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

原始陨石基质中小矿物颗粒的起源和演化过程

基本信息

批准号：
09440187
负责人：
TOMEOKA Kazushige
金额：
$ 7.94万
依托单位：
Kobe University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (B)
财政年份：
1997
资助国家：
日本
起止时间：
1997 至 1999
项目状态：
已结题

项目摘要

(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

(1) Vigarano CV3 碳质球粒陨石中的黑色包裹体几乎完全由富铁橄榄石小颗粒（直径 <5 μm）组成。这种黑色包裹体显示出由弓形带网络组成的不寻常纹理。大致平行，形成一组连续的平行带，其中一些带相互交叉，这些带含有稍多量的相对较小（<1μm）的橄榄石颗粒，因此比其他区域更密集。条带中的橄榄石颗粒比其他区域的橄榄石颗粒略富铁，由于小富铁橄榄石颗粒的丰度减少，条带通常在凹侧呈现渐变。从结构上看，弓形条带与“盘状结构”，常见于地球上的粉砂岩和砂岩中。盘状结构的特点是薄、深色、近水平到凹向上的层理，富含相对丰富的在地球上，盘状结构是在未固结的细粒沉积物的压实和脱水过程中形成的，它们是细粒流化形成的特征沉积结构之一，因此，维加拉诺的暗色包裹体提供了这种结构。第一个证据表明，由于水迁移而导致的沉积过程可能发生在星子内，并进一步表明流态化可能在碳质球粒陨石的形成中发挥了重要作用。(2)结果对 Murchison Cm 球粒陨石进行的一系列冲击实验表明，球粒在冲击锋平面上被压扁，大致与高达约 25 GPa 的峰值冲击压力强度成正比，但在 25-30 GPa 时，它们不再变平但在约 20 GPa 时基质中开始发生涉及破裂和粉碎的变化，并且这些变化随着 25 至 30 之间压力的增加而急剧推进。 GPa ；因此，在 20-30 GPa 的压力下，基质也会发生 10-50 μm 的局部熔化，形成熔脉和熔袋，默奇森会对冲击做出灾难性的破坏。伴随着气体的广泛熔化、碎裂、粉碎、脱挥发分和剧烈膨胀，我们对撞击时内能增加的计算表明，冲击热效应产生于每个实验都可以通过以比峰值冲击压力低得多的冲击压力撞击自然目标（默奇森母体的表面材料）来实现。这些结果表明，如果 CM 母体在高于峰值冲击压力的压力下冲击表面。 ~25 GPa，压力释放时，细颗粒会发生爆炸性扩散，水和流动的挥发物会随这些颗粒从母体中逸出。因此，结果支持了 CM 球粒陨石在 20-30 以上冲击的假设。 GPa 从小行星逃逸出来，形成的颗粒太小，无法作为陨石存活下来，这可能解释了为什么我们没有达到 S3 级别以上的 CM 球粒陨石，这也可能为长期存在的问题提供线索。化学成分最原始的陨石陨石，如CI、CM球粒陨石相对较少，而主带小行星中C型陨石则较多。（3）绝大多数前人赞成：球粒周围的细粒边缘起源很原始，是由太阳星云中的球粒和 CAI 表面的尘埃直接吸积而形成的。最近，我们发现球粒周围的细粒边缘有一小部分（约 5%）。 Vigarano CV3 球粒陨石主要由含水层状硅酸盐组成，富含层状硅酸盐的边缘和层状硅酸盐。它们所包围的球粒显示出大量的水蚀变证据，球粒边缘的低钙辉石和橄榄石已部分被富含页硅酸盐的物质所取代，从而在球粒表面产生了铁镍金属、硫化铁和磁铁矿。由于优先取代低钙辉石和橄榄石、硫化物，已部分被氢氧化铁取代。磁性和氢氧化铁仍然暴露在球粒边缘，部分分解并分布在边缘中，矿物学和文本证据无法与太阳星云中的吸积形成边缘相一致，但最合理的解释是陨石上的水蚀变和角砾化。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。这是第一个解释碳质球粒陨石中细粒边缘通过母体过程形成的自洽模型。