Screening apatites for (U-Th)/He thermochronometry via continuous ramped heating: He age components and implications for age dispersion
Screening apatites for (U-Th)/He thermochronometry via continuous ramped heating: He age components and implications for age dispersion
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通过连续斜坡加热筛选磷灰石 (U-Th)/He 热测时法：He 年龄成分及其对年龄分散的影响
DOI：
10.1016/j.gca.2017.11.031
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发表时间：
2018
影响因子：
5
通讯作者：
Fayon, Annia K.
中科院分区：
文献类型：
--
作者：
McDannell, Kalin T.;Zeitler, Peter K.;Janes, Darwin G.;Idleman, Bruce D.;Fayon, Annia K.
古老的缓慢冷却的磷灰石通常会产生分散的（U-Th）/He年龄，原因有很多，有些是很好理解的，有些则不是。像仔细选择颗粒这样的分析方案可以减少这种分散的影响，但会增加时间和资源的成本，而且往往被证明是不够的。我们评估了一种新的分析协议，利用静态气体测量连续升温（CRH）作为一种手段，以快速筛选磷灰石样品。在约常规总气体分析所需的时间，这种方法可以区分样品显示预期的体积扩散行为和那些显示异常的释放模式不一致，其直接使用在热年代学应用。这种方法似乎也能够区分样品释放的放射性和非放射性4 He部分，可能允许年龄被校正。表现良好的例子，如杜兰戈标准和其他磷灰石具有良好的年龄再现性显示预期的平滑，S形气体释放曲线预测的体积扩散使用典型的磷灰石动力学，以20 °C/min的速度线性加热，到900 °C时完全耗尽。次要因素，如U和Th分区和α损失分布对此类剖面的影响相对较小。相比之下，具有更大的年龄分散的样品显示出显着的他释放的形式的释气尖峰和他释放推迟到更高的温度。一系列样品的筛选结果使我们能够评估CRH筛选可以识别行为不端的谷物的程度，深入了解外来He的来源，并表明在某些情况下，可能可以纠正这些成分存在的年龄。
Old slowly-cooled apatites often yield dispersed (U-Th)/He ages for a variety of reasons, some well understood and some not. Analytical protocols like careful grain selection can reduce the impact of this dispersion but add costs in time and resources and too often have proven insufficient. We assess a new analytical protocol that utilizes static-gas measurement during continuous ramped heating (CRH) as a means to rapidly screen apatite samples. In about the time required for a conventional total-gas analysis, this method can discriminate between samples showing expected volume-diffusion behavior and those showing anomalous release patterns inconsistent with their direct use in thermochronologic applications. This method also appears able to discriminate between the radiogenic and extraneous4He fractions released by a sample, potentially allowing ages to be corrected.Well-behaved examples such as the Durango standard and other apatites with good age reproducibility show the expected smooth, sigmoidal gas-release curves predicted for volume diffusion using typical apatite kinetics, with complete exhaustion by ∼900 °C for linear heating at 20 °C/min. Secondary factors such as U and Th zoning and alpha-loss distribution have a relatively minor impact on such profiles. In contrast, samples having greater age dispersion show significant He release in the form of outgassing spikes and He release deferred to higher temperatures. Screening results for a range of samples permit us to assess the degree to which CRH screening can identify misbehaving grains, give insight into the source of extraneous He, and suggest that in some cases it may be possible to correct ages for the presence of such components.
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