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Laser ablation (U-Th-Sm)/He dating of detrital apatite
Chemical Geology ( IF 3.9 ) Pub Date : 2020-08-01 , DOI: 10.1016/j.chemgeo.2020.119683
Julia Pickering , William Matthews , Eva Enkelmann , Bernard Guest , Chris Sykes , Brett M. Koblinger

Abstract Laser ablation (U-Th-Sm)/He dating of apatite (LA-aHe) is an emerging technique that presents a number of advantages over conventional whole-grain methods. Increased analytical efficiency, through the elimination of He re-extractions and acid digestion, increases sample throughput and facilitates larger datasets. The spatially resolved selection of helium extraction locations eliminates the need for alpha-ejection corrections and allows for imperfect grains with inclusions, fractures, unusual morphologies, rounding, and surface frosting or coatings to be measured. These advantages allow the effective analysis of detrital samples, which opens new avenues for geologic applications. Time-resolved parent isotope concentration data, collected by LA-ICP-MS, enables the screening of grains for chemical zonation, a commonly cited cause of over-dispersion in (U-Th-Sm)/He dates. Simultaneous collection of other chemical (e.g., anion or trace-element compositions) or isotopic (U Pb age) information for the grain may enhance provenance interpretations. We demonstrate the accuracy and uncertainty of the measurement method using two measurement sessions. We determine the age of the Fish Canyon Tuff apatite reference material to be 27.9 ± 0.6 Ma (n = 45; MSWD 1.5), which is in agreement with previous determinations. Our data reveal that zonation is a common feature in Fish Canyon Tuff apatite and can be resolved by LA-aHe methods to reduce dispersion. LA-aHe methods are ideally suited to detrital studies where a large number of measurements and a reduction in grain-selection bias compared to whole-grain methods should result in the sample better reproducing the source population. The utility of the method is demonstrated using a modern sand from the Wenatchee River, Washington, USA, previously characterized by conventional whole-grain aHe methods. Our LA-aHe dataset (n = 74) better reproduces the aHe ages of bedrock samples within the drainage in comparison to the age distribution derived from conventional aHe dating.

中文翻译:

碎屑磷灰石的激光烧蚀 (U-Th-Sm)/He 测年

摘要 激光烧蚀 (U-Th-Sm)/磷灰石 He 测年 (LA-aHe) 是一种新兴技术,与传统的全谷物方法相比具有许多优势。通过消除 He 再提取和酸消解,提高分析效率,增加样品通量并促进更大的数据集。氦提取位置的空间解析选择消除了对 alpha 喷射校正的需要,并允许测量包含夹杂物、裂缝、异常形态、倒圆和表面磨砂或涂层的不完美颗粒。这些优势允许对碎屑样品进行有效分析,为地质应用开辟了新的途径。LA-ICP-MS 收集的时间分辨母体同位素浓度数据能够筛选用于化学分带的颗粒,(U-Th-Sm)/He 日期过度分散的常见原因。同时收集谷物的其他化学(例如,阴离子或微量元素成分)或同位素(U Pb 年龄)信息可能会增强对来源的解释。我们使用两个测量会话来证明测量方法的准确性和不确定性。我们确定 Fish Canyon Tuff 磷灰石参考材料的年龄为 27.9 ± 0.6 Ma(n = 45;MSWD 1.5),这与之前的测定结果一致。我们的数据显示,分带是 Fish Canyon 凝灰磷灰石的一个共同特征,可以通过 LA-aHe 方法解决以减少分散。LA-aHe 方法非常适合碎屑研究,在这种研究中,与全谷物方法相比,大量测量和谷物选择偏差的减少应该会导致样本更好地再现源种群。使用来自美国华盛顿州韦纳奇河的现代沙子证明了该方法的实用性,该沙子以前以传统的全谷物 aHe 方法为特征。与传统 aHe 测年得出的年龄分布相比,我们的 LA-aHe 数据集 (n = 74) 更好地再现了排水系统内基岩样本的 aHe 年龄。
更新日期:2020-08-01
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