The origin and history of the metal and sulfide components of chondrules

Fourteen siderophile and other non-lithophile elements determined in 31 Semarkona (LL3.0) chondrules by neutron activation analysis are severely fractionated relative to lithophile elements. Their chondrule/whole-rock abundance ratios vary by factors of up to 1000; the mean ratio is ~0.2. Non-refractory siderophile abundance patterns in Ni-rich chondrules are smooth functions of volatility and in Ni-poor chondrules patterns are more irregular. Refractory siderophile elements are often fractionated from Ni; they covary, confirming the presence of a refractory metal component. The chalcophile element Se correlates with Br and siderophile elements. Zinc is uniformly low and uncorrelated with other elements.Most metal and sulfide in chondrules was probably present in the solar nebula before chondrule formation; most siderophile and chalcophile elements were in these materials. Some Fe was also in silicates, as were minor amounts of Ni, Co, Au, Ge and possibly Se. The amount of metal formed by reduction during chondrule melting was minor. The common metal component in chondrules is similar to, and may be the same as the common component involved in the metal/silicate fractionation of the ordinary chondrite groups.Chondrules are depleted in metal chiefly because they sampled metal-poor precursor assemblages. Metal segregation during the molten period and subsequent loss was a minor process that may be responsible for most surface craters on chondrules.     

Metal and associated phases in Krymka and Chainpur: Nebular formational processes

The highly unequilibrated LL3 chondrites Krymka and Chainpur preserve a relatively unaltered record of formation in the solar nebula in the texture and chemistry of their opaque mineral assemblages. A moderate degree of diversity among these meteorites and Bishunpur is apparently associated with formation under differing conditions.Spheroidal kamacite, some Cr-bearing, is present in chondrule interiors. Fine-grained metal within the Fe-rich opaque matrix of Krymka consists exclusively of taenite and minor tetrataenite; kamacite occurs inside metal-sulfide nodules. These nodules are surrounded by an inner layer of FeO-rich, fine-grained silicate material (FeO/(FeO + MgO) > 80%) and an outer troilite-rich layer, and contain variable amounts of a hydrated Fe-oxide phase. It appears that the nodules were melted, often incompletely, possibly during the chondrule formation process. Some nodule metal is Si- and Cr-bearing, indicating little reaction with nebular H₂O. Nodules are much less common in Chainpur than in Krymka and rare in Bishunpur.Most metal-poor chondrules in Krymka, Bishunpur and Chainpur appear to have formed from precursors that had acquired significant amounts of FeO as a result of reaction with the nebular gas down to low temperatures; metal-rich chondrules seem to have derived from aggregates of coarse, high-temperature Fe-poor silicates. Low Ni concentrations (34–41 mg/g) in chondrule kamacite may largely result from dilution by Fe reduced from the silicates during chondrule formation.The opaque silicate matrix of Krymka is considerably more oxidized than that of Bishunpur and Chainpur, it contains no kamacite and its composition is very uniform. This may either reflect the growth of silicate grains during incipient recrystallization in the matrices of Bishunpur and Chainpur or, more likely, a lower mean grain size of the Krymka matrix components, possibly indicating later formation of the Krymka parent planetesimal.     

Trace elements in rims and interiors of Chainpur chondrules

Trace elements were measured in the rims and interiors of nine chondrules separated from the Chainpur LL-3 chondrite. Whole rock samples of Chainpur and samples of separated rims were also measured. Chondrule rims are moderately enriched in siderophile and volatile elements relative to the chondrule interiors. The enriched volatile elements include the lithophilic volatile element Zn. The moderate enrichment of volatiles in chondrule rims and the lack of severe depletion in chondrules can account for the complete volatile inventory in Chainpur. These results support a three-component model of chondrite formation in which metal plus sulfide, chondrules plus rims and matrix silicates are mixed to form chondrites.     

Matrix material in type 3 chondrites—occurrence,heterogeneity and relationship with chondrules

Matrix material in type 3 chondrites forms rims on chondrules, metal-sulfide aggregates, Ca,Al-rich inclusions and chondritic clasts; it also forms lumps up to a millimeter in size, which may contain coarser silicates. Chondrules of all types were found with internal matrix lumps that appear to have entered the chondrules before the latter had crystallized. Mean concentrations of Mg, Na, Al and Ca in matrix occurrences show up to fivefold variations in a single chondrite. Variations between mean matrix compositions of individual type 3 ordinary chondrites are almost as large and partly reflect systematic differences between H, L and LL matrices. Such variations are probably a result of nebular separation of feldspathic material and ferromagnesian silicates.Compositions of chondrules and their matrix rims are normally unrelated, although rim compositions are correlated with those of matrix lumps inside chondrules. A single chondrule was found with a composition nearly identical to that of its internal matrix lump, suggesting that some chondrules may have formed from matrix material. Matrix lumps are as heterogeneous as chondrules, but mean chondrule and matrix compositions differ, even allowing for possible loss of metallic Fe,Ni during chondrule formation. Since bulk compositions of matrix lumps and rims have probably not changed significantly since their formation except for Fe-Mg exchange, our matrix samples cannot represent typical chondrule precursor materials.     

Metal and associated phases in Bishunpur,a highly unequilibrated ordinary chondrite

It appears that the highly unequilibrated Bishunpur ordinary chondrite preserves phase relations acquired during solar nebular processes to a relatively high degree; metamorphic temperatures may not have exceeded 300–350°C. The major categories of metal are: 3 kinds of metal in the metal matrix, three kinds in chondrule interiors and 2 kinds in chondrule rims. The fine-grained matrix metal is highly variable in composition: the kamacite Co content (7.8 ± 2.0 mg/g) is within the L-group range (6.7–8.2 mg/g) but extends well above and below; its Ni content (38 ± 5 mg/g) is considerably lower than in more equilibrated chondrites and taenite is Ni-rich ( > 450 mg/g) and unzoned. These compositions imply equilibration at very low temperatures of about 300–350°C. It seems unlikely that volume diffusion could account for the observed relatively unzoned phases; a better model involves mass transport by grain boundary diffusion and grain growth at the indicated temperatures. We find no evidence that the matrix was ever at higher temperatures. Large (50–650 μm) polycrystalline metal aggregates consisting of individually zoned crystals are also found in the matrix; they probably represent clusters formed in the solar nebula. A few large (50–250 μm) round monocrystalline grains are also present in the matrix.Metal-bearing chondrules tend to be highly reduced; they contain low-Ni metal that occasionally contains Si and/or Cr. Silicates in these chondrules tend to have low $\text{FeO}\text{(FeO + MgO)}$ ratios. The Si-rich metal grains are never in contact with silicates and are always surrounded by troilite with a poorly characterized Ca, Cr-sulfide at the metal-troilite interface; they appear to be high temperature nebular condensates that avoided oxidation even during the chondrule forming process. Silicon contents drop below our detection limit when the sulfide coating is absent. Much more common in chondrule interiors are Si-free spheroidal metal grains not associated with sulfides. These have Ni and Co contents very similar to the Si-bearing grains, and appear to be oxidized variants of the same material. The third class of chondrule metal is fine ( ~1 μm) dusty grains inside individual olivine grains. These seem to reflect high temperature in situ reduction of FeO from the olivine.The composition of kamacite is different in sulfide-rich and sulfide-poor chondrule rims and in both cases it is dissimilar to the compositions in the chondrule interiors and matrix; this indicates that chondrule rims could not have resulted from reactions with the matrix, but are primary features acquired prior to accretton.     

Refractory precursor components of Semarkona chondrules and the fractionation of refractory elements among chondrites

Chondrules from the Semarkona (LL3.0) chondrite show refractory and common lithophile fractionation trends similar to those observed among the chondrite groups. It appears that chondrules are mixtures of a small number of pre-existing solid components, and we infer that chondrule precursor materials were related to the nebular components involved in the lithophile element fractionations recognized in ordinary chondrites. Compositional trends among the chondrules can be used to deduce the compositions of these components.We use instrumental neutron activation analysis to measure many (~20) of the lithophile elements in 30 chondrules. The amounts of oxidized iron were calculated from other compositional parameters; concentrations of Si were estimated using mass-balance considerations. The data were corrected for the diluting effects of non-lithophile constituents. Plots of lithophile elements versus a reference refractory element such as Al show that there were two major chondrule silicate precursor components: a refractory, olivine-rich, FeO-free one, and a non-refractory, SiO₂-, FeO-rich one.The refractory component probably forms from olivine-enriched condensates formed above the condensation temperature of enstatite. The non-refractory component must have formed from fine-grained materials that were able to equilibrate down to lower nebular temperatures. Chondrite matrix may have had an origin similar to that of the non-refractory material, and constitutes a third lithophile-bearing component that took part in chondrite fractionation processes. The low abundance of refractories and Mg in ordinary and enstatite chondrites was produced by the loss of materials having a higher refractory-element/Mg ratio than that in the refractory component of chondrules.     

Spinel group minerals in LL3.00-6 chondrites: Indicators of nebular and parent body processes

We carried out a systematic study of spinel group minerals in LL3.00-3.9 and LL4-6 chondrites. With increasing petrologic type, the size and abundance of spinel increase. The compositions of spinel group minerals in type 3 chondrites depend on the occurrence; Mg-Al-rich spinel occurs mainly in chondrules. Some chromite occurs in chondrules and matrix, and nearly pure chromite is exclusively encountered in the matrix. The occurrence of nearly pure chromite and the wide compositional variations distinguish spinel group minerals in types 3.00-3.3 from those in the other types. Spinel group minerals in types 3.5-3.9 show a narrower range of compositions, and those in types 4-6 are homogeneous. The changes in composition and abundance of spinel in type 3 chondrites are most likely due to thermal metamorphism. Therefore, the chemistry of spinel group minerals could be used as a sensitive indicator of metamorphic conditions, not only for type 3-6, but also 3.00-3.9. They can be applied to identify the most primitive (least metamorphosed) chondrites. The bulk compositions of spinel-bearing chondrules and the textural setting of the spinel indicate that most spinel group minerals crystallized directly from chondrule melts. However, some spinel grains, especially those enclosed in olivine phenocrysts, can not be explained by in situ crystallization in the chondrule. We interpret these spinel grains to be relic phases that survived chondrule melting. This is supported by the oxygen isotopic composition of a spinel grain, which has significantly lighter oxygen than the coexisting olivine. The oxygen isotopic composition of this spinel is similar to those of Al-rich chondrules. Our discovery of relic spinel in chondrules is an indication of the complexities in the early solar nebular processes that ranged from formation of refractory inclusion, through Al-rich chondrule, to ferromagnesian chondrules, and attests to the recycling of earlier formed materials into the precursors of later formed materials. The characteristic features of spinel group minerals are not only sensitive to thermal metamorphism, but also shed light on chondrule formation processes.     

Metal in CR chondrites

In section many low-FeO CR chondrules are surrounded by rings of metal; this metal-cladding seems to have formed during chondrule melting events as films of metal that wetted the surface. Electron microprobe studies show that in each ring the metal is very uniform in composition, consistent with efficient mixing during formation of the metal film. In contrast the mean Ni contents of 13 different rings vary by up to a factor of 2. There is no FeS associated with ring metal. Ring metal Co is positively correlated with Ni but the Co/Ni ratio seems to decrease with increasing Ni. We observed a weak negative correlation between ring metal Ni and the fayalite content of the host olivine. Coarse interior metal has higher Ni contents than that in the surrounding rings. At any specific chondrule location, smaller grains tend to have lower Ni contents than larger grains. These trends in Ni seem to reflect two processes: (1) The mean Ni content of metal (and easily reduced sulfides or oxides) in chondrule precursor materials seems to have decreased with the passage of time; on average, the metal in earlier-formed chondrules had higher Ni contents than the metal in later-formed chondrules. (2) Some oxidized Fe was reduced during chondrule formation leading to lower Ni contents in small grains compared to large grains; prior to reduction the Fe was in FeS or in FeO in accessible (fine-grained) sites. We suggest that the compositional evolution of nebular solids was responsible for the interchondrule variations whereas reduction of minor amounts of FeS or FeO was responsible for the size-related small variations in Ni content. We suggest that, during chondrule formation events, CR chondrules experienced relatively long thermal pulses that were responsible for the thorough loss of FeS and the common granoblastic texture observed in low-FeO chondrules. The preservation of the structures of internal rings shows, however, that even though high temperatures occurred in the secondary chondrule, temperatures in the centers of large (>20 μm) metal and silicate grains in the primary chondrule did not get high enough to cause appreciable melting.     

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

Chondrite groups (CV, CK, CR) with large average chondrule sizes have low proportions of RP plus C chondrules, high proportions of enveloping compound chondrules, high proportions of chondrules with (thick) igneous rims, and relatively low proportions of type-I chondrules containing sulfide. In contrast, chondrite groups (CM, CO, OC, R, EH, EL) with smaller average chondrule sizes have the opposite properties. Equilibrated CK chondrites have plagioclase with relatively low Na; equilibrated OC, R, EH and EL chondrites have more sodic plagioclase. Enveloping compound chondrules and chondrules with igneous rims formed during a remelting event after the primary chondrule was incorporated into a dustball. Repeated episodes of remelting after chondrules were surrounded by dust would tend to produce large chondrules. RP and C chondrules formed by complete melting of their precursor assemblages; remelting of RP and C chondrules surrounded by dust would tend to produce porphyritic chondrules as small dust particles mixed with the melt, providing nuclei for crystallizing phenocrysts. This process would tend to diminish the numbers of RP and C chondrules. Correlations among these chondrule physical properties suggest that chondrite groups with large chondrules were typically surrounded by thick dust-rich mantles that formed in locally dusty nebular environments. Chondrules that were surrounded by thick dust mantles tended to cool more slowly because heat could not quickly radiate away. Slow cooling led to enhanced migration of sulfide to chondrule surfaces and more extensive sulfide evaporation. These chondrules also lost Na; the plagioclase that formed from equilibrated CK chondrites was thus depleted in Na.     

Sulfur isotope composition of putative primary troilite in chondrules from Bishunpur and Semarkona

The sulfur isotopic compositions of putative primary troilite grains within 15 ferromagnesian chondrules (10 FeO-poor and 5 FeO-rich chondrules) in the least metamorphosed ordinary chondrites, Bishunpur and Semarkona, have been measured by ion microprobe. Some troilite grains are located inside metal spherules within chondrules. Since such an occurrence is unlikely to be formed by secondary sulfidization processes in the solar nebula or on parent bodies, those troilites are most likely primary, having survived chondrule-forming high-temperature events. If they are primary, they may be the residues of evaporation at high temperatures during chondrule formation and may have recorded mass-dependent isotopic fractionations. However, the supposed primary troilites measured in this study do not show any significant sulfur isotopic fractionations (<1 ‰/amu) relative to large troilite grains in matrix. Among other chondrule troilites that we measured, only one (BI-CH22) apparently has a small excess of heavy isotopes (2.7 ± 1.4 ‰/amu) consistent with isotopic fractionation during evaporation. All other grains have isotopic fractionations of <1 ‰/amu. Because sulfur is so volatile that evaporation during chondrule formation is probably inevitable, non-Rayleigh evaporation most likely explains the lack of isotopic fractionation in putative primary troilite inside chondrules. Evaporation through the surrounding silicate melt would have suppressed the isotopic fractionation after silicate dust grains melted. At lower temperatures below extensive melting of silicates, a heating rate of >10⁴-10⁶ K/h would be required to avoid a large degree of sulfur isotopic fractionation in the chondrule precursors. This heating rate may provide a new constraint on the chondrule formation processes.     

Earth and Mars – Distinct inner solar system products

Composition of terrestrial planets records planetary accretion, core–mantle and crust–mantle differentiation, and surface processes. Here we compare the compositional models of Earth and Mars to reveal their characteristics and formation processes. Earth and Mars are equally enriched in refractory elements (1.9 × CI), although Earth is more volatile-depleted and less oxidized than Mars. Their chemical compositions were established by nebular fractionation, with negligible contributions from post-accretionary losses of moderately volatile elements. The degree of planetary volatile element depletion might correlate with the abundances of chondrules in the accreted materials, planetary size, and their accretion timescale, which provides insights into composition and origin of Mercury, Venus, the Moon-forming giant impactor, and the proto-Earth. During its formation before and after the nebular disk's lifetime, the Earth likely accreted more chondrules and less matrix-like materials than Mars and chondritic asteroids, establishing its marked volatile depletion. A giant impact of an oxidized, differentiated Mars-like (i.e., composition and mass) body into a volatile-depleted, reduced proto-Earth produced a Moon-forming debris ring with mostly a proto-Earth's mantle composition. Chalcophile and some siderophile elements in the silicate Earth added by the Mars-like impactor were extracted into the core by a sulfide melt (∼0.5% of the mass of the Earth's mantle). In contrast, the composition of Mars indicates its rapid accretion of lesser amounts of chondrules under nearly uniform oxidizing conditions. Mars’ rapid cooling and early loss of its dynamo likely led to the absence of plate tectonics and surface water, and the present-day low surface heat flux. These similarities and differences between the Earth and Mars made the former habitable and the other inhospitable to uninhabitable.     

Fine,nickel-poor Fe-Ni grains in the olivine of unequilibrated ordinary chondrites

Fine (?2 μm), Ni-poor (? 10 mg/g) Fe-Ni grains are common inclusions in the olivine in porphyritic chondrules in unequilibrated ordinary chondrites. The olivine grains appear to be relicts that survived chondrule formation without melting. The most common occurrence of this “dusty” metal is in the core of olivine grains having clear Fe-poor rims and surrounded either by small euhedral clear olivine grains zoned with FeO increasing toward the border of the grains or by large elongated Fe-poor orthopyroxenes oriented parallel to the chondrule surface and enclosing small round olivine grains. Various amounts of Ca, Al-rich glass are always present. The dusty metal is occasionally found in the rims of olivine grains either isolated in the matrix or included in chondrules. A rare occurrence is as bands in highly deformed olivines.This dusty metal appears to be the product of in situ reduction of FeO from the host olivine. Among the possible reductants H₂ or carbonaceous matter (CH₂)_n seem the most likely. Hydrogen may have been implanted by solar-wind or solar-flare irradiation, but this requires that dissipation of nebular gas occurred before the end of the chondrule formation process. Carbonaceous matter may have been implanted by shock. Less likely reductants are nebular CO or C dissolved in the olivine lattice. The large relict olivine grains may be nebular condensates or, more likely, fragments broken off earlier generations of chondrules.     

Formation processes of compound chondrules in CV3 carbonaceous chondrites: Constraints from oxygen isotope ratios and major element concentrations

We found thirty compound chondrules in two CV3 carbonaceous chondrites. The abundance in each meteorite relative to single chondrules is 29/1846 (1.6%) in Allende and 1/230 (0.4%) in Axtell. We examined petrologic features, major element concentrations and oxygen isotopic compositions. Textural, compositional and isotopic evidence suggests that multiple, different mechanisms are responsible for the formation of compound chondrules.Seven compound chondrules are composed of two conjoined porphyritic chondrules with a blurred boundary. At the boundary region of this type of compounds, a poikilitic texture is commonly observed. This suggests that the two chondrules were melted when they came to be in contact. On the other hand, seventeen compound chondrules consist of two conjoined chondrules with a discrete boundary. The preservation of spherical boundary planes of an earlier-formed chondrule of this type implies that it already solidified before fusing with a later-formed chondrule that was still melted. Six samples out of 17 compound chondrules of this type are composed of two BO chondrules. The BO-BO compound chondrules have a unique textural feature in common: the directions of the barred olivines are mostly parallel between two chondrules. This cannot be explained by a simple collision process and forces another mechanism to be taken into consideration.The remaining six compound chondrules differ from the others; they consist of an earlier-formed chondrule enclosed by a later-formed chondrule. A large FeO enrichment was observed in the later-formed chondrules and the enrichment was much greater than that in the later-formed chondrules of other types of compounds. This is consistent with the relict chondrule model, which envisages that the later-formed chondrule was made by a flash melting of a porous FeO-rich dust clump on an earlier-formed chondrule. The textural evidence of this type of compound shows that the earlier-formed chondrule has melted again to varying degrees at the second heating event. This implies that FeO concentrations in bulk chondrules increases during the second heating event if an earlier-formed chondrule was totally melted together with the FeO-rich dust aggregates.Silicate minerals such as olivine and low-Ca pyroxene in compound chondrules have oxygen isotope compositions similar to those in single chondrules from CV3 chondrites. The oxygen isotope composition of each part of the compound chondrule is basically similar to their chondrule pair, but silicates in some chondrules show varying degrees of ¹⁶O-enrichment down to −15‰ in δ¹⁸O, while those in their partners have ¹⁶O-poor invariable compositions near 0 ‰ in δ¹⁸O. This implies that the two chondrules in individual compounds formed in the same environments before they became conjoined and the heterogeneous oxygen isotope compositions in some chondrules resulted from incomplete exchange of oxygen atoms between ¹⁶O-rich chondrule melts and ¹⁶O-poor nebular gas.     

The conditions of chondrule formation, Part I: Closed system

Mixing was an important process in the early solar nebula and is often used as an argument to explain the compositional scatter among chondrules—mm-sized, once molten silicate spherules in chondritic meteorites. If it is hypothesized that chondrules only acted as closed systems and the scatter in chondrule bulk chemical compositions is only the result of mixing heterogeneous precursor grains—the basic components of chondrules—, it is in turn possible to determine the sizes of the precursor grains using statistical calculations. In order to reproduce the observed compositional scatter in chondrules not more than ∼10 precursor grains should contribute to a single chondrule, each with a diameter of several 100 μm. This finding has important implications for the conditions of chondrule formation and replaces the so far widely accepted model that chondrules formed from fine-grained “dust-balls”. Chondrules rather formed from coarse-grained precursor aggregates with variable amounts of μm-fine matrix material. As a consequence, only chondrite matrix or interstellar material winnows as precursor material. Large grains of variable composition serving as precursor grains must have been formed prior to chondrule formation. Chondrules probably have not been their immediate precursors, as only 1-2 chondrule recycling steps would have homogenized bulk chondrule compositions. Chondrule recycling can therefore only have occurred to a limited extent. Chondrule formation needed at least three steps: (1) production of large and heterogeneous chondrule precursor grains, (2) agglomeration of large precursor grains and fine-grained precursors into aggregates, (3) formation of chondrules during transient heating events. Al-rich chondrules can in this context be explained by the admixture of CAIs to either chondrule precursors or a population of existing chondrules.     

普通球粒陨石的物理和岩石学性质及其分类参数

不同球粒陨石群的物理和岩石学性质,包括球粒的平均大小、球粒结构类型、复合球粒、带火成边球粒及含硫化物的比例、化学组成及矿物学特征等可用以划分球粒陨石的化学-岩石类型和小行星类型,这些性质提供了不同球粒陨石群有用的分类参数及其形成环境的信息.由于不同球粒陨石群的△17O与日心距离存在有相关关系,因此,依据不同球粒陨石群形...     

Mass independently fractionated sulfur components in chondrites

We report high precision sulfur isotopic data obtained by sequential extraction from various physically separated phases (bulk, matrix, and chondrules) from chondrites. A significant excess of ³³S (up to Δ³³S of 0.112‰ for Dhajala Chondrule) has been observed and is most likely carried by chondrule rims, though chondrule interiors cannot be ruled out as a carrier. Stellar nucleosynthesis and spallation are ruled out as a cause for this anomaly. Photochemical irradiation of sulfur gaseous species in the early solar nebula has, most likely, produced this anomaly. Observations of mass independent sulfur of photochemical origin suggest that chondrules and their rims must have formed in an optically thin nebular region. This also suggests that the chondrules were formed near the protoSun when it was active in ultraviolet light emission.     

Chondrules in H3 chondrites: textures,compositions and origins

Major and minor element bulk compositions of 90 individual chondrules and 16 compound chondrule sets in unequilibrated (type 3) H-group chondrites were determined in polished thin sections by broad beam electron probe analysis and the chondrules were classified petrographically into six textural types (barred olivine, porphyritic olivine, porphyritic pyroxene, barred pyroxene, radiating pyroxene, fine-grained). Although analyses of individual chondrules scatter widely, the mean composition of each textural type (except barred pyroxene) is rather distinct, as verified by discriminant function analysis. Al₂O₃, TiO₂ and Na₃O are correlated in chondrules, but Al₂O₃ and CaO do not correlate. Compound chondrule sets were found to consist almost entirely of chondrules or partial chondrules of similar texture and composition.The data suggest that composition played a conspicuous role in producing the observed textures of chondrules, though other factors such as cooling rates and degrees of supercooling prior to nucleation were also important. If compound chondrules formed and joined when they were still molten or plastic, then the data suggest that chondrules of each textural type could have formed together in space or time. The correlation of Al₂O₃ and TiO₂ with Na₂O and not with CaO appears to rule out formation of chondrules by direct equilibrium condensation from the nebula. We conclude that the most reasonable model for formation of the majority of chondrules is that they originated from mixtures of differing fractions of high-, intermediate- and low-temperature nebular condensates that underwent melting in space. A small percentage of chondrules might have formed by impacts in meteorite parent-body regoliths.     

Chemistry of glass inclusions in olivines of the CR chondrites Renazzo, Acfer 182, and El Djouf 001

Glass inclusions in olivines of the Renazzo, El Djouf 001, and Acfer 182 CR-type chondrites are chemically divers and can be classified into Al-rich, Al-poor, and Na-rich types. The chemical properties of the glasses are independent of the occurrence of the olivine (isolated or part of an aggregate or chondrule) and its composition. The glasses are silica-saturated (Al-rich) or oversaturated (Al-poor, 24% normative quartz). All glasses have chondritic CaO/Al₂O₃ ratios, unfractionated CI-normalized abundances of refractory trace elements and are depleted in moderately volatile and volatile elements. Thus the glasses are likely to be of a primitive condensate origin whose chemical composition has been established before chondrule formation and accretion, rather then the product of either crystal fractionation from chondrule melts or part melting of chondrules. Rare Na-rich glasses give evidence for elemental exchange between the glass and a vapor phase. Because they have Al₂O₃ contents and trace element abundances very similar to those of the Al-rich glasses, they likely were derived from the latter by Ca exchange (for Na) with the nebula. Elemental exchange reactions also have affected practically all olivines (e.g., exchange of Mg of olivine for Fe²⁺, Mn²⁺, and Cr³⁺). Glasses formed contemporaneously with the host olivine. As the most likely process for growing nonskeletal olivines from a vapor we consider the VLS (vapor-liquid-solid) growth process, or liquid-phase epitaxy. Glasses are the possible remnants of the liquid interface between growing crystal and the vapor. Such liquids can form stably or metastably in regions with enhanced oxygen fugacity as compared to that of a nebula of solar composition.     

An experimental study of the formation of metallic iron in chondrules

The abundance of metallic iron is highly variable in different kinds of chondrites. The precise mechanism by which metal fractionation occurred and its place in time relative to chondrule formation are unknown. As metallic iron is abundant in most Type I (FeO-poor) chondrules, determining under what conditions metal could form in chondrules is of great interest. Assuming chondrules were formed from low temperature nebular condensate, we heated an anhydrous CI-like material at 1580°C in conditions similar to those of the canonical nebula (P_H2 = 1.3 × 10⁻⁵ atm). We reproduced many of the characteristics of Type IA and IIA chondrules but none of them contained any iron metal. In these experiments FeO was abundant in charges that were heated for as long as 6 h. At a lower temperature, 1350°C, dendritic/cellular metal crystallized from Fe-FeS melts during the evaporation of S. However, the silicate portion consisted of many relict grains and vesicles, not typical of chondrules.Evaporation experiments conducted at P_H2 = 1 atm and 1565°C produced charges containing metallic iron both as melt droplets and inclusions in olivine, similar to those found in chondrules. Formation of iron in these experiments was primarily the result of desulfurization of FeS. With long heating times Fe° was lost by evaporation. Apart from some reduction of FeO by kerogen to make metal inclusions within olivine grains, reduction of FeO to make Fe° in these charges was not observed.This study shows that under canonical nebular conditions FeS and iron-metal are extremely volatile so that metal-rich Type I chondrules could not form by melting “CI.” Under these conditions FeO is lost predominantly by hydrogen stripping and, due to the relative low abundance of hydrogen at low pressures, remains in the melt for as long as 6 h. Conversely, at higher total pressures (1-atm H₂) iron metal (produced mainly by the desulfurization of troilite) is less volatile and remains in the melt for longer times (at least 6 h). In addition, due to elevated pressures of hydrogen, FeO is stripped away much faster. These results suggest that chondrule formation occurred in environments with elevated pressures relative to the canonical nebula for iron metal to be present.