LPSC XXIII 153 MINERALOGY OF FINE-GRAINED MATRIX IN THE IVUNA CI CARBONACEOUS
LPSC XXIII 153 MINERALOGY OF FINE-GRAINED MATRIX IN THE IVUNA CI CARBONACEOUS
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LPSC XXIII 153 IVUNA CI 碳质岩中细粒基质的矿物学
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发表时间：
2009
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通讯作者：
C. Adrian
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作者：
C. Adrian
在宇宙化学上，CI碳质陨石被认为是最原始的陨石物质，因为它们的组成与太阳光球的组成密切相关[I]。尽管它们的组成是未分级的，但现在广泛认识到，存在于Cl方解石中的矿物不是原始缩合产物，而是复杂过程的结果，该过程可能涉及富水流体相与Cl母体上的无水前体材料的相互作用[2]。这些矿物-流体反应导致硫化物、氧化物、碳酸盐、硫酸盐和超细层状硅酸盐的形成[3]。直到最近，人们对氯镁石基质中细粒矿物的确切性质还知之甚少，因此蚀变条件和蚀变反应机制还不清楚。最近对CI基质的TEM研究[4，5，6]已经开始揭示CI球粒陨石细尺度矿物学的差异，这为CI球粒陨石母体上发生的蚀变过程提供了重要的见解。为了扩大数据库的良好特性的CI矩阵我进行了详细的扫描电镜，微探针和TEM研究基质中的Ivuna CI球粒陨石，在不断努力了解这些复杂的陨石的演变。虽然已经描述了Ivuna的矿物学和岩石学的几个方面[7，8，9，10]，但迄今为止尚未对基质的细粒矿物学进行详细研究。宽束（10 μ m直径）电子显微探针分析和扫描电镜研究的Ivuna矩阵进行了薄切片，随后通过TEM研究。用电子探针测得的基体组成与McSween和Richardson的数据非常吻合[7]。元素比率模式，标准化为Si和Cl体值，显示Ca、Ti、Mn、Na和S的特征性贫化和K的富集，这是典型的Cl球粒陨石基质[6]。电子探针数据和BSE图像都表明，至少在所研究的样品中，基质在成分上是相对均匀的。没有发现像Orgueil [4]那样的具有高体积Fe的碎屑，尽管Ivuna也是角砾岩[11]。硫化物、氧化物和碳酸盐散布在整个基质中。与Orgueil一样，Ivuna基质中存在具有多种不同形态的磁铁矿，包括片状和草莓状形态。硫化物（硫铁矿和镍黄铁矿）往往具有半自形形态。颗粒大小通常<10 μ m，但也有罕见的高达100 μ m的大颗粒。与Orgueil基质相比，Ivuna基质含有相对较少的细粒磁铁矿，Orgueil基质中含有大量直径<1 μ m的细粒磁铁矿颗粒。伊武纳的TEM研究证实，细粒磁铁矿稀疏分布在基质中。基质主要由具有两种不同总体形态的层状硅酸盐矿物大区域组成。在Orgueil中观察到了类似的形态[4]。伊武纳的大部分基质由随机取向的、非常细的颗粒（<30 nm宽）、结晶不良的层状硅酸盐组成。这些细粒层状硅酸盐的高分辨率TEM成像和分析电子显微镜显示，它们主要由蛇纹石（0.7 nm的基底间距）与少量皂石蒙皂石（1.1-1.3 nm的基底间距）夹层。这种类型的共生似乎是CI基质层状硅酸盐的独特特征[4，6]。细粒蛇纹岩通常具有板状或略微弯曲的形态，尽管也观察到了卷状和圆柱状。与Orgueil不同的是，没有证据表明氧化铁相，如水铁矿，与这些细粒层状硅酸盐密切共生。第二组层状硅酸盐以离散的、不规则形状的簇（尺寸为1-4 μ m）出现，其由辐射状或亚平行的片状层状硅酸盐晶体组成。这些
Cosmochemically, the CI carbonaceous chondrites are regarded as the most primitive examples of meteoritic material, because their compositions correlate closely with that of the photosphere of the sun [I]. Despite their unfractionated compositions, it is now widely recognized that the minerals present in CI chondrites are not primitive condensation products, but are the result of complex processes which probably involved the interaction of a water-rich fluid phase with an anhydrous precursor material on the CI parent body [2]. These mineral-fluid reactions resulted in the formation of sulfides, oxides, carbonates, sulfates and ultrafinegrained phyllosilicates [3]. Until recently the exact nature of the fine-grained minerals in the matrix of CI chondrites was poorly understood and as a result the conditions of alteration and the mechanisms of the alteration reactions were not well-defined. Recent TEM studies of CI matrix [4,5,6] have begun to reveal differences in the fine-scale mineralogy of CI chondrites, which provide important insights into alteration processes which occurred on the CI chondrite parent body. To expand the database of well-characterized CI matrices I have carried out a detail SEM, microprobe and TEM study of matrix in the Ivuna CI chondrite, in a continuing effort to understand the evolution of these complex meteorites. Although several aspects of the mineralogy and petrology of Ivuna have been described [7,8,9,10], no detailed study of the finegrained mineralogy of the matrix has been undertaken to date. Broad beam (10 pm diameter) electron microprobe analyses and SEM studies of Ivuna matrix were performed on a thin section which was subsequently studied by TEM. The matrix composition obtained by electron microprobe is in excellent agreement with the data of McSween and Richardson [7]. Elemental ratio patterns, normalized to Si and CI bulk values, show the characteristic depletions in Ca, Ti, Mn, Na and S and an enrichment in K, typical of CI chondrite matrices [6]. Both electron microprobe data and BSE images indicate that, at least in the sample studied, the matrix is compositionally relatively homogeneous. No clasts with high bulk Fe, such as those found in Orgueil [4] were found, although Ivuna is also a breccia [ l l ] . Disseminated throughout the matrix are sulfides, oxides and carbonates. Like Orgueil, magnetite with several different morphologies occurs in Ivuna matrix, including platelet and framboidal morphologies. Sulfides (pyrihotite and pentlandite) tend to have subhedral morphologies. Grain sizes are typically <10pm, but rare, large grains up to 100pm in size also occur. In comparison with Orgueil matrix, which contains a myriad of fine-grained magnetite particles <1pm in diameter throughout the matrix, Ivuna matrix contains relatively few fine-grained magnetites. TEM studies of Ivuna have confirmed that fine-grained magnetite is sparsely distributed in the matrix. The matrix is predominated by large regions of phyllosilicate minerals with two distinct overall morphologies. Comparable morphologies have been observed in Orgueil [4]. The bulk of the matrix in Ivuna consists of randomly oriented, very fine-grained (<30nm wide), poorly crystalline phyllosilicates. High resolution TEM imaging and analytical electron microscopy of these fine-grained phyllosilicates shows that they consist largely of serpentine (0.7 nm basal spacing) interlayered with minor saponitic smectite (1.1-1.3 nm basal spacing). This type of intergrowth appears to be a unique characteristic of CI matrix phyllosilicates [4,6]. The fine-grained serpentines typically have platy or slightly curved morphologies, although rolled and cylindrical types have also been observed. Unlike Orgueil, there is no evidence of iron oxide phases, such as ferrihydrite, occurring intimately intergrown with these fine-grained phyllosilicates. The second group of phyllosilicates occurs as discrete, irregularly-shaped clusters (1-4 pm in size), which consist of radiating or subparallel platy phyllosilicate crystals. These