Electron probe microanalysis and microscopy: Principles and applications in characterization of mineral inclusions in chromite from diamond deposit
Electron probe microanalysis and microscopy: Principles and applications in characterization of mineral inclusions in chromite from diamond deposit
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DOI：
10.1016/j.oregeorev.2014.09.020
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发表时间：
2015-03
影响因子：
3.3
通讯作者：
Donggao Zhao;Youxue Zhang;E. Essene
中科院分区：
文献类型：
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作者：
Donggao Zhao;Youxue Zhang;E. Essene
电子探针微分析和显微技术是一种广泛应用的现代分析技术，主要用于固体材料化学成分的定量分析，以及在微米或纳米尺度上对样品的元素分布或表面形貌进行测绘或成像。这项技术使用由电子枪产生的电磁透镜聚焦电子束来轰击样品。当电子束与样品相互作用时，从相互作用体积中产生二次电子、背散射电子和特征x射线等信号。这些信号随后由探测器检测，以获取样品的化学和成像信息。电子探针的独特之处在于它配备了多个WDS x射线光谱仪，每个光谱仪都有多个衍射晶体，以便同时分析多种元素。电子探针能够分析几乎所有的元素（从Be到U），其空间分辨率在微米级或以下，检测极限低至几ppm。本文以辽宁瓦房店金伯利岩铬铁矿中的矿物包裹体为例，介绍了电子探针显微分析和显微技术在矿床伴生矿物，特别是金刚石矿床伴生矿物表征中的应用。化学分析和SE、BSE成像表明，铬铁矿中的矿物包裹体包括无水硅酸盐、含水硅酸盐、碳酸盐和硫化物，以离散或单一矿物包裹体或复合多矿物包裹体的形式出现。铬-橄榄石对是电子探针分析橄榄石中铬的一个重要问题。铬铁矿中的铬被橄榄石中的铁所二次荧光，使铬铁矿中橄榄石包裹体中cr2o3的表观含量显著增加。根据电子探针测得的化学成分，利用适用的地温计计算出铬铁矿及其矿物包裹体的形成温度和形成压力在46 kbar和980℃至53 kbar和1130℃之间，处于金刚石稳定场范围内，因此富铬铬铁矿是寻找金伯利岩和金刚石矿床的有用指示矿物。由硅酸盐和碳酸盐矿物组成的铬铁矿复合包裹体的总体成分为33.2% SiO2、2.5 wt.% Al2O3、22.0% MgO、7.5 wt.% CaO、2.5 wt.% BaO、0.8 wt.% K2O、25.5 wt.% CO2和0.8 wt.% H2O，与瓦房店金伯利岩的化学成分相似，表明其为圈闭金伯利岩岩浆。
Electron probe microanalysis and microscopy is a widely used modern analytical technique primarily for quantifying chemical compositions of solid materials and for mapping or imaging elemental distributions or surface morphology of samples at micrometer or nanometer-scale. This technique uses an electromagnetic lens-focused electron beam, generated from an electron gun, to bombard a sample. When the electron beam interacts with the sample, signals such as secondary electron, backscattered electron and characteristic X-ray are generated from the interaction volume. These signals are then examined by detectors to acquire chemical and imaging information of the sample. A unique part of an electron probe is that it is equipped with multiple WDS spectrometers of X-ray and each spectrometer with multiple diffracting crystals in order to analyze multiple elements simultaneously. An electron probe is capable of analyzing almost all elements (from Be to U) with a spatial resolution at or below micrometer scale and a detection limit down to a few ppm.Mineral inclusions in chromite from the Wafangdian kimberlite, Liaoning Province, China were used to demonstrate the applications of electron probe microanalysis and microscopy technique in characterizing minerals associated with ore deposits, specifically, in this paper, minerals associated with diamond deposit. Chemical analysis and SE and BSE imaging show that mineral inclusions in chromite include anhydrous silicates, hydrous silicates, carbonates, and sulfides, occurring as discrete or single mineral inclusions or composite multiple mineral inclusions. The chromite–olivine pair poses a serious problem in analysis of Cr in olivine using electron probe. Secondary fluorescence of Cr in chromite by Fe in olivine drastically increases the apparent Cr2O3content of an olivine inclusion in a chromite. From the chemical compositions obtained using electron probe, formation temperatures and pressures of chromite and its mineral inclusions calculated using applicable geothermobarometers are from 46 kbar and 980 °C to 53 kbar and 1130 °C, which are within the stability field of diamond, thus Cr-rich chromite is a useful indication mineral for exploration of kimberlite and diamond deposit. A composite inclusion in chromite composed of silicate and carbonate minerals has a bulk composition of 33.2 wt.% SiO2, 2.5 wt.% Al2O3, 22.0 wt.% MgO, 7.5 wt.% CaO, 2.5 wt.% BaO, 0.8 wt.% K2O, 25.5 wt.% CO2, and 0.8 wt.% H2O, similar to the chemical composition of the Wafangdian kimberlite, suggesting that it is trapped kimberlitic magma.