Postshield stage transitional volcanism on Mahukona Volcano, Hawaii
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Age spectra from 40Ar/39Ar incremental heating experiments yield ages of 298 ± 25 ka and 310 ± 31 ka for transitional composition lavas from two cones on submarine Mahukona Volcano, Hawaii. These ages are younger than the inferred end of the tholeiitic shield stage and indicate that the volcano had entered the postshield alkalic stage before going extinct. Previously reported elevated helium isotopic ratios of lavas from one of these cones were incorrectly interpreted to indicate eruption during a preshield alkalic stage. Consequently, high helium isotopic ratios are a poor indicator of eruptive stage, as they occur in preshield, shield, and postshield stage lavas. Loihi Seamount and Kilauea are the only known Hawaiian volcanoes where the volume of preshield alkalic stage lavas can be estimated.
KeywordsHawaii Volcanism Ar–Ar ages Eruptive stages He isotopes
We thank Gabi Laske, chief scientist of cruise TUIM01 on the R/V Melville in 2005 for providing the time to complete the dredge after the PLUME seismometer array was deployed. Alice Davis provided the microprobe analyses of the D6 glasses, which were completed at the US Geological Survey in Menlo Park, CA. Argon geochronology was funded by the USGS Volcano Hazards Program, and James Saburomaru assisted with the analyses. The TUIM01 cruise was supported by NSF grant OCE-00-02470. DAC was supported by a grant to the Monterey Bay Aquarium Research Institute from the David and Lucile Packard Foundation. Careful reviews by Tom Sisson, Bob Fleck, Tekeshi Hanyu, and associate editor Setsuya Nakada improved the manuscript.
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