高温熔融研制钾长石玻璃标准物质初探

摘要

激光剥蚀多接收等离子体质谱( LA - MC - ICPMS)是进行原位微区分析微量元素和同位素的重要技术之一,标准样品与样品之间的基体匹配是解决影响该技术准确分析的基体效应和分馏效应的首选方案.长石(特别是长石微区)的Pb同位素组成是示踪岩石形成和演化历史的重要途径,而LA - MC - ICPMS技术则是进行长石Pb同位素原位微区分析的关键技术,然而目前国内外尚没有合适的长石Pb同位素分析标准.文章研究探讨了利用高温炉进行原位微区分析钾长石中Pb同位素组成所用外部标准物质合成条件,结果表明,常规的74 μm(200目)碎样无法得到均一的钾长石玻璃,需要将初始钾长石粉末研磨至1300目以下；高温炉合成温度为1680℃；熔融时间为2h;采用液氮方式淬火.制成的钾长石玻璃除表面具有轻微的不均一性外,内部的Pb同位素比值为1.90779±0.00009(208Pb/206Pb,2s),0.75899±0.00004(207 Pb/206 Pb,2s),20.909±0.002(206 Pb/204 Pb,2s),15.871±0.002(207 Pb/204 Pb,2s)和39.888±0.005( 208 Pb/204 Pb,2s),相应的相对标准偏差(RSD)分别为0.007％、0.008％、0.016％、0.016％和0.021％.表明利用本研究方法合成的钾长石玻璃可作为潜在的钾长石中Pb同位素组成原位微区分析外部校准物质.%Laser ablation Quadruple and multiple collector ICP-MS (LA-Q & MC-ICPMS) is one of the most important analytical technique in terms of in situ analysis of trace elements and isotopic compositions. A matrix-matched standard is the principal way to obtain precise measurements by reducing mass fractionation and matrix interferences. Lead isotopic composition of K-feldspar is one of the important proxies to trace the history of rock formation and evolution. However, there is no appropriate external standard for in situ Pb isotope analysis using LA-MC-ICPMS. This paper reports on work done on the synthesis of K-feldspar glass in a high temperature furnace. The final experimental conditions of the synthesis were to melt the 1300 mesh K-feldspar powders at 1680°C for 2 hours followed by liquid nitrogen quenching. Routine 74 μm (200 mesh) K-feldspar is not good enough to produce homogeneous K-feldspar standard glass. The surface glass is slightly heterogeneous due to lead evaporation at high temperature, while the inside of the glass is homogenous. Results showed that the lead isotopic compositions of the inner glass are 1. 90779 ±0.00009 (208pb/206pb, 2s), 0.75899 ±0.00004 (207Pb/206Pb, 2s), 20.909 ±0.002 (206Pb/204Pb, 2a), 15.871 ±0.002(207Pb/204Pb, 2s) and 39. 888 ± 0. 005 (208 Pb/204 Pb, 2s), and the RSDs were 0. 007%, 0. 008% , 0. 016% , 0.016% and 0.021%, respectively. Therefore, synthesized K-feldspar glass could potentially serve as an external calibration standard for in situ lead isotope measurements.