Analysis and application of yttrium element in garnet by electron micro-probe analyzer: A case study of garnet-mica schist from Foziling Group

Abstract

石榴石是最重要的造岩矿物之一，通常能够保留早期的矿物结构和物质并记录较为晚期的变形和变质反应。石榴石钇（Y）元素环带特征丰富、复杂，不同的环带特征通常暗示不同的形成环境或经历了不同的变质事件，是变质演化历史研究的重要媒介之一。以往的研究中，多以LA-ICP-MS作为石榴石Y元素的主要分析手段，EPMA主要用于主量元素的分析。但是，LA-ICP-MS的束斑尺寸（44μm）和基底效应较EPMA （0~5μm）大，当石榴石颗粒小、包体和裂隙发育或成分环带以微区尺寸内存在较大变化时，大束斑更容易覆盖某些特殊信息。通过对石榴石Y元素测试参数的调试和标样验证，最终确定峰位测试时长和背景测试时长分别为140s和70s，并进行了PHA谱峰干扰剥离，降低检测限至54×10^-6。本文将通过对比佛子岭石榴云母片岩（LD025）4颗石榴石的EPMA主、微量原位分析（Ca、Mg、Mn、Fe、Y、Al、Si、Cr、Ti、Na）和LA-ICP-MS石榴石Y元素分析结果，论证EPMA分析Y元素的可行性。石榴石X-ray Mapping和主量成分剖面揭示该4颗石榴石均为生长环带，Mn呈钟形分带，Y与XSps呈强烈正相关性，与XGrs、XAlm、XPrp相关性不清晰。EPMA和LA-ICP-MS分析结果显示Y含量曲线在核部和幔部具有良好的一致性，Grt1~Grt3中Y均表现出自核部（500×10^-6~1200×10^-6）向幔部（200×10^-6~500×10^-6）逐渐降低，极边部Y含量低（20×10^-6~200×10^-6）且变化复杂；Grt4中Y含量差异相对较小（180×10^-6~450×10^-6），仅在边部出现不同程度的升降。因EPMA对于Y元素含量较低（<200×10^-6）时灵敏度不够或者LA-ICP-MS束斑尺寸大容易掩盖边部窄带成分真实变化等原因，二者在边部Y元素差异较大。分别对EPMA和LA-ICP-MS的分析结果应用Grt-Xtm温度计和Grt单矿物压力计获得的变质PT结果显示Grt1~Grt3（核-幔-边）和Grt4（核-边）均记录较为完整、统一的温压演化过程。M1→M2→M3的变质温压变化分别为T=530~544℃、P=0.78~0.82GPa→T=577~616℃、P=0.89~0.98GPa→T=631~661℃、P=1.01~1.07GPa，表现为顺时针演化型式，M1至M3反映的是一个暖俯冲过程。根据温度评价结果，Grt1~Grt3（1.2~1.4mm，自形程度高）形成时间应早于Grt4（0.8mm，自形程度低）。由此可知，大颗粒的石榴石Y元素含量及变化特征通常更容易揭示相对完整的变质演化历史。本次研究为变泥质岩演化历史、变质温压评价等研究提供了不同视角和思路，结合EPMA主量（矿物成分、X-ray mapping、BSE分析）和微量元素（Y等）分析能够更加精准、全面地解读地质信息。;Garnet is one of the most rock-forming mineral in the earth, it can record not only the former mineral's composition and structure, but also later deformation history and metamorphic reactions of rocks. In previous studies, LA-ICP-MS was mostly used as the main analysis method of yttrium (Y) in garnet, and EPMA was mainly used for analysis of major elements. However, the beam spot size (44μm) and basement effect of LA-ICP-MS are larger than those of EPMA (0~5μm). When garnet particles are small, inclusions and fractures are developed, or there are large changes in the micro area size of the composition ring, the large beam spot is easier to cover some special information. Through the debugging of garnet Y analysis parameters and standard sample verification, it is finally determined that the peak position and background counting time are 140s and 70s respectively, and the PHA spectrum peak interference stripping is carried out to reduce the detection limit to 54×10^-6. The paper will demonstrate the feasibility of EPMA analysis of Y by comparing the results of EPMA main and micro in-situ analysis (Ca, Mg, Mn, Fe, Y, Al, Si, Cr, Ti, Na) and LA-ICP-MS garnet Y content analysis. Garnet X-ray mapping and component profile reveal that the four garnets are characterize by growth ring, and the bell shaped zoning of Mn is typical. Among them, Y has a strong positive correlation with XSps, and the correlation with XGrs, XAlm and XPrp is not clear. The results of EPMA and LA-ICP-MS analysis show that the Y content curves are in good agreement both core and rim. Yttrium in Grt1~Grt3 is characterized by a gradual decrease from the core (500×10^-6~1200×10^-6) to the mantle (200×10^-6~500×10^-6), the extreme rim with low Y (20×10^-6~200×10^-6) and the change is complex. The difference of Y content in Grt4 is relatively small (180×10^-6~450×10^-6), and there are only different degrees of rise and fall at the outer edge. Due to the insufficient sensitively of EPMA to low Y content (<200×10^-6) or the large beam spot size of LA-ICP-MS, which is easy to cover up the real change of narrow rim Y composition, the two have great differences Y contents in the rim section. Based on the analysis results of EPMA and LA-ICP-MS, the metamorphic PT conditions obtained by Grt-Xtm thermometer and Grt single mineral geobarometer show that Grt1~Grt3 (core-mantle-rim) and Grt4 (core-rim) record a relatively complete and unified metamorphic PT evolution process. The metamorphic PT changes of M1→M2→M3 are T=530~544℃and P=0.78~0.82GPa→T=577~616℃ and P=0.89~0.98GPa→T=631~661℃ and P=1.01~1.07GPa respectively, showing a clockwise evolution pattern. M1 to M3 reflect a warm subduction process. According to the PT results, the formation time of Grt1~Grt3 (1.2~1.4mm, high automorphism) should be earlier than Grt4 (0.8mm, lower automorphism). Therefore, the content and variations characteristics of Y in large grain garnet are usually easier to reveal the relatively complete metamorphic evolution process. This study provides different perspectives and ideas for the study of metamorphic argillaceous rock evolution history and metamorphic temperature. Combined with EPMA major element (composition, X-ray mapping and BSE analysis) and trace element (Y, etc.) analysis, the geological information can be interpreted more accurately and comprehensively.