| Budget Amount *help |
¥12,400,000 (Direct Cost: ¥12,400,000)
Fiscal Year 1999: ¥1,300,000 (Direct Cost: ¥1,300,000)
Fiscal Year 1998: ¥1,300,000 (Direct Cost: ¥1,300,000)
Fiscal Year 1997: ¥9,800,000 (Direct Cost: ¥9,800,000)
|
|---|
| Research Abstract |
(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
|
