安德氏石榴石和金红石的剥蚀行为研究及其对U-Pb地质年代学的启示

IF 2.7 2区 地球科学 Q2 GEOCHEMISTRY & GEOPHYSICS Geostandards and Geoanalytical Research Pub Date : 2023-01-12 DOI:10.1111/ggr.12478
Kate Jenkins, Karsten Goemann, Ivan Belousov, Maxwell Morissette, Leonid Danyushevsky
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引用次数: 0

摘要

利用LA-ICP-MS对钙铝榴石-粗石榴石(granite)和金红石进行U-Pb定年,可用于约束各种变质、交代和火成地质过程。在本研究中,我们检查并比较了不同分析条件(注量、脉冲宽度、激光束尺寸和烧蚀频率)对不同成分的浮石和金红石样品的烧蚀坑形态、烧蚀速率、井下分馏和U-Pb年龄的影响。巨大烧蚀坑的形状表明,矿物是通过经典蒸发烧蚀的,具有显著的熔化,即使在刚好高于烧蚀阈值的通量下也无法消除。具有更高钙铝榴石比例的石榴石具有更快的烧蚀速率。浮石的总体低U含量需要使用大的激光束尺寸来获得可接受的U-Pb年龄精度。在这样的条件和弹坑深度<;10μm,2.1和3.5J cm-2的通量,5ns和20ns的激光脉冲宽度,以及3.5和6.5Hz之间的消融频率,当grossular的比例<;35%。金红石烧蚀坑形态显示了熔体飞溅和热应力开裂的证据。它们具有显著的坑底特征,随着较低的通量和较高的激光发射次数,起伏增加,这表明这些特征可能与能量有关,并且在使用193nm准分子激光器时,产生均匀烧蚀所需的较高通量,如5J cm-2。低通量下的缓慢消融速率,然后在2.0 J cm-2左右急剧增加,表明消融机制从剥离转变为经典蒸发。陨石坑底部特征和其他烧蚀行为因样本而异,这可能与它们的颜色差异有关。尽管样品的井下分馏模式在5 J cm-2时相似,但一些样品的U-Pb年龄随着不同的分析条件和/或测量阶段而显著变化,特别是当使用30μm的激光束时,这表明质量偏差和更可变的烧蚀行为存在差异。建议使用尺寸至少为60μm的激光束对金红石进行可重复的U-Pb定年。
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Investigation of the Ablation Behaviour of Andradite-Grossular Garnets and Rutile with Implications for U-Pb Geochronology

U-Pb dating of andradite-grossular garnet (grandite) and rutile by LA-ICP-MS can be used to constrain various metamorphic, metasomatic and igneous geological processes. In this study, we examine and compare the impact of different analytical conditions (fluence, pulse width, laser beam size and ablation frequency) on the ablation crater morphology, ablation rates, down-hole fractionation and U-Pb ages of grandite and rutile samples of different compositions. The shapes of grandite ablation craters suggest the mineral ablates by classical evaporation with significant melting that cannot be eliminated even at fluences just above the ablation threshold. Grandite garnets with higher andradite proportions have faster ablation rates. The overall low U contents of grandite require using large laser beam sizes to obtain acceptable precision of U-Pb ages. At such conditions and crater depths < 10 μm, fluences of 2.1 and 3.5 J cm-2, laser pulse width of 5 ns and 20 ns, and ablation frequencies between 3.5 and 6.5 Hz, obtain similar and reproducible ages when the proportion of grossular is < 35%. Rutile ablation crater morphology shows evidence of melt splashing and thermal stress cracking. They have significant crater bottom features, which increase in relief with lower fluences and a higher number of laser shots, indicating the features are probably energy-related and making higher fluences, such as 5 J cm-2, necessary for uniform ablation when using 193 nm excimer lasers. The slow ablation rate at low fluences and then steep increase at around 2.0 J cm-2 suggests a transition in the ablation mechanism from exfoliation to classical vaporisation. Crater bottom features and other ablation behaviour vary between samples, which could be related to their difference in colour. Although the down-hole fractionation patterns of the samples are similar at 5 J cm-2, the U-Pb ages of some samples vary significantly with different analytical conditions and/or measurement sessions, particularly when using laser beam sizes of 30 μm, suggesting differences in mass bias and more variable ablation behaviour. A laser beam size of at least 60 μm is recommended for reproducible U-Pb dating of rutile.

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来源期刊
Geostandards and Geoanalytical Research
Geostandards and Geoanalytical Research 地学-地球科学综合
CiteScore
7.10
自引率
18.40%
发文量
54
审稿时长
>12 weeks
期刊介绍: Geostandards & Geoanalytical Research is an international journal dedicated to advancing the science of reference materials, analytical techniques and data quality relevant to the chemical analysis of geological and environmental samples. Papers are accepted for publication following peer review.
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