THE AR-40 AR-39 AND K/AR DATING OF LAVAS FROM THE HILO 1-KM CORE-HOLE, HAWAII SCIENTIFIC DRILLING PROJECT/

Mauna Kea lava flows cored in the Hilo hole range in age from <200 ka to about 400 ka based on Ar-40/Ar-39 incremental heating and K-Ar anal yses of 16 groundmass samples and one coexisting plagioclase. The lava s, all subaerially deposited, include a lower section consisting only of tholeiitic basalts and an upper section of interbedded alkalic, tra nsitional tholeiitic, and tholeiitic basalts. The lower section has yi elded predominantly complex, discordant Ar-40/Ar-39 age spectra that r esult from mobility of Ar-40 and perhaps K, the presence of excess Ar- 40, and redistribution of Ar-39 by recoil. Comparison of K-Ar ages wit h Ar-40/Ar-39 integrated ages indicates that some of these samples hav e also lost Ar-39. Nevertheless, two plateau ages of 391 +/- 30 and 40 0 +/- 26 ka from deep in the hole, combined with data from the upper s ection, show that the tholeiitic section accumulated at an average rat e of about 7 to 8 m/kyr and has an mean recurrence interval of 0.5 kyr /flow unit. Samples from the upper section yield relatively precise Ar -40/Ar-39 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. Wit hin their uncertainty, these ages define a linear relationship with de pth, with an average accumulation rate of 0.9 m/kyr and an average rec urrence interval of 4.8 kyr/flow unit. The top of the Mauna Kea sequen ce at -280 m must be older than the plateau age of 132 +/- 32 ka, obta ined for the basal Mauna Loa flow in the corehole. The upward decrease in lava accumulation rate is a consequence of the decreasing magma su pply available to Mauna Kea as it rode the Pacific plate away from its magma source, the Hawaiian mantle plume. The age-depth relation in th e core hole may be used to test and refine models that relate the grow th of Mauna Kea to the thermal and compositional structure of the mant le plume.