A critical evaluation of young (near-zero) K–Ar ages

• Published in: Earth and planetary science letters Vol. 220; no. 3; pp. 265 - 275
• Main Authors: Scaillet, Stéphane, Guillou, Hervé
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.04.2004; Elsevier
• Subjects: Continental interfaces, environment; historical age; Ocean, Atmosphere; positivity constraint; resolution; Sciences of the Universe; unspiked K–Ar dating; historical age; positivity constraint; unspiked K–Ar dating; resolution
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/S0012-821X(04)00069-X

An evaluation of the precision and resolution of the unspiked K–Ar dating method is presented with particular regard to the statistical significance of ages that are measured near or at the detection limit of the technique. Near-zero (historical) ages can be measured by the unspiked K–Ar technique with a precision that is essentially controlled by the precision with which the 40Ar/ 36Ar of the sample can be resolved from the present-day atmospheric value of 295.5. The best analytical precision on the isotopic ratio is ±0.05% (1σ) by this technique, which currently limits the lower detection limit of unspiked K–Ar ages to samples featuring at least 0.14% of radiogenic 40Ar. The corresponding youngest resolvable K–Ar age depends on the K content and atmospheric contamination of the sample. Total-fusion analysis of high-K refractory minerals like sanidine is not practicable via K–Ar, and the lowest resolvable age for medium-K samples more amenable to complete fusion is around 1.5 ka (on a single-run basis). It is argued that near-zero age measured with a probability density straddling or narrowing the time-origin cannot be handled without accounting for the non-negativity constraint imposed by the physical requirement of a positive age. The pertinent equations are derived both for the single-run case and for the case of independent replicates made on a single sample. We show that pooled K–Ar replicates can theoretically reduce the nominal uncertainty of individual unspiked ages (typically ±1.5 ka, 2σ) to a value that is close to the smallest 40Ar/ 39Ar isochron age uncertainty achievable on sanidine in the 0–2 ka range (±0.2 ka, 2σ). However, this performance is obtained at the cost of prohibitively large-sample statistics ( n≥15) for medium-K feldspars datable via K–Ar. Coupled with the inability of the K–Ar approach to obviate the problems of excess/fractionated 40Ar and/or xenocrystic contamination, this makes the 40Ar/ 39Ar technique the method of choice for dating historical events by the K–Ar scheme.