Age, geology, geophysics, and geochemistry of Mahukona Volcano, Hawai`i
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The size, shape, and magmatic history of the most recently discovered shield volcano in the Hawaiian Islands, Mahukona, have been controversial. Mahukona corresponds to what was thought to be a gap in the paired sequence (Loa and Kea trends) of younger Hawaiian volcanoes (< 4 Ma). Here, we present the results of marine expeditions to Mahukona where new bathymetry, sidescan sonar, gravity data, and lava samples were collected to address these controversies. Modeling of bathymetric and gravity data indicate that Mahukona is one of the smallest Hawaiian volcanoes (similar to 6,000 km(3)) and that its magmatic system was not focused in a long-lived central reservoir like most other Hawaiian volcanoes. This lack of a long-lived magmatic reservoir is reflected by the absence of a central residual gravity high and the random distribution of cones on Mahukona Volcano. Our reconstructed subsidence history for Mahukona suggests it grew to at least similar to 270 m below sea level but probably did not form an island. New Ar-40-Ar-39 plateau ages range from 350 to 654 ka providing temporal constraints for Mahukona’s post-shield and shield stages of volcanism, which ended prematurely. Mahukona post-shield lavas have high He-3/He-4 ratios (16-21 Ra), which have not been observed in post-shield lavas from other Hawaiian volcanoes. Lava compositions range widely at Mahukona, including Pb isotopic values that straddle the boundary between Kea and Loa sequences of volcanoes. The compositional diversity of Mahukona lavas may be related to its relatively small size (less extensive melting) and the absence of a central magma reservoir where magmas would have been homogenized.