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Matrix dependency of baddeleyite U–Pb geochronology by femtosecond-LA-ICP-MS and comparison with nanosecond-LA-ICP-MS†
Journal of Analytical Atomic Spectrometry ( IF 3.4 ) Pub Date : 2018-03-20 00:00:00 , DOI: 10.1039/c7ja00403f
Cora C. Wohlgemuth-Ueberwasser 1, 2, 3, 4 , Jan A. Schuessler 1, 2, 3, 4 , Friedhelm von Blanckenburg 1, 2, 3, 4 , Andreas Möller 5, 6, 7, 8
Affiliation  

Baddeleyite is a key mineral in geochronology of mafic rocks as it crystallizes in silica-undersaturated systems that do not grow zircon. It has been shown that nanosecond (ns-)LA-ICP-MS U–Pb analysis requires matrix-matched calibration due to significantly stronger element downhole fractionation in baddeleyite compared to zircon. Using zircon as external standard for downhole fractionation correction produces reverse discordant results with low precision intercept ages (≥5%). In contrast it has been shown that femtosecond (fs)-LA-ICP-MS can produce accurate and precise data for a variety of difficult matrices that require matrix-matching with ns-LA-ICP-MS. Here we compare U–Pb data obtained by ns-LA-ICP-MS and fs-LA-ICP-MS. We conducted spot as well as line scan analyses with both systems applying Plešovice zircon, Duluth zircon and Duluth baddeleyite as reference materials, and the well-characterized Phalaborwa baddeleyite as unknown sample. If the cause for previously observed reverse discordance is only downhole elemental fractionation, then raster analyses should remedy this even with ns-LA-ICP-MS. Our results show that elemental fractionation occurs in both fs- and ns-LA-ICP-MS and needs to be corrected for by application of baddeleyite as reference material. Although raster analyses are not affected by downhole fractionation, discordant ages result nevertheless. The underlying elemental fractionation process might be caused by ablation of material previously ablated and deposited along the raster path. Deposition of such pre-ablated material can undergo fractionation during condensation of which the material is incorporated later. In summary, spot analysis of matrix-matched calibration is the preferred method to obtain concordant high-precision U–Pb ages by both ns- and fs-LA-ICP-MS.

中文翻译:

飞秒-LA-ICP-MS对坏死者U-Pb年代学的基质依赖性以及与纳秒-LA-ICP-MS的比较

Baddeleyite是镁铁质岩石年代学中的关键矿物,因为它在不会生长锆石的二氧化硅欠饱和系统中结晶。已经显示,纳秒(ns-)LA-ICP-MS U–Pb分析需要基质匹配的校准,因为与锆石相比,在Baddeleyite中元素的井下分级明显更强。使用锆石作为井下分馏校正的外标,会产生反向不一致的结果,且截距年龄较低(≥5%)。相反,已经证明,飞秒(fs)-LA-ICP-MS可以为需要与ns-LA-ICP-MS进行矩阵匹配的各种困难矩阵产生准确而精确的数据。在这里,我们比较通过ns-LA-ICP-MS和fs-LA-ICP-MS获得的U–Pb数据。我们使用这两个使用Plešovice锆石的系统进行了点扫描和线扫描分析,以Duluth锆石和Duluth Baddeleyite为参考材料,以及特征明确的Phalaborwa Baddeleyite为未知样品。如果先前观察到的反向不一致的原因仅仅是井下元素分级,那么即使使用ns-LA-ICP-MS,栅格分析也应对此进行补救。我们的结果表明,在fs-和ns-LA-ICP-MS中均发生元素分馏,需要通过使用Baddeleyite作为参比材料进行校正。尽管栅格分析不受井下分割的影响,但仍会导致年龄不一致。潜在的元素分离过程可能是由先前沿光栅路径烧蚀和沉积的材料烧蚀引起的。这种预烧蚀的材料的沉积可在冷凝期间经历分级分离,随后将其并入该材料中。总之,
更新日期:2018-03-20
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