FIB-TEM Study of Mineral Inclusions in Chromite
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摘要: 铬铁矿矿物包裹体可以记录其成岩成矿时物理化学条件（温度、压力等）、化学成分演化、熔/流体富集活动规律等关键信息.然而传统的二维分析方法无法全面获取形态微小、成分复杂的包裹体信息.通过聚焦离子束-透射电镜（FIB-TEM）联用对华北克拉通遵化豆荚状铬铁矿中矿物包裹体进行观察测试，在三维空间上发现包裹体的矿物种类丰富（硅酸盐、铂族、碳酸盐等），矿物形态复杂多变，且包裹体矿物中发育位错、部分开放晶界/相界、熔流体痕迹等显微-超显微结构.因此综合矿物种类、形态、显微结构等信息推断遵化铬铁矿具有复杂的形成条件和演化过程.Abstract: Mineral inclusions in chromite can record key information of physicochemical conditions (pressure, temperature, etc.), evolution of chemical composition, mechanics of fluid enrichment activities when chromite and its host rock form. However, traditional two-dimensional analytical methods cannot fully reveal the information in tiny and complex inclusions. In this paper, mineral inclusions in the Zunhua podiform chromite from the North China craton were observed and tested by the focused ion beam combined with transmission electron microscopy (FIB-TEM). The mineral types of inclusions are various and mainly include silicate, platinum group mineral, carbonate, and so on. The morphology of these mineral inclusions is also various and complex. Many microstructures and ultra-microstructures are developed in these mineral inclusions such as dislocations, partially open grain/phase boundaries, fluid- or melt-present marks. Therefore, it can be inferred that the Zunhua chromite has complex formation conditions and evolution processes based on the information of mineral types, morphology, and microstructures.
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图 1 华北克拉通构造简图(a)；研究区详细地质图(b)
a图改自Kusky et al.(2016)，图中红框代表研究区所在的位置；豆荚状铬铁矿产出在图b的超镁铁质岩石中
Fig. 1. (a) Tectonic units of the North China Craton and the location of the study area (red square), (b) detailed geological map of the study area
图 2 图 2遵化蛇绿混杂岩中豆荚状铬铁矿的豆状、环状和浸染状结构照片
a.豆状、环状铬铁矿的手标本照片；b.豆状、浸染状结构的岩片抛光图；c.豆状、环状结构铬铁矿的镜下照片(单偏光)；d.豆状、环状结构铬铁矿的镜下照片(反射光)
Fig. 2. Images of disseminated, nodular and orbicular textures of podiform chromitite from the Zunhua ophiolitic mélange
图 3 亥姆霍兹波茨坦研究中心-德国地球科学研究中心FEI FIB200聚焦离子束系统(Richard Wirth短期课程.中国地质大学, 武汉)
Fig. 3. Focused ion beam (FEI FIB200) in Helmholtz-Centre Potsdam-GFZ German Research Centre for Geosciences (from short course by Richard Wirth at CUG, Wuhan)
图 4 FIB制备TEM观察薄膜的主要步骤
图改自Wirth(2009)；a.样品表面的背散射图像，显示了待制样的位置；b.两个“×”以及之间的铂沉积保护层；c.图像显示样品已被切割，待取出；d.配备显微操作器的光学显微镜；e.将FIB制作的样品放置在已喷碳的TEM标准样品铜网载片上
Fig. 4. Major steps of TEM foil prepared by FIB
图 5 TEM鉴定矿物相的主要步骤
a.获取待测矿物相高分辨率电子显微镜(HREM)图像；b.HREM图像经过快速傅里叶转换(FFT)获得的衍射图样；c.在衍射图样上测得晶体参数(a，b，c，α，β)；d.依据已知矿物化学组成和晶体参数与矿物晶体数据库(例如ICSD)中已知矿物相进行匹配，最终标定矿物相
Fig. 5. Major steps of phase identification by TEM
图 6 遵化蛇绿混杂岩豆荚状铬铁矿中不同类型矿物包裹体的典型背散射电子图像(BSE)
a.橄榄石(Ol)；b.角闪石(Amp)；c.铂族矿物(PGM)；d.菱镁矿(Mgs)；e.金红石(Rt)；f.两矿物相-角闪石(Amp)+磷灰石(Ap)；g.三矿物相-单斜辉石(Cpx)+黑云母(Bt)+磷灰石(Ap)；h.四矿物相-黑云母(Bt)+单斜辉石(Cpx)+磷灰石(Ap)+白云石(Dol)；i.五矿物相-单斜辉石(Cpx)+黑云母(Bt)+硬石膏(Anh)+角闪石(Amp)+白云石(Dol)
Fig. 6. Typical backscattered electron (BSE) images of different kinds of mineral inclusions in podiform chromite from Zunhua ophiolitic mélange
图 7 铬铁矿中顽火辉石等硅酸盐矿物的光学显微镜照片(a)，扫描电镜下的背散射照片(b)以及透射电镜下的高角度环形暗场(HAADF)照片(c)
图b中白线为聚焦离子束(FIB)的切样位置.图c中上部发亮部分为FIB制样过程中保留的铂保护层
Fig. 7. Photomicrograph (a), BSE under scanning electron microscopy (b) and high angle annular dark field (HAADF) image under transmission electron microscopy (c) of silicate minerals such as pyroxene in chromite
图 8 铬铁矿中PGM包体扫描电镜下的背散射照片(a)，透射电镜下的高角度环形暗场(HAADF)照片(b)以及PGM包裹体的EDS分析数据(c)
图a中白线代表聚焦离子束(FIB)的切样位置.EDS的分析位置标记在图b
Fig. 8. BSE under scanning electron microscopy(a), high angle annular dark field (HAADF) image under transmission electron microscopy (b) and EDS analyses data (c) of PGM inclusion in chromite
图 9 铬铁矿中贱金属硫化物及其他矿物的光学显微镜照片(a)，扫描电镜下的背散射照片(b)以及透射电镜下的高角度环形暗场(HAADF)照片(c)
图b中白线为聚焦离子束(FIB)的切样位置
Fig. 9. Photomicrograph (a), BSE under scanning electron microscopy (b) and high angle annular dark field (HAADF) image under transmission electron microscopy (c) of base metal minerals and other mineral in chromite
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