DATING OF QUATERNARY BASALTS USING THE COSMOGENIC HE-3 AND C-14 METHODS WITH IMPLICATIONS FOR EXCESS AR-40

Concordant radiocarbon and cosmogenic He-3 dates of 3 and 11 ha on bas alt flows and associated scoria deposits from the Zuni-Bandera volcani c field in New Mexico further substantiate published production rates and latitudinal and altitudinal corrections for He-3. Identical He-3 c oncentrations in coexisting olivine and clinopyroxene confirm that the se two minerals have very similar production rates. A cosmogenic He-3 date of 57 ha on another flow is concordant with a U-series date and i s younger than two K-Ar dates, suggesting that this flow contains exce ss Ar-40. This excess Ar-40 is also evident in gases released by vacuu m crushing of olivine-clinopyroxene fractions from all three flows.