石榴子石Fe^(3+)含量电子探针原位分析:Flank Method方法的实例应用

Microprobe analysis of ferric iron in garnet: The Flank Method and case application

电子探针分析技术主要应用于对物质进行快速无损的定性、定量分析,是材料科学和地质学领域最基础和重要的研究手段。而利用电子探针对矿物中不同价态Fe的含量进行原位定量分析,更是具有非常重要的科学意义。近来发展起来的可进行Fe^(3+) 含量定量分析的电子探针Flank Method测试方法,相比早期的实验方法具有更高的准确度和精确性。该方法主要是通过定义Fe的特征X射线L能级谱线峰位的峰侧位置(flank position),对应了不同价态Fe吸收谱线的差值线谱(difference spectrum)中强度计数最大值(FeLα*)和最小值(FeLβ*)的位置,从而避免了在早期实验中,一般通过直接测量Fe的L能级峰位(FeLα和FeLβ)可能导致的测试不确定性。同时,该测试方法通过对电子探针谱仪系统进行实时的位移误差校正,确保其在某一个稳定的实验条件(周期)下具有可重现性(精确度)。在本文中,我们根据北京大学造山带与地壳演化教育部重点实验室的JEOL JXA-8100型号电子探针的实际性能状态,在前人建立的实验步骤基础上,进行了测试流程的自适性改进和修正,建立了更符合本实验室实际情况的电子探针Flank Method测试方法,用于分析石榴子石中的Fe^(3+) 含量。通过对已知Fe^(3+) 含量的标准石榴子石样品进行检测验证,确定了该方法的可行性、可靠性以及可重复性。在对天然样品的分析中,我们选择了来自我国天山、拉萨和北祁连的洋壳俯冲类型中低温(超)高压榴辉岩中的石榴子石样品,对其进行了Fe^(3+) 含量(环带)的分析。结果显示,石榴子石中Fe^(3+) 含量可作为一个可靠的氧逸度指示剂,其(环带)变化很好地对应了榴辉岩寄主岩石的变质演化轨迹,反映了在板片俯冲过程中,榴辉岩的氧逸度具有降低的趋势。

The electron microprobe(EMP)analysis is a quick and non-destructive method for qualitative and quantitative analyses upon samples from material science and geoscience,which plays a fundamental and vital role in these subjects.It is of great scientific significance in the geological research to use EMP to determine the in-situ contents of Fe with different valences(ferric and ferrous)in minerals.The newly developed microprobe-based Flank Method measurement to quantify the Fe3+content in minerals is comparatively of much higher precision and greater credibility than the early experimental approaches.The Flank Method measures the defined flank positions instead of the actual peaks of the iron’s X-ray characteristic L emission spectra(FeL);those flank positions correspond to the maxima(Feα*)and minima(FeLβ*)values of the iron’s absorption difference spectrum.Meanwhile,a spectrometer calibration procedure is designed for the peak shift error correction prior to the Flank Method measurement in order to maintain the analytical reliability and reproducibility.This calibration procedure should be performed in each working session within a proper time duration.In this work,we propose a modified and improved Flank Method measuring procedure after the early suggestions,to analyze the Fe3+content in the garnet by using JEOL JXA-8100 microprobe in our facility.The feasibility,reliability and repeatability of this method has been well tested on several standard garnets with known Fe3+contents in our laboratory,and then it is applied on some natural garnet samples.The analyzed garnets are from the typical oceanic subduction type medium-low temperature and(ultra-)highpressure eclogites of SW Tianshan(Habutengsu),Lhasa terrane(Songduo area)and North Qilian(Baijingsi foliate zone).The results show that the Fe3+content serves a good indicator of the oxidation state,and its(zonal)change in garnet corresponds to the metamorphic P-T path,suggesting that the oxygen-fugacity is probably prone to decline in the subduction process.