2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 32
Presentation Time: 1:30 PM-5:30 PM


STONE, John, Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, WA 98195-1310, HUNGERFORD, Jefferson, Department of Geosciences, Oregon State University, 104 Wilkinson Hall, Corvallis, OR 97331-5503, FIFIELD, L. Keith, Department of Nuclear Physics, Australian National University, Canberra, A.C.T, 0200, Australia, TRUSDELL, Frank A., Hawaiian Volcano Observatory, United States Geol Survey, 1 Crater Rim Drive, Hawaii Volcanoes National Park, HI 96718-0051 and KENT, Adam J.R., Department of Geosciences, Oregon State University, 104 Wilkinson Hall, Corvallis, OR 97331-5506, stone@geology.washington.edu

We have obtained cosmogenic 36Cl exposure ages of 24 ± 19 years and 20 ± 19 years for lava samples from the 1984 and 1975 summit eruptions of Mauna Loa volcano, Hawaii, respectively. These estimates are within 1σ uncertainty limits of the actual eruption ages of the lavas at the time the samples were collected in 2002. The analytical sensitivity developed in dating these rocks will allow us to use historic (post AD 1855) lavas to calibrate cosmogenic 36Cl production rates, and to date older flows and pyroclastic deposits high on Mauna Loa, where the absence of vegetation precludes 14C dating of subvolcanic soil carbon.

These age estimates are corrected for the presence of magmatic 36Cl assuming a magmatic 36Cl/Cl ratio of 3 ± 4 x 10-16, the average of three measurements on zero-age glass, collected while molten from Kilauea Iki. We note that the ratio of 36Cl, produced by neutron capture, relative to target 35Cl, is approximately a factor of 5 lower than the equilibrium ratio supported by U and Th concentrations in the Kilauea lavas. This indicates that U and Th enrichment during mantle melting and differentiation occurred less than 160 kyr prior to eruption of the lavas.