Ordinary and carbonaceous chondrites of the lowest petrologic types were surveyed by Xray mapping techniques A variety of metamorphic effects were noted and subjected to detailed analysis using electron microprobe, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and cathodoluminescence (CL) methods The distribution of Cr in FeO-rich olivine systematically changes as metamorphism increases between type 30 and type 32 Igneous zoning patterns are replaced by complex ones and Cr-rich coatings develop on all grains Cr distributions in olivine are controlled by the exsolution of a Cr-rich phase, probably chromite Cr in olivine may have been partly present as tetrahedrally coordinated Cr3+ Separation of chromite is nearly complete by petrologic type 32 The abundance of chondrules showing an inhomogeneous distribution of alkalis in mesostasis also increases with petrologic type TEM shows this to be the result of crystallization of albite Residual glass compositions systematically change during metamorphism, becoming increasingly rich in K Glass in type I chondrules also gains alkalis during metamorphism Both types of chondrules were open to an exchange of alkalis with opaque matrix and other chondrules The matrix in the least metamorphosed chondrites is rich in S and Na The S is lost from the matrix at the earliest stages of metamorphism due to coalescence of minute grains Progressive heating also results in the loss of sulfides from chondrule rims and increases sulfide abundances in coarse matrix assemblages as well as inside chondrules Alkalis initially leave the matrix and enter chondrules during early metamorphism Feldspar subsequently nucleates in the matrix and Na re-enters from chondrules These metamorphic trends can be used to refine classification schemes for chondrites Cr distributions in olivine are a highly effective tool for assigning petrologic types to the most primitive meteorites and can be used to subdivide types 30 and 31 into types 300 through 315 On this basis, the most primitive ordinary chondrite known is Semarkona, although even this meteorite has experienced a small amount of metamorphism Allan Hills (ALH) A77307 is the least metamorphosed CO chondrite and shares many properties with the ungrouped carbonaceous chondrite Acfer 094 Analytical problems are significant for glasses in type II chondrules, as Na is easily lost during microprobe analysis As a result, existing schemes for chondrule classification that are based on the alkali content of glasses need to be revised

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1945-5100.2005.tb00366.x

The onset of metamorphism in ordinary and carbonaceous chondrites

Physical properties of chondrules in different chondrite groups: Implications for multiple melting events in dusty environments

The examination of the physical properties of chondrules has generally received less emphasis than other properties of meteorites such as their mineralogy, petrology, and chemical and isotopic compositions. Among the various physical properties of chondrules, chondrule size is especially important for the classification of chondrites into chemical groups, since each chemical group possesses a distinct size-frequency distribution of chondrules. Knowledge of the physical properties of chondrules is also vital for the development of astrophysical models for chondrule formation, and for understanding how to utilize asteroidal resources in space exploration. To examine our current knowledge of chondrule sizes, we have compiled and provide commentary on available chondrule dimension literature data. We include all chondrite chemical groups as well as the acapulcoite primitive achondrites, some of which contain relict chondrules. We also compile and review current literature data for other astrophysically-relevant physical properties (chondrule mass and density). Finally, we briefly examine some additional physical aspects of chondrules such as the frequencies of compound and “cratered” chondrules. A purpose of this compilation is to provide a useful resource for meteoriticists and astrophysicists alike.

Chondrule size and related physical properties: A compilation and evaluation of current data across all meteorite groups

 Planetesinial bow shocks would have existed during the chondrule forniation epoch if proto-Jupiter formed within a few Myr of CAI forniation and if the protoplanetary nebula persisted for a few Myr afterwards As planetesimal orbits evolved inward I t   1 1 , they would have passed through Jovian resonances y increased eccentricities leading to supersonic velocities relative gas Previous studies have demonstrated that s would have been heated to - an one bow shock would have 1 planetesimal radius from the ugh bow shocks with scale sizes < id to be consistent with of this distance, formed been decelerated to low e planetesimal This would 1 A near-susace boundary layer o ed solids-to- as niass ratios ecelerating incident particles If mean chondrule size - c calculations indicate that forniation bow - shock would have resulted in 4 x gni/cni* An approximate ow shocks in roducin the large chondrites inxcates tiat, within observational uncertainties, the bow shock niechanisni can not easily be excluded as a possible chondrule formation model

/pdf/particle-gas-dynamics-and-primary-accretion-joaxp0dtdk.pdf

Particle-Gas Dynamics and Primary Accretion

We present a mechanism for chondrules to stick together by means of compaction of a porous dust rim they sweep up as they move through the dusty nebula gas. It is shown that dust aggregates formed out of micron-size grains stick to chondrules, forming a porous dust rim. When chondrules collide, this dust can be compacted by means of rolling motions within the porous dust layer. This mechanism dissipates the collisional energy, compacting the rim and allowing chondrules to stick. The structure of the obtained chondrule-dust agglomerates (referred to as compounds) then consists of three phases: chondrules, porous dust, and dust that has been compacted by collisions. Subsequently, these compounds accrete their own dust and collide with other compounds. The evolution of the compound size distribution and the relative importance of the phases is calculated by a Monte Carlo code. Growth ends, and a simulation is terminated when all the dust in the compounds has been compacted. Numerous runs are performed, reflecting the uncertainty in the physical conditions at the chondrule formation time. It is found that compounds can grow by 1-2 orders of magnitudes in radius, up to dm sizes when turbulence levels are low. However, relative velocities associated with radial drift form a barrier for further growth. Earlier findings that the dust sweep-up by chondrules is proportional to their sizes are confirmed. We contrast two scenarios regarding how this dust evolved further toward the densely packed rims seen in chondrites.

Co-Accretion of Chondrules and Dust in the Solar Nebula

We measured the sizes and textural types of 719 intact chondrules and 1322 chondrule fragments in thin sections of Semarkona (LL3.0), Bishunpur (LL3.1), Krymka (LL3.1), Piancaldoli (LL3.4) and Lewis Cliff 88175 (LL3.8). The mean apparent diameter of chondrules in these LL3 chondrites is 0.80 Φ units or 570 um, much smaller than the previous rough estimate of ~900 μm. Chondrule fragments in the five LL3 chondrites have a mean apparent cross-section of 1.60 phi units or 330 μm. The smallest fragments are isolated olivine and pyroxene grains; these are probably phenocrysts liberated from disrupted porphyritic chondrules. All five LL3 chondrites have fragment/chondrule number ratios exceeding unity, suggesting that substantial numbers of the chondrules in these rocks were shattered. Most fragmentation probably occurred on the parent asteroid. Porphyritic chondrules (porphyritic olivine + porphyritic pyroxene + porphyritic olivine-pyroxene) are more readily broken than droplet chondrules (barred olivine + radial pyroxene + cryptocrystalline). The porphyritic fragment/chondrule number ratio (2.0) appreciably exceeds that of droplet-textured objects (0.9). Intact droplet chondrules have a larger mean size than intact porphyritic chondrules, implying that large porphyritic chondrules are fragmented preferentially. This is consistent with the relatively low percentage of porphyritic chondrules within the set of the largest chondrules (57%) compared to that within the set of the smallest chondrules (81%). Differences in mean size among chondrule textural types may be due mainly to parent-body chondrule-fragmentation events and not to chondrule-formation processes in the solar nebula.

/pdf/size-frequency-distributions-of-chondrules-and-chondrule-27ktttmzoe.pdf

Victoria E. Nelson

Papers

Size-frequency distributions of chondrules and chondrule fragments in LL3 chondrites: Implications for parent-body fragmentation of chondrules