UBIQUITOUS EXSOLUTION OF PENTLANDITE AND TROILITE IN PYRRHOTITE FROM THE TIL 91722 CM2 CARBONACEOUS CHONDRITE: A RECORD OF LOW TEMPERATURE SOLID STATE
UBIQUITOUS EXSOLUTION OF PENTLANDITE AND TROILITE IN PYRRHOTITE FROM THE TIL 91722 CM2 CARBONACEOUS CHONDRITE: A RECORD OF LOW TEMPERATURE SOLID STATE
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TIL 91722 CM2 碳质球粒陨石中镍黄铁矿和硫铁矿在磁黄铁矿中的普遍溶出：低温固态记录
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发表时间：
2010
期刊：
影响因子：
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通讯作者：
C. Martínez
中科院分区：
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作者：
A. Brearley;C. Martínez
简介：CM陨石是一个非常多样化的陨石群，其早期太阳系的记录是所有陨石群中最复杂的。认识到这些陨石表现出不同程度的水蚀变，导致了相当大的努力，以发展水蚀变程度的指标[1-4]。了解CM钙钛矿在水蚀变过程中的演化的一个关键组成部分是解决不同的元素如何被水流体动员和重新分配的问题。硫在这一过程中起着核心作用，因为它存在于CM钙钛矿中的各种矿物中，包括托基利尼石、硫化物和硫酸盐。硫化物一直是人们关注的焦点[5-7]，并且已经认识到存在的关键矿物相是镍黄铁矿和磁黄铁矿[5-9]。然而，这些阶段之间的关系及其起源仍然知之甚少。在这里，我们提出了新的扫描电镜和透射电镜观察硫化物从CM提供额外的见解，这些颗粒的起源。技术：使用FEI Quanta 3D FEGSEM对CM球粒陨石TIL 91722的薄切片进行SE和BSE成像研究。在JEOL 5200 LV SEM上从单个颗粒获得定性EDS光谱。使用FEI Quanta 3D DualBeam® FEGSEM/FIB制备来自单个硫化物晶粒的TEM切片。使用Omniprobe 200显微操作器从薄切片中取出FIB制备的样品。结果：我们研究了20个来自TIL 91722的磁黄铁矿颗粒，大小从30到60 μm不等。鉴定出两种不同的矿点：1）磁黄铁矿和镍黄铁矿的聚集体，由两相的许多单个颗粒组成; 2）更常见的结晶良好的磁黄铁矿颗粒，通常是磁黄铁矿和镍黄铁矿的复合物。磁黄铁矿是这些颗粒中体积最丰富的相。镍黄铁矿通常以不规则形状的颗粒存在，尺寸可达20 μm，沿着磁黄铁矿晶体的外围分布。在某些磁黄铁矿颗粒内部也有细长的、近圆形的镍黄铁矿。这些产状是其他CM磁铁矿中磁黄铁矿的典型产状，例如[5-9]。TIL 91722中的磁黄铁矿颗粒均未显示任何蚀变迹象。FEGSEM研究表明，在所研究的每个磁黄铁矿颗粒中，镍黄铁矿的亚微米出溶是明显的（图1）。镍黄铁矿以晶体取向的薄片形式存在，长度可达6 μm，宽度<0.1 μm，以及通常长<0.2 μm，宽<0.1 μm的细长气泡。镍黄铁矿的出溶已被记录在CM磁铁矿的磁黄铁矿中[7-9]，但这种现象的广泛发生尚未得到普遍认可。除了镍黄铁矿包裹体外，磁黄铁矿颗粒还含有不规则域，其显示出比磁黄铁矿主体更低的BSE对比度（图2）。这些域具有正弦或火焰状纹理，并且在几百纳米的尺度上发生。它们似乎分布在整个磁黄铁矿颗粒，并存在于所有的粗粒单晶进行了研究。从磁黄铁矿颗粒中取出的FIB样品的TEM观察揭示了复杂的显微结构。贯穿磁黄铁矿的是从~0.5 μm到<0.1 μm的伸长的、出溶的镍黄铁矿气泡。AEM分析表明镍黄铁矿含Co。虽然在常规TEM图像中基本上不可见，但DF STEM Z衬度图像显示，该晶粒呈现出显著的火焰状微观结构（图3），由更富Fe的硫化物与较低Z相共生的薄片组成。从这种共生的电子衍射图案表明，占主导地位的（低Z）相是单斜磁黄铁矿共生六角硫铁矿。
Introduction: CM chondrites are a remarkably diverse group of meteorites whose early solar system record is among the most complex of all the meteorite groups. The recognition that these meteorites exhibit variable degrees of aqueous alteration has led to considerable efforts to develop indicators of the extent of aqueous alteration [1-4]. A key component of understanding the evolution of CM chondrites during aqueous alteration is addressing the issue of how different elements may be mobilized and redistributed by aqueous fluids. Sulfur plays a central role in this process, because it is present in a wide range of minerals in CM chondrites, including tochilinite, sulfides and sulfates. Sulfides have been the focus of considerable interest [5-7], and it has been recognized that the key mineral phases that are present are pentlandite and pyrrhotite [5-9]. The relationships between these phases and their origins still remain poorly understood, however. Here we present new SEM and TEM observations of sulfides from a CM that provide additional insights into the origin of these grains. Techniques: A thin section of the CM chondrite TIL 91722 was studied with SE and BSE imaging using a FEI Quanta 3D FEGSEM. Qualitative EDS spectra were obtained from individual grains on a JEOL 5200LV SEM. TEM sections from individual sulfide grains were prepared using a FEI Quanta 3D DualBeam® FEGSEM/FIB. FIB-prepared samples were removed from the thin sections using the in situ lift out technique with an Omniprobe 200 micromanipulator. Results: We studied 20 pyrrhotite grains from TIL 91722, ranging in size from 30 to 60 μm. Two different occurrences were identified: 1) aggregates of pyrrhotite and pentlandite consisting of numerous individual grains of both phases and 2) more common, well-crystallized grains of pyrrhotite that are usually composites of pyrrhotite and pentlandite. Pyrrhotite is volumetrically the most abundant phase in these grains. Pentlandite usually occurs as irregular-shaped grains up to 20 μm in size along the periphery of the pyrrhotite crystals. Elongate, subrounded pentlandite also occurs in the interior of some pyrrhotite grains. These occurrences are typical of pyrrhotite in other CM chondrites, e.g. [5-9]. None of the pyrrhotite grains in TIL 91722 show any evidence of alteration. FEGSEM studies show that in every pyrrhotite grain studied, submicron exsolution of pentlandite is evident (Figure 1). The pentlandite occurs as both crystallographically-oriented lamellae up to 6 μm in length and <0.1 μm wide and as elongate blebs that are typically <0.2 μm long and <0.1 μm wide. Exsolution of pentlandite has been documented in pyrrhotite in CM chondrites [7-9], but the widespread occurrence of this phenomenon has not been generally recognized. In addition to inclusions of pentlandite, the pyrrhotite grains also contain irregular domains which show lower BSE contrast than the pyrrhotite host (Fig 2). These domains have a sinusoidal or flame-like texture and occur on the scale of a few 100s of nanometers. They appear to be distributed throughout pyrrhotite grains and are present in all the coarse-grained single crystals that were studied. TEM observations of a FIB sample removed from a pyrrhotite grain reveal a complex microstructure Elongate, exsolved pentlandite blebs ranging from ~0.5 μm down to <0.1 μm are distributed throughout the pyrrhotite. AEM analyses show that the pentlandite is Co-bearing. Although essentially invisible in conventional TEM images, DF STEM Z-contrast images show that the grain exhibits a remarkable flame-like microstructure (Figure 3) consisting of lamellae of a more Fe-rich sulfide intergrown with a lower Z phase. Electron diffraction patterns from this intergrowth show that the dominant (lower Z) phase is monoclinic pyrrhotite intergrown with hexagonal troilite.