We present high-precision mass spectrometric measurements of (226Ra/230Th) and (238U/230Th) disequilibria in a suite of historic or young, dated Hawaiian lava samples from Kilauea, Mauna Loa, Hualalai and Haleakala which range from tholeiitic to basanitic in chemical composition. This range of lava chemistry represents most of the range exhibited by Hawaiian lavas and is believed to stem from variations in the melting parameters as the volcanoes, carried by the moving pacific plate, traverse the Hawaiian mantle plume. (238U/230Th) disequilibria for these samples are consistent with simple equilibrium batch melting of a garnet peridotite [Sims et al., Science, 267, 1995]. The addition of (226Ra/230Th) activity ratios to this suite of samples allows us to model the data in terms of porous flow melt extraction models and thereby obtain estimates of porosity, melting rate and melt velocity during Hawaiian basalt petrogenesis. The measurements also limit magma chamber residence time. To compare Ocean Island basalt petrogenesis with MORB petrogenesis, the Hawaiian data are compared to a suite of MORB samples from the Juan de Fuca and Gorda ridges for which high precision mass spectrometric data have recently been published [Volpe and Goldstein, GCA 57, 1993].
The average porosity in the melting zone beneath Hawaii is 0.4 ± 0.2% and does not vary with basalt type. Melting rate varies from 3 x 10-3 kg m-3yr-1 for tholeiites to 7 x 10-5 kg m-3yr-1 for basanite. Melt separation velocities, expressed as a multiple of the upwelling velocity of the solid matrix, vary from about 50 for tholeiite to about 5 for basanite. Garnet is required as a residual phase in the magma sources for all of the lavas. The (230Th/238U) and (230Th/226Ra) disequilibria in mid-ocean ridge basalts are much different from those of Hawaiian lavas, and indicate a wide range of porosities and a narrower range of melting rates.