Accurate analyses of key petrogenetic minor and trace elements in olivine by electron microprobe
Accurate analyses of key petrogenetic minor and trace elements in olivine by electron microprobe
复制标题
通过电子探针对橄榄石中关键的岩石成因微量和痕量元素进行精确分析
DOI：
10.1016/j.chemgeo.2022.121199
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发表时间：
2022-11
期刊：
影响因子：
3.9
通讯作者：
Peng Jiang;M. Perfit;D. A. Foster;A. Trucco
中科院分区：
文献类型：
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作者：
Peng Jiang;M. Perfit;D. A. Foster;A. Trucco
橄榄石中微量元素丰度越来越多地被用作地幔源岩性、地幔交代历史、地幔位温和岩浆分异的成岩指标。由于橄榄石在精细尺度上存在复杂的分带现象，因此开发了高空间分辨率下EPMA （electronic microprobe microanalyzer）痕量元素分析的高精度分析方法。然而，以前的研究更多地关注分析精度，而较少检查数据的准确性。利用Cameca sx5场发射（FE） EPMA，以蒙古sh11 - 2橄榄石为基准，全面评价了光束设置、背景偏移和背景回归模型以及主要标定标准对10种关键岩石成因元素（Na、Al、P、Ca、Ti、Cr、Mn、Co、Ni和Zn）数据精度的影响。研究结果表明，高电压、高光束电流和长计数时间不仅可以提高数据精度，而且可以提高数据精度，特别是对于Zn和Cr等P/B（峰/背景）比低的元素。重要的是，需要通过对每个目标元素进行高分辨率WDS相对扫描或步进扫描来获得仔细的背景偏移和背景回归模型。Co元素分析需要特别注意，以避免或纠正Fe - k - β的峰干扰。在10种微量元素中，Al、Mn和Ti元素需要采用指数背景回归模型，其他元素需要采用线性背景回归模型。此外，为了避免铝和锌表面因氧化铝抛光或黄铜的存在而受到污染，强烈建议在每个中间抛光步骤之间进行超声波清洗，并在EPMA实验之前立即进行等离子清洗。需要避免橄榄石或邻近富钙相中的微包裹体，如铬铁矿和尖晶石，以尽量减少与Al、Cr或Ca有关的初级或次级荧光污染。作为挥发性元素，Na元素需要首先进行分析，并在适当的计数时间内将高光束条件下的Na损失降到最低。值得一提的是，主要元素（Mg、Fe和Si）的分析最好使用MongOlSh11-2或San Carlos橄榄石作为校准的主要标准，由于改进了基体校正，可以产生更准确的主要元素和微量元素数据。使用我们推荐的分析方案，我们通过Ca、Ti、Ni、Co和Mn丰度成功地从东太平洋隆起（EPR）北部的EMORB中区分出“枯竭”的地幔橄榄石岩芯。来自Siqueiros Transform和附近8°20′N海山的橄榄石数据也有助于揭示EPR北部下方的交代橄榄岩地幔。总的来说，本研究提出的方案可以作为准确的EPMA橄榄石微量元素分析的指南，这可能有助于在全球范围内建立一个可比较的橄榄石数据库。
Abundances of minor and trace elements in olivine are increasingly used as petrogenetic indicators for mantle source lithologies, mantle metasomatism history, mantle potential temperatures, and magmatic differentiation. As it is common for olivine to be complexly zoned on a fine-scale, high precision analytical methods for EPMA (electron microprobe microanalyzer, or Electron Microprobe) trace element analysis under high spatial resolution have been developed. However, previous studies have focused more on analytical precision with fewer efforts in examining the accuracy of the data. In this study, we used the Cameca SXFive field emission (FE) EPMA to fully evaluate the effects of beam settings, background offsets and background regression models, and primary calibration standards on the data accuracy of 10 key petrogenetic elements (Na, Al, P, Ca, Ti, Cr, Mn, Co, Ni, and Zn) using MongOlSh11–2 olivine as a reference material. Our results indicate that high voltage, high beam current and long counting time not only improve data precision, but also improve data accuracy, especially on elements with low P/B (peak/background) ratios such as Zn and Cr. Importantly, careful background offsets and background regression models need to be obtained via high resolution WDS relative scans or step scans on each target element. Special care needs to be paid to Co element analysis to avoid or correct for peak interference of Fe Kβ. Among 10 minor and trace elements, exponential background regression models need to be applied to Al, Mn, and Ti elements, whereas other elements require linear background regression. Furthermore, to avoid Al and Zn surface contamination due to alumina polishing or brass presence, ultrasonic cleaning between each intermediate polishing steps and plasma cleaning immediately prior to EPMA experiments is highly recommended. Micro-inclusions such as chromite and spinel in olivine or adjacent Ca-rich phases need to be avoided to minimize primary or secondary fluorescence-related contamination on Al, Cr, or Ca. As a volatile element, Na element needs to be analyzed first with appropriate counting time to minimize the Na loss under high beam conditions. It needs mentioning that major elements (Mg, Fe, and Si) are best analyzed using MongOlSh11–2 or San Carlos olivine as primary standards for calibrations, which can yield more accurate data for both major elements and trace elements because of the improved matrix-corrections. Using our recommended analytical protocols, we have successfully discriminated “depleted” mantle olivine cores from an EMORB in northern East Pacific Rise (EPR) via Ca, Ti, Ni, Co, and Mn abundances. Our olivine data from Siqueiros Transform and the nearby 8°20′ N seamounts also help reveal a metasomatized peridotite mantle beneath the northern EPR. Overall, the protocols proposed in this study can serve as a guide for accurate EPMA olivine trace element analyses, which potentially contributes to the efforts of fostering a comparable olivine database worldwide.