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 fl ows in 1949 and 1954, and avalanche deposits in 1975. Potassium-argon “dating” of fi ve of these fl ows 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 fl ow, the explanation being variations in excess 40Ar* 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 40Ar* 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 signifi cant implications for the validity of K-Ar and 40Ar/39Ar “dating.” Introduction Mt. Ngauruhoe is an andesite stratovolcano of 2291 m elevation, rising above the Tongariro volcanic massif within the Tongariro Volcanic Center of the Taupo Volcanic Zone, North Island, New Zealand (Fig. 1).1, 2 Though not as well publicized as its neighbor, Mt. Ruapehu (about 12 km to the south), Ngauruhoe is an imposing, almost perfect cone that rises more than 1000 m above the surrounding landscape. Eruptions from a central 400 m diameter crater have constructed the steep (33°), outer slopes of the cone.3, 4 Geologic Setting The Taupo Volcanic Zone, a volcanic arc and marginal basin of the Taupo-Hikurangi arc-trench (subduction) system,5 is a southward extension on the Tonga-Kermadec arc into the continental crustal environment of New Zealand’s North Island. It has been interpreted as oblique subduction of the Pacifi c plate beneath the Australian plate. The zone extends approximately 300 km north-northeast across the North Island from Ohakune to White Island (Fig. 1) and is up to 50 km wide in the central part, narrowing northwards and southwards. This volcano-tectonic depression (Taupo-Rotorua depression6) comprises four rhyolitic centers (Rotorua, Okataina, Maroa, and Taupo), plus the calc-alkaline Tongariro Volcanic Center, part of a young (<0.25 Ma) andesitedacite volcanic arc with no associated rhyolitic volcanism extending along the eastern side of the zone.7 The Tongariro Volcanic Center extends for 65 km south-southwest from Lake Taupo at the southern end of the Taupo Volcanic Zone (Fig. 1) and consists of four large predominantly andesite volcanoes Kakaramea, Pihanga, Tongariro, and Ruapehu (Fig. 2); two smaller eroded centers at Maungakatote and Hauhungatahi; a satellite cone and associated fl ows at Pukeonake and four craters at Ohakune (Fig. 2).8, 9 Most vents lie close to the axis of a large graben in which Quaternary volcanic rocks overlie a basement of Mesozoic greywacke and Tertiary sediments.10, 11 North-northeast-trending normal faults with throws up to 30 m cut the volcanoes within the graben. Nearly all vents active within the last 10 ka lie on a gentle arc which extends 25 km north-northeast from the Rangataua vent on the southern slopes of Ruapehu through Ruapehu summit and north fl ank vents, Tama Lakes, Ngauruhoe, Red Crater, Blue Lake, and Te Mari craters. None of the young vents lie on the mapped faults, which mostly downthrow towards the axis of the graben. The vent lineation lies above this axis, which is considered to mark a major basement fracture12–14 that allows the intrusion of andesite dikes. © 2009 Answers in Genesis