Allanite U-Th-Pb geochronology by ion microprobe
Allanite U-Th-Pb geochronology by ion microprobe
复制标题
离子探针 U-Th-Pb 年代学
DOI：
10.1039/c9ja00426b
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发表时间：
2020
影响因子：
3.4
通讯作者：
Liu Yu
中科院分区：
文献类型：
--
作者：
Xin Liao;Li Qiu-li;Whitehouse M. J.;Yang Yue-heng;Liu Yu
尿绿石是绿帘石类矿物，是火成岩、变质岩和热液矿床中常见的副矿物。它含有放射性元素Th和U，使其成为各种地质过程中有价值的U-Th-Pb地质计时仪。由于褐柱石的结构和成分的复杂性，当其呈非均质或多期结晶时，原位测年方法是较好的选择。然而，化学成分的广泛变化增加了基质效应的可能性，并使尿囊石U-Th-Pb年代学的原位方法的使用复杂化。为了解决这个问题，我们利用二次离子质谱仪(SIMS)对尿囊石的U-Th-Pb年代学进行了全面的研究。使用了5个不同元素组成和常见铅浓度的尿囊石样品(CAP、DaiBosatsu、SQ-51、Toba OTT、Tara Alanite)。具有不同FeO(12.8wt%至16.1wt%)和相似ThO2含量(约1-2%)的Allanite不表现出明显的基质效应，正如先前的研究所表明的那样。而Th含量范围较大(300ppm~2.0wt%)的尿安石，其校正参数(UO2+/U+，ThO2+/Th+)发生了明显的变化。采用适当指数的Pb2+/U+与UO2+/U+、Pb2+/Th+与ThO2+/Th+之间的幂定律关系，我们没有观察到明显的基质效应随ThO2浓度的变化。研究表明，褐柱石可以作为热液矿床多期成矿的良好地质年代学指标，为锆石和独居石提供了有价值的补充信息。
Allanite, an epidote group mineral, occurs as a common accessory mineral in igneous, metamorphic rocks and hydrothermal deposits. It contains radioactive elements Th and U, making it a valuable U–Th–Pb geochronometer for various geological processes. Due to the structural and compositional complexity of allanite, in situ dating method is a better choice when the allanite is heterogeneous or has crystallized in multi-stage events. The wide variation of chemical compositions, however, raises the potential for matrix effects and complicates the use of in situ methods for allanite U–Th–Pb chronology. To address this issue, we made a comprehensive investigation on allanite U–Th–Pb chronology using Secondary Ion Mass Spectrometry (SIMS). Five allanite samples (CAP, Daibosatsu, SQ-51, Toba OTT, TARA allanite) with variable elemental compositions and common lead concentrations have been used. Allanites with variations in FeO (from 12.8 to 16.1 wt%) and similar ThO2 contents (around 1–2%) do not show significant matrix effects, as suggested by a previous study. While, obvious changes in calibration parameters (UO2+/U+, ThO2+/Th+) were noticed for allanites with a large range of Th contents (from 300 ppm to 2.0 wt%). By employing power law relationships between Pb+/U+versus UO2+/U+, Pb+/Th+versus ThO2+/Th+ with suitable exponentials, we do not observe obvious matrix effects with ThO2 concentrations variations. This study demonstrates that allanite can be a good geochronometer for multi-stage mineralization of hydrothermal deposits, providing valuable complementary information to zircon and monazite.