Analysis and Application of Yttrium Element in Garnet by Electron Micro-Probe Analyzer: A Case Study of Garnet-Mica Schist from Foziling Group

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 (44 mu m) and basement effect of LA-ICP-MS are larger than those of EPMA (0 similar to 5 mu m). 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 x 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 similar to Grt3 is characterized by a gradual decrease from the core (500 x 10(-6)similar to 1200 x 10(-6)) to the mantle (200 x 10(-6)similar to 500 x 10(-6), the extreme rim with low Y (20 x 10(-6)similar to 200 x 10(-6)) and the change is complex. The difference of Y content in Grt4 is relatively small (180 x 10(-6)similar to 450 x 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 x 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 similar to 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 similar to 544 degrees C and P = 0. 78 similar to 0. 82GPa -> T = 577 similar to 616 degrees C and P = 0. 89 similar to 0. 98GPa -> T = 631 similar to 661 degrees C and P = 1. 01 similar to 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 similar to Grt3 (1. 2 similar to 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.