Macrochondrules in chondrites—Formation by melting of mega‐sized dust aggregates and/or by rapid collisions at high temperatures?

Abstract– Seventy‐four macrochondrules with sizes >3 mm were studied. Considering the extraordinary size of the chondrules (occasionally achieving a mass of 1000 times (and more) the mass of a normal‐sized chondrule), the conditions in the formation process must have been somewhat different compared with the conditions for the formation of the common chondrules. Macrochondrules are typically rich in olivine and texturally similar to specific chondrule types (barred, radial, porphyritic, and cryptocrystalline) of normal‐sized chondrules. However, our studies show that most of the macrochondrules are fine‐grained or have elongated crystals (mostly BO, RP, and C), which lead to the assumption that they were once totally molten and cooled quite rapidly. Porphyritic chondrules belong to the least abundant types of macrochondrules. This distribution of chondrule types is highly unusual and just a reverse of the distribution of chondrule types among the typical‐sized chondrules in most chondrite groups except for the CH and CB chondrites. New chondrule subtypes (like radial‐olivine [RO] or multi‐radial [MR] chondrules) are defined to better describe the textures of certain large chondrules. Macrochondrules may have formed due to melting of huge precursor dust aggregates or due to rapid collisions of superheated melt droplets, which led to the growth of large molten spherules in regions with high dust densities and high electrostatic attraction.     

An unusual compound object in Yamato 793408 (H3.2‐an): The missing link between compound chondrules and macrochondrules?

We found a large (~2 mm) compound object in the primitive Yamato 793408 (H3.2‐an) chondrite. It consists mostly of microcrystalline material, similar to chondrule mesostasis, that hosts an intact barred olivine (BO) chondrule. The object contains euhedral pyroxene and large individual olivine grains. Some olivine cores are indicative of refractory forsterites with very low Fe‐ and high Ca, Al‐concentrations, although no ¹⁶O enrichment. The entire object is most likely a new and unique type, as no similar compound object has been described so far. We propose that it represents an intermediate stage between compound chondrules and macrochondrules, and formed from the collision between chondrules at low velocities (below 1 m s^?1) at high temperatures (around 1550 °C). The macrochondrule also trapped and preserved a smaller BO chondrule. This object appears to be the first direct evidence for a genetic link between compound chondrules and macrochondrules. In accordance with previous suggestions and studies, compound chondrules and macrochondrules likely formed by the same mechanism of chondrule collisions, and each represents different formation conditions, such as ambient temperature and collision speed.     

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.     

RELATIVE ABUNDANCES OF CHONDRULE PRIMARY TEXTURAL TYPES IN ORDINARY CHONDRITES AND THEIR BEARING ON CONDITIONS OF CHONDRULE FORMATION

A petrographic survey of > 1600 chondrules in thin-sections of 12 different mildly to highly unequilibrated H-, L-, and LL-chondrites, as well as morphological and textural study of 141 whole chondrules separated from 11 of the same chondrites, was used to determine the relative abundances of definable chondrule primary textural types. Percentage abundances of various chondrule types are remarkably similar in all chondrites studied and are ～ 47–52 porphyritic olivine-pyroxene (POP), 15–27 porphyritic olivine (PO), 9–11 porphyritic pyroxene (PP), 3–4 barred olivine (BO), 7–9 radial pyroxene (RP), 2–5 granular olivine-pyroxene (GOP), 3–5 cryptocrystalline (C), and ≤ 1 metallic (M). Neither chondrule size nor shape is strongly correlated with textural type. Compound and cratered chondrules, which are interpreted as products of collisions between plastic chondrules, comprise ～ 2–28% of nonporphyritic (RP, GOP, C) but only ～ 2–9% of porphyritic (POP, PO, PP, BO) chondrules, leading to a model-dependent implication that nonporphyritic chondrules evolved at number densities (chondrules per unit volume of space) which were 10² to 10⁴ times greater than those which prevailed during porphyritic chondrule formation (total range of ～ 1 to ～ 10⁶ m^?3). Distinctive “rims” of fine-grained sulfides and/or silicates occur on both porphyritic and nonporphyritic types and appear to post-date chondrule formation. Apparently, either the same process(es) contributed chondrules to all unequilibrated ordinary chondrites or, if genetically different, the various chondrule types were well mixed before incorporation into chondrites. Melting of pre-existing materials is the mechanism favored for chondrule formation.     

Chondrules: Precursors and interactions with the nebular gas

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.     

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

Abstract— 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 μm, 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 φ 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.     

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.     

The formation and alteration of the Renazzo‐like carbonaceous chondrites III: Toward understanding the genesis of ferromagnesian chondrules

To better understand the formation conditions of ferromagnesian chondrules from the Renazzo‐like carbonaceous (CR) chondrites, a systematic study of 210 chondrules from 15 CR chondrites was conducted. The texture and composition of silicate and opaque minerals from each observed FeO‐rich (type II) chondrule, and a representative number of FeO‐poor (type I) chondrules, were studied to build a substantial and self‐consistent data set. The average abundances and standard deviations of Cr₂O₃ in FeO‐rich olivine phenocrysts are consistent with previous work that the CR chondrites are among the least thermally altered samples from the early solar system. Type II chondrules from the CR chondrites formed under highly variable conditions (e.g., precursor composition, redox conditions, cooling rate), with each chondrule recording a distinct igneous history. The opaque minerals within type II chondrules are consistent with formation during chondrule melting and cooling, starting as S‐ and Ni‐rich liquids at 988–1350 °C, then cooling to form monosulfide solid solution (mss) that crystallized around olivine/pyroxene phenocrysts. During cooling, Fe,Ni‐metal crystallized from the S‐ and Ni‐rich liquid, and upon further cooling mss decomposed into pentlandite and pyrrhotite, with pentlandite exsolving from mss at 400–600 °C. The composition, texture, and inferred formation temperature of pentlandite within chondrules studied here is inconsistent with formation via aqueous alteration. However, some opaque minerals (Fe,Ni‐metal versus magnetite and panethite) present in type II chondrules are a proxy for the degree of whole‐rock aqueous alteration. The texture and composition of sulfide‐bearing opaque minerals in Graves Nunataks 06100 and Grosvenor Mountains 03116 suggest that they are the most thermally altered CR chondrites.     

Size-frequency distributions of chondrules in CO3 chondrites

Abstract— The size-frequency distributions of chondrules in 11 CO3 chondrites were determined by petrographic analysis of thin sections. CO chondrites have the smallest chondrules of any major chondrite group. In order of decreasing chondrule size, chondrite groups can be arranged as CV ≥ LL > L > H ≥ CM ≥ EH > CO. Chondrule size varies significantly among different CO chondrites; there is a tendency for chondrules to increase in average size with increasing metamorphic grade of the whole-rock. Different chondrule types in CO chondrites have distinct size-frequency distributions: in order of decreasing chondrule size, BO > PO > PP > POP > RP = C. The large size of BO chondrules is problematic; however, PO chondrules are among the largest because ～20% of them contain very coarse relict olivine grains that constitute 40–90 vol.% of the individual chondrules. PP chondrules may be larger than POP chondrules because some of them contain coarse relict pyroxene grains; a compound object consisting of a POP chondrule attached to a large relict pyroxene grain occurs in Lancé. The mean proportions of chondrule types in CO chondrites are estimated to be 69% POP, 18% PP, 8% PO, 2% BO, 2% RP, 1% C and <0.1% GOP. CO chondrites thus contain a smaller proportion of nonporphyritic chondrules than ordinary or EH chondrites, but a larger proportion than CV chondrites. Relative proportions of chondrule types vary with size interval: PO chondrules decrease fairly regularly in abundance with decreasing chondrule size, and RP chondrules appear to be most abundant in the smallest size intervals.     

Cover

Cover: Within the carbonaceous chondrite Allende (CV3) a very complex compound chondrule of ~1 mm in size was found consisting of at least 16 sub‐chondrules. Concerning the formation of this compound chondrule calculations suggest the existence of a super‐dense chondrule‐forming region with an extremely high solid‐to‐gas mass ratio. For details see the article by Addi Bischoff et al. on p. 906 . Image courtesy of A. Bischoff.     

A comparison of FeO-rich,porphyritic olivine chondrules in unequilibrated chondrites and experimental analogues

Abstract Experimentally produced analogues of porphyritic olivine (PO) chondrules in ordinary chondrites provide an important insight into chondrule formation processes. We have studied experimental samples with PO textures grown at three different cooling rates (2, 5 and 100 *C/h), and samples that have been annealed at high temperatures (1000–1200 °C) subsequent to cooling. These are compared with natural chondrules of similar composition and texture from the ordinary chondrites Semarkona (LL3.0) and ALH 81251 (LL3.3). Zoning properties of olivine grains indicate that the Semarkona chondrules cooled at comparable rates to the experiments. Zoning in olivine from chondrules in ALH 81251 is not consistent with cooling alone but indicates that the chondrules underwent an annealing process. Chromium loss from olivine is very rapid during annealing and calculated diffusion coefficients for Cr in olivine are very similar to those of Fe-Mg interdiffusion coefficients under the same conditions. Annealed experimental samples contain an aluminous, low-Ca pyroxene which forms by reaction of olivine and liquid. No similar reaction texture is observed in ALH 81251 chondrules, and this may be evidence that annealing of the natural samples took place at considerably lower temperatures than the experimental analogues. The study supports the model of chondrule formation in a cool nebula and metamorphism of partly equilibrated chondrites during reheating episodes on the chondrite parent bodies.     

Pb isotopic age of the Allende chondrules

Abstract— We have studied Pb‐isotope systematics of chondrules from the oxidized CV3 carbonaceous chondrite Allende. The chondrules contain variably radiogenic Pb with a ²⁰⁶Pb/²⁰⁴Pb ratio between 19.5–268. Pb‐Pb isochron regression for eight most radiogenic analyses yielded the date of 4566.2 ± 2.5 Ma. Internal residue‐leachate isochrons for eight chondrule fractions yielded consistent dates with a weighted average of 4566.6 ± 1.0 Ma, our best estimate for an average age of Allende chondrule formation. This Pb‐Pb age is consistent with the range of model ²⁶Al‐²⁶Mg ages of bulk Allende chondrules reported by Bizzarro et al. (2004) and is indistinguishable from Pb‐Pb ages of Ca‐Al‐rich inclusions (CAIs) from CV chondrites (4567.2 ± 0.6 Ma) (Amelin et al. 2002) and the oldest basaltic meteorites. We infer that chondrule formation started contemporaneously with or shortly after formation of CV CAIs and overlapped in time with formation of the basaltic crust and iron cores of differentiated asteroids. The entire period of chondrule formation lasted from 4566.6 ± 1.0 Ma (Allende) to 4564.7 ± 0.6 Ma (CR chondrite Acfer 059) to 4562.7 ± 0.5 Ma (CB chondrite Gujba) and was either continuous or consisted of at least three discrete episodes. Since chondrules in CB chondrites appear to have formed from a vapor‐melt plume produced by a giant impact between planetary embryos after dust in the protoplanetary disk had largely dissipated (Krot et al. 2005), there were possibly a variety of processes in the early solar system occurring over at least 4–5 Myr that we now combine under the umbrella name of “chondrule formation.”     

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.     

Axtell,a new CV3 chondrite find from Texas

Abstract— We describe a previously unreported meteorite found in Axtell, Texas, in 1943. Based on the mineralogical composition and texture of its matrix and the sizes and abundance of chondrules, we classify it as a CV3 carbonaceous chondrite. The dominant opaque phase in the chondrules is magnetite, and that in refractory inclusions is Ni-rich NiFe metal (awaruite). Axtell, therefore, belongs to the oxidized subgroup of CV3 chondrites, although unlike Allende it escaped strong sulfidation. The meteorite bears a strong textural resemblance to Allende, and its chondrule population and matrix appear to be quite similar to those of Allende, but its refractory inclusions, thermoluminescence properties, and cosmogenic ⁶⁰Co abundances are not. Our data are consistent with a terrestrial age for Axtell of ～100 years and a metamorphic grade slightly lower than that of Allende.     

Anorthite‐rich chondrules in CR and CH carbonaceous chondrites: Genetic link between calcium‐aluminum‐rich inclusions and ferromagnesian chondrules

Abstract— Anorthite‐rich chondrules in CR and CH carbonaceous chondrites consist of magnesian low‐Ca pyroxene and forsterite phenocrysts, FeNi‐metal nodules, interstitial anorthite, Al‐Ti‐Cr‐rich low‐Ca and high‐Ca pyroxenes, and crystalline mesostasis composed of silica, anorthite and high‐Ca pyroxene. Three anorthite‐rich chondrules contain relic calcium‐aluminum‐rich inclusions (CAIs) composed of anorthite, spinel, ±Al‐diopside, and ± forsterite. A few chondrules contain regions which are texturally and mineralogically similar to magnesian (type I) chondrules and consist of forsterite, low‐Ca pyroxene and abundant FeNi‐metal nodules. Anorthite‐rich chondrules in CR and CH chondrites are mineralogically similar to those in CV and CO carbonaceous chondrites, but contain no secondary nepheline, sodalite or ferrosilite. Relatively high abundances of moderately‐volatile elements such as Cr, Mn and Si in the anorthite‐rich chondrules suggest that these chondrules could not have been produced by volatilization of the ferromagnesian chondrule precursors or by melting of the refractory materials only. We infer instead that anorthite‐rich chondrules in carbonaceous chondrites formed by melting of the reduced chondrule precursors (olivine, pyroxenes, FeNi‐metal) mixed with the refractory materials, including relic CAIs, composed of anorthite, spinel, high‐Ca pyroxene and forsterite. The observed mineralogical and textural similarities of the anorthite‐rich chondrules in several carbonaceous chondrite groups (CV, CO, CH, CR) may indicate that these chondrules formed in the region(s) intermediate between the regions where CAIs and ferromagnesian chondrules originated. This may explain the relative enrichment of anorthite‐rich chondrules in ¹⁶O compared to typical ferromagnesian chondrules (Russell et al., 2000).     

Impact-induced chondrule flattening in the Allende CV3 carbonaceous chondrite: Shock experiments

Abstract— We have carried out shock-recovery experiments on the Allende CV3 carbonaceous chondrite using a single-stage propellant gun and succeeded in reproducing oriented, flattened chondrules like those observed in some natural CV3 chondrites. The Allende samples were shocked at equilibrium pressures of 11 and 21 GPa, which are close to the highest values in shock stages S2 and S3, respectively (Stöffler et al., 1991). Chondrules are flattened nearly perpendicular to the compaction axis with mean aspect ratios of 1.34 and 1.62 at pressures of 11 and 21 GPa, respectively; thus, the degree of chondrule flattening is proportional to the shock intensity. The chondrule flattening and foliation are mainly due to collapse of pores in the matrix under shock pressure. High matrix abundance of CV3 chondrites could result in much apparent chondrule flattening relative to ordinary chondrites. Optical and electron microscope observations show that textural and mineralogical characteristics of chondrules and matrix in the shock-loaded samples are very similar to those observed in naturally shocked CV3 chondrites. Our results provide strong support for the interpretation that the chondrule flattening and foliation in CV3 chondrites were caused by shock-induced pressure due to hypervelocity impacts on the meteorite parent bodies.     

Oxygen isotope characteristics of chondrules from the Yamato‐82094 ungrouped carbonaceous chondrite: Further evidence for common O‐isotope environments sampled among carbonaceous chondrites

High‐precision secondary ion mass spectrometry (SIMS) was employed to investigate oxygen three isotopes of phenocrysts in 35 chondrules from the Yamato (Y) 82094 ungrouped 3.2 carbonaceous chondrite. Twenty‐one of 21 chondrules have multiple homogeneous pyroxene data (?¹⁷O 3SD analytical uncertainty: 0.7‰); 17 of 17 chondrules have multiple homogeneous pyroxene and plagioclase data. Twenty‐one of 25 chondrules have one or more olivine data matching coexisting pyroxene data. Such homogeneous phenocrysts (1) are interpreted to have crystallized from the final chondrule melt, defining host O‐isotope ratios; and (2) suggest efficient O‐isotope exchange between ambient gas and chondrule melt during formation. Host values plot within 0.7‰ of the primitive chondrule mineral (PCM) line. Seventeen chondrules have relict olivine and/or spinel, with some δ¹⁷O and δ¹⁸O values approaching ?40‰, similar to CAI or AOA‐like precursors. Regarding host chondrule data, 22 of 34 have Mg#s of 98.8–99.5 and ?¹⁷O of ?3.9‰ to ?6.1‰, consistent with most Acfer 094, CO, CR, and CV chondrite chondrules, and suggesting a common reduced O‐isotope reservoir devoid of ¹⁶O‐poor H₂O. Six Y‐82094 chondrules have ?¹⁷O near ?2.5‰, with Mg#s of 64–97, consistent with lower Mg# chondrules from Acfer 094, CO, CR, and CV chondrites; their signatures suggest precursors consisting of those forming Mg# ~99, ?¹⁷O: ?5‰ ± 1‰ chondrules plus ¹⁶O‐poor H₂O, at high dust enrichments. Three type II chondrules plot slightly above the PCM line, near the terrestrial fractionation line (?¹⁷O: ~+0.1‰). Their O‐isotopes and olivine chemistry are like LL3 type II chondrules, suggesting they sampled ordinary chondrite‐like chondrule precursors. Finally, three Mg# >99 chondrules have ?¹⁷O of ?6.7‰ to ?8.1‰, potentially due to ¹⁶O‐rich refractory precursor components. The predominance of Mg# ~99, ?¹⁷O: ?5‰ ± 1‰ chondrules and a high chondrule‐to‐matrix ratio suggests bulk Y‐82094 characteristics are closely related to anhydrous dust sampled by most carbonaceous chondrite chondrules.     

Silica‐rich igneous rims around magnesian chondrules in CR carbonaceous chondrites: Evidence for condensation origin from fractionated nebular gas

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.     

Refractory forsterite in primitive meteorites: Condensates from the solar nebula?

Abstract— All groups of chondritic meteorites contain discrete grains of forsteritic olivine with FeO contents below 1 wt% and high concentrations of refractory elements such as Ca, Al, and Ti. Ten such grains (52 to 754 μg) with minor amounts of adhering matrix were separated from the Allende meteorite. After bulk chemical analysis by instrumental neutron activation analysis (INAA), some samples were analyzed with an electron microprobe and some with an ion microprobe. Matrix that accreted to the forsterite grains has a well‐defined unique composition, different from average Allende matrix in having higher Cr and lower Ni and Co contents, which implies limited mixing of Allende matrix. All samples have approximately chondritic relative abundances of refractory elements Ca, Al, Sc, and rare‐earth elements (REE), although some of these elements, such as Al, do not quantitatively reside in forsterite; whereas others (e.g., Ca) are intrinsic to forsterite. The chondritic refractory element ratios in bulk samples, the generally high abundance level of refractory elements, and the presence of Ca‐Al‐Ti‐rich glass inclusions suggest a genetic relationship of refractory condensates with forsteritic olivine. The Ca‐Al‐Ti‐rich glasses may have acted as nuclei for forsterite condensation. Arguments are presented that exclude an origin of refractory forsterite by crystallization from melts with compositions characteristic of Allende chondrules: (a) All forsterite grains have CaO contents between 0.5 and 0.7 wt% with no apparent zoning, requiring voluminous parental melts with 18 to 20 wt% CaO, far above the average CaO content of Allende chondrules. Similar arguments apply to Al contents. (b) The low FeO content of refractory forsterite of 0.2‐0.4 wt% imposes an upper limit of ～1 wt% of FeO on the parental melt, too low for ordinary and carbonaceous chondrule melts, (c) The Mn contents of refractory forsterites are between 30 to 40 ppm. This is at least one order of magnitude below the Mn content of chondrule olivines in all classes of meteorites. The observed Mn contents of refractory forsterite are much too low for equilibrium between olivine and melts of chondrule composition, (d) As shown earlier, refractory forsterites have O‐isotopic compositions different from chondrules (Weinbruch et al., 1993a). Refractory olivines in carbonaceous chondrites are found in matrix and in chondrules. The compositional similarity of both types was taken to indicate that all refractory forsterites formed inside chondrules (e.g., Jones, 1992). As refractory forsterite cannot have formed by crystallization from chondrule melts, we conclude that refractory forsterite from chondrules are relic grains that survived chondrule melting and probably formed in the same way as refractory forsterite enclosed in matrix. We favor an origin of refractory forsterite by condensation from an oxidized nebular gas.