|
|||||
|
|
Combined U-Th/He and Ar/Ar geochronology of post-shield lavas from the Mauna Kea and Kohala volcanoes, Hawaii |
|
| Authors: | SM Aciego F Jourdan DJ DePaolo PR Renne |
| Institution: | a Institute for Isotope Geochemistry and Mineral Resources, ETH-Zurich, Clausiusstrasse 25, NW C83.1, Zurich 8092, Switzerland b Center for Isotope Geochemistry, Lawrence Berkeley National Laboratory, Berkeley, CA, USA c Western Australian Argon Isotope Facility, Department of Applied Geology & JdL Center, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, USA d Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA e Dept. of Geology and Geophysics, Woods Hole Oceanographic Institute, Woods Hole, MA 02543, USA f Dept. of Geology and Geophysics, University of Wyoming, Laramie, WY 82071, USA |
| 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 (2σ 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. |
| Keywords: | |
| 本文献已被 ScienceDirect 等数据库收录！ | |