The Cause of Anomalous Potassium-Argon Ages for Recent Andesite Flows at Mt. Ngauruhoe, New Zealand, and the Implications for Potassium-Argon Dating

New Zealand's newest and most active volcano, Mt Ngauruhoe in the Taupo Volcanic Zone, produced andesite flows in 1949 and 1954, and avalanche deposits in 1975. Potassium-argon "dating" of five of these flows and deposits yielded K-Ar model "ages" from <0.27 Ma to 3.5 ± 0.2 Ma. "Dates" could not be reproduced, even from splits of the same samples from the same flow, the explanation being variations in excess "'Ar* content. A survey of anomalous K-Ar "dates" indicates they are common, particularly in basalts, xenoliths and xenocrysts such as diamonds that are regarded as coming from the upper mantle. In fact, it is now well established that there are large quantities of excess "'Ar* in the mantle, which in part represent primordial argon not produced by in situ radioactive decay of .⁴⁰K and not yet outgassed. And there are mantle-crust domains between, and within, which argon circulates during global tectonic processes, magma genesis and mixing of crustal materials. This has significant implications for the validity of K-Ar and ⁴⁰Ar/³⁹Ar "dating".