Geochronology, geochemistry and geophysics of Mahukona Volcano, Hawai`i

Mahukona is an extinct submarine volcano that fills a gap in the Loa-trend of paired Hawaiian volcanoes between Hualalai and Kaho`olawe. A new marine survey of the seamount was undertaken in an attempt to resolve the location of the volcano’s summit. The multibeam bathymetry showed no clear summit. The gravity data reveals a central oval-shaped residual gravity anomaly with a maximum density 85 kg/m3 greater than the surrounding edifice, which could be the frozen magmatic center of Mahukona. Eighteen weakly to strongly olivine-phyric samples were collected by submersible from the shallower parts (>2 km) of the volcano to supplement previous dredged samples. These fresh, mostly glassy samples vary from low-silica tholeiites to weakly alkali basalts. Ar-Ar weighted plateau ages range from 653 ka for a tholeiite to 479 and 351 ka for transitional basalts. These ages straddle the predicted age for the end of shield building (435 ka) and are older than previous ages for transitional basalts (310-298 ka; Clague and Calvert, 2008). Trace elements show a moderate range of variability (33% for Ba and Nb) and parallel primitive mantle normalized patterns suggesting variable degrees of melting of a similar source. Zr/Nb ratios for this Loa chain volcano (11-14) span the Loa-Kea boundary. Pb, Sr, Nd and Hf isotope ratios for 12 samples are distinct from adjacent Kohala volcano with Loihi-like values, although they are slightly higher in Hf and Nd at a given Pb isotope value. Most samples have Loa-like Pb isotope ratios, although two tholeiites have Kea-like ratios but high, Loa-like Zr/Nb. Sr isotopes are well correlated with the other isotopic systems indicating no ancient carbonate-rich sediment source component is needed. Mahukona He isotope ratios overlap with those found at Lo`ihi Seamount. Higher values are found in transitional basalts and lower in the tholeiites (16-21 vs. 12-14 Ra), which is opposite to other Hawaiian volcanoes. With high-precision data sets for more volcanoes along the Hawaiian chain in multiple stages of growth, we will be able to resolve the fine structure and evolution of the Hawaiian plume. Clague, D. and Calvert, A. 2008. Bull. Volcanol.