Igneous rims on micrometeorites
Igneous rims on micrometeorites
复制标题
微陨石上的火成岩边缘
DOI：
10.1016/j.gca.2006.02.005
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发表时间：
2006
影响因子：
5
通讯作者：
M. Genge
中科院分区：
文献类型：
--
作者：
M. Genge
微陨石（mm）由于大气进入加热而熔化，导致颗粒的质地、矿物学和成分发生重大变化，从而模糊了它们的主要性质，使对这些外星物质的解释变得非常复杂。尽管有大量熔化的mm，但这些材料的熔化过程的性质却受到了很差的限制。本文分析了77个具有火成岩边缘的微陨石的矿物学、结构和成分数据，表明微陨石在进入大气层时的融合最初是通过表面熔融发生的。纹理和矿物学证据明确地表明，火成岩的边缘从围绕着一个很大程度上未熔化的岩心的熔体中结晶，并确立了熔化是一个渐进的过程。细粒岩心（fgMMs）上火成岩岩心的组成与未熔化岩心的组成大致相似，除了部分蒸发产生的挥发性和中度挥发性元素的消耗，这表明岩心是由细粒基质岩心熔化形成的。火成岩边缘Fe/Si、Ni/Si和Mn/Si的富集与fgMMs内未熔化的岩心相比，可能是由于Fe - s共晶液体从颗粒岩心迁移到表面熔体层。硫化物液体可能是在碳质物质热解的还原条件下在颗粒的核心产生的。由于层状硅酸盐的热分解，fgMMs上存在火成岩边缘，因此表明fgMMs在进入大气之前是水合颗粒。相比之下，粗粒岩心（cgMMs）上的火成岩边缘的成分很难与未熔化岩心的成分相协调，而与fgMMs的成分非常相似。因此，cgMMs的火成岩边缘被认为是由在进入大气之前存在于这些颗粒外部的细粒基质熔化形成的。因此，具有火成岩边缘的粗粒mm最初是由细粒和粗粒材料组成的复合微流星体，被认为是球粒样物体的样本。从复合mm到cgMMs的丰度，可以初步估计mm母体内的平均球粒半径为~ 625μm。
Melting of micrometeorites (MMs) due to atmospheric entry heating causes significant changes in the textures, mineralogies and compositions of particles that obscure their primary properties and greatly complicate interpretation of these extraterrestrial materials. Despite the abundance of melted MMs, the nature of melting processes in these materials is poorly constrained. In this study, mineralogical, textural, and compositional data on 77 MMs with igneous rims are presented, which suggest that fusion of micrometeorites during atmospheric entry occurs initially by surface melting. Textural and mineralogical evidence are presented that demonstrate unequivocally that igneous rims crystallized from a melt surrounding a largely unmelted core and establish melting as a gradational process. The compositions of igneous rims on fine-grained MMs (fgMMs) are broadly similar to those of the unmelted core except for depletions in volatile and moderately volatile elements produced by partial evaporation and suggest the formation of the rims by melting of the fine-grained matrix core. Enrichments in Fe/Si, Ni/Si, and Mn/Si in igneous rims compared with unmelted cores within fgMMs are suggested to occur due to the migration of Fe–S eutectic liquids from the core of the particle into the surface melt layer. The sulphide liquids are probably generated in the core of the particle under reducing conditions resulting by the pyrolysis of carbonaceous materials. The presence of igneous rims on fgMMs is enabled by the thermal decomposition of phyllosilicates and, therefore, indicates that fgMMs were hydrated particles prior to atmospheric entry. In contrast the compositions of igneous rims on coarse-grained MMs (cgMMs) are difficult to reconcile with those of the unmelted core and instead closely resemble those of fgMMs. The igneous rims of cgMMs are, therefore, suggested to form by melting of fine-grained matrix that was present on the exterior of these particles prior to atmospheric entry. Coarse-grained MMs with igneous rims thus were originally composite micrometeoroids, which consist of both fine-grained and coarse-grained materials, and are thought to be samples of chondrule-like objects. The abundance of composite MMs to cgMMs allows a first estimate of the mean chondrule radius within the parent bodies of MMs of ∼625μm to be made.