The effects of secular disequilibrium on (U-Th)/He systematics and dating of Quaternary volcanic zircon and apatite
The effects of secular disequilibrium on (U-Th)/He systematics and dating of Quaternary volcanic zircon and apatite
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DOI：
10.1016/s0012-821x(02)00659-3
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发表时间：
2002-07-15
影响因子：
5.3
通讯作者：
Pillans, B
中科院分区：
文献类型：
--
作者：
Farley, KA;Kohn, BP;Pillans, B
（U-Th）/He定年法适用于富铀相，如锆石和磷灰石，具有足够的灵敏度和精度，可用于相对较近的地质事件（如火山爆发）定年。然而，在结晶年龄小于类似于1 Ma的相中，U-238衰变系列中的化学分馏可能会改变He的长入速率，导致从长期平衡年龄方程计算出的He年龄不正确。由此产生的系统误差取决于喷发时定年相的[Th-230/U-238]活度比和喷发年龄。锆石在排除Th相对于U的情况下，其长期平衡He“年龄”可能会低估喷发年龄高达几十个百分点，并随着喷发年龄的增加而减小。磷灰石倾向于容纳U和Th，分选作用较小，因此磷灰石长期平衡He年龄与喷发年龄基本一致。如果矿物在结晶后立即喷发，则根据Th/U分系统可以合理地解释不平衡效应。然而，结晶可能会在岩浆房中存在未知的但可能很长时间，这样在He积聚开始之前，长期不平衡的程度将会降低。（U-Th）/He分析的同成因相，如磷灰石和锆石，其U/Th比值不同，可以更好地约束喷发年龄，并提供对岩浆室停留时间的见解。我们用新西兰更新世Rangitawa Tephra的锆石和磷灰石的(U-Th)/He分析来说明这种方法。(C) 2002年Elsevier Science B.V.出版
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 similar to1 Ma, chemical fractionation within the U-238 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 [Th-230/U-238] 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. (C) 2002 Published by Elsevier Science B.V.