Combined U–Th/He and 40Ar/ 39Ar geochronology of post-shield lavas from the Mauna Kea and Kohala volcanoes, Hawaii

Abstract

Late Quaternary, post-shield lavas from the Mauna Kea and Kohala volcanoes on the Big Island of Hawaii have been dated using the 40Ar/ 39Ar and U–Th/He methods. The objective of the study is to compare the recently demonstrated U–Th/He age method, which uses basaltic olivine phenocrysts, with 40Ar/ 39Ar ages measured on groundmass from the same samples. As a corollary, the age data also increase the precision of the chronology of volcanism on the Big Island. For the U–Th/He ages, U, Th and He concentrations and isotopes were measured to account for U-series disequilibrium and initial He. Single analyses U–Th/He ages for Hamakua lavas from Mauna Kea are 87 ± 40 to 119 ± 23 ka (2 σ uncertainties), which are in general equal to or younger than 40Ar/ 39Ar ages. Basalt from the Polulu sequence on Kohala gives a U–Th/He age of 354 ± 54 ka and a 40Ar/ 39Ar age of 450 ± 40 ka. All of the U–Th/He ages, and all but one spurious 40Ar/ 39Ar ages conform to the previously proposed stratigraphy and published 14C and K–Ar ages. The ages also compare favorably to U–Th whole rock–olivine ages calculated from 238U– 230Th disequilibria. The U–Th/He and 40Ar/ 39Ar results agree best where there is a relatively large amount of radiogenic 40Ar (>10%), and where the 40Ar/ 36Ar intercept calculated from the Ar isochron diagram is close to the atmospheric value. In two cases, it is not clear why U–Th/He and 40Ar/ 39Ar ages do not agree within uncertainty. U–Th/He and 40Ar/ 39Ar results diverge the most on a low-K transitional tholeiitic basalt with abundant olivine. For the most alkalic basalts with negligible olivine phenocrysts, U–Th/He ages were unattainable while 40Ar/ 39Ar results provide good precision even on ages as low as 19 ± 4 ka. Hence, the strengths and weaknesses of the U–Th/He and 40Ar/ 39Ar methods are complimentary for basalts with ages of order 100–500 ka.