The onset of metamorphism in ordinary and carbonaceous chondrites
The onset of metamorphism in ordinary and carbonaceous chondrites
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DOI：
10.1111/j.1945-5100.2005.tb00366.x
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发表时间：
2005-01-01
影响因子：
2.2
通讯作者：
Brearley, AJ
中科院分区：
文献类型：
--
作者：
Grossman, JN;Brearley, AJ
用x射线测图技术对岩石学类型最低的普通球粒陨石和碳质球粒陨石进行了测量。利用电子探针、扫描电子显微镜（SEM）、透射电子显微镜（TEM）和阴极发光（CL）等方法对各种变质效应进行了详细的分析。3.0 ~ 3.2型之间富feo橄榄石中Cr的分布随着变质作用的增加而发生系统的变化。火成岩分带模式被复杂的分带模式所取代，所有晶粒上都发育富铬涂层。铬在橄榄石中的分布受富铬相（可能是铬铁矿）的析出控制。铬在橄榄石中可能部分以四面体配位的Cr3+形式存在。铬铁矿的岩石学类型为3.2型，分离基本完成。显示中平衡状态中碱分布不均匀的球粒丰度随岩石类型的增加而增加。透射电镜显示这是钠长石结晶的结果。在变质作用过程中，残余玻璃成分发生了系统的变化，钾含量越来越丰富，I型球粒中的玻璃也在变质作用过程中获得碱。两种类型的球粒都与不透明的基质和其他球粒进行碱交换。最小变质球粒陨石的基体富含S和Na。在变质作用的最初阶段，由于微小颗粒的合并，S从基体中丢失。逐渐加热还会导致硫化物从球粒边缘流失，并增加粗基质组合以及球粒内部的硫化物丰度。在早期变质作用中，碱性物质最初离开基质并进入球粒。长石随后在基体中成核，钠从球粒中重新进入。这些变质趋势可以用来完善球粒陨石的分类方案。橄榄石中的Cr分布是最原始陨石岩石学类型划分的有效工具，可将3.0和3.1型划分为3.00 ~ 3.15型。在此基础上，已知最原始的普通球粒陨石是Semarkona，尽管即使是这种陨石也经历了少量的变质作用。Allan Hills (ALH) A77307是变质程度最小的CO球粒陨石，与未分类的碳质球粒陨石Acfer 094具有许多相同的性质。对于II型球粒中的玻璃来说，分析问题是重要的，因为在微探针分析过程中Na很容易丢失。因此，现有的基于玻璃碱含量的球粒分类方案需要修改。
Ordinary and carbonaceous chondrites of the lowest petrologic types were surveyed by Xray mapping techniques. A variety of metamorphic effects were noted and subjected to detailed analysis using electron microprobe, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and cathodoluminescence (CL) methods. The distribution of Cr in FeO-rich olivine systematically changes as metamorphism increases between type 3.0 and type 3.2. Igneous zoning patterns are replaced by complex ones and Cr-rich coatings develop on all grains. Cr distributions in olivine are controlled by the exsolution of a Cr-rich phase, probably chromite. Cr in olivine may have been partly present as tetrahedrally coordinated Cr3+. Separation of chromite is nearly complete by petrologic type 3.2. The abundance of chondrules showing an inhomogeneous distribution of alkalis in mesostasis also increases with petrologic type. TEM shows this to be the result of crystallization of albite. Residual glass compositions systematically change during metamorphism, becoming increasingly rich in K. Glass in type I chondrules also gains alkalis during metamorphism. Both types of chondrules were open to an exchange of alkalis with opaque matrix and other chondrules. The matrix in the least metamorphosed chondrites is rich in S and Na. The S is lost from the matrix at the earliest stages of metamorphism due to coalescence of minute grains. Progressive heating also results in the loss of sulfides from chondrule rims and increases sulfide abundances in coarse matrix assemblages as well as inside chondrules. Alkalis initially leave the matrix and enter chondrules during early metamorphism. Feldspar subsequently nucleates in the matrix and Na re-enters from chondrules. These metamorphic trends can be used to refine classification schemes for chondrites. Cr distributions in olivine are a highly effective tool for assigning petrologic types to the most primitive meteorites and can be used to subdivide types 3.0 and 3.1 into types 3.00 through 3.15. On this basis, the most primitive ordinary chondrite known is Semarkona, although even this meteorite has experienced a small amount of metamorphism. Allan Hills (ALH) A77307 is the least metamorphosed CO chondrite and shares many proper-ties with the ungrouped carbonaccous chondrite Acfer 094. Analytical problems are significant for glasses in type II chondrules, as Na is easily lost during microprobe analysis. As a result, existing schemes for chondrule classification that are based on the alkali content of glasses need to be revised.