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The effects of secular disequilibrium on (U-Th)/He systematics and dating of Quaternary volcanic zircon and apatite

Farley, K. A. and Kohn, B. P. and Pillans, B. (2002) The effects of secular disequilibrium on (U-Th)/He systematics and dating of Quaternary volcanic zircon and apatite. Earth and Planetary Science Letters, 201 (1). pp. 117-125. ISSN 0012-821X.

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The (U–Th)/He dating method applied to U-rich phases such as zircon and apatite has sufficient sensitivity and precision to be of potential use for dating relatively recent geologic events such as volcanic eruptions. However, in phases with crystallization ages less than ∼1 Ma, chemical fractionation within the ^(238)U decay series may modify the He ingrowth rate, causing He ages computed from the secular equilibrium age equation to be incorrect. The resulting systematic error depends on the [^(230)Th/^(238)U] activity ratio of the dated phase when it is erupted, and on the eruption age. Zircons, which exclude Th relative to U, will likely have secular equilibrium He ‘ages’ that underestimate the eruption age by up to a few tens of %, decreasing with increasing eruption age. Apatites tend to accommodate U and Th with little fractionation, so apatite secular equilibrium He ages will be nearly concordant with eruption age. If minerals are erupted immediately after crystallization, the disequilibrium effect can be reasonably accounted for based on Th/U systematics. However, crystals are likely to reside for unknown but potentially long periods in a magma chamber, such that the degree of secular disequilibrium will be reduced prior to the onset of He accumulation. (U–Th)/He analyses of co-genetic phases that fractionate the U/Th ratio differently, like apatite and zircon, can be used to better constrain eruption age, as well as to provide insights into magma chamber residence time. We illustrate this approach with (U–Th)/He analyses of zircons and apatites of the Pleistocene-age Rangitawa Tephra, New Zealand.

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Farley, K. A.0000-0002-7846-7546
Additional Information:© 2002 Published by Elsevier Science B.V. Received 19 December 2001; received in revised form 25 March 2002; accepted 5 April 2002. This study was supported by the Australian Research Council and a fellowship to K.A.F. from the David and Lucille Packard Foundation. K.A.F. thanks A. Gleadow and the University of Melbourne for hosting a sabbatical leave. Mary Reid, Ian Fletcher, Tim Elliot and Fin Stuart provided helpful comments on the manuscript. [BARD]
Funding AgencyGrant Number
Australian Research CouncilUNSPECIFIED
David and Lucile Packard Foundation FellowshipUNSPECIFIED
Subject Keywords:Th/U; zircon; apatite; Quaternary; volcanic rocks; uranium disequilibrium
Issue or Number:1
Record Number:CaltechAUTHORS:20121115-080342768
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Official Citation:K.A. Farley, B.P. Kohn, B. Pillans, The effects of secular disequilibrium on (U–Th)/He systematics and dating of Quaternary volcanic zircon and apatite, Earth and Planetary Science Letters, Volume 201, Issue 1, 15 July 2002, Pages 117-125, ISSN 0012-821X, 10.1016/S0012-821X(02)00659-3. (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:35470
Deposited By: Ruth Sustaita
Deposited On:15 Nov 2012 16:57
Last Modified:03 Mar 2020 13:01

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