ACCRETION TEXTURES, IRON EVAPORATION AND RE-CONDENSATION IN RENAZZO
ACCRETION TEXTURES, IRON EVAPORATION AND RE-CONDENSATION IN RENAZZO
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RENAZZO 中的吸积纹理、铁蒸发和再凝结
DOI：
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发表时间：
2001
期刊：
影响因子：
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通讯作者：
A. J. Campbell
中科院分区：
文献类型：
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作者：
Chondrules B. Zanda;M. Bourot;R. Hewins;B. A. Cohen;J. S. Delaney;M. Humayun;A. J. Campbell
简介：Renazzo球粒陨石是不平衡的，高度还原的，含有丰富的金属（7.3体积%），主要与球粒有关[1，2]。因此，它提供了一个独特的机会，了解之间的遗传关系的硅酸盐和金属。通常认为，CR碳酸盐中的金属是冷凝物来源，原因有两个：（i）其P和Cr含量[3];（ii）其Co和Ni遵循冷凝趋势，即宇宙比为0.046 [3，2]。因此，与位于球粒边缘的颗粒相比，球粒内部颗粒中Ni（和Co）的倾向更高，这被解释为核心金属颗粒在较高温度下凝聚的证据，表明球粒在逐渐降低的温度下增生生长[2]，但[4]认为这是由于邻近硅酸盐的FeO减少，使球粒边缘上的颗粒中加入了后期的Fe。然而，Zanda等人[5]表明，熔融程度最低的球粒（基于硅酸盐粒度和球粒轮廓）的金属颗粒中的Ni和Co没有遵循冷凝趋势，并且由于Fe的氧化或还原，在球粒熔融期间逐渐建立了Ni-Co相关性。此外，Zanda等人[6]表明，金属中Cr和Si的存在可以通过在球粒形成时与周围硅酸盐的反应来解释。最近，关于球粒陨石金属起源的争论再次被归因于冷凝的CH-球粒陨石中的分区金属颗粒的发现所激起[例如7]，而[8]分析了CR球粒陨石中的PGE分布，并认为球粒内部的金属起源于熔融期间通过硅酸盐熔体的还原，而边缘金属是从耐火亲铁矿物中耗尽的蒸汽再冷凝的产物。目前的工作是[6]的后续工作，在[6]中，我们试图将金属和硅酸盐织构与它们的成分联系起来。硅酸盐和金属的纹理：Renazzo的球粒通常不是球形的。它们有着复杂的轮廓，暗示着在凝聚过程中熔化的颗粒。(Fig. 1a）。熔化程度最低的物体是由细粒硅酸盐和细粒金属分散在各处。随着熔化过程的进行，物体的轮廓变得更加平滑，可以看到内部金属开始融合，同时开始形成金属边缘（图1b）。更广泛融化的物体几乎是球形的。它们在硅酸盐中几乎没有残留的金属，但表现出发达的金属边缘，有时可以看到含有一个或两个大的球形金属颗粒（图1c）。图1：Renazzo的三个陨石球粒从熔化程度较低（a）到熔化程度较高（c）排列。一
Introduction: The Renazzo chondrite is unequilibrated, highly reduced and contains abundant metal (7.3 vol%) mostly associated with chondrules [1,2]. It thus provides a unique opportunity to understand the genetic relationship between chondritic silicates and metal. Metal in CR chondrites has often been believed to be of condensate origin for two reasons: (i) its P and Cr content [3]; (ii) its Co and Ni following a condensation trend i.e. having the cosmic ratio of 0.046 [3,2]. A tendency for Ni (and Co) to be higher in the grains in chondrule interiors compared to grains sitting on chondrule margins was thus interpreted as evidence that the core metal grains had condensed at higher temperatures, suggesting accretionary growth of chondrules at progressively lower temperatures [2], but [4] attributed this to late Fe addition to the grains sitting on the chondrule margins due to FeO reduction from the adjoining silicates. Zanda et al. [5] however showed that Ni and Co in the metal grains of the least melted chondrules (based on the silicate grain sizes and the chondrule outline) did not follow a condensation trend and that the Ni-Co correlation was gradually established during chondrule melting due to Fe oxidation or reduction. In addition, Zanda et al. [6] showed that the presence of Cr and Si in the metal could be explained by a reaction with the surrounding silicates at the time of chondrule formation. Recently, the debate on the origin of chondritic metal was stirred again by the finding of zoned metal grains in CH-chondrites attributed to condensation [e.g. 7], whereas [8] analyzed PGE distribution in CR chondrites and argued that metal inside chondrules originated during melting via reduction from the silicate melt whereas rim metal was the product of recondensation from a vapor depleted in refractory siderophiles. The present work is a follow up on [6] in which we tried to correlate metal and silicate textures together with their compositions. Textures of silicates and metal: Chondrules in Renazzo are often not spherical. They have convoluted outlines that suggest melted particles in the process of coalescing. (Fig. 1a). The least melted objects have fine-grained silicates and fine-grained metal dispersed throughout. As the melting progresses, the outline of the objects becomes smoother and the internal metal can be seen to coalesce while a metallic rim starts to develop (Fig. 1b). More extensively melted objects are almost spherical. They have very little remaining metal dispersed within the silicates, but exhibit a well developed metallic rim and sometimes can be seen to contain one or 2 large spherical metal grains (Fig. 1c). Figure 1: Three chondrules in Renazzo ranked from less melted (a) to more melted (c). a