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Abstract

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".

Keywords

Andesite, 1949-1975 flows, Mt Ngauruhoe, New Zealand, potassium-argon dating, anomalous model "ages", excess "'Ar*, excess "'Ar* in rocks and minerals, upper mantle, geochemical reservoirs, mantlecrust domains, crustal mixing, magma genesis

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Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.

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