碎屑锆石U-Pb数据不一致的处理

IF 2.7 Q2 GEOCHEMISTRY & GEOPHYSICS Geochronology Pub Date : 2020-12-11 DOI:10.5194/gchron-2020-38
P. Vermeesch
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引用次数: 39

摘要

摘要锆石U-Pb年代学是地壳演化研究和沉积物源分析的重要内容。对于一致的206Pb/238U和207Pb/206Pb组成,构建(碎屑)U-Pb年龄谱是简单的。但不幸的是,许多U-Pb数据集包含了很大比例的不一致分析。本文研究了在分析这种不一致的U-Pb数据时必须做出的两个决定。首先,分析人员必须选择是使用206Pb/238U还是207Pb/206Pb日期。206Pb/238U法对年轻样品更精确,而207Pb/206Pb法更适合于老样品。然而,对于应该使用哪种“截止”来在两者之间切换,并没有一致意见。这种主观决定可以通过使用单粒协和年龄来避免。这代表了206Pb/238U和207Pb/206Pb方法之间的一种加权平均值,其精度优于后两种方法。第二个主观决定是如何定义“好”和“坏”数据之间的不一致界限。不一致通常定义为(1)206Pb/238U和207Pb/206Pb日期的相对年龄差。然而,本文表明,其他几个定义也是可能的,包括(2)绝对年龄差异;(3)根据Stacey-Kramers地幔演化模型计算的共pb分数;(4)一致性的p值;(5)到concordia线的垂直对数比(或“艾奇逊”)距离;(6)与Concordia线上最大似然成分的对数比距离。对70 869粒锆石U-Pb数据进行分析,发现(1)相对年龄不一致滤波抑制了U-Pb年龄谱中的年轻年龄成分,而膨胀了老年年龄成分;(ii)斯泰西-克莱默斯不协调过滤器更有可能拒绝旧的谷物,而不太可能拒绝年轻的谷物;(iii)基于p值的不一致滤波器会使结果偏向最不精确的测量值;(iv)基于对数比的不协调滤波对元古代颗粒最严格,对显生宙和太古宙年龄成分较宽松;(v)在所有方法中,与Concordia成分的对数比距离产生的结果最好,因为它产生的年龄谱与未过滤数据的年龄谱最接近:它使年龄谱锐化,但不改变其形状。根据这一标准,流行的相对年龄定义表现最差。本文提出的所有方法都已在IsoplotR地质年代学工具箱中实现。
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On the treatment of discordant detrital zircon U–Pb data
Abstract. Zircon U–Pb geochronology is a staple of crustal evolution studies and sedimentary provenance analysis. Constructing (detrital) U–Pb age spectra is straightforward for concordant 206Pb/238U and 207Pb/206Pb compositions. But unfortunately, many U–Pb datasets contain a significant proportion of discordant analyses. This paper investigates two decisions that must be made when analysing such discordant U–Pb data. First, the analyst must choose whether to use the 206Pb/238U or the 207Pb/206Pb date. The 206Pb/238U method is more precise for young samples, whereas the 207Pb/206Pb method is better suited for old samples. However there is no agreement which “cutoff” should be used to switch between the two. This subjective decision can be avoided by using single-grain concordia ages. These represent a kind of weighted mean between the 206Pb/238U and 207Pb/206Pb methods, which offers better precision than either of the latter two methods. A second subjective decision is how to define the discordance cutoff between “good” and “bad” data. Discordance is usually defined as (1) the relative age difference between the 206Pb/238U and 207Pb/206Pb dates. However, this paper shows that several other definitions are possible as well, including (2) the absolute age difference; (3) the common-Pb fraction according to the Stacey–Kramers mantle evolution model; (4) the p value of concordance; (5) the perpendicular log ratio (or “Aitchison”) distance to the concordia line; and (6) the log ratio distance to the maximum likelihood composition on the concordia line. Applying these six discordance filters to a 70 869-grain dataset of zircon U–Pb compositions reveals that (i) the relative age discordance filter tends to suppress the young age components in U–Pb age spectra, whilst inflating the older age components; (ii) the Stacey–Kramers discordance filter is more likely to reject old grains and less likely to reject young ones; (iii) the p-value-based discordance filter has the undesirable effect of biasing the results towards the least precise measurements; (iv) the log-ratio-based discordance filters are strictest for Proterozoic grains and more lenient for Phanerozoic and Archaean age components; (v) of all the methods, the log ratio distance to the concordia composition produces the best results, in the sense that it produces age spectra that most closely match those of the unfiltered data: it sharpens age spectra but does not change their shape. The popular relative age definition fares the worst according to this criterion. All the methods presented in this paper have been implemented in the IsoplotR toolbox for geochronology.
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来源期刊
Geochronology
Geochronology Earth and Planetary Sciences-Paleontology
CiteScore
6.60
自引率
0.00%
发文量
35
审稿时长
19 weeks
期刊最新文献
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