碳酸盐岩常用主微量元素、同位素分析测试结果差异性探讨:基于川中下寒武统龙王庙组实例研究

Variations in analytical results of commonly used major and trace elements and isotopic analyses in carbonate studies:a case study on the Lower Cambrian Longwangmiao Formation in central Sichuan Basin

碳酸盐岩研究中存在多种微量元素、同位素测试方法,为进一步探究各测试手段实际获取地化信息之间是否存在差异?能否进行比对?以川中下寒武统龙王庙组碳酸盐岩样品为例,通过对比研究中常用的主微量元素测试(电子探针[EPMA]、激光剥蚀—等离子质谱[LA-ICP-MS]、溶液法微量[ICP-OES])、氧同位素测定(原位离子探针[in-situ SIMS]、酸溶粉末)结果,揭示讨论各测试结果的差异性,旨在为降低地化信息的多解性提供经验依据。研究发现:(1)各测试手段(EPMA、LA-ICP-MS、ICP-OES)之间的差异性客观存在,但测试结果偏差整体随着所测元素实际浓度的增高而降低。对于主量元素(>10%)各测试结果偏差小于2%;富集元素(>1000μg/g)各测试结果处于测试误差之内,结果偏差小于6%;微量元素浓度区间(100~1000μg/g)各测试结果偏差显著增大,并且LA-ICP-MS与ICP-OES结果偏差要小于LA-ICP-MS与EPMA结果偏差,前者偏差幅度由6%增至45%,后者偏差幅度由9.1%增至151%;在低于100μg/g元素浓度区间,受矿物内非均质性影响LA-ICP-MS与ICP-OES的测试结果可相差几倍。(2)微区原位限定下,EPMA测试结果在邻近检测线区间(100~300μg/g)与LA-ICP-MS结果偏差逐渐加大,推测此偏差变化是由EPMA结果矫正过程中对低含量元素的矫正补偿机制所造成。(3)in-situ SIMS氧同位素值揭示了矿物微区尺度上的δ;O值差异,但其整体测试结果与传统酸溶法测试的结果存在0.5‰~2.5‰V-PDB的负偏偏差,推测此偏差可能来自于标样矫正转换误差。(4)实例样品中可见阴极发光特征与Fe、Mn含量无关的现象,故成岩流体判定更需结合岩石学、地化证据探讨。

There are multiple methods of analyzing trace elements and isotopes in studies on carbonate. Are there any differences in the actual geochemical information obtained by different methods?Are those results possibly to compare with each other?In this paper, we compare the results of the main trace element tests(electron probe micro-analyzer[EPMA], laser ablation-plasma mass spectrometry[LA-ICP-MS], and solution method trace[ICP-OES])and oxygen isotope measurements(in-situ ion probe[in-situ SIMS],and acid soluble powder)commonly used in carbonate studies. Taking the carbonates in the Lower Cambrian Longwangmiao Formation in central Sichuan Basin as an example, we reveal the variability in the results of different test methods, in order to provide references for reducing multi-resolution of geochemical information. The study reveals:(1)the variability among the test means(EPMA,LA-ICP-MS and ICP-OES)objectively exists, but the overall bias of the test results decreases with the increase of concentration of measured elements. For the major elements(> 10%),the standard deviation of each test result is less than 2%;For the enriched trace elements(> 1000 μg/g),the results are within the test error and the standard deviation of the results is less than 6%;For the concentration range of trace elements(100-1000 μg/g),the standard deviation of each test result increases significantly. The deviations of LA-ICP-MS and ICP-OES results are smaller than that of LA-ICP-MS and EPMA,with the deviations of the former increasing from 6% to 45% and those of the latter increasing from 9.1% to 151%. In the concentration range below 100 μg/g, the deviations of LA-ICP-MS and ICP-OES results can be several times different due to the heterogeneity within the minerals.(2)It was found that the deviation of the EPMA results from the LA-ICP-MS results in the adjacent detection line interval(ranging from 100 to 300 μg/g)gradually increases under the micro-area limitation, and it was speculated that this deviation was caused by the correction compensation mechanism for low content elements in the EPMA result correction process.(3)The in-situ SIMS oxygen isotope values reveal the differences in δ;O values at the micro-regional scale of minerals, but the overall test results are 0.5‰~2.5‰ V-PDB negative bias. Presumably, this deviation may come from the specimen correction conversion errors.(4)The cathodoluminescence characteristics of the samples are not related to the Fe or Mn contents, and thus it needs to explore the fluid source in conjunction with lithological and geochemical evidence.