共查询到20条相似文献，搜索用时 31 毫秒
1.
Hydrated minerals occur in accretionary rims around chondrules in CM chondrites. Previous models suggested that these phyllosilicates did not form by gas-solid reactions in the canonical solar nebula. We propose that chondrule-forming shock waves in icy regions of the nebula produced conditions that allowed rapid mineral hydration. The time scales for phyllosilicate formation are similar to the time it takes for a shocked system to cool from the temperature of phyllosilicate stability to that of water ice condensation. This scenario allows for simultaneous formation of chondrules and their fine-grained accretionary rims. 相似文献
2.
Chromium, silicon, and phosphorus concentrations of 0.1 to 1 percent by weight are common in metal grains in the least metamorphosed ordinary and carbonaceous chondrites. These concentrations are fairly uniform within single chondrules (but different from chondrule to chondrule) and are inversely correlated with the fayalite concentrations of the chondrule olivines. This relation shows that these chromium, silicon, and phosphorus concentrations could not have been established by condensation or equilibration in the solar nebula but are the result of metal-silicate equilibration within chondrules. Two generations of inclusions made by the exsolution of those elements have been identified: One formed during chondrule cooling and the other formed during metamorphism. The distribution and composition of the latter in type 3 to type 5 chondrites are consistent with increasing metamorphism relative to type 2 and type 3.0 material. 相似文献
3.
High-precision magnesium isotope measurements of whole chondrules from the Allende carbonaceous chondrite meteorite show that some aluminum-rich Allende chondrules formed at or near the time of formation of calcium-aluminum-rich inclusions and that some others formed later and incorporated precursors previously enriched in magnesium-26. Chondrule magnesium-25/magnesium-24 correlates with [magnesium]/[aluminum] and size, the aluminum-rich, smaller chondrules being the most enriched in the heavy isotopes of magnesium. These relations imply that high gas pressures prevailed during chondrule formation in the solar nebula. 相似文献
4.
Certain opaque inclusions within primitive meteorites exhibit textures that suggest chondrules formed during intense, short-duration radiative heating episodes in the early solar system. Experimental support for this interpretation is provided by the textures produced when chondrule-like assemblages are heated with visible laser light. Computer simulations of radiative heating provide additional evidence for the role of electromagnetic energy in heating nebular solids by offering an explanation for the size distributions of chondrules and the presence of dusty chondrule rims. Nebular lightning and magnetic reconnection flares are possible sources of electromagnetic energy for these transient heating events. 相似文献
5.
Chondrules, which are roughly millimeter-sized silicate-rich spherules, dominate the most primitive meteorites, the chondrites. They formed as molten droplets and, judging from their abundances in chondrites, are the products of one of the most energetic processes that operated in the early inner solar system. The conditions and mechanism of chondrule formation remain poorly understood. Here we show that the abundance of the volatile element sodium remained relatively constant during chondrule formation. Prevention of the evaporation of sodium requires that chondrules formed in regions with much higher solid densities than predicted by known nebular concentration mechanisms. These regions would probably have been self-gravitating. Our model explains many other chemical characteristics of chondrules and also implies that chondrule and planetesimal formation were linked. 相似文献
6.
Krot AN Meibom A Russell SS Alexander CM Jeffries TE Keil K 《Science (New York, N.Y.)》2001,291(5509):1776-1779
Chondrules in the metal-rich meteorites Hammadah al Hamra 237 and QUE 94411 have recorded highly energetic thermal events that resulted in complete vaporization of a dusty region of the solar nebula (dust/gas ratio of about 10 to 50 times solar). These chondrules formed under oxidizing conditions before condensation of iron-nickel metal, at temperatures greater than or equal to 1500 K, and were isolated from the cooling gas before condensation of moderately volatile elements such as manganese, sodium, potassium, and sulfur. This astrophysical environment is fundamentally different from conventional models for chondrule formation by localized, brief, repetitive heating events that resulted in incomplete melting of solid precursors initially residing at ambient temperatures below approximately 650 K. 相似文献
7.
Existence of an 16O-rich gaseous reservoir in the solar nebula 总被引：1，自引：0，他引：1
Krot AN McKeegan KD Leshin LA MacPherson GJ Scott ER 《Science (New York, N.Y.)》2002,295(5557):1051-1054
Carbonaceous chondrite condensate olivine grains from two distinct petrographic settings, calcium-aluminum-rich inclusion (CAI) accretionary rims and amoeboid olivine aggregates (AOAs), are oxygen-16 (16O) enriched at the level previously observed inside CAIs. This requires that the gas in the nebular region where these grains condensed was 16O-rich. This contrasts with an 16O-poor gas present during the formation of chondrules, suggesting that CAIs and AOAs formed in a spatially restricted region of the solar nebula containing 16O-rich gas. The 16O-rich gas composition may have resulted either from mass-independent isotopic chemistry or from evaporation of regions with enhanced dust/gas ratios, possibly in an X-wind environment near the young Sun. 相似文献
8.
Chondrulelike objects in short-period comet 81P/Wild 2 总被引：1，自引：0，他引：1
Nakamura T Noguchi T Tsuchiyama A Ushikubo T Kita NT Valley JW Zolensky ME Kakazu Y Sakamoto K Mashio E Uesugi K Nakano T 《Science (New York, N.Y.)》2008,321(5896):1664-1667
The Stardust spacecraft returned cometary samples that contain crystalline material, but the origin of the material is not yet well understood. We found four crystalline particles from comet 81P/Wild 2 that were apparently formed by flash-melting at a high temperature and are texturally, mineralogically, and compositionally similar to chondrules. Chondrules are submillimeter particles that dominate chondrites and are believed to have formed in the inner solar nebula. The comet particles show oxygen isotope compositions similar to chondrules in carbonaceous chondrites that compose the middle-to-outer asteroid belt. The presence of the chondrulelike objects in the comet suggests that chondrules have been transported out to the cold outer solar nebula and spread widely over the early solar system. 相似文献
9.
SS Russell G Srinivasan GR Huss GJ Wasserburg GJ MacPherson 《Science (New York, N.Y.)》1996,273(5276):757-762
A search was made for 26Mg (26Mg*) from the decay of 26Al (half-life = 0.73 million years) in Al-rich objects from unequilibrated ordinary chondrites. Two Ca-Al-rich inclusions (CAIs) and two Al-rich chondrules (not CAIs) were found that contained 26Al when they formed. Internal isochrons for the CAIs yielded an initial 26Al/27Al ratio [(26Al/27Al)0] of 5 x 10(-5), indistinguishable from most CAIs in carbonaceous chondrites. This result shows that CAIs with this level of 26Al are present throughout the classes of chondrites and strengthens the notion that 26Al was widespread in the early solar system. The two Al-rich chondrules have lower 26Mg*, corresponding to a (26Al/27Al)0 ratio of approximately 9 x 10(-6). Five other Al-rich chondrules contain no resolvable 26Mg*. If chondrules and CAIs formed from an isotopically homogeneous reservoir, then the chondrules with 26Al must have formed or been last altered approximately2 million years after CAIs formed; the 26Mg*-free chondrules formed >1 to 3 million years later still. Because 26Mg*-containing and 26Mg*-free chondrules are both found in Chainpur, which was not heated to more than approximately400°C, it follows that parent body metamorphism cannot explain the absence of 26Mg* in some of these chondrules. Rather, its absence indicates that the lifetime of the solar nebula over which CAIs and chondrules formed extended over approximately5 million years. 相似文献
10.
Electron-transparent sections of the Allende meteorite, a carbonaceous chondrite, have been prepared by ion-thinning and examined by high-voltage (800-kilovolt) transmission electron microscopy. The matrix crystals, mainly olivine, range in size from approximately 5 to approximately 0.01 micrometers; carbon is present as intergranular films of poorly crystalline graphite. The chondrules exhibit extensive radiation damage, a feature lacking in the matrix. In addition, both chondrules and matrix are undeformed and contain negative crystals; submicroscopic exsolution lamellae are present in pyroxenes. Comparison of the substructure in the Allende meteorite with that in the Parnallee meteorite and in lunar and selected terrestrial rocks leads to the conclusion that chondrule irradiation preceded cold accretion during formation of the solar system and that the meteorite has since been undisturbed. 相似文献
11.
The primary mineral assemblages and initial (26)Al/(27)Al ratios of rare calcium-aluminum-rich inclusions (CAIs) from enstatite (E) chondrites are similar to those of CAIs from other chondrite classes. CAIs from all chondrite classes formed under oxidizing conditions that are much different from the reducing conditions under which the E chondrites formed. Either CAIs formed at an earlier, more oxidizing epoch in the region where E chondrites ultimately formed, or they formed at a different place in the solar nebula and were transported into the E chondrite formation region. 相似文献
12.
A barred chondrule in the Ngawi meteorite contains a magnetite spherule embedded in it. The collision between these two objects fractured and partially remelted the chondrule, an indication that the impact velocity was 10(5) to 10(6) centimeters per second. This observation supports Cameron's and Whipple's recent predictions that grains achieved high velocities in the nebula and that the resulting impacts provide a suitable chondrule-forming mechanism. 相似文献
13.
Meibom A Desch SJ Krot AN Cuzzi JN Petaev MI Wilson L Keil K 《Science (New York, N.Y.)》2000,288(5467):839-841
Chemical zoning patterns in some iron, nickel metal grains from CH carbonaceous chondrites imply formation at temperatures from 1370 to 1270 kelvin by condensation from a solar nebular gas cooling at a rate of approximately 0.2 kelvin per hour. This cooling rate requires a large-scale thermal event in the nebula, in contrast to the localized, transient heating events inferred for chondrule formation. In our model, mass accretion through the protoplanetary disk caused large-scale evaporation of precursor dust near its midplane inside of a few astronomical units. Gas convectively moved from the midplane to cooler regions above it, and the metal grains condensed in these parcels of rising gas. 相似文献
14.
Whipple FL 《Science (New York, N.Y.)》1966,153(3731):54-56
The millimeter-sized, sometimes glassy spheroids called chondrules that occur abundantly in stony meteorites may have been produced by lightning in the primitive Laplaciantype nebula while earthy materials were condensing and collecting to form the asteroids and the terrestrial planets. 相似文献
15.
Kieffer SW 《Science (New York, N.Y.)》1975,189(4200):333-340
I have proposed that droplet chondrules were formed by jetting during collision of meteoritic particles with diameters ranging in order of magnitude from 0.5 mm to 20 cm. This conclusion, based on a dynamic model for the collision process, supports the hypotheses of Wasson (2) (based on geochemical considerations) and Whipple (35) and Cameron (36) (based on dynamic model considerations) that chondrules were formed from objects less than 1 m in radius. In this model, the formation of chondrules is viewed as a textural, but not substantial chemical, change in the material of the early solar system. Droplets of melt produced by jetting are mixtures of material derived from two parent grains. Jets are probably not appreciably fractionated (except in volatile elements) either in the short duration of the shock events (several microseconds) or in subsequent cooling. This model for the formation of droplet chondrules implies that they were formed at a time in the history of the solar system when particle sizes were small. The most likely time for this condition is early in the process of accretion of nebular dust to planetary matter. Since velocities less than approximately 1.5 km/sec are required for the agglomeration and accretion of particles (37), the relatively higher velocities indicated for droplet chondrule-forming collisions indicate an early high-velocity destructive epoch amidst the general trend toward accretion of material. 相似文献
16.
Complete chondrules and fragments of chondrules have been found within silicate inclusions from the octahedrite iron meteorite Netschaevo. The bulk chemical composition, mineralogy, and mineral chemistry indicate that this chondritic material has properties intermediate between those of the H-group chondrites and those of the enstatite chondrites. 相似文献
17.
Rare earth element (REE) data from two ordinary chondrite chondrules show distinct negative chondrite-normalized concentration anomalies of samarium, europium, and ytterbium. The peculiar patterns may be the result of REE gas/solid fractionation at an oxygen fugacity lower than has been assumed for the canonical solar nebula. We suggest that the two ordinary chondrite chondrules acquired the fractionated REE patterns by incorporation of highly reduced, ultrarefractory condensates in their precursors. This interpretation implies that high-temperature condensation processes occurred in nebular environments with a strong deficit in oxygen, such as regions with an enhanced carbon/oxygen ratio. 相似文献
18.
Magnetite from the Orgueil C1 chondrite is only 2.0 +/- 2.4 million years older by the iodine-xenon method than the next oldest meteorite, the Karoonda C4 chondrite. This age ties the primitive C1 chondrites to the extensive iodine-xenon chronology of normal chrondrites. If Karoonda and Orgueil magnetite formed from similar material, then the age difference is an upper limit to the formation time of these meteorites-and by customary extension, the solar system. Condensation, chondrule formation, accretion, and metamorphism of the Karoonda parent body all seem to have been completed within a few million years. 相似文献
19.
Chondrules have been observed in several breccia samples returned by the Apollo 14 mission. These lunar chondrules are believed to have formed during a large impact event, perhaps the one that formed the Imbrian Basin. This suggests that some meteoritic chondrules are also formed by impact processes such as crystallization after shock melting and abrasion and diffusion in base-surge and fall-back deposits generated by impacts on planetary surfaces. 相似文献
20.
Graphite whiskers (GWs), an allotrope of carbon that has been proposed to occur in space, have been discovered in three CV-type carbonaceous chondrites via Raman imaging and electron microscopy. The GWs are associated with high-temperature calcium-aluminum inclusion (CAI) rims and interiors, with the rim of a dark inclusion, and within an inclusion inside an unusual chondrule that bears mineralogy and texture indicative of high-temperature processing. Current understanding of CAI formation places their condensation, and that of associated GWs, relatively close to the Sun and early in the condensation sequence of protoplanetary disk materials. If this is the case, then it is a possibility that GWs are expelled from any young solar system early in its history, thus populating interstellar space with diffuse GWs. Graphite whiskers have been postulated to play a role in the near-infrared (near-IR) dimming of type Ia supernovae, as well as in the thermalization of both the cosmic IR and microwave background and in galactic center dimming between 3 and 9 micrometers. Our observations, along with the further possibility that GWs could be manufactured during supernovae, suggest that GWs may have substantial effects in observational astronomy. 相似文献