锆石化学研磨对 SIMS U-Pb、δ18O、微量元素以及 LA-ICPMS 微量元素和 Lu-Hf 同位素分析的影响

IF 2.7 Q2 GEOCHEMISTRY & GEOPHYSICS
Cate Kooymans, Charles W. Magee Jr., Kathryn Waltenberg, Noreen J. Evans, Simon Bodorkos, Y. Amelin, Sandra L Kamo, T. Ireland
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引用次数: 0

摘要

摘要本研究评估了化学磨损对锆石原位质谱同位素和元素分析的影响。化学磨损改善了参考锆石的 SIMS(二次离子质谱)分析的 U-Pb 系统学,而其他同位素系统则基本保持不变。经过化学研磨的参考材料 TEMORA-2、91500、QGNG 和 OG1 的 SIMS 206Pb/238U 年龄精确度在 0.25 % 至 0.4 % 之间,与经过化学研磨的 TIMS(热电离质谱法)参考年龄的不确定性在同一范围内,而未经处理的锆石的 SIMS 206Pb/238U 年龄与未使用化学研磨的 TIMS 参考年龄的不确定性在同一范围内。化学研磨锆石和未经处理的锆石似乎可以在不确定的范围内进行交叉校准,但参考材料的一种可能排列方式除外,前提是对相应的材料采用相应的化学研磨或未经处理的参考年代。在参考锆石 QGNG 和 OG1 略有不一致的情况下,化学研磨和未处理材料的 SIMS U-Pb 年龄差异超出了各自 95 % 的置信区间。对具有多个生长阶段的化学研磨锆石进行的 SIMS U-Pb 分析更难以解释。来自 S 型佩因特火山岩的锆石晶体上经过处理的火成岩边缘的常见铅含量比未经处理的锆石颗粒上的边缘低得多。然而,对化学研磨材料的分析却显示出过多的分散性。化学磨蚀还改变了从沉积原岩中继承下来的锆石核心年龄的相对丰度,这可能是由于某些群体比其他群体更有可能在化学磨蚀过程中存活下来。我们认为这些从继承的 S 型锆石岩芯中得到的结果可以作为未熔融沉积物中的碎屑锆石颗粒结果的参考。我们还对这些锆石进行了微量元素、δ18O 和 εHf分析。这些系统都没有因化学磨损而发生实质性变化。最不一致的参考材料 OG1 在化学磨损后出现了 OH 的损失,这可能是由于水合元胞的溶解或化学磨损退火步骤中的热脱水造成的。在化学磨损过程的高频部分溶解阶段,锆石均未因晶格结合的取代 OH 或其他阴离子与氟交换而获得氟。众所周知,OG1、QGNG 和 TEMORA-2 锆石样品的微量元素组成不均匀,因此点与点之间的差异在微量元素结果中占主导地位。即使是 91500 块巨晶锆石也表现出很大的片间差异。经化学研磨的 OG1 和 TEMORA-2 中的轻稀土元素 (LREE) 低于未经处理的样品。所有样品中的钛浓度和磷饱和度((Y + REE) / P)基本保持不变。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Effect of chemical abrasion of zircon on SIMS U–Pb, δ18O, trace element, and LA-ICPMS trace element and Lu–Hf isotopic analyses
Abstract. This study assesses the effect of chemical abrasion on in situ mass spectrometric isotopic and elemental analyses in zircon. Chemical abrasion improves the U–Pb systematics of SIMS (secondary ion mass spectrometry) analyses of reference zircons, while leaving other isotopic systems largely unchanged. SIMS 206Pb/238U ages of chemically abraded reference materials TEMORA-2, 91500, QGNG, and OG1 are precise to within 0.25 % to 0.4 % and are within uncertainty of chemically abraded TIMS (thermal ionization mass spectrometry) reference ages, while SIMS 206Pb/238U ages of untreated zircons are within uncertainty of TIMS reference ages where chemical abrasion was not used. Chemically abraded and untreated zircons appear to cross-calibrate within uncertainty using all but one possible permutation of reference materials, provided that the corresponding chemically abraded or untreated reference age is used for the appropriate material. In the case of reference zircons QGNG and OG1, which are slightly discordant, the SIMS U–Pb ages of chemically abraded and untreated material differ beyond their respective 95 % confidence intervals. SIMS U–Pb analysis of chemically abraded zircon with multiple growth stages is more difficult to interpret. Treated igneous rims on zircon crystals from the S-type Mount Painter Volcanics are much lower in common Pb than the rims on untreated zircon grains. However, the analyses of chemically abraded material show excess scatter. Chemical abrasion also changes the relative abundance of the ages of zircon cores inherited from the sedimentary protolith, presumably due to some populations being more likely to survive the chemical abrasion process than others. We consider these results from inherited S-type zircon cores to be indicative of results for detrital zircon grains from unmelted sediments. Trace element, δ18O, and εHf analyses were also performed on these zircons. None of these systems showed substantial changes as a result of chemical abrasion. The most discordant reference material, OG1, showed a loss of OH as a result of chemical abrasion, presumably due to dissolution of hydrous metamict domains or thermal dehydration during the annealing step of chemical abrasion. In no case did zircon gain fluorine due to exchange of lattice-bound substituted OH or other anions with fluorine during the HF partial dissolution phase of the chemical abrasion process. As the OG1, QGNG, and TEMORA-2 zircon samples are known to be compositionally inhomogeneous in trace element composition, spot-to-spot differences dominated the trace element results. Even the 91500 megacrystic zircon pieces exhibited substantial chip-to-chip variation. The light rare earth elements (LREEs) in chemically abraded OG1 and TEMORA-2 were lower than in the untreated samples. Ti concentration and phosphorus saturation ((Y + REE) / P) were generally unchanged in all samples.
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来源期刊
Geochronology
Geochronology Earth and Planetary Sciences-Paleontology
CiteScore
6.60
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35
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19 weeks
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