Bulk Composition and MIneralogy of Antarctic Micrometeorites
Bulk Composition and MIneralogy of Antarctic Micrometeorites
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南极微陨石的块体成分和矿物学
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发表时间：
1992
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通讯作者：
S. Sutton
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作者：
W. Kloeck;W. Beckerling;B. Spettel;G. Flynn;S. Sutton
近年来，在格陵兰岛和南极洲收集的微陨石（1，2）提供了大量的外星物质。与深海球体相比，来自这些地点的宇宙尘埃似乎不太受蚀变和采样偏差的影响。在大于100 μ m的尺寸范围内的大多数颗粒是球粒。其中最有可能的是来自较大陨石的消融球，以及在进入大气层时熔化的真正的微陨石（3）。在50 pm到100 um大小范围内的微陨石主要是细粒、多孔、不规则的颗粒，它们可能因为质量较小而在大气中逃脱了完全熔化。所研究的颗粒由M. 1988年，他在南极洲收集了Maurette和Maurette。图1给出了微陨石球的组成范围，标准化为CI丰度。该图包含来自格陵兰岛和南极球体的数据。这两个地方的球体在主要元素组成上是相同的。图1中绘制了大约150个分析。矩形包含所有数据点，除了Al和Ca（Cr、Mn和Fe的数据点较少），其中6%和10%的所有分析落在框外，并单独绘制。球体的Na、K和S含量通常在几百ppm的量级。镍含量变化很大，丰度可达0.01 x CI。图2给出了南极洲未熔化的圆锥花序的主要元素丰度。在组成上最显著的差别是在不规则型颗粒中存在Na、S和K。钙含量的20个分析的26个不规则的微陨石低于0.6 x CI-球粒陨石丰度，而90%的所有领域的钙丰度从0.6 x到3.5 x CI-球粒陨石。只有高钙亚类的未熔化颗粒是一个可能的前体材料的高钙球。90%的熔融微陨石的来源物质在化学上与不规则的钙贫化颗粒不同，除非钙贫化是南极环境中水蚀变的结果。CM 2基质成分（图2中的方框区域）与一些诊断元素（如未熔化颗粒的Na、Al和Ca）相当吻合。硫和镍的相关贫化表明硫化物矿物有一些损失，可能是由于陆地水蚀变（4）或由于大气加热，尽管大气进入加热对“未熔化”颗粒来说应该是最小的。似乎有一些证据表明，在未熔化的南极颗粒中，Ni、Co和Se的损失要比Cl的损失要大（图3）。分析的所有不规则颗粒的Nillr比均低于Cl颗粒（图4）。然而，大多数熔化的微陨石都沿着南极尼尔比（图5）。图4中较低Nillr比值的数据点可以用硫化物的陆源蚀变和溶解来解释。一些球的低Nillr比可能是由微小的铂金属块造成的，除了一些蚀变效应。未熔化颗粒的NiICo比（图6）从Ni/Co 10分散到NiICo 20。虽然大多数熔化的微陨石具有铁镍比，但大多数球体的镍钴比（图7）约为10或更小。几个分析的微陨石进行了更详细的研究，通过TEM技术。具有最高Ni含量的未熔化颗粒完全由页硅酸盐组成。高分辨率图像显示1.0-1.4 nm的良好定义的基底间距。另外三颗镍含量在0.13%至0.44%之间的微陨石主要由橄榄石、辉石和磁铁矿的小晶体（<100 nm）组成，镶嵌在玻璃中。其中一个颗粒含有可能残留的层状晶格硅酸盐的区域。低镍粒子在进入大气过程中表现出严重的加热效应。事实上，改变最少的穗是高镍和更热的颗粒较低的镍可能表明，镍耗尽是大气进入加热的结果，而不是陆地水蚀变。在50-100 pm大小范围内的微陨石被认为主要是彗星起源（3）。在未熔化的南极颗粒中发现页硅酸盐，支持在微陨石中存在相当数量的小行星物质。参考编号：（1）M. Maurette等人，（1987）Nature 328，699-702;（2）：M. Maurene等人，（1991）Nature 351，44-47;（3）：G.M. Raisbeck和F. Yiou（1987）Meteoritics 22，485-486;（4）：G. W. Kallemeyn等人，等（1991）GCA 55，881 - 892。
In recent years, the collection of micrometeorites in Greenland and Antarctica (1,2) provided plenty of extraterrestrial material. Cosmic dust from these locations seems to be less effected by alteration and sampling biases compared to Deep Sea Spheres. The majority of particles in the size range larger than 100 pm are spherules. Among them are most likely ablation spheres from larger meteorites as well as true micrometeorites which melted during atmospheric entry (3). The population of micrometeorites in the 50 pm to 100 ,urn size range is dominated by finegrained, porous, irregular particles, which might have escaped total melting in the atmosphere because of their smaller masses. The particles studied were provided by M. Maurette and were collected by him in 1988 in Antarctica. Ranges of compositions of micrometeorite-spheres, normalized to CI abundances, are given in Fig.1. The graph contains data from Greenland as well as from Antarctic spheres. Spheres from the two locations are identical in major element compositions. About 150 analyses are plotted in Fig.1. The rectangle contains all the data points, except for Al and Ca (and less so for Cr, Mn and Fe), where 6% and 10% of all analyses fall outside the box and are plotted separately. Na, K and S contents of spheres are generally on the order of a few hundred ppm. Ni contents are very variable and extend to abundances of 0.01 x CI. Major element abundances of unmelted panicles from Antarctica are given in Fig.2. The most striking difference in composition is the presence of Na, S and K in the irregular-type particles. Ca contents of 20 out of 26 analyzed irregular micrometeorites are below 0.6 x CI-chondrite abundances, whereas 90% of all spheres have Ca abundances from 0.6 x to 3.5 x CI-chondrites. Only the high-Ca subclass of unmelted particles is a likely precursor material of high-Ca spheres. The source material of 90% of melted micrometeorites is chemically unlike the irregular, Ca-depleted particles, unless the Ca-depletion is a consequence of aqueous alteration in the Antarctic environment. CM2 matrix compositions (boxed area in Fig.2) agree reasonably well with some diagnostic elements, like Na, Al, and Ca of unmelted particles. The correlated depletion of S and Ni argues for some loss of sulfide minerals, probably by terrestrial aqueous alteration (4) or due to atmospheric heating, though atmospheric entry heating should be minimal for the "unmelted" particles. There seems to be some evidence for the preferential loss of Ni, Co and Se compared to CI composition in unmelted Antarctic particles (Fig.3). All irregular particles analyzed have Nillr ratios lower than CI chondrites (Fig.4). Most of the melted micrometeorites, however, plot along the chondritic Nillr ratio (Fig.5). The data points at lower Nillr ratios in Fig.4 could be explained by terrestrial alteration and dissolution of sulfides. Low Nillr ratios of some spheres might be caused by tiny platinum metal nuggets in addition to some alteration effects. NiICo ratios of unmelted particles (Fig.6) scatter from Ni/Co 10 to NiICo 20. Though most of the melted micrometeorites have chondritic Nillr ratios, NiICo ratios of the majority of spheres (Fig.7) are approximately 10 or smaller. Several of the analyzed micrometeorites were studied in more detail by TEM techniques. The unmelted particle with the highest Ni content consists entirely of phyllosilicates. High resolution images show well defined basal spacings of 1.0-1.4 nm. Magnetites were not found. Three other micrometeorites, having Ni contents between 0.13% and 0.44% are composed mainly of small (< 100nm) crystals of olivine, pyroxene and magnetites embedded in glass. One of these particles contains areas of probable relict layer-lattice-silicates. The low-Ni particles show severe effects of heating during atmospheric entry. The fact that the least altered panicle is high in Ni and the more heated particles are lower in Ni could indicate that the Ni depletion is a consequence of atmospheric entry heating rather than terrestrial aqueous alteration. Micrometeorites in the 50-100 pm size range were believed to be mainly of cometary origin (3). The identification of phyllosilicates among the unmelted Antarctic particles supports the existence of a sizeable population of asteroidal material among micrometeorites. Ref.: (1 ): M. Maurette et a1.,(1987) Nature 328, 699-702; (2): M. Maurene et a1.,(1991) Nature 351, 44-47; (3): G.M. Raisbeck and F. Yiou (1987) Meteoritics 22, 485-486; (4): G. W. Kallemeyn et a1.,(1991) GCA 55, 881 -892.