Re: Argon-Argon & Pompeii


Автор сообщения: gorm
Дата и время сообщения: 17 October 2005 at 14:52:27:

В ответ на сообщение: Re: Argon-Argon & Pompeii

Этот метод используется чрезвычайно широко, но обычно на другом масштабе, для гораздо более древних дат. Им датикуются породы, содержащие много калия. Собственно, данная статья была не столько статьей о датировке, а о проверке насколько точным может быть этот метод для недавнего прошлого при благоприятных условиях. И с этой статьи можно считать и начинается отсчет развития метода в приложении к историческому времени.

Вот совсем недавняя работу на эту тему, в которой содержится оценка возможностей метода.

Title: A critical evaluation of young (near-zero) K-Ar ages
Author(s): Scaillet S, Guillou H
Source: EARTH AND PLANETARY SCIENCE LETTERS 220 (3-4): 265-275 APR 15 2004
Document Type: Article
Language: English
Cited References: 28 Times Cited: 1 Find Related Records Information

Abstract: 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 Ar-40/Ar-36 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% (1sigma) by this technique, which currently limits the lower detection limit of unspiked K-Ar ages to samples featuring at least 0.14% of radiogenic Ar-40. 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, 2sigma) to a value that is close to the smallest Ar-40/Ar-39 isochron age uncertainty achievable on sanidine in the 0-2 ka range (+/-0.2 ka, 2sigma). However, this performance is obtained at the cost of prohibitively large-sample statistics (n greater than or equal to 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 Ar-40 and/or xenocrystic contamination, this makes the Ar-40/Ar-39 technique the method of choice for dating historical events by the K-Ar scheme. (C) 2004 Elsevier B.V. All rights reserved.
Author Keywords: unspiked K-Ar dating; resolution; historical age; positivity constraint
KeyWords Plus: AR-40-AR-39 LASER PROBE; EXCESS ARGON; XENOCRYSTIC CONTAMINATION; MASS FRACTIONATION; VOLCANIC-ROCKS; AR-40/AR-39; PLEISTOCENE; FELDSPAR; HOLOCENE; PLAGIOCLASE
Addresses: Scaillet S (reprint author), CEA, CNRS UMR 1572, Lab Sci Climat & Environm, Domaine CNRS Bat 12,Ave Terrasse, Gif Sur Yvette, F-91198 France
CEA, CNRS UMR 1572, Lab Sci Climat & Environm, Gif Sur Yvette, F-91198 France
E-mail Addresses: stephane.scaillet@lsce.enrs-gif.fr
Publisher: ELSEVIER SCIENCE BV, PO BOX 211, 1000 AE AMSTERDAM, NETHERLANDS
Subject Category: GEOCHEMISTRY & GEOPHYSICS
IDS Number: 811YT
ISSN: 0012-821X


2128. Argon-Argon & Pompeii - mv 09:15 09.10.05 (8)
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