Unspiked K-Ar dating of young volcanic rocks from Loihi and Pitcairn hot spot seamounts

A geochronological study of volcanically active seamounts from the Pitcairn and Hawaii hot spots successfully utilized the unspiked K-Ar technique to obtain ages for young volcanic rocks. These ages were used to calculate lava production rates for the youthful stage for the Hawaiian volcanic chain. The lavas from Pitcairn seamounts have ages ranging from 3 +/- 1 to 344 +/- 32 ka. The reliably dated lavas from Loihi are from a 500 m thick section on the east flank of the volcano and they range in age from 5 +/- 4 to 102 +/- 13 ka. These lavas are all alkalic and are from the preshield stage of volcanism. These ages yield lava accumulation rates that increase from 3.5 mm/a for the dominantly alkalic lower section to 7.8 mm/a for the predominantly tholeiitic upper part of the volcano. These values are consistent with those obtained for other Hawaiian volcanoes. The duration of the transition from tholeiitic to alkalic lava during the preshield stage is about 17 to 40 ka, which is similar to the tholeiitc to alkalic transition during the postshield stage of Hawaiian volcanism. The total duration of the preshield 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.