Total fusion ⁴⁰Ar/³⁹Ar ages using Hifar reactor

Abstract

Total fusion ⁴⁰Ar/³⁹Ar ages have been determined on 22 samples, including biotite, muscovite, sanidine, hornblende, plagioclase, pyroxene, and whole rock samples of volcanics. The samples have conventional K‐Ar ages ranging from 14 m.y. to 2000 m.y. The neutron irradiations for the ⁴⁰Ar/³⁹Ar dating were made in the HIFAR reactor, using the standard biotite GA1550 as the flux monitor. The ⁴⁰Ar/³⁹Ar total fusion ages of 14 samples agree to within 2 percent with the conventional isotope dilution K‐Ar ages. The remaining 8 give measured ⁴⁰Ar/³⁹Ar ages from 3 to 9.5 percent greater than the conventional K‐Ar ages. These systematic discrepancies are due to significant neutron flux gradients in the reactor which become apparent because larger amounts of sample, compared with the amount of flux monitor, were used in these cases. With samples laid out in the reactor can as in the present study the precision of the ⁴⁰Ar/³⁹Ar ages is about 2 percent (standard deviation) in the ideal case. Correction factors for interfering argon istopes in HIFAR have been determined: (³⁹Ar/³⁷Ar)c_a = 7.44 (± 0.24) x 10^‐4; (³⁶Ar/³⁷Ar)c_a = 3.05 (± 0.10) x 10‐⁴; (⁴⁰Ar/³⁹Ar)_K = 0.28 ± 0.03.