Origin and evolutionary processes of small mineral grains in the primitive meteorite matrix

(1)Vigarano CV3碳质球粒陨石中的黑色包裹体几乎完全由小颗粒(直径&lt；5μm)的富铁橄榄石组成。这一黑色包裹体显示了一种不同寻常的质地，由弧形带子组成的网络。两个或两个以上的带大致平行，形成一组相继的平行带，其中一些相互交叉。这些带含有略高数量的相对较小(&lt；1μm)的橄榄石颗粒，因此比其他地区更密集。条带中的橄榄石颗粒比其他地区的橄榄石颗粒略富铁。由于小的富铁橄榄石颗粒的丰度降低，这些条带通常在凹陷的一侧显示出渐变。从结构上看，这些弧形带与地球上粉砂岩和砂岩中常见的“碟形结构”非常相似。碟形结构的特点是较薄，颜色较深，呈近水平或向上凹的层状，富含相对细粒的材料。地球上的DISH Structure…疏松细粒沉积物在压实脱水过程中形成的S较多，是细粒流态化形成的特征沉积构造之一。因此，Vigarano中的暗色包裹体首次提供了证据，表明由于水的迁移而导致的沉积过程可能发生在行星体内，并进一步表明流态化作用可能在碳质球粒陨石的形成中发挥了重要作用。(2)对Murchison Cm球粒陨石进行的一系列冲击实验结果表明，球粒陨石在激波锋面上被压扁，大致与峰值冲击压力的强度成正比，最高可达~25 Gpa，但在25-30 Gpa时，它们不再被压扁，而是日益破碎。在~20 GPa时，基质开始发生破碎和粉碎的变化，在25~30 GPa时，这些变化随着压力的增加而急剧增加，因此，基质在10~50μm的尺度上被致密地粉碎，在20~30 GPa时，局部熔融也以熔脉和口袋的形式发生。在高于30-35 GPA的压力下，Murchison对冲击的反应是灾难性的破坏，伴随着广泛的熔化、破碎、粉化、挥发和气体的强烈膨胀。我们对撞击时内能增加的计算表明，每次实验产生的激波热效应可以通过在比峰值激波压力低得多的激波压力下撞击自然目标(Murchison母体中的表面材料)来实现。这些结果表明，如果CM母体在高于~25 GPA的压力下冲击表面，在压力释放时，细小颗粒将发生爆炸性分散，水分和可活化的挥发物将随这些颗粒从母体中逃逸。因此，这一结果支持了CM球粒陨石在20-30 GPA以上受到冲击时从母星上逃逸并形成的粒子太小而不能作为陨石生存的假设。这可能解释了为什么我们没有震级高于S3级的CM球粒陨石。这也可能提供一个长期存在的问题的线索，即为什么化学最原始的陨石坠落，如CI和CM球粒陨石，相对较少，而在主带小行星中，被归类为C型的小行星数量很多。(3)绝大多数以前的研究人员认为，球粒陨石周围的细粒边缘是原始的，是由尘埃直接吸积到太阳星云中的球粒陨石和球粒陨石表面形成的。最近，我们发现在Vigarano CV3球粒陨石中，有一小部分(~5%)球粒陨石周围的细粒边缘主要由水合层状硅酸盐组成。富含层状硅酸盐的边缘和包裹体中的球粒陨石显示出大量的水蚀变证据。球粒边缘的低钙辉石和橄榄石部分被富含层状硅酸盐的物质所取代，在球粒表面产生凹陷。球粒中的铁镍金属、铁硫化物和磁铁矿也被氢氧化铁部分取代。由于低钙辉石和橄榄石的优先交代，硫化物、磁性和氢氧化铁仍暴露在球粒边缘，并部分解离并分布在轮缘中。矿物学和结构证据不能与太阳星云中的吸积作用形成边缘相一致，但可以用陨石母体上的水蚀变和角砾化作用来解释。根据这些观察，我们认为球粒/边缘组合和碎屑是同一前体球粒陨石的一部分，该陨石是在陨石母体上角砾化时破碎的。这是第一个解释碳质球粒陨石中球粒陨石通过母体过程在球粒陨石上形成细粒边缘的自洽模型。这项工作仍在进行中。较少

(1)A dark inclusion in the Vigarano CV3 carbonaceous chondrite consists almost exclusively of small (<5 μm in diameter) grains of Fe-rich olivine. This dark inclusion shows an unusual texture comprising a network of arcuate bands. Two or more bands occur roughly parallel, forming a set of succesive parallel bands, some crosscutting one another. The bands contain slightly higher amounts of relatively small (<1 μm) olivine grains and so are more densely packed than other areas. The olivine grains in the bands are slightly more Fe-rich than those in other areas. The bands commonly show gradation on the concave side due to a decrease in the abundance of the small Fe-rich olivine grains. Texturally, the arcuate bands closely resemble "dish structures" that are commonly observed in siltstones and sandstones on Earth. Dish structures are characterized by thin, dark-colored, subhorizontal to concave-upward laminations which are rich in relatively fine-grained material. On Earth, dish structure … More s from during compaction and dewatering of unconsolidated fine-grained sediments and they are one of the characteristic sedimentary structures formed through fluidization of fine grains. The dark inclusion in Vigarano, therefore, provides the first evidence that sedimentary processes due to water migration may have taken place within planetesimals, and further suggests that fluidization may have played a significant role in the formation of the carbonaceous chondrites.(2)The results of a series of shock experiments on the Murchison Cm chondrite reveal that chondrules are flattened in the plane of the shock front roughly in proportion to the intensity of peak shock pressure up to 〜25 GPa, but at 25-30 GPa they are no longer flattened but are increasingly disrupted. Changes involving fracturing and comminution begin to occur in matrix at 〜20 GPa, and these changes drastically advance with increasing pressure between 25 and 30 GPa ; thus, the matrix is densely comminuted on scales of 10-50 μm. Local melting also occurs as melt veins and pockets at 20-30 GPa. At pressures higher than 30-35 GPa, Murchison responds to shock with catastrophic disruption accompanied by extensive melting, fragmentation, pulverization, devolatilization and intense expansion of gas. Our calculations of internal energy increase upon impact suggest that the shock thermal effects produced at each experiment may be attained by impact on a natural target (surface material in the Murchison parent body) at a considerably lower shock pressure than the peak shock pressure. These results suggest that if the CM parent body were shocked on the surface at pressures higher than 〜25 GPa, on pressure release there would be explosive dispersal of fine grains, and water and mobilized volatiles would escape with these grains from the parent body. Therefore, the results support the hypothesis that CM chondrites shocked above 20-30 GPa escaped from the parent asteroids and formed particles that are too small to survive as meteorites. This probably explains why we have no CM chondrites shocked to the stages higher than S3 levels. It may also provide a clue to the long-standing question of why the most chemically primitive meteorite falls, such as CI and CM chondrites, are relatively few, while among the main belt asteroids those classified as C-type are abundant.(3)The vast majority of previous workers favor that fine-grained rims surrounding chondrules are primitive in origin and formed by direct accretion of dust onto the surfaces of chondrules and CAIs in the solar nebula. Recently, we found that a small proportion (〜5%) of the fine-grained rims surrounding chondrules in the Vigarano CV3 chondrite consists mainly of hydrous phyllosilicates. The phyllosilicate-rich rims and the chondrules that they enclose show abundant evidence of aqueous alteration. Low-Ca pyroxene and olivine at chondrule margins have been partially replaced by phyllosilicate-rich materials, producing embayments on chondrule surfaces. Fe-Ni metal, Fe sulfide and magnetite in the chondrules have also been partially replaced by Fe hydroxide. Due to preferential replacement of low-Ca pyroxene and olivine, sulfide, magnetic and Fe hydroxide remain exposed at chondrule margins and are partly disaggregated an distributed into rims. The mineralogical and textural evidence can not be reconciled with rim formation by accretion in the solar nebula but can be most plausible explained by aqueous alteration and brecciation on the meteorite parent body. From these observations, we suggest that the chondrule/rim assemblages and the clasts are a part of the same precursor chondrite, which was fragmented during brecciation on the meteorite parent body. This is the first self-consistent model to explain the formation of fine-grained rims on chondrules in a carbonaceous chondrite by parent-body processes. This work is still in progress. Less