HAWAII SCIENTIFIC DRILLING PROJECT: A UNIQUE VIEW OF MANTLE PLUME VOLCANISM

The HSDP began with a workshop and proposal in 1986 and is nearing completion with a final phase of deep coring in late 2006. In 1993 a 1.04 km pilot hole was cored just east of Hilo, Hawaii, in lavas of the Mauna Loa and Mauna Kea volcanoes. In 1999 a 3.11 km hole was cored at a site 2 km south of the pilot hole; this hole was deepened to 3.35 km in 2005, and will be deepened further this year. The drilling, supported by NSF Continental Dynamics and ICDP, has produced a nearly continuous 3.1 km-thick (500 ky) record of the lava output of the Mauna Kea volcano as it drifted over the Hawaiian hotspot. The stratigraphy and volcanology of the core have been documented, and hundreds of samples have been measured for mineralogical, chemical, and isotopic composition. The results are reported in 48 publications involving 80 investigators and students from the U.S., Canada, and Europe. The Mauna Kea lavas were found to be twice as old as inferred from surface studies; the Hawaiian volcanoes grow more slowly and are active for a longer period of time than previously thought. The age-depth data can be accounted for with a relatively simple model of mantle plume magma supply which accords with geophysical data. Magma generation and transport parameters are constrained by trace and major element chemistry and are consistent with plume models. The isotopic geochemistry of Mauna Kea lavas changes systematically with age (depth), which corresponds to volcano position with respect to the hotspot. Large changes in He-3 and Pb-208 isotopes are accompanied by only slight changes in Nd, Sr, Pb-206, Pb-207, and Hf isotope ratios. The isotopic stratigraphy maps to structure at the base of the Earth's mantle where the plume originates. The He-3 isotope signal is apparently restricted to a layer, perhaps only 10 km thick, at the base of the mantle. The anomalous helium may be leaking into this layer from the Earth's core. The hydrology of Hawaii is more complex than had previously been recognized. At a depth of 1 km, the temperature in the hole is only about 12°C due to cold seawater circulating through the flanks of the volcano. The well intersected a number of pressurized aquifers - some fresh, and some salty -down to 3 km depth, which was unexpected and has implications for water resources. There are traces of microbiota in the rocks down to 2600 meters.