COSMOGENIC CHLORINE-36 AGES FOR THE 1975 AND 1984 ERUPTIONS OF MAUNA LOA, HAWAII

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.