用于化学Th-U-Pb年代测定的独居石电子探针显微分析 (EPMA) 规程

IF 2.2 4区 地球科学 Q2 GEOCHEMISTRY & GEOPHYSICS
Minerals Pub Date : 2024-08-12 DOI:10.3390/min14080817
Bernhard Schulz, Joachim Krause, Wolfgang Dörr
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引用次数: 0

摘要

利用电子探针显微分析仪(EPMA)对独居石(LREE,Y,Th,U,Si,Ca)PO4 进行原位Th-U-Pb 测定的方法涉及到一个合适的参照独居石。通过 TIMS-U-Pb 同位素分析确定了几种潜在参照独居石的年龄。EPMA 方案基于 REE 正磷酸盐和同质富钍参比独居石的校准,束流条件为 20 kV、50 nA 和 5 µm,以获得最佳基质匹配并避免死区时间偏差。在 20 kV、100 nA 和 5 µm 的光束条件下对样品进行 EPMA 测量,并对参比独居石进行重复分析。在 PETL 晶体上分析铅和铀需要进行 YLg-on-PbMa 和 ThMz-on-UMb 干涉校正。离线重新校准富钍参比独居石上的钍校准,以符合其标称年龄,是协议的重要组成部分。EPMA-Th-U-Pb 数据按照 ThO2*-PbO 坐标进行检查,以确保等时线与强制回归零点相匹配。独居石年龄群的误差计算通过加权平均例程进行。根据分析次数和 ThO2*-PbO 坐标的分布情况,古生代独居石年龄的最小误差可能小于 10 Ma。对 Erbendorf-Vohenstrauß 古生代变质区(东北-巴伐利亚,波希米亚丘陵西部)的两个石榴石元青石样本进行了测试。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
A Protocol for Electron Probe Microanalysis (EPMA) of Monazite for Chemical Th-U-Pb Age Dating
A protocol for the monazite (LREE,Y,Th,U,Si,Ca)PO4 in situ Th-U-Pb dating by electron probe microanalyser (EPMA) involves a suitable reference monazite. Ages of several potential reference monazites were determined by TIMS-U-Pb isotope analysis. The EPMA protocol is based on calibration with REE-orthophosphates and a homogeneous Th-rich reference monazite at beam conditions of 20 kV, 50 nA, and 5 µm for best possible matrix matches and avoidance of dead time bias. EPMA measurement of samples and repeated analysis of the reference monazite are performed at beam conditions of 20 kV, 100 nA, and 5 µm. Analysis of Pb and U on a PETL crystal requires YLg-on-PbMa and ThMz-on-UMb interference corrections. Offline re-calibration of the Th calibration on the Th-rich reference monazite, to match its nominal age, is an essential part of the protocol. EPMA-Th-U-Pb data are checked in ThO2*-PbO coordinates for matching isochrones along regressions forced through zero. Error calculations of monazite age populations are performed by weighted average routines. Depending on the number of analyses and spread in ThO2*-PbO coordinates, minimum errors <10 Ma are possible and realistic for Paleozoic monazite ages. A test of the protocol was performed on two garnet metapelite samples from the Paleozoic metamorphic Zone of Erbendorf-Vohenstrauß (NE-Bavaria, western Bohemian Massif).
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来源期刊
Minerals
Minerals MINERALOGY-MINING & MINERAL PROCESSING
CiteScore
4.10
自引率
20.00%
发文量
1351
审稿时长
19.04 days
期刊介绍: Minerals (ISSN 2075-163X) is an international open access journal that covers the broad field of mineralogy, economic mineral resources, mineral exploration, innovative mining techniques and advances in mineral processing. It publishes reviews, regular research papers and short notes. Our aim is to encourage scientists to publish their experimental and theoretical results in as much detail as possible. There is no restriction on the length of the papers. The full experimental details must be provided so that the results can be reproduced.
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