Bo Xian (LL3.9): Oxygen‐isotopic and mineralogical characterisation of separated chondrules

Abstract— We report the results of a mineralogical and O‐isotopic study of 362 chondrules disaggregated from the Bo Xian chondrite. The range of mineral compositions (Fa = 0.8–31.2%, mean = 23.5%, mode = 27–28%) are consistent with a reclassification of this meteorite from LL4 to LL3.9. Chondrule diameters range from 0.20 to 3.40 mm (mean = 0.74 mm) in the disaggregated population. A lower mean diameter (0.64 mm) calculated from thin‐section measurements partly reflects the high proportion of chondrule fragments. The chondrule size distribution, which is approximately log‐normal, is consistent with size‐sorting mechanisms. This sorting could be linked to the fragmentation of many chondrules on the parent body. However, in detail, the variation in diameter of different chondrule types and a hiatus in the size distribution at 0.6 mm indicate that there may have been complex controls perhaps partly being determined by the chondrule formation mechanism. Seven percent of the sectioned chondrules (102) contain chemically fractionated mineral assemblages: cristobalite‐bearing and Al‐rich. This significant degree of chemical heterogeneity probably resulted from both igneous and volatility controls. Oxygen‐isotopic compositions were determined on mineral separates and 16 of the sectioned chondrules. Three separate isotopic exchange events have been identified. The dominant one is a low‐temperature hydrous gas‐solid exchange event between ¹⁶O‐rich solid and ¹⁶O‐poor gas reservoirs that lay along a slope 1.0 line on three‐isotope plots. Partial equilibration with the gas by feldspar and cristobalite, which exchanged more rapidly than olivine or pyroxene, led to formation of a slope 0.77 mixing line for Bo Xian and other LL chondrites. Mineralogy is the dominant control on the extent of this exchange; no relationship between isotopic composition and chondrule texture or size was identified. The feldspar separate and cristobalite‐rich chondrules have the most ¹⁶O‐poor compositions. Subsequently, thermal metamorphism in the parent body led to partial isotopic equilibration between the different mineral phases. A third exchange event, predating the other two events, is probably shown by one of the Al‐rich chondrules. This has an ¹⁶O‐rich composition, lying below the terrestrial fractionation line. Another Al‐rich chondrule has a normal ordinary chondrite isotopic composition. It is not clear whether the isotopic fractionation recorded in some Al‐rich chondrules can be achieved by the dominant gas‐solid exchange. Instead, the precursor O to the mineral phases may have become ¹⁶O‐rich during an earlier phase of mass‐independent fractionation.     

A Shock-Wave Heating Model for Chondrule Formation: Effects of Evaporation and Gas Flows on Silicate Particles

A shock-wave heating model is one of the possible models for chondrule formation. We examine, within the framework of a shock-wave heating model, the effects of evaporation on the heating of chondrule precursor particles and the stability of their molten state in the postshock flow. We numerically simulate the heating process in the flow taking into account evaporation. We find that the melting criterion and the minimum radius criterion do not change significantly. However, if the latent heat cooling due to the evaporation dominates the radiative cooling from the precursor particle, the peak temperature of the precursor particle is suppressed by a few hundred Kelvins. We also find that the total gas pressure (ram plus static) acting on the precursor particle exceeds the vapor pressure of the molten precursor particle. Therefore, it is possible to form chondrules in the shock-wave heating model if the precursor temperature increases up to the melting point.     

Compound chondrule formation in the shock-wave heating model: Three-dimensional hydrodynamics simulation of the disruption of a partially-molten dust particle

We carried out three-dimensional hydrodynamics simulations of the disruption of a partially-molten dust particle exposed to high-speed gas flow to examine the compound chondrule formation due to mutual collisions between the fragments (fragment-collision model; [Miura, H., Yasuda, S., Nakamoto, T., 2008a. Icarus194, 811-821]).In the shock-wave heating model, which is one of the most plausible models for chondrule formation, the gas friction heats and melts the surface of the cm-sized dust particle (parent particle) and then the strong gas ram pressure causes the disruption of the molten surface layer. The hydrodynamics simulation shows details of the disruptive motion of the molten surface, production of many fragments and their trajectories parting from the parent particle, and mutual collisions among them. In our simulation, we identified 32 isolated fragments extracted from the parent particle. The size distribution of the fragments was similar to that obtained from the aerodynamic experiment in which a liquid layer was attached to a solid core and it was exposed to a gas flow. We detected 12 collisions between the fragments, which may result in the compound chondrule formation. We also analyzed the paths of all the fragments in detail and found the importance of the shadow effect in which a fragment extracted later blocks the gas flow toward a fragment extracted earlier. We examined the collision velocity and impact parameter of each collision and found that 11 collisions should result in coalescence. It means that the ratio of coalescent bodies to single bodies formed in this disruption of a parent particle is R_coa=11/(32-11)=0.52. We concluded that compound chondrule formation can occur just after the disruption of a cm-sized molten dust particle in shock-wave heating.     

SIZE-DISTRIBUTIONS OF CHONDRULE TYPES IN THE INMAN AND ALLAN HILLS A77011 L3 CHONDRITES

A petrographc study of 9 thin sections of Inman (L3) and 18 thin sections of ALHA77011 (L3) served to determine the size-distributions of different chondrule textural types. Inman chondrules are significantly larger than those in ALHA77011, but in each chondrite, there is no statistically significant difference between the size-distributions of barred olivine and radial pyroxene plus cryptocrystalline chondrules. In ALHA77011, barred olivine chondrules outnumber radial pyroxene plus cryptocrystalline chondrules, whereas in Inman, the reverse is true. Because compound and cratered chondrules were formed by the collision of similarly-sized objects, the dustball precursors of chondrules must have been size-sorted prior to chondrule formation. The region of dustball size-sorting in the solar nebula must have been very large, similarly affecting the physically-separated precursors of different chondrule types. Size-sorting was probably accomplished by aerodynamic particle-gas interactions. Zones of dustball melting (i.e., chondrule formation) were relatively small, generally affecting only dustballs of one compositional type and relatively uniform size. Different chondrule types were then mixed together in somewhat variable ratios. Within the region where chondrites of a particular compositional group agglomerated, there were sub-reservoirs that contained (roughly) uniformly large or uniformly small chondrules with different mixtures of textural types.     

Opaque minerals in chondrules and fine‐grained chondrule rims in the Bishunpur (LL3.1) chondrite

Abstract— We present a detailed petrographic and electron microprobe study of metal grains and related opaque minerals in the chondrule interiors and rims of the Bishunpur (LL3.1) ordinary chondrite. There are distinct differences between metal grains that are completely encased in chondrule interiors and those that have some portion of their surface exposed outside of the chondrule boundary, even though the two types of metal grains can be separated by only a few microns. Metal grains in chondrule interiors exhibit minor alteration in the form of oxidized P‐, Cr‐, and Si‐bearing minerals. Metal grains at chondrule boundaries and in chondrule rims are extensively altered into troilite and fayalite. The results of this study suggest that many metal grains in Bishunpur reacted with a type‐I chondrule melt and incorporated significant amounts of P, Cr, and Si. As the system cooled, some metal oxidation occurred in the chondrule interior, producing metal‐associated phosphate, chromite, and silica. Metal that migrated to chondrule boundaries experienced extensive corrosion as a result of exposure to the external atmosphere present during chondrule formation. It appears that chondrule‐derived metal and its corrosion products were incorporated into the fine‐grained rims that surround many type‐I chondrules, contributing to their Fe‐rich compositions. We propose that these fine‐grained rims formed by a combination of corrosion of metal expelled from the chondrule interior and accretion of fine‐grained mineral fragments and microchondrules.     

Deformation and thermal histories of chondrules in the Chainpur (LL3.4) chondrite

Abstract— Transmission-electron-microscopy (TEM) and optical data suggest that chondrules in the Chainpur (LL3.4) chondrite experienced varied thermal and deformation histories prior to the final agglomeration of the meteorite. Chainpur may be regarded as an agglomerate or breccia that experienced little deformation or heating during and after the final accumulation and compaction of its constituents. One chondrule in Chainpur was impact-shocked to high pressures (～ 20–50 GPa), almost certainly prior to final agglomeration, either while it was an independent entity in space or while it was in the regolith of a parent body. However, most (>85%) of the chondrules in Chainpur were evidently not significantly shock-metamorphosed subsequent to their formation. The dearth of shock effects implies that most chondrules in Chainpur did not form by shock melting, although some chondrules may have formed by this process. Dusty-metal-bearing olivine grains, which are widely interpreted to have escaped melting during chondrule formation, contain moderate densities of dislocations (～ 10⁸ cm^?2). The dislocations in these grains were introduced before or during the last episode of melting in at least one chondrule. This observation can be explained if olivine was impact-deformed before or during chondrule formation, or if olivine was strained by reduction or thermally-induced processes during chondrule formation. Low-Ca pyroxene grains in chondrules are often strained. In most cases this strain probably arose as a by-product of polytype transformations (protoenstatite → clinoenstatite/orthoenstatite and clinoenstatite → orthoenstatite) that occurred during the igneous crystallization and static annealing of chondrules. Droplet chondrules with glassy mesostases were minimally annealed, consistent with an origin as relatively rapidly cooled objects in an unconfined, cold environment. Some irregular chondrules and at least one droplet chondrule were thermally metamorphosed prior to final agglomeration, either as a result of moderately slow cooling (～ 100 °C/hr) from melt temperatures (during autometamorphism) or as a result of reheating episodes. Two of the most annealed chondrules contain relatively abundant plagioclase feldspar, and one of these has a uniform olivine composition appropriate to that of an LL4 chondrite.     

Chondrule and metal grain size sorting from jet flows

Abstract— We examine the size sorting of chondrules and metal grains within the context of the jet flow model for chondrule/CAI formation. In this model, chondrules, CAIs, AOAs, metal grains, and related components of meteorites are assumed to have formed in the outflow region of the innermost regions of the solar nebula and then were ejected, via the agency of a bipolar jet flow, to outer regions of the nebula. We wish to see if size sorting of chondrules and metal grains is a natural consequence of this model. To assist in this task, we used a multiprocessor system to undertake Monte Carlo simulations of the early solar nebula. The paths of a statistically significant number of chondrules and metal grains were analyzed as they were ejected from the outflow and travelled over or into the solar nebula. For statistical reasons, only distances ≤3 AU from the Sun were examined. Our results suggest that size sorting can occur provided that the solar nebula jet flow had a relatively constant flow rate as function of time. A constant flow rate outflow produces size sorting, but it also produces a sharp size distribution of particles across the nebula and a metal‐rich Fe/Si ratio. When the other extreme of a fully random flow rate was examined, it was found that size sorting was removed, and the initial material injected into the flow was simply spread over most of the the solar nebula. These results indicate that the outflow can act as a size and density classifier. By simply varying the flow rate, the outflow can produce different types of proto‐meteorites from the same chondrule and metal grain feed stock. As a consequence of these investigations, we observed that the number of particles that impact into the nebula drops off moderately rapidly as a function of distance r from the Sun. We also derive a corrected form of the Epstein stopping time.     

In situ determination of iodine content and iodine-xenon systematics in silicates and troilite phases in chondrules from the LL3 chondrite Semarkona

Abstract— Iodine concentrations in small domains (～10 μm) of silicates and troilite (FeS) phases in three chondrules from the Semarkona (LL3) meteorite were determined by an ion microprobe. Independent determination of I content in some of these phases was accomplished by in situ laser probe mass spectrometric analysis of I-derived ¹²⁸Xe in one of these neutron-irradiated chondrules. The ion microprobe data suggest low I content for olivines (20–45 ppb) and relatively higher values for pyroxene and glass (mesostasis) (40–160 ppb). The broad similarity in the measured I contents in pyroxenes in a porphyritic pyroxene chondrule by ion microprobe (42–138 ppb) and by laser probe (37–76 ppb) demonstrate the feasibility of in situ determination of I content in silicate phases via ion microprobe. The I contents in troilite measured by ion microprobe, however, are prone to uncertainty because of the lack of a sulfide standard. The ion microprobe data suggest I content of > 1 ppm in troilite, if the calibration from our silicate standard is used. However, the noble gas data suggest that the I content in troilite is comparable to that in silicates. We attribute this apparent discrepancy to an enhanced sputter ion yield of I from sulfides. Iodine-derived ¹²⁹Xe excesses were observed in both pyroxene and troilite within this chondrule. The I-Xe model ages of these selected phases are consistent with the I-Xe studies of the bulk chondrule. The individual data points fall on or near the isochron obtained from the bulk chondrule, although all except the most radiogenic data point contain evidence of low-temperature uncorrelated iodine.     

Origin of a unique impact-melt rock—the L-chondrite Ramsdorf

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.     

Quantifying the error of 2D bulk chondrule analyses using a computer model to simulate chondrules (SIMCHON)

Bulk chondrule compositions are important to many questions in cosmochemistry, however, the number of available bulk chondrule data sets is still small. A main reason for this is the difficulties of determining bulk chondrule compositions. A commonly used technique is to obtain 2D bulk chondrule compositions from meteorite sections. This technique has an error that we quantify here for the first time using a mathematical model of a chondrule called SIMCHON. The theoretically calculated errors are compared to errors that we determined from serial sectioning of eight Efremovka chondrules. The errors obtained from both approaches are in excellent agreement, proving that our mathematical model produces reliable errors that can be assigned to 2D bulk chondrule compositions. These errors allow a much better interpretation of 2D bulk chondrule data. We provide a table that contains typical errors for 2D bulk compositions of porphyritic chondrules. The errors are in the range of ±<1–30 relative‐%. This should be acceptable for many problems in cosmochemistry. The effect of a chemical layering inside chondrules and the occurrence of a rim around them, as well as the occurrence of opaque and other accessory phases have been studied. A spreadsheet is provided that enables the calculation of errors for any desired chondrule mineral composition. BO chondrules have a negligible error, but it is impossible to provide reasonable error estimates for BO chondrules with an igneous rim. Radial pyroxene chondrules have negligible errors.     

I‐Xe measurements of CAIs and chondrules from the CV3 chondrites Mokoia and Vigarano

Abstract— I‐Xe analyses were carried out for chondrules and refractory inclusions from the two CV3 carbonaceous chondrites Mokoia and Vigarano (representing the oxidized and reduced subgroups, respectively). Although some degree of disturbance to the I‐Xe system is evident in all of the samples, evidence is preserved of aqueous alteration of CAIs in Mokoia 1 Myr later than the I‐Xe age of the Shallowater standard and of the alteration of a chondrule (V3) from Vigarano ～0.7 Myr later than Shallowater. Other chondrules in Mokoia and Vigarano experienced disturbance of the I‐Xe system millions of years later and, in the case of one Vigarano chondrule (VS1), complete resetting of the I‐Xe system after decay of essentially all 129I, corresponding to an age more than 80 Myr after Shallowater. Our interpretation is that accretion and processing to form the Mokoia and Vigarano parent bodies must have continued for at least 4 Myr and 80 Myr, respectively. The late age of a chondrule that shows no evidence for any aqueous alteration or significant thermal processing after its formation leads us to postulate the existence of an energetic chondrule‐forming mechanism at a time when nebular processes are not expected to be important.     

Breakup of liquids by high velocity flow and size distribution of chondrules

As a new approach to understanding the chondrule formation process, we carried out aerodynamic experiments in which a liquid layer was attached to solid cores, and the breakup of this layer occurred by means of the interaction with a high-velocity gas flow. The size distribution of the dispersed droplets was investigated and compared with the size distributions of chondrules. Both distributions had an exponential form. Using the experimental results, the hydrodynamic pressure to produce the chondrule size distributions was estimated to be ∼ 10⁴ Pa.     

Chondrules born in plasma? Simulation of gas–grain interaction using plasma arcs with applications to chondrule and cosmic spherule formation

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.     

Magnesium isotopic fractionations in barred olivine chondrules from the Allende meteorite

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.     

Kinetic stability of a melted iron globule during chondrule formation. I. Non‐rotating model

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.     

Multiple melting in a four‐layered barred‐olivine chondrule with compositionally heterogeneous glass from LL3.0 Semarkona

Chondrule K7p from LL3.0 Semarkona consists of four nested barred‐olivine (BO) chondrules. The innermost BO chondrule (chondrule 1) formed by complete melting of an olivine‐rich dustball. After formation, the chondrule was incorporated into another olivine‐rich dustball. A second heating event caused this second dustball to melt; the mesostasis and some of the olivine in chondrule 1 were probably also melted at this time, but the chondrule 1 structure remained largely intact. At this stage, the object was an enveloping compound BO chondrule. This two‐step process of melting and dustball enshrouding repeated two more times. The different proportions of olivine and glass in chondrules 1–4 suggest that the individual precursor dustballs differed in the amounts of chondrule fragments they contained and the mineral proportions in those fragments. The final dustball (which ultimately formed chondrule 4) was somewhat more ferroan; after melting, crystallizing, and quenching, chondrule 4 contained olivine and glass with higher FeO and MnO contents than those of the earlier formed chondrules. Subsequent aqueous alteration on the LL parent body transformed the abundant metal blebs and stringers at the chondrule surface into carbide, iron oxide, and minor Ni‐rich metal. Portions of the mesostasis underwent dissolution, producing holes and adjacent blades of more resistant material. Much of the glass in the chondrule remained isotropic, even after minor hydration and leaching. The sharp, moderately lobate boundary between the extensively altered mesostasis and the isotropic glass represents the reaction front beyond which there was little or no glass dissolution.     

Properties of chondrules in EL3 chondrites,comparison with EH3 chondrites,and the implications for the formation of enstatite chondrites

Abstract— The study of chondrules provides information about processes occurring in the early solar system. In order to ascertain to what extent these processes played a role in determining the properties of the enstatite chondrites, the physical and chemical properties of chondrules from three EL3 chondrites and three EH3 chondrites have been examined by optical, cathodoluminescence (CL), and electron microprobe techniques. Properties examined include size, texture, CL, and composition of both individual phases and bulk chondrules. The textures, distribution of textures, and composition of silicates of the EL3 chondrules resemble those of EH3 chondrules. However, the chondrules from the two classes differ in that (1) the size distribution of the EL chondrules is skewed to larger values than EH chondrules, (2) the enstatite in EL chondrules displays varying shades of red CL due to the presence of fine‐grained sulfides and metal in the silicates, and (3) the mesostasis of EH chondrules is enriched in Na relative to that of EL chondrules. The similarities between the chondrules of the two classes suggest similar precursor materials, while the differences suggest that there was not a single reservoir of meteoritic chondrules, but that their origin was fairly local. The differences in the size distribution of chondrules in EH and EL chondrites may be explained by aerodynamic and gravitational sorting during accumulation of the meteoric material, while differences in CL and mesostasis properties may reflect differences in formation conditions and cooling rate following chondrule formation. We argue that our observations are consistent with the formation of enstatite chondrites in a thick dynamic regolith on their parent body.     

Elemental redistribution in Tieschitz and the origin of white matrix

Abstract— Two types of mesostasis coexist within some porphyritic chondrules in Tieschitz. One type is smooth. The other, confined to chondrule margins, is blocky on a 5–10 μm scale. Mesostases in one porphyritic olivine-pyroxene (POP) chondrule and one porphyritic olivine (PO) chondrule were analysed by scanning electron microscopy (SEM) and energy-dispersive x-ray spectrometry (EDS), as was white matrix nearby. Mesostases in the PO chondrule and in four others were analysed by ion probe. Pyroxene phenocrysts or dendrites extend across contacts between smooth and blocky mesostasis with no compositional change. Relative to smooth mesostasis, blocky mesostasis is enriched in Al, alkalis, Ba, F, and Cl but depleted in Si, Fe, and Ca. White matrix fills channels between the chondrules. It is physically and chemically similar to blocky mesostasis, but three ion probe analyses indicate that, unlike the mesostases, it is poor in Sc and has variable and fractionated rare earth elements (REEs). Smooth mesostasis is interpreted as solidified primary chondrule liquid; whereas blocky mesostasis is its alteration product or, less likely, a precipitate replacing smooth mesostasis leached out by aqueous fluid. White matrix may have formed by secondary alteration or replacement of mesostases that had been expelled from chondrules during accretion, or as a precipitate filling interchondrule voids. Iron may have been lost from the bulk meteorite, but most other elements merely underwent internal redistribution. Disturbed isotopic systems indicate that aqueous fluid may have been active on the Tieschitz parent body only 2 Ga ago. If correct, this would be the first evidence that an ordinary chondrite parent body underwent internal reprocessing significantly later than 4.5 Ga ago.     

Shape,metal abundance,chemistry, and origin of chondrules in the Renazzo (CR) chondrite

Abstract— We used synchrotron X‐ray microtomography to image in 3‐dimensions (3D) eight whole chondrules in a ?1 cm³piece of the Renazzo (CR) chondrite at ?17 μm per volume element (voxel) edge. We report the first volumetric (3D) measurement of metal/silicate ratios in chondrules and quantify indices of chondrule sphericity. Volumetric metal abundances in whole chondrules range from 1 to 37 volume % in 8 measured chondrules and by inspection in tomography data. We show that metal abundances and metal grain locations in individual chondrules cannot be reliably obtained from single random 2D sections. Samples were physically cut to intersect representative chondrules multiple times and to verify 3D data. Detailed 2D chemical analysis combined with 3D data yield highly variable whole‐chondrule Mg/Si ratios with a supra‐chondritic mean value, yet the chemically diverse, independently formed chondrules are mutually complementary in preserving chondritic (solar) Fe/Si ratios in the aggregate CR chondrite. These results are consistent with localized chondrule formation and rapid accretion resulting in chondrule + matrix aggregates (meteorite parent bodies) that preserve the bulk chondritic composition of source regions.     

The effect of multiple particle sizes on cooling rates of chondrules produced in large‐scale shocks in the solar nebula

Chondrules represent one of the best probes of the physical conditions and processes acting in the early solar nebula. Proposed chondrule formation models are assessed based on their ability to match the meteoritic evidence, especially experimental constraints on their thermal histories. The model most consistent with chondrule thermal histories is passage through shock waves in the solar nebula. Existing models of heating by shocks generally yield a good first‐order approximation to inferred chondrule cooling rates. However, they predict prolonged heating in the preshock region, which would cause volatile loss and isotopic fractionation, which are not observed. These models have typically included particles of a single (large) size, i.e., chondrule precursors, or at most, large particles accompanied by micron‐sized grains. The size distribution of solids present during chondrule formation controls the opacity of the affected region, and significantly affects the thermal histories of chondrules. Micron‐sized grains evaporate too quickly to prevent excessive heating of chondrule precursors. However, isolated grains in chondrule‐forming regions would rapidly coagulate into fractal aggregates. Preshock heating by infrared radiation from the shock front would cause these aggregates to melt and collapse into intermediate‐sized (tens of microns) particles. We show that inclusion of such particles yields chondrule cooling rates consistent with petrologic and isotopic constraints.