UNSPIKED K-AR DATING OF YOUNG VOLCANIC-ROCKS FROM LOIHI AND PITCAIRN HOT-SPOT SEAMOUNTS

A geochronological study of volcanically active seamounts from the Pit cairn and Hawaii hot spots successfully utilized the unspiked K-Ar tec hnique to obtain ages for young volcanic rocks. These ages were used t o calculate lava production rates for the youthful stage for the Hawai ian volcanic chain. The lavas from Pitcairn seamounts have ages rangin g from 3 +/- 1 to 344 +/- 32 ka. The reliably dated lavas from Loihi a re from a 500 m thick section on the east flank of the volcano and the y range in age from 5 +/- 4 to 102 +/- 13 ka. These lavas are all alka lic and are from the preshield stage of volcanism. These ages yield la va accumulation rates that increase from 3.5 mm/a for the dominantly a lkalic lower section to 7.8 mm/a for the predominantly tholeiitic uppe r part of the volcano. These values are consistent with those obtained for other Hawaiian volcanoes. The duration of the transition from tho leiitic to alkalic lava during the preshield stage is about 17 to 40 k a, which is similar to the tholeiitc to alkalic transition during the postshield stage of Hawaiian volcanism. The total duration of the pres hield stage of Hawaiian volcanism is estimated to be at least 250,000 years, which is more than twice previous estimates. The overall length of magmatic activity, for a typical Hawaiian volcano is now estimated to be similar to 1.4 Ma. (C) 1997 Elsevier Science B.V.