首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 576 毫秒
1.
We are investigating chondrule formation by nebular shock waves, using hot plasma as an analog of the heated gas produced by a shock wave as it passes through the protoplanetary environment. Precursor material (mainly silicates, plus metal, and sulfide) was dropped through the plasma in a basic experimental set‐up designed to simulate gas–grain collisions in an unconstrained spatial environment (i.e., no interaction with furnace walls during formation). These experiments were undertaken in air (at atmospheric pressure), to act as a “proof‐of‐principle”—could chondrules, or chondrule‐analog objects (CAO), be formed by gas–grain interaction initiated by shock fronts? Our results showed that if accelerating material through a fixed plasma field is a valid simulation of a supersonic shock wave traveling through a cloud of gas and dust, then CAO certainly could be formed by this process. Melting of and mixing between starting materials occurred, indicating temperatures of at least 1266 °C (the olivine‐feldspar eutectic). The production of CAO with mixed mineralogy from monomineralic starting materials also shows that collisions between particles are an important mechanism within the chondrule formation process, such that dust aggregates are not necessarily required as chondrule precursors. Not surprisingly, there were significant differences between the synthetic CAO and natural chondrules, presumably mainly because of the oxidizing conditions of the experiment. Results also show similarity to features of micrometeorites like cosmic spherules, particularly the dendritic pattern of iron oxide crystallites produced on micrometeorites by oxidation during atmospheric entry and the formation of vesicles by evaporation of sulfides.  相似文献   

2.
Abstract— We have investigated the kinematics of the separation of iron globules from chondrules during chondrule formation. A simple model, which assumes that the system has no angular momentum, was used to calculate the energy of a system with an iron globule and a chondrule. The energies of three different states were calculated: 1) a melted iron globule fully embedded in a melted chondrule, 2) a melted iron globule on the surface of a melted chondrule, and 3) a melted iron globule being separated from a melted chondrule. We also calculated the lowest energy shape for a melted iron globule on the surface of a melted chondrule, and compared our result with the shapes of four natural samples of chondrules and iron globules in thin sections. The shapes were calculated using an assumed value for the interface energy between the four couples of melted chondrules and the iron globules, and agree well with the natural shapes of chondrules and iron globules. The results of our calculations show that the iron globules of these four samples would be strongly bound to the surface of the melted chondrule during chondrule formation, and separation would be difficult, if the iron globules had been on the surface of precursors of these chondrules. Our results also show that if these iron globules were initially inside and transported to the surface of the melted chondrule, most of them would be ejected from the inside to outside because of surface tension forces, as long as the energy losses due to viscous dissipation when the globules pass through the surface of melted chondrules were sufficiently small. Although further improvement of the model is required, our results demonstrate that this ejection process may be responsible for the depletion of siderophile elements in natural chondrules.  相似文献   

3.
Abstract— We investigate the possible formation of chondrules by planetesimal bow shocks. The formation of such shocks is modeled using a piecewise parabolic method (PPM) code under a variety of conditions. The results of this modeling are used as a guide to study chondrule formation in a one‐dimensional, finite shock wave. This model considers a mixture of chondrule‐sized particles and micron‐sized dust and models the kinetic vaporization of the solids. We found that only planetesimals with a radius of ?1000 km and moving at least ?8 km/s with respect to the nebular gas can generate shocks that would allow chondrule‐sized particles to have peak temperatures and cooling rates that are generally consistent with what has been inferred for chondrules. Planetesimals with smaller radii tend to produce lower peak temperatures and cooling rates that are too high. However, the peak temperatures of chondrules are only matched for low values of chondrule wavelength‐averaged emissivity. Very slow cooling (<?100s of K/hr) can only be achieved if the nebular opacity is low, which may result after a significant amount of material has been accreted into objects that are chondrule‐sized or larger, or if chondrules formed in regions of the nebula with small dust concentrations. Large shock waves of approximately the same scale as those formed by gravitational instabilities or tidal interactions between the nebula and a young Jupiter do not require this to match the inferred thermal histories of chondrules.  相似文献   

4.
We have carried out a systematic study involving SEM, EPMA, and TEM analyses to determine the textures and compositions of sulfides and sulfide–metal assemblages in a suite of minimally to weakly altered CM and CR carbonaceous chondrites. We have attempted to constrain the distribution and origin of primary sulfides that formed in the solar nebula, rather than by secondary asteroidal alteration processes. Our study focused primarily on sulfide assemblages associated with chondrules, but also examined some occurrences of sulfides within the matrices of these meteorites. Although sulfides are a minor phase in carbonaceous chondrites, we have determined that primary sulfide grains are actually a major proportion of the sulfide grains in weakly altered CM chondrites and have survived aqueous alteration relatively unscathed. In minimally altered CR chondrites, we have determined that essentially all of the sulfides are of primary origin, confirming the observations of Schrader et al. ( 2015 ). The pyrrhotite–pentlandite intergrowth (PPI) grains formed from crystallization of monosulfide solid solution (mss) melts, while sulfide-rimmed metal (SRM) grains formed from sulfidization of Fe,Ni metal. Micron-sized metal inclusions in some PPI grains may have formed by co-crystallization of metal and sulfide from a sulfide melt that experienced S volatilization during the chondrule formation event, or alternatively, may be a remnant of sulfidization of Fe,Ni metal that also occurred during chondrule formation. Sulfur fugacity for SRM grains ranged from −18 to −10 (log units) largely in agreement with predicted solar nebular values. Our observations show that understanding the formation mechanisms of primary sulfide grains provides clues to solar nebular conditions, such as the sulfur fugacity during chondrule formation.  相似文献   

5.
Abstract— The outer portions of many type I chondrules (Fa and Fs <5 mol%) in CR chondrites (except Renazzo and Al Rais) consist of silica‐rich igneous rims (SIRs). The host chondrules are often layered and have a porphyritic core surrounded by a coarse‐grained igneous rim rich in low‐Ca pyroxene. The SIRs are sulfide‐free and consist of igneously‐zoned low‐Ca and high‐Ca pyroxenes, glassy mesostasis, Fe, Ni‐metal nodules, and a nearly pure SiO2 phase. The high‐Ca pyroxenes in these rims are enriched in Cr (up to 3.5 wt% Cr2O3) and Mn (up to 4.4 wt% MnO) and depleted in Al and Ti relative to those in the host chondrules, and contain detectable Na (up to 0.2 wt% Na2O). Mesostases show systematic compositional variations: Si, Na, K, and Mn contents increase, whereas Ca, Mg, Al, and Cr contents decrease from chondrule core, through pyroxene‐rich igneous rim (PIR), and to SIR; FeO content remains nearly constant. Glass melt inclusions in olivine phenocrysts in the chondrule cores have high Ca and Al, and low Si, with Na, K, and Mn contents that are below electron microprobe detection limits. Fe, Ni‐metal grains in SIRs are depleted in Ni and Co relative to those in the host chondrules. The presence of sulfide‐free, SIRs around sulfide‐free type I chondrules in CR chondrites may indicate that these chondrules formed at high (>800 K) ambient nebular temperatures and escaped remelting at lower ambient temperatures. We suggest that these rims formed either by gas‐solid condensation of silica‐normative materials onto chondrule surfaces and subsequent incomplete melting, or by direct SiO(gas) condensation into chondrule melts. In either case, the condensation occurred from a fractionated, nebular gas enriched in Si, Na, K, Mn, and Cr relative to Mg. The fractionation of these lithophile elements could be due to isolation (in the chondrules) of the higher temperature condensates from reaction with the nebular gas or to evaporation‐recondensation of these elements during chondrule formation. These mechanisms and the observed increase in pyroxene/olivine ratio toward the peripheries of most type I chondrules in CR, CV, and ordinary chondrites may explain the origin of olivine‐rich and pyroxene‐rich chondrules in general.  相似文献   

6.
Abstract— The Kobe CK4 chondrite, like most metamorphosed CK chondrites, exhibits pronounced silicate darkening of matrix and chondrule mesostases. Our petrographic and scanning electron microscopic study reveals that the matrix of Kobe consists mostly of intermixtures of two types of fine‐grained olivine. One forms subhedral to anhedral normal crystals. The other fills interstices of the subhedral to anhedral olivine crystals, exhibiting a complex network of veinlets. The latter type of olivine contains high densities of small spherical vesicles (<0.05‐3 μm in diameter) and grains (<0.05‐5 μm) of magnetite and pentlandite as well as round to anhedral grains (1–10 μm) of plagioclase, low‐Ca pyroxene, diopside and chlorapatite. The vesicular olivine is particularly abundant in regions of matrix that exhibit a relatively high degree of darkening and commonly fills chondrule mesostases. The vesicular olivine is clearly the principal cause of the silicate darkening in Kobe. The internal texture of the vesicular olivine closely resembles those of local melts produced from the matrices of experimentally and naturally shocked carbonaceous chondrites. The occurrence and texture of the vesicular olivine suggest that it resulted from recrystallization of partially melted matrix olivine by shock. Kobe exhibits light shock effects in olivine that are consistent with shock stage S2 that is too low to explain the occurrence of olivine melting. We suggest that the vesicular olivine in Kobe was produced by shock metamorphism at a relatively mild shock pressure (<25 GPa) and a high temperature (>600 °C). Thus, it is probable that the shock effects in olivine, manifest as fracturing and deformation, were relatively minor, but heating was strong enough to cause partial melting of matrix olivine.  相似文献   

7.
Abstract— The primordial asteroid belt contained at least several hundred and possibly as many as 10,000 bodies with diameters of 1000 km or larger. Following the formation of Jupiter, nebular gas drag combined with passage of such bodies through Jovian resonances produced high eccentricities (e = 0.3‐0.5), low inclinations (i < 0.5°), and, therefore, high velocities (3–10 km/s) for “resonant” bodies relative to both nebular gas and non‐resonant planetesimals. These high velocities would have produced shock waves in the nebular gas through two mechanisms. First, bow shocks would be produced by supersonic motion of resonant bodies relative to the nebula. Second, high‐velocity collisions of resonant bodies with non‐resonant bodies would have generated impact vapor plume shocks near the collision sites. Both types of shocks would be sufficient to melt chondrule precursors in the nebula, and both are consistent with isotopic evidence for a time delay of ?1‐1.5 Myr between the formation of CAIs and most chondrules. Here, initial simulations are first reported of impact shock wave generation in the nebula and of the local nebular volumes that would be processed by these shocks as a function of impactor size and relative velocity. Second, the approximate maximum chondrule mass production is estimated for both bow shocks and impact‐generated shocks assuming a simplified planetesimal population and a rate of inward migration into resonances consistent with previous simulations. Based on these initial first‐order calculations, impact‐generated shocks can explain only a small fraction of the minimum likely mass of chondrules in the primordial asteroid belt (?1024‐1025g). However, bow shocks are potentially a more efficient source of chondrule production and can explain up to 10–100 times the estimated minimum chondrule mass.  相似文献   

8.
The importance of experiments: Constraints on chondrule formation models   总被引：1，自引：0，他引：1  
Abstract— We review a number of constraints that have been placed on the formation of chondrules and show how these can be used to test chondrule formation models. Four models in particular are examined: the “X‐wind” model (sudden exposure to sunlight <0.1 AU from the proto‐Sun, with subsequent launching in a magnetocentrifugal outflow); solar nebula lightning; nebular shocks driven by eccentric planetesimals; and nebular shocks driven by diskwide gravitational instabilities. We show that constraints on the thermal histories of chondrules during their melting and crystallization are the most powerful constraints and provide the least ambiguous tests of the chondrule formation models. Such constraints strongly favor melting of chondrules in nebular shocks. Shocks driven by gravitational instabilities are somewhat favored over planetesimal bow shocks.  相似文献   

9.
Abstract— We have studied a unique impact-melt rock, the Ramsdorf L chondrite, using optical and scanning microscopy and electron microprobe analysis. Ramsdorf contains not only clast-poor impact melt (Begemann and Wlotzka, 1969) but also a chondritic portion (>60 g) with what appears at low magnification to be a normal, well-defined chondritic texture. However, detailed studies at high magnification show that >90 vol% of the crystals in the chondritic portion were largely melted by the impact: the chondrules lack normal microtextures and are ghosts of the original features. The only relics from the precursor chondrules are olivine crystals, which have the highest melting temperature (～1620 °C). Pyroxene-rich chondrules were so extensively melted that no phenocrysts were preserved and the melt crystallized in situ before significant mixing with exterior olivine-rich melts. Fine-grained pyroxene chondrule ghosts have sharper boundaries with the matrix than porphyritic olivine and pyroxene chondrule ghosts, probably because pyroxene-rich melts are significantly more viscous. Complex textures that formed by injection of melt along cracks and fractures in relic olivines suggest that the chondritic portion of Ramsdorf formed directly from petrologic type 3–4 material by strong shock. We infer that Ramsdorf was largely melted by shock pressures of ～75–90 GPa and that chondrule ghosts and relic olivine phenocrysts were locally preserved by rapid cooling. Quenching was not due to the addition of cold clasts into the melt but to heterogeneous shock heating that only caused internal melting of large olivines and pyroxenes. Ramsdorf appears to be one of the most heavily shocked meteorites that has retained some trace of its original texture.  相似文献   

10.
Abstract– Chondrule compositions suggest either ferroan precursors and evaporation, or magnesian precursors and condensation. Type I chondrule precursors include granoblastic olivine aggregates (planetary or nebular) and fine‐grained (dustball) precursors. In carbonaceous chondrites, type I chondrule precursors were S‐free, while type II chondrules have higher Fe/Mn than in ordinary chondrites. Many type II chondrules contain diverse forsteritic relicts, consistent with polymict dustball precursors. The relationship between finer and coarser grained type I chondrules in ordinary chondrites suggests more evaporation from more highly melted chondrules. Fe metal in type I, and Na and S in type II chondrules indicate high partial pressures in ambient gas, as they are rapidly evaporated at canonical conditions. The occurrence of metal, sulfide, or low‐Ca pyroxene on chondrule rims suggests (re)condensation. In Semarkona type II chondrules, Na‐rich olivine cores, Na‐poor melt inclusions, and Na‐rich mesostases suggest evaporation followed by recondensation. Type II chondrules have correlated FeO and MnO, consistent with condensation onto forsteritic precursors, but with different ratios in carbonaceous chondrites and ordinary chondrites, indicating different redox history. The high partial pressures of lithophile elements require large dense clouds, either clumps in the protoplanetary disk, impact plumes, or bow shocks around protoplanets. In ordinary chondrites, clusters of type I and type II chondrules indicate high number densities and their similar oxygen isotopic compositions suggest recycling together. In carbonaceous chondrites, the much less abundant type II chondrules were probably added late to batches of type I chondrules from different O isotopic reservoirs.  相似文献   

11.
Abstract— Research on chondrules during the past decade and a half has produced a number of constraints on the processes that formed these enigmatic objects. Although some chondrules may have formed in exceptional ways, it now seems clear that the vast majority did not form by condensation or by any other process that resulted in an extended (>100 s) period of heating; viable models of chondrule formation must generate brief (1–10 s) “flash” heating events. Many chondrules were incompletely melted, indicating that the heat source was marginal; coarse-grained rims are probably the result of heating by the same source that heated chondrules. Although some chondrules are enriched in refractories and poor in volatiles, most chondrules contain FeS in their interiors, implying that the last generation of chondrules formed after the local nebula had cooled below 650 K. The very small weight fraction of chondrules haying small (<50 μm in ordinary chondrites) radii requires either that (a) the formational process destroyed small chondrules by volatilizing them or efficiently recycling them into larger chondrules, or (b) that nebular size-sorting occurred and the fine fraction is not well represented in our known set of chondrites. Our recent studies of compound chondrules show that about 60% are siblings that formed together in the same heating event, and about 40% are independents that originated in different events. Independent compound chondrules tend to have similar FeO/(FeO + MgO) ratios, a possible indication of a high degree of compositional homogeneity in nebular subrogions defined by location or time. About 8% of barred olivine chondrules are the primaries of independently formed compound chondrules, and have thus been subjected to at least two flash-heating events. Allowance for observational biases suggests that a sizable fraction of chondrules have experienced two thermal events strong enough to produce major melting as well as many additional events that could produce minor melting, sintering, and crystal growth.  相似文献   

12.
Abstract— We have studied the CB carbonaceous chondrites Queen Alexandra Range (QUE) 94411, Hammadah al Hamra (HH) 237, and Bencubbin with an emphasis on the petrographical and mineralogical effects of the shock processing that these meteorite assemblages have undergone. Iron‐nickel metal and chondrule silicates are the main components in these meteorites. These high‐temperature components are held together by shock melts consisting of droplets of dendritically intergrown Fe,Ni‐metal/sulfide embedded in silicate glass, which is substantially more FeO‐rich (30–40 wt%) than the chondrule silicates (FeO <5 wt%). Fine‐grained matrix material, which is a major component in most other chondrite classes, is extremely scarce in QUE 94411 and HH 237, and has not been observed in Bencubbin. This material occurs as rare, hydrated matrix lumps with major and minor element abundances roughly similar to the ferrous silicate shock melts (and CI). We infer that hydrated, fine‐grained material, compositionally similar to these matrix lumps, was originally present between the Fe,Ni‐metal grains and chondrules, but was preferentially shock melted. Other shock‐related features in QUE 94411, HH 237, and Bencubbin include an alignment and occasionally strong plastic deformation of metal and chondrule fragments. The existence of chemically zoned and metastable Fe,Ni‐metal condensates in direct contact with shock melts indicates that the shock did not substantially increase the average temperature of the rock. Because porphyritic olivine‐pyroxene chondrules are absent in QUE 94411, HH 237, and Bencubbin, it is difficult to determine the precise shock stage of these meteorites, but the shock was probably relatively light (S2–S3), consistent with a bulk temperature increase of the assemblages of less than ?300 °C. The apparently similar shock processing of Bencubbin, Weatherford, Gujba (CBa) and QUE 94411/HH 237 (CBb) supports the idea of a common asteroidal parent body for these meteorites.  相似文献   

13.
The origin of three-dimensional shapes of chondrules is an important information to identify their formation mechanism in the early solar nebula. The measurement of their shapes by using X-ray computed topography suggested that they are usually close to perfect spheres, however, some of them have rugby-ball-like (prolate) shapes [Tsuchiyama, A., Shigeyoshi, R., Kawabata, T., Nakano, T., Uesugi, K., Shirono, S., 2003. Lunar Planet. Sci. 34, 1271-1272]. We considered that the prolate shapes reflect the deformations of chondrule precursor dust particles when they are heated and melted in the high velocity gas flow. In order to reveal the origin of chondrule shapes, we carried out the three-dimensional hydrodynamics simulations of a rotating molten chondrule exposed to the gas flow in the framework of the shock-wave heating model for chondrule formation. We adopted the gas ram pressure acting on the chondrule surface of in a typical shock wave. Considering that the chondrule precursor dust particle has an irregular shape before melting, the ram pressure causes a net torque to rotate the particle. The estimated angular velocity is for the precursor radius of r0=1 mm, though it has a different value depending on the irregularity of the shape. In addition, the rotation axis is likely to be perpendicular to the direction of the gas flow. Our calculations showed that the rotating molten chondrule elongates along the rotation axis, in contrast, shrinks perpendicularly to it. It is a prolate shape. The reason why the molten chondrule is deformed to a prolate shape was clearly discussed. Our study gives a complementary constraint for chondrule formation mechanisms, comparing with conventional chemical analyses and dynamic crystallization experiments that have mainly constrained the thermal evolutions of chondrules.  相似文献   

14.
Abstract— The Mg‐isotopic compositions in five barred olivine (BO) chondrules, one coarse‐grained rim of a BO chondrule, a relic spinel in a BO chondrule, one skeletal olivine chondrule similar to BO chondrules in mineralogy and composition, and two non‐BO chondrules from the Allende meteorite have been measured by thermal ionization mass spectrometry. The Mg isotopes are not fractionated and are within terrestrial standard values (±2.0%o per amu) in seven of the eight analyzed ferromagnesian chondrules. A clump of relic spinel grain and its host BO chondrule R‐11 give well‐resolvable Mg fractionations that show an enrichment of the heavier isotopes, up to +2.5%‰ per amu. The Mg‐isotopic compositions of coarse‐grained rim are identical to those of the host chondrule with BO texture. The results imply that ferromagnesian and refractory precursor components of the Allende chondrule may have been formed from isotopically heterogeneous reservoirs. In the nebula region where Allende chondrules formed, recycling of chondrules and multiple high‐temperature heating did not significantly alter the chemical and isotopic memory of earlier generations. Chemical and isotopic characteristics of refractory precursors of carbonaceous chondrite chondrules and CAIs are more closely related than previously thought. One of the refractory chondrule precursors of CV Allende is enriched in the heavier Mg isotopes and different from those of more common ferromagnesian chondrule precursors. The most probable scenario at the location where chondrule R‐11 formed is as follows. Before chondrule formation, several high‐temperature events occurred and then RPMs, refractory oxides, and silicates condensed from the nebular gas in which Mg isotopes were fractionated. Then, this CAI was transported into the chondrule formation region and mixed with more common, ferromagnesian precursors with normal Mg isotopes, and formed the BO chondrule. Because Mg isotope heterogeneity among silicates and spinel are found in some CAIs (Esat and Taylor, 1984), we cannot rule out the possibility that Mg isotopes of a melted portion of the refractory precursor (i.e., outer portion of CAI) are normal or enriched in the light isotope. Magnesium isotopes in the R‐11 host are also enriched in the heavier isotopes, +2.5%o per amu, which suggests that effects of isotopic heterogeneity among silicates and spinel, if they existed, are not considered to be large. It is possible that CAI precursor silicates partially dissolved during the chondrule forming event, contributing Mg to the melt and producing a uniform Mg‐isotopic signature but enriched in the heavier Mg isotopes, +2.5%‰ per amu. Most Mg isotopes in more common ferromagnesian chondrules represent normal chondritic material. Chemical and Mg‐isotopic signatures formed during nebular fractionations were not destroyed during thermal processes that formed the chondrule, and these were partly preserved in relic phases. Recycling of Allende chondrules and multiple heating at high temperature did not significantly alter the chemical and Mg‐isotopic memory of earlier generations.  相似文献   

15.
Abstract— We examined partially molten dust particles that have a solid core and a surrounding liquid mantle, and estimated the maximal size of chondrules in a framework of the shock wave heating model for chondrule formation. First, we examined the dynamics of the liquid mantle by analytically solving the hydrodynamics equations for a core‐mantle structure via a linear approximation. We obtained the deformation, internal flow, pressure distribution in the liquid mantle, and the force acting on the solid core. Using these results, we estimated conditions in which liquid mantle is stripped off from the solid core. We found that when the particle radius is larger than about 1–2 mm, the stripping is expected to take place before the entire dust particle melts. So chondrules larger than about 1–2 mm are not likely to be formed by the shock wave heating mechanism. Also, we found that the stripping of the liquid mantle is more likely to occur than the fission of totally molten particles. Therefore, the maximal size of chondrules may be determined by the stripping of the liquid mantle from the partially molten dust particles in the shock waves. This maximal size is consistent with the sizes of natural chondrules.  相似文献   

16.
Dar al Gani (DaG) 978 is an ungrouped type 3 carbonaceous chondrite. In this study, we report the petrography and mineralogy of Ca,Al‐rich inclusions (CAI), amoeboid olivine aggregates (AOAs), chondrules, mineral fragments, and the matrix in DaG 978. Twenty‐seven CAIs were found: 13 spinel‐diopside‐rich inclusions, 2 anorthite‐rich inclusions, 11 spinel‐troilite‐rich inclusions, and 1 spinel‐melilite‐rich inclusion. Most CAIs have a layered texture that indicates a condensation origin and are most similar to those in R chondrites. Compound chondrules represent a high proportion (approximately 8%) of chondrules in DaG 978, which indicates a local dusty chondrule‐forming region and multiple heating events. Most spinel and olivine in DaG 978 are highly Fe‐rich, which corresponds to a petrologic type of >3.5 and a maximum metamorphic temperature of approximately 850–950 K. This conclusion is also supported by other observations in DaG 978: the presence of coarse inclusions of silicate and phosphate in Fe‐Ni metal, restricted Ni‐Co distributions in kamacite and taenite, and low S concentrations in the matrix. Mineralogic records of iron‐alkali‐halogen metasomatism, such as platy and porous olivine, magnetite, hedenbergite, nepheline, Na‐rich in CAIs, and chlorapatite, are present, but relatively limited, in DaG 978. The fine‐grained, intergrowth texture of spinel‐troilite‐rich inclusions was probably formed by reaction between pre‐existing Al‐rich silicates and shock‐induced, high‐temperature S‐rich gas on the surface of the parent body of DaG 978. A shock‐induced vein is present in the matrix of DaG 978, which indicates that the parent body of DaG 978 at least experienced a shock event with a shock stage up to S3.  相似文献   

17.
Abstract— Primary minerals in calcium‐aluminum‐rich inclusions (CAIs), Al‐rich and ferromagnesian chondrules in each chondrite group have δ18O values that typically range from ?50 to +5%0. Neglecting effects due to minor mass fractionations, the oxygen isotopic data for each chondrite group and for micrometeorites define lines on the three‐isotope plot with slopes of 1.01 ± 0.06 and intercepts of ?2 ± 1. This suggests that the same kind of nebular process produced the 16O variations among chondrules and CAIs in all groups. Chemical and isotopic properties of some CAIs and chondrules strongly suggest that they formed from solar nebula condensates. This is incompatible with the existing two‐component model for oxygen isotopes in which chondrules and CAIs were derived from heated and melted 16O‐rich presolar dust that exchanged oxygen with 16O‐poor nebular gas. Some FUN CAIs (inclusions with isotope anomalies due to fractionation and unknown nuclear effects) have chemical and isotopic compositions indicating they are evaporative residues of presolar material, which is incompatible with 16O fractionation during mass‐independent gas phase reactions in the solar nebula. There is only one plausible reason why solar nebula condensates and evaporative residues of presolar materials are both enriched in 16O. Condensation must have occurred in a nebular region where the oxygen was largely derived from evaporated 16O‐rich dust. A simple model suggests that dust was enriched (or gas was depleted) relative to cosmic proportions by factors of ～10 to >50 prior to condensation for most CAIs and factors of 1–5 for chondrule precursor material. We infer that dust‐gas fractionation prior to evaporation and condensation was more important in establishing the oxygen isotopic composition of CAIs and chondrules than any subsequent exchange with nebular gases. Dust‐gas fractionation may have occurred near the inner edge of the disk where nebular gases accreted into the protosun and Shu and colleagues suggest that CAIs formed.  相似文献   

18.
Abstract– We investigate the hypothesis that many chondrules are frozen droplets of spray from impact plumes launched when thin‐shelled, largely molten planetesimals collided at low speed during accretion. This scenario, here dubbed “splashing,” stems from evidence that such planetesimals, intensely heated by 26Al, were abundant in the protoplanetary disk when chondrules were being formed approximately 2 Myr after calcium‐aluminum‐rich inclusions (CAIs), and that chondrites, far from sampling the earliest planetesimals, are made from material that accreted later, when 26Al could no longer induce melting. We show how “splashing” is reconcilable with many features of chondrules, including their ages, chemistry, peak temperatures, abundances, sizes, cooling rates, indented shapes, “relict” grains, igneous rims, and metal blebs, and is also reconcilable with features that challenge the conventional view that chondrules are flash‐melted dust‐clumps, particularly the high concentrations of Na and FeO in chondrules, but also including chondrule diversity, large phenocrysts, macrochondrules, scarcity of dust‐clumps, and heating. We speculate that type I (FeO‐poor) chondrules come from planetesimals that accreted early in the reduced, partially condensed, hot inner nebula, and that type II (FeO‐rich) chondrules come from planetesimals that accreted in a later, or more distal, cool nebular setting where incorporation of water‐ice with high Δ17O aided oxidation during heating. We propose that multiple collisions and repeated re‐accretion of chondrules and other debris within restricted annular zones gave each chondrite group its distinctive properties, and led to so‐called “complementarity” and metal depletion in chondrites. We suggest that differentiated meteorites are numerically rare compared with chondrites because their initially plentiful molten parent bodies were mostly destroyed during chondrule formation.  相似文献   

19.
John T. Wasson 《Icarus》2008,195(2):895-907
Studies of matrix in primitive chondrites provide our only detailed information about the fine fraction (diameter <2 μm) of solids in the solar nebula. A minor fraction of the fines, the presolar grains, offers information about the kinds of materials present in the molecular cloud that spawned the Solar System. Although some researchers have argued that chondritic matrix is relatively unaltered presolar matter, meteoritic chondrules bear witness to multiple high-temperature events each of which would have evaporated those fines that were inside the high-temperature fluid. Because heat is mainly transferred into the interior of chondrules by conduction, the surface temperatures of chondrules were probably at or above 2000 K. In contrast, the evaporation of mafic silicates in a canonical solar nebula occurs at around 1300 K and FeO-rich, amorphous, fine matrix evaporates at still lower temperatures, perhaps near 1200 K. Thus, during chondrule formation, the temperature of the placental bath was probably >700 K higher than the evaporation temperatures of nebular fines. The scale of chondrule forming events is not known. The currently popular shock models have typical scales of about 105 km. The scale of nebular lightning is less well defined, but is certainly much smaller, perhaps in the range 1 to 1000 m. In both cases the temperature pulses were long enough to evaporate submicrometer nebular fines. This interpretation disagrees with common views that meteoritic matrix is largely presolar in character and CI-chondrite-like in composition. It is inevitable that presolar grains (both those recognized by their anomalous isotopic compositions and those having solar-like compositions) that were within the hot fluid would also have evaporated. Chondrule formation appears to have continued down to the temperatures at which planetesimals formed, possibly around 250 K. At temperatures >600 K, the main form of C is gaseous CO. Although the conversion of CO to CH4 at lower temperatures is kinetically inhibited, radiation associated with chondrule formation would have accelerated the conversion. There is now evidence that an appreciable fraction of the nanodiamonds previously held to be presolar were actually formed in the solar nebula. Industrial condensation of diamonds from mixtures of CH4 and H2 implies that high nebular CH4/CO ratios favored nanodiamond formation. A large fraction of chondritic insoluble organic matter may have formed in related processes. At low nebular temperatures appreciable water should have been incorporated into the smoke that condensed following dust (and some chondrule) evaporation. If chondrule formation continued down to temperatures as low as 250 K this process could account for the water concentration observed in primitive chondrites such as LL3.0 and CO3.0 chondrites. Higher H2O contents in CM and CI chondrites may reflect asteroidal redistribution. In some chondrite groups (e.g., CR) the Mg/Si ratio of matrix material is appreciably (30%) lower than that of chondrules but the bulk Mg/Si ratio is roughly similar to the CI or solar ratio. This has been interpreted as a kind of closed-system behavior sometimes called “complementarity.” This leads to the conclusion that nebular fines were efficiently agglomerated. Its importance, however is obscured by the observation that bulk Mg/Si ratios in ordinary and enstatite chondrites are much lower than those in carbonaceous chondrites, and thus that complementarity did not hold throughout the solar nebula.  相似文献   

20.
Group‐IIIE iron meteorites can be ordered into four categories reflecting increasing degrees of shock alteration. Weakly shocked samples (Armanty, Colonia Obrera, Coopertown, Porto Alegre, Rhine Villa, Staunton, and Tanokami Mountain) have haxonite within plessite, unrecrystallized kamacite grains containing Neumann lines or possessing the ? structure, and sulfide inclusions typically consisting of polycrystalline troilite with daubréelite exsolution lamellae. The only moderately shocked sample is NWA 4704, in which haxonite has been partially decomposed to graphite; the majority of the kamacite in NWA 4704 is recrystallized, and its sulfide inclusions were partly melted. Strongly shocked samples (Cachiyuyal, Kokstad, and Paloduro) contain graphite and no haxonite, suggesting that pre‐existing haxonite fully decomposed. Also present in these rocks are recrystallized kamacite and melted troilite. Residual heat from the impact caused annealing and recrystallization of kamacite as well as the decomposition of haxonite into graphite. Severely shocked samples (Aliskerovo and Willow Creek) have sulfide‐rich assemblages consisting of fragmental and subhedral daubréelite crystals, 1–4 vol% spidery troilite filaments, and 30–50 vol% low‐Ni kamacite grains, some of which contain up to 6.0 wt% Co; haxonite in these inclusions has fully decomposed to graphite. The wide range of impact effects in IIIE irons is attributed to one or more major collision(s) on the parent asteroid that affected different group members to different extents depending on their proximity to the impact point.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号