As对SIMS测定黄铁矿34S/32S的基质效应

34S/32S (δ34S) of pyrite has extensive applications in a wide range of research areas of Earth sciences. In-situ high-resolution and high-precision determination of 34S/32S in pyrite by SIMS has been widely used. Electron microprobe and LA-ICP-MS have discovered a variety of trace elements of variable abundance in pyrites, and the concentration of arsenic can be up to ca. 20 wt.%. Unlike the analysis of sulphur mass independent fractionation Δ33S and Δ36S that is independent of sulphide composition, analyzing δ34S with SIMS can be offset by the matrix effects resulting from compositional discrepancies between reference materials and unknowns. However, such bias has not been investigated systematically yet. This project aims to quantify the matrix effects of arsenic in pyrite. Pyrites containing arsenic of five different abundances (10-1, 10-2, 10-3, 10-4, 10-5) will firstly be synthesized experimentally. After confirmation of the structure and composition of these synthesized pyrite qualitatively and quantitatively, they will be measured for 34S/32S using SIMS. Conventional bulk analysis will also be performed, the results of which will be compared to the counterparts of SIMS (calibrated by reference material) and the bias will thus be obtained. By regression of the data of arsenic contents and δ34S offsets (Δ34S), in addition to appropriate extrapolations, a mathmatical equation can be obtained and the matrix effects of arsenic can thus be quantified. The results of this project can be applied to the correction of 34S/32S measured by SIMS, thereby improving the accuracy.

SIMS原位分析黄铁矿34S/32S已广泛使用。EPMA和LA-ICP-MS已揭示出黄铁矿含有多种不同浓度的微量元素，其中As含量可高达20wt.%。不同于硫同位素非质量分馏Δ33S和Δ36S不受硫化物成分影响，SIMS原位分析δ34S会受到参照物质与未知样之间成分差异的影响，即基质效应。然而As对SIMS测定黄铁矿34S/32S的基质效应还未被系统地研究过。本项目拟将As的基质效应定量化。首先在实验室合成五种不同数量级As含量的含As黄铁矿。然后用SIMS测定每种As含量黄铁矿的34S/32S。再用传统全硫化物方法测定每种As含量黄铁矿的34S/32S（真实值），将其与SIMS测得值相减得到δ34S偏移量（Δ34S）。拟合不同As含量与相应Δ34S，得到二者的数学关系式，从而将As的基质效应定量化。

该项目旨在定量化镍对SIMS测定黄铁矿34S/32S的基质效应。为此，项目首先通过化学气体传输法合成了不同镍含量的黄铁矿。实验产物通过激光拉曼光谱和电子探针确定为黄铁矿；镍含量由电子探针和LA-ICPMS测得，变化范围为0–21700 ppm；黄铁矿的原位硫同位素组成δ34S值由SHRIMP-II测得，不同颗粒和单颗粒的δ34S值较为均一；黄铁矿的全硫化物δ34S值由MAT253、Flash EA 2000型元素分析仪以及Conflo IV连续流装置获得，气体质谱全硫化物δ34S值较为一致，且与原位δ34S值吻合。黄铁矿中镍元素所引起的基质效应为负Δ34S，即SIMS测得的δ34S值比气体质谱测得的真实值略小。拟合本次测得数据，得到镍引起的基质效应Δ34S与对应镍含量的数学表达式为Δ34S=−0.26C(Ni)−0.40（R2=0.13；C(Ni)是用电子探针测得的黄铁矿中的镍含量，单位是10−2）或Δ34S=−3×10−5C(Ni)−0.41（R2=0.12；C(Ni)是用LA-ICPMS测得的黄铁矿中的镍含量，单位是ppm。