wsharp@bgc.org
The Hawaiian Scientific Drilling Project located near Hilo, 40 km east of the summit of Mauna Kea, recovered nearly continuous core from 776 m of Mauna Kea lavas (Hawaii Scientific Drilling Project, 1994). The lavas, all subaerially deposited, include a lower section consisting only of tholeiitic basalts, and an upper section of interbedded alkalic, transitional tholeiitic and tholeiitic basalts (Rhodes, 1996). Ages of the Mauna Kea lavas from the Hilo corehole are of interest for several reasons. 1) The ages show that the core samples fill an age gap in other Mauna Kea lavas sampled from the surface and by dredging. 2) The rate of lava accumulation varies systematically with age, an observation that can be used to test models linking the supply of magma to Mauna Kea with its transport history across the Hawaiian hot spot (DePaolo and Stolper, 1996). 3) The age-depth curve for the corehole may be compared to subsidence rates for the Hawaiian Ridge. With allowances for eustatic sea level changes, this yields the age-elevation history of the drillsite, and by inference that of the Mauna Kea shield surface.
4) Ages permit correlation of paleomagnetic excursions and intensity shifts observed in the core samples (Holt, 1996) with those in other Pleistocene paleomagnetic records, addressing the question of whether these are global or regional events.
Precise dating of the core samples is challenging. They lack separable high K minerals, forcing the use of "whole-rocks" (microcrystalline groundmass with phenocrysts removed). The youthfulness, low K contents, and substantial atmospheric 40Ar concentrations of the core samples result in low whole rock 40Ar*/40Ar total ratios. This hampers the precision of K-Ar age determinations. Also, K-Ar analyses cannot detect 40Ar* loss, or the presence of excess 40Ar. Incremental heating 40Ar/39Ar analyses can, in many cases, solve these problems. Such analyses can resolve the atmospheric, radiogenic and excess 40Ar contributions of interstitial material, plagioclase, pyroxene and olivine because of the contrasting Ca/K ratios and Ar retentivities of these phases. In samples with small grain sizes (10-15 microns), however, redistribution of 39Ar and 37Ar by recoil during irradiation can cause significant experimental artifacts in the age spectra.
Mauna Kea lava flows in the Hilo hole range in age from <200 ka to 400 ka, based on 40Ar/39Ar incremental heating and K-Ar analyses of 16 groundmass samples and one coexisting plagioclase. The lower, tholeiitic section has yielded predominantly complex, discordant 40Ar/39Ar age spectra that result from mobility of 40Ar and perhaps K, the presence of excess 40Ar and redistribution of 39Ar by recoil. Nevertheless, two plateau ages of 391 ± 40 and 400 ± 26 ka from deep in the hole show that the tholeiitic section accumulated at an average rate of about 8 m/k.y. and has a mean recurrence interval of 0.5 k.y./flow unit. Alkalic basalts from the upper section yield relatively precise 40Ar/39Ar plateau and isotope correlation ages of 326 ± 23, 241 ± 5, 232 ± 4 and 199 ± 9 ka for depths of -415.7 m to -299.2 m. Within their uncertainties, these ages define a linear relationship with depth, with an average accumulation rate of 0.9 m/k.y. and an average recurrence interval of 4.8 k.y./flow unit.
The lava accumulation rate decreases upward in the
corehole as a consequence of the reduced magma supply available to Mauna Kea as it rode the Pacific plate away from its magma source, the Hawaiian mantle plume. The age-depth data are consistent with a quantitative model for the growth of Mauna Kea (DePaolo and Stolper, 1996) that assumes the volcano has a simple geometric form with a circular magma capture area, that melt production varies radially within the mantle plume, and that the volcano moves across the plume with the Pacific plate at a velocity of 10 cm/yr. The initial appearance of transitional tholeiitic-alkalic lavas coincides with a near 10x reduction in accumulation rate. Allowing for regional subsidence and eustatic sea level change, the drillsite emerged above sea level before 400 ka, and rapid lava accumulation increased the drillsite elevation to a maximum of ~400 m at about 325 ka. Subsequently, subsidence outpaced growth and the drillsite sank close to sea level by 130 ka. A paleomagnetic excursion at -320m depth is dated at 226 ± 7 ka, indicating that it correlates with the Pringle Falls event observed in the western United States, providing evidence that this excursion is a global event (Sharp, 1995).
DePaolo, D.J. & Stolper, E.M., J. Geophys. Res. Spec. Sec., (in press) (1996).
Hawaii Scientific Drilling Project, Core-logs (Stolper, E. & Baker, M., eds.) 471pp. (CalTech, Pasadena, 1994).
Holt JW., Kirschvink, J.L. & Garnier, F., J. Geophys. Res. Spec. Sec., (in press) (1996).
Laj et al., J. Geophys. Res. Spec. Sec., (in press) (1996).
Rhodes J.M., J. Geophys. Res. Spec. Sec., (in press) (1996).
Sharp, W.D., Holt, J.W., Renne, P.R. & Turrin, B.D., EOS 76, F176 (1995)