. Thus, asthenospheric melting by flexural decompression is a viable mechanism of intraplate volcanism, which can explain many general characteristics of secondary Hawaiian volcanism.