Anomalous "Ages" (Sidebar 3)

Anomalous "Ages" for Australian Volcanic Rocks
Snelling made an analysis of rock samples from five different lava flows since 1949 at Mt. Ngauruhoe on North Island in New Zealand. This mountain is an andesite stratovolcano of 2,291 meters elevation. The volcano is thought to be the result of an oblique subduction of the Pacific plate beneath the Australian plate. The lava flows overlie late Miocene marine siltstones with variable dates of 0.22-0.26 Ma.

Eleven samples were used to form thin sections for petrographic analysis and whole rock analyses at labs in Adelaid, Australia, and Boston, Massachusetts.

The results indicated anomalous K-Ar model "ages" due to excess ⁴⁰Ar. This implies a residual concentration of ⁴⁰Ar in the lavas before congealing. There are many other examples of excess ⁴⁰Ar in crustal and mantle rock determinations. It is postulated that the ⁴⁰Ar is ancient or primordial in origin and not strictly all radiogenic at the time of the congealing of lava flows. The evidence points to an origin of the excess ⁴⁰Ar in the parent basaltic magma that formed in the upper mantle. It has a concentration of 150 times more ⁴⁰Ar in magma than the atmospheric level relative to ⁴⁰Ar.

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 plus/minus 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 mantel. In fact, it is now well established that there are large quantities of excess Ar* in the mantel, 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."

Snelling, Andrew A., 1998. "The Cause of Anomalous Potassium-Argon 'Ages' for Recent Andesite Flows at Mt. Ngauruhoe, New Zealand, and The Implications for Potassium-Argon 'Dating'." Proc. of the Fourth International Conference on Creation, Technical Symposium Sessions, Creation Science Fellowship, Inc. Pittsburgh, PA. pp. 503-525.