普通球粒陨石 Semarkona中球粒的 Pb-Pb法年龄

陨石年代学研究中常用定年工具包括短半衰期和长半衰期放射性同位素体系,其中前者可以给出陨石形成的相对年龄,而后者则可以给出绝对形成年龄。在长半衰期体系中,PbP-b法是目前能获得高精度可靠年龄的最有效方法。普通球粒陨石Sem arkona是最不平衡的LL3.0型陨石,受后期热变质的影响很小,因此其年龄研究对反演陨石起源有重要意义。在本文中,对Sem arkona中球粒用不同的化学浸洗流程,并测定浸洗溶液和残渣中UT-hP-b同位素组成,其中浸洗后的残渣均给出相对较高的206Pb/204Pb比值,表明其中含有较多的放射成因Pb同位素组成,这些残渣构成PbP-b等时线,给出年龄为(4566.9±5.8)M a,M SW D=26,与207Pb/206Pb模式年龄在误差范围内一致。尽管Sem arkona陨石可能经历了后期蚀变的影响,但前人对陨石变质温度的研究结果表明,热变质温度并未足以使球粒中Pb同位素体系重置,因此获得的年龄代表Sem arkona陨石球粒的形成年龄。     

荷叶塘陨石:一个L3型普通球粒陨石的岩石学和地球化学特征

荷叶塘为一块我国降落的原始3型普通球粒陨石,因此具有重要研究意义。本文对荷叶塘陨石光薄片及粉末样品的岩石学、矿物学和全岩组成地球化学特征进行研究,为这块陨石的深入研究提供重要基础数据。研究表明荷叶塘陨石具L3型陨石岩石学特征,具典型的球粒陨石结构,球粒清晰,球粒结构类型多,基质重结晶程度低,组成模式为:球粒80vol%,金属和硫化物含量为5vol%,基质15vol%。矿物化学成分表明,该陨石球粒以Ⅰ型(贫铁型)球粒为主,橄榄石Fa0.41-34.1(PMD=51),低钙辉石Fs1.82-27.2(PMD=88),Wo0.18-3.13(PMD=103),铁纹石中Co含量平均0.62%(PMD=20),矿物成份不均一程度高,橄榄石矿物结晶颗粒内部化学成分变化大,呈正环带分布,与岩浆型结晶顺序一致,球粒与基质及间隙物成分明显不同,表现为不同物质来源。化学成分全岩分析结果显示,荷叶塘陨石亲石、亲铁元素含量均为L型陨石特征。依据以上岩石矿物学和化学组成特征,依照陨石亚分类参数,将其类型划分为L3.4型普通球粒陨石。冲击变质程度S2,风化程度W1。研究结果表明荷叶塘陨石为一块受后期水、热蚀变和风化影响较少的原始类型陨石。组成矿物成分极不均一,在矿物晶体内部,球粒内部及球粒与基质间均有明显变化。     

The petrology of chondrules in the sharps meteorite

Correlated petrographic and microprobe studies of 96 chondrules in the Sharps (H-3) chondrite indicate that chondritic material had a highly varied pre-accumulation history. Some chondrules, chiefly excentroradial and barred types, appear to be quenched droplets. Others, including most of the metal poor microporphyritic type, appear to have crystallized more slowly and are thought to be fragments of pre-existing rock. Although chondrules of all types show various effects similar to those produced by shock, such effects are most conspicuous in metal-rich chondrules and least conspicuous in spherical chondrules. It is concluded that shock was involved in the origin of chondrules and not simply a secondary effect.It is proposed that chondrules were formed by shock processes during the accumulation of nebular dust into asteroid-sized bodies. Olivine-rich microporphyritic chondrules are thought to be due to complete melting of large masses of target material; metal-rich chondrules represent shock melting and partial vaporization; and spherical, pyroxene-rich chondrules are interpreted as condensates from shock-generated vapor.     

The petrology of chondrules in the Hallingeberg meteorite

Petrographic study of 124 chondrules in the Hallingeberg (L-3) chondrite and electron probe microanalyses of olivine and low-Ca pyroxene in 96 of them reveal patterns of variation like those encountered previously in Sharps (H-3). Chondrule mineralogy, mineral composition, and the incidence of shock-related textures vary systematically with chondrule type. This fact and evidence of recrystallization in at least a fourth of the chondrules studied indicate that the pre-accretion histories of chondrules included complex and overlapping episodes of magmatic crystallization, burial, metamorphism and exhumation, in which impact shock was heavily involved. Data for Hallingeberg and Sharps suggest that orthopyroxene accompanies or replaces clinoenstatite in some chondrules and that its presence is due, in part at least, to pre-accretion recrystallization. A comparison of modes for chondrules in Sharps and Hallingeberg shows the former to contain more olivine, on the average, than the latter. It appears that the mean compositions of chondrules in H- and L-group chondrites reflect bulk chemical differences between the two groups, and that chondrule formation followed the siderophile fractionation which differentiated H-, L- and LL-group ordinary chondrites.     

Microchondrule-bearing clast in the Piancaldoli LL3 meteorite: a new kind of type 3 chondrite and its relevance to the history of chondrules

In the Piancaldoli LL3 chondrite, we found a mm-sized clast containing ~100 chondrules 0.2–64 μm in apparent diameter (much smaller than any previously reported) that are all of the same textural type (radial pyroxene; FS_1–17). This clast, like other type 3 chondrites, has a fine-grained Ferich opaque silicate matrix, sharply defined chondrules, abundant low-Ca clinopyroxene and minor troilite and Si- and Cr-bearing metallic Fe,Ni. However, the very high modal matrix abundance (63 ± 8 vol. %), unique characteristics of the chondrules, and absence of microscopically-observable olivine indicate that the clast is a new kind of type 3 chondrite. Most chondrules have FeO-rich edges, and chondrule size is inversely correlated with chondrule-core FeO concentration (the first reported correlation of chondrule size and composition). Chondrules acquired Fe by diffusion from Fe-rich matrix material during mild metamorphism, possibly before final consolidation of the rock. Microchondrules (those chondrules ? 100 μm in diameter) are also abundant in another new kind of type 3 chondrite clast in the Rio Negro L chondrite regolith breccia. In other type 3 chondrite groups, microchondrule abundance appears to be anticorrelated with mean chondrule size, viz. 0.02–0.04 vol. % in H and CO chondrites and ?0.006 vol. % in L, LL, and CV chondrites.Microchondrules probably formed by the same process that formed normal-sized droplet chondrules: melting of pre-existing dustballs. Because most compound chondrules in the clast and other type 3 chondrites formed by collisions between chondrules of the same textural type, we suggest that dust grains were mineralogically sorted in the nebula before aggregating into dustballs. The sizes of compound chondrules and chondrule craters, which resulted from collisions of similarly-sized chondrules while they were plastic, indicate that size-sorting (of dustballs) occurred before chondrule formation, probably by aerodynamic processes in the nebula. We predict that other kinds of type 3 chondrites exist which contain chondrule abundances, size-ranges and proportions of textural types different from known chondrite groups.     

Oxygen-isotopic compositions of low-FeO relicts in high-FeO host chondrules in Acfer 094, a type 3.0 carbonaceous chondrite closely related to CM

With one exception, the low-FeO relict olivine grains within high-FeO porphyritic chondrules in the type 3.0 Acfer 094 carbonaceous chondrite have Δ¹⁷O (= δ¹⁷O − 0.52 × δ¹⁸O) values that are substantially more negative than those of the high-FeO olivine host materials. These results are similar to observations made earlier on chondrules in CO3.0 chondrites and are consistent with two independent models: (1) Nebular solids evolved from low-FeO, low-Δ¹⁷O compositions towards high-FeO, more positive Δ¹⁷O compositions; and (2) the range of compositions resulted from the mixing of two independently formed components. The two models predict different trajectories on a Δ¹⁷O vs. log Fe/Mg (olivine) diagram, but our sample set has too few values at intermediate Fe/Mg ratios to yield a definitive answer.Published data showing that Acfer 094 has higher volatile contents than CO chondrites suggest a closer link to CM chondrites. This is consistent with the high modal matrix abundance in Acfer 094 (49 vol.%). Acfer 094 may be an unaltered CM chondrite or an exceptionally matrix-rich CO chondrite. Chondrules in Acfer 094 and in CO and CM carbonaceous chondrites appear to sample the same population. Textural differences between Acfer 094 and CM chondrites are largely attributable to the high degree of hydrothermal alteration that the CM chondrites experienced in an asteroidal setting.     

Evidence for primitive nebular components in chondrules from the Chainpur chondrite

The least equilibrated ordinary chrondrites contain chondrules which have experienced little change since the time of their formation in the early solar system. These chondrules are excellent indicators of the physical and chemical nature of the solar nebula. We separated 36 chondrules from the Chainpur (LL3.4) chondrite and analyzed each for 20 elements and petrographic properties. Sampling biases were minimized as far as possible.Chondrules seem to have formed through the melting of random mixtures of grains comprising a limited number of nebular components. The identity of these components can be deduced from chondrule compositions. The dominant components appear to be: 1) a mixture of metal and sulfide with composition similar to whole-rock metal and sulfide; 2) refractory (Ir-rich) metal; 3) refractory, olivine-rich silicates; 4) low-temperature, pyroxene-rich silicates, and, possibly, 5) a component containing the more volatile lithophiles.Most of the textural types of chondrules formed from the same set of precursor components. In some cases chondrules having different textures are almost identical in composition. A few, unusual chondrule types seem to mainly consist of uncommon nebular components, possibly indicating different modes of formation.Etching experiments confirm that chondrule rims are enriched in metal, troilite and moderately volatile elements relative to the bulk chondrules. However, a large fraction of the volatiles remains in the unetched interior.     

Mineralogy and petrology of chondrules and inclusions in the Mokoia CV3 chondrite

All objects >100 μm in apparent diameter in five polished thin sections of the Mokoia CV3 chondrite were studied and classified. Number and volume percentages and mean apparent size of each type of chondrule and inclusion were determined. Three major types of olivine chondrules were observed: igneous chondrules, recrystallized chondrules, and chondrules that appear to be accretional aggregates. Coarse-grained CAI's have igneous textures and mineral parageneses, while fine-grained CAI's are aggregates containing varying proportions of Al-rich concentric objects, Ca-rich chaotic material, and inclusion matrix. Chondrules and refractory inclusions in Mokoia and Allende are broadly similar in texture and mineral chemistry, but Mokoia refractory inclusions contain phyllosilicates rather than feldspathoids, and melilite-rich CAI's are more abundant in Allende.We think that most CAI's formed during the metamorphism, partial melting, and incomplete distillation of primitive dust aggregates when they were heated in the solar nebula. In the process, Ca-rich melt appears to have been physically separated from Al-rich residues, producing the observed fractionation of Ca from Al into distinct constituents of CAI's. Some CAI's may be aggregates of devitrified, amorphous metastable condensates. Inclusion matrix may have condensed from silicate-rich vapors produced during distillation. Mokoia inclusion matrix contains phyllosilicates that are probably primitive nebular material.     

Oscillatory zoned fayalitic olivine from the Lewis Cliff 90500 CM2 meteorite

A grain of fayalitic olivine with oscillatory Fe-rich and Fe-poor zoning occurs in the Antarctic Lewis Cliff 90500 CM meteorite, suggesting reaction via diffusion. This grain is continuously enclosed by a fine-grained rim ranging from 20 to 100 μm in thickness. Phyllosilicates and other hydrous phases occur at three broken edges of the grain as well as within its surrounding rim, suggesting in situ alteration via aqueous processes. This olivine grain plus its rim provides evidence for both anhydrous reaction that predated the accretion of the meteorite parent body and hydrous alteration that presumably occurred on the parent body.     

Systematic compositional variations in the Cape York iron meteorite

Concentrations of Re, Ir and Au are nearly constant within individual masses of the Cape York IIIAB iron meteorite, but differences between masses can be as large as a factor of 2, the extremes being Savik (5.1 μg/g Ir) and Agpalilik (2.7 μg/g Ir). The S concentration shows a still larger range from 13 mg/g in Agpalilik to 1.4 mg/g in Savik. A relatively large compositional hiatus between Dog and Agpalilik probably reflects inadequate sampling of the original material.Concentrations of Ir vary by ~10% and Au by ~3% between the ends of an 85-cm section from the Agpalilik mass of Cape York, but other sections through Agpalilik show smaller variations. These concentration ranges are much larger than expected from radial crystallization of a moderately large (radius 10 s of km) core. These variations in the Agpalilik mass may reflect dendritic crystallization, or they may have resulted from the process that produced the large concentration range among the Cape York masses.Large gradients in Re and Ir and small gradients in Ni and Au were also observed in samples within 2 cm of a large (100 cm³) troilite nodule. These gradients may reflect rapidly changing solid/liquid distribution coefficients during the final crystallization of S-rich liquid.The compositional trends among the various masses can either be explained by mixing of disparate end members followed by diffusive homogenization on a scale of m, or by dendritic crystallization on the ceiling of the IIIAB magma chamber. The mixing of a solid similar in composition to Savik with a liquid in equilibrium with this solid yields a good match to the observed trends, in which case Agpalilik consists of a mixture of 64% liquid and 36% solids. The bulk S content of the IIIAB core is calculated to be 14 mg/g on the basis of this model.     

Non-nebular origin of dark mantles around chondrules and inclusions in CM chondrites

Our examination of nine CM chondrites that span the aqueous alteration sequence leads us to conclude that compact dark fine mantles surrounding chondrules and inclusions in CM chondrites are not discrete fine-grained rims acquired in the solar nebula as modeled by Metzler et al. [Accretionary dust mantles in CM chondrites: evidence for solar nebula processes. Geochim. Cosmochim. Acta56, 1992, 2873-2897]. Nebular processes that lead to agglomeration produce materials with porosities far higher than those in the dark mantles. We infer that the mantles were produced from porous nebular materials on the CM parent asteroid by impact-compaction (a process that produces the lowest porosity adjacent to chondrules and inclusions). Compaction was followed by aqueous alteration that formed tochilinite, serpentine, Ni-bearing sulfide, and other secondary products in voids in the interchondrule regions. Metzler et al. reported a correlation between mantle thickness and the radius of the enclosed object. In Yamato 791198 we find no correlation when all sizes of central objects and dark lumps are included but a significant correlation (r² = 0.44) if we limit consideration to central objects with radii >35 μm; a moderate correlation is also found in QUE 97990. We suggest that impact-induced shear of a plum-pudding-like precursor produced the observed “mantles”; these were shielded from comminution during impact events by the adjacent stronger chondrules and inclusions. Some mantles in CM chondrites with low degrees of alteration show distinct layers that may largely reflect differences in porosity. Typically, a gray, uniform inner layer is surrounded by an outer layer consisting of darker silicates with BSE-bright speckles. The CM-chondrite objects characterized as “primary accretionary rocks” by Metzler et al. did not form in the nebula, but rather on the parent body. The absence of solar-flare particle tracks and solar-wind-implanted rare gases in these clasts reflect their lithified nature and low surface/volume ratios during the period when they resided in the regolith and were subject to irradiation by solar particles. The clasts are analogous to the light-colored metamorphosed clasts in ordinary-chondrite regolith breccias (which also lack solar-flare particle tracks and solar-wind gas).     

Chemical and crystallographic study of cronstedtite in the matrix of the Cochabamba (CM2) carbonaceous chondrite

Summary The principal mineral component in the matrix of the Cochabamba carbonaceous chondrite is a phyllosilicate, which is identified as cronstedtite mainly on the basis of its chemical composition. Its approximate idealized formula is given by M₆ Fe _0.7 ³⁺ Al_0.5Si_2.7O₁₀ (OH)₈ with M=Fe²⁺, Fe³⁺, and Mg in somewhat variable amounts. TEM studies reveal the presence of three polytypes, and show a high degree of stacking disorder parallel to (001) with the displacement vector ±b/3 or ±2b . Crumpled amorphous masses in the matrix may contain structural building blocks of phyllosilicates. They, rather than the anhydrous minerals, seem to be the most likely progenitors of cronstedtite. Some constraints on its origin are reviewed. In addition to cronstedtite, observations on some other matrix phases are also reported.

---

Chemische und kristallographische Untersuchung von Cronstedtit in der Matrix des kohligen Chondrits (CM2) Cochabamba

Zusammenfassung Der Hauptbestandteil der Matrix im kohligen Chondrit Cochabamba ist ein Schichtsilikat, das hauptsächlich aufgrund seiner chemischen Zusammensetzung als Cronstedtit identifiziert wurde. Die idealisierte Formel entspricht ungefähr M₆Fe _0.7 ⁺³ Al_0.5Si_2.7O₁₀(OH)₈ mit M=Fe²⁺, Fe³⁺ und Mg in wechselnden Mengen. TEM-Untersuchungen zeigen das Vorkommen von drei Modifikationen, sowie einen hohen Grad von Versetzungsfehlern parallel zu (001), mit dem Versetzungsfaktor ±b/3 oder ±2b/3. Deformierte amorphe Aggregate in der Matrix scheinen primitive Bausteine der Schichtsilikate zu sein. Sie (und nicht die wasserfreien Mineralien) dürften das Material darstellen, aus dem Cronstedtit gebildet wurde. Die Bildungsbedingungen von Cronstedtit werden diskutiert. Außerdem wird über Beobachtungen an anderen Matrixmineralien berichtet.

---

---

With 4 Figures     

A study of chondrules in the Boxian meteorite from Anhui Province,China

The types of chondrules in the Boxian meteorite have been determined. Electron microprobe analysis of various textural types of chondrules in the meteorite has revealed the composition of glassy chondrules and of their glassy matrix. Also presented in this paper are the related explanations. In addition, the genesis, textural diversity and evolution of the chondrules as well as the olivine girdles observed in the chondrules are discussed.     

Metamorphism of the H-group chondrites: implications from compositional and textural trends in chondrules

Major and minor element bulk compositions of 373 individual chondrules from 18 H3 to H6 chondrites were determined in polished thin sections by broad-beam electron probe analysis. Bulk chondrule FeO and Al₂O₃ increase and TiO₂ and Cr₂O₃ decrease with increasing petrologic type; normative fayalite, albite and plagioclase increase through the petrologic sequence. Chondrule diameters correlate with phenocryst sizes in porphyritic chondrules of type 3 chondrites, but this correlation is diminished in the higher petrologic types. Furthermore, for a given chondrule diameter, phenocryst sizes are larger in the higher petrologic types. We attribute most compositional trends in chondrules through the petrologic sequence to diffusion and equilibration among chondrules and between chondrules and matrix in response to increasing degrees of thermal metamorphism. Increased phenocryst sizes in the higher petrologic types are probably the result of grain growth during metamorphism.We suggest that H-group chondrites formed by accretion of high-temperature (chondrules) and low-temperature (matrix) materials. Parent materials of each of the petrologic types resembled type 3 chondrites, but had slight compositional differences (e.g. volatiles, rare gases, total iron) inherited during accretion. These differences were predominantly functions of decreasing temperature in the nebula as accretion progressed. Internal reheating of the parent materials to different temperatures and (probably) for different times, as a function of depth in the parent body, caused compositional equilibration, grain coarsening, and reduction of FeO to Fe° by carbon.     

Chondrules in the CB/CH-like carbonaceous chondrite Isheyevo: Evidence for various chondrule-forming mechanisms and multiple chondrule generations

The recently discovered metal-rich carbonaceous chondrite Isheyevo consists of Fe, Ni-metal grains, chondrules, heavily hydrated matrix lumps and rare refractory inclusions. It contains several lithologies with mineralogical characteristics intermediate between the CH and CB carbonaceous chondrites; the contacts between the lithologies are often gradual. Here we report the mineralogy and petrography of chondrules in the metal-rich (70 vol%) and metal-poor (20 vol%) lithologies. The chondrules show large variations in textures [cryptocrystalline, skeletal olivine, barred olivine, porphyritic olivine, porphyritic olivine-pyroxene, porphyritic pyroxene], mineralogy and bulk chemistry (magnesian, ferrous, aluminum-rich, silica-rich). The porphyritic magnesian (Type I) and ferrous (Type II) chondrules, as well as silica- and Al-rich plagioclase-bearing chondrules are texturally and mineralogically similar to those in other chondrite groups and probably formed by melting of mineralogically diverse precursor materials. We note, however, that in contrast to porphyritic chondrules in other chondrite groups, those in Isheyevo show little evidence for multiple melting events; e.g., relict grains are rare and igneous rims or independent compound chondrules have not been found. The magnesian cryptocrystalline and skeletal olivine chondrules are chemically and mineralogically similar to those in the CH and CB carbonaceous chondrites Hammadah al Hamra 237, Queen Alexandra Range 94411 (QUE94411) and MacAlpine Hills 02675 (MAC02675), possibly indicating a common origin from a vapor–melt plume produced by a giant impact between planetary embryos; the interchondrule metal grains, many of which are chemically zoned, probably formed during the same event. The magnesian cryptocrystalline chondrules have olivine–pyroxene normative compositions and are generally highly depleted in Ca, Al, Ti, Mn and Na; they occasionally occur inside chemically zoned Fe, Ni-metal grains. The skeletal olivine chondrules consist of skeletal forsteritic olivine grains overgrown by Al-rich (up to 20 wt% Al₂O₃) low-Ca and high-Ca pyroxene, and interstitial anorthite-rich mesostasis. Since chondrules with such characteristics are absent in ordinary, enstatite and other carbonaceous chondrite groups, the impact-related chondrule-forming mechanism could be unique for the CH and CB chondrites. We conclude that Isheyevo and probably other CH chondrites contain chondrules of several generations, which may have formed at different times, places and by different mechanisms, and subsequently accreted together with the heavily hydrated matrix lumps and refractory inclusions into a CH parent body. Short-lived isotope chronology, oxygen isotope and trace element studies of the Isheyevo chondrules can provide a possible test of this hypothesis.     

Organic compounds in the Forest Vale, H4 ordinary chondrite

We have analyzed the H4 ordinary chondrite Forest Vale for polycyclic aromatic hydrocarbons (PAHs) using two-step laser mass spectrometry (L2MS) and for amino acids using a standard chromatographic method. Indigenous PAHs were identified in the matrices of freshly cleaved interior faces but could not be detected in pulverized silicates and chondrules. No depth dependence of the PAHs was found in a chipped interior piece. Amino acids, taken from the entire sample, consisted of protein amino acids that were nonracemic, indicating that they are terrestrial contaminants. The presence of indigenous PAHs and absence of indigenous amino acids provides support for the contention that different processes and environments contributed to the synthesis of the organic matter in the solar system.     

Derivation of a heterogeneous lithic fragment in the Bovedy L-group chondrite from impact-melted porphyritic chondrules

The Bovedy L-group chondrite contains a light-colored poikilitic lithic fragment with olivine, low-Ca pyroxene and kamacite compositions characteristic of porphyritic chondrules from unequilibrated ordinary chondrites. Its texture, compositional similarities to porphyritic chondrules, and low Na₂O, K₂O and P₂O₅ content indicate that the fragment represents a solidified, slightly fractionated impact melt formed from a source that was rich in porphyritic chondrules. The fragment is heterogeneous, with a progressive increase in the bulk $\text{MgO}\text{FeO}$ ratio and in MgO content of olivines and low-Ca pyroxenes across its length. ${}^{39}\text{Ar}{}^{40}\text{Ar}$ analyses of the fragment and host indicate that the meteorite experienced extensive degassing due to reheating. The approximate age of 0.5–0.94 Byr dates the reheating event and not the formation of the lithic fragment or the Bovedy breccia. This reheating event renders the fragment's and host's metallographic cooling rate of ~ 5 C/Myr (through 500°C) imprecise. However, the absence of martensite and the presence of kamacite. zoned taenite and tetrataenite in the fragment and host are consistent with such slow cooling through 500°C. This cooling rate must have resulted from burial of the fragment-host assemblage beneath insulating material on the Bovedy parent body. If the thermal diffusivity (κ) of this overburden was approximately comparable to that of the lunar regolith (10^?4cm²/sec), then the fragment was buried at a depth ≌ 6.5 km; if K = 10^?2 cm²/sec (similar to chondritic material), then the fragment was buried at a depth ?65 km.     

Indicators of parent-body processes: Hydrated chondrules and fine-grained rims in the Mokoia CV3 carbonaceous chondrite

A petrographic and electron microscopic study of the Mokoia CV3 carbonaceous chondrite shows that all of the chondrules and inclusions (>400 μm in diameter) and most of their fine-grained rims studied (referred to as chondrules/rims) contain various amounts of hydrous phyllosilicates (mostly saponite) formed by aqueous alteration of anhydrous silicates. The rims mainly consist of fine-grained olivine and saponite in varying proportions and contain crosscutting veins of Fe-rich olivine. The boundaries between the chondrules and their rims are irregular and show abundant evidence of aqueous alteration interactions between them. In contrast, the host matrix contains very minor amounts of saponite and shows no evidence of such extensive aqueous alteration. The boundaries between the chondrules/rims and the matrix are sharp and show no traces of the matrix having been involved in the alteration of the chondrules/rims. These observations indicate that the aqueous alteration in the chondrules/rims did not occur in the present setting.We suggest that the chondrules/rims are actually clasts transported from a location on the meteorite parent body different from where the Mokoia meteorite was from. The aqueous alteration of the chondrules/rims probably occurred there. The veins in the rims were originally fractures produced in an interchondrule matrix by impacts; these were later filled by Fe-rich olivine during aqueous activity. This location was then involved in impact brecciation, and individual chondrules were ejected as clasts with remnants of the matrix surrounding them. During the continuing brecciation, those chondrule/rim clasts were transported, mixed with anhydrous matrix grains, and finally lithified to the present meteorite. Therefore, the rims are fragmented remnants of a former matrix.Textures characterized by fine-grained rims surrounding chondrules in chondrites have been widely thought to have formed in the solar nebula before they accreted into their parent bodies. However, our results suggest that some textures may not be explained by such an accretionary model; instead, the multi-stage parent-body process modeled for the Mokoia rim formation may be a more plausible explanation.