Type IA Chondrules Research Articles

Forging inner-disk Al-rich chondrules by interactions of CAI-like melt and ambient gas

The mechanism of gas-melt interactions and the compositions of precursors are key to understanding the formation of chondrules. To shed light on the two enigmas, we studied the petrography, chemistry, and oxygen isotopes of six Al-rich chondrules (ARCs, five glassy and one plagioclase-bearing) in unequilibrated ordinary chondrites (OCs, petrologic subtype: 3.05). The plagioclase-bearing ARC was also investigated with Al-Mg chronology. Elemental zonation and inter-element correlations in glassy mesostasis of two ARCs indicate the condensation of gaseous Mg, SiO, Fe, and Na onto chondrule melt. The plagioclase-bearing ARC appears to display internal mass-independent oxygen isotope fractionation with δ18O increasing following the order of mineral crystallization, suggesting partial oxygen isotope exchange with ambient gas during crystallization. Oxygen isotopes of the six ARCs are distributed along a mixing line of slope = 0.99 ± 0.05, which intersects with calcium-aluminum-rich inclusions (CAIs), consistent with a small portion of OC type IA chondrules, but deviates from other OC ferromagnesium chondrules (FMCs) towards higher δ17O, suggesting that OC ARCs and some IA chondrules were established by interactions between CAI-like melts and 16O-poor ambient gas, rather than simply remelting solid mixtures of CAI and FMC materials.All six ARCs have unfractionated refractory lithophile element patterns with bulk concentrations ranging from ∼7 × CI to ∼15 × CI, indicating ∼ 30–100 % of CAI-like materials in their precursors. Their bulk compositions are linearly evolved toward the Mg: SiO ∼ 3:2 to 2:1 (in atomic) apex, consistent with adding gaseous Mg and SiO to the chondrule bulk via gas–melt interactions. The back-calculated compositions of the recycled CAI-like materials closely overlap with pyroxene-anorthite-rich CAIs, suggesting that extensive interactions between the melt of pyroxene-anorthite-rich CAI-like materials and ambient gas could make OC ARCs. The Al-Mg age of the plagioclase-bearing ARC is ∼2.2 Ma after CAIs, similar to typical OC FMCs, suggesting that the refractory component arrived at the OC reservoirs in the late stage of the chondrule heating events.

Differences in bulk Fe content and density between type I and type II ordinary chondrite chondrules: Implications for parent body heterogeneities in oxidation state and O‐isotopic composition

AbstractType II chondrules have higher oxidation states than type I chondrules; in ordinary chondrites (OC), type II chondrules tend to be larger, richer in bulk Fe, and have higher densities than type I chondrules. Magnesian type IA chondrules tend to be richer in 16O than type II chondrules. Because the aerodynamic behavior of a particle is a function of the product of its size and density, type I and type II chondrules (or their precursors) were partly separated in the ordinary chondrite zone of the solar nebula prior to the accretion of OC parent asteroids. LL chondrites acquired a chondrule population with the highest type II/type I ratios, L chondrites acquired chondrules with an intermediate ratio, and H chondrites acquired chondrules with the lowest type II/type I ratios. This contributed to the observed differences among OC groups in oxidation state and O‐isotopic composition: in going from H to L to LL, mean oxidation state increases and mean Δ17O values increase. Higher oxidation is marked by increases in the FeO contents of olivine, low‐Ca pyroxene, chromite, and ilmenite; increases in the TiO2 content of chromite; and increases in the Co content of kamacite.

Refractory inclusions as Type IA chondrule precursors: Constraints from melting experiments

The formation of chondrules involved major processes in the protoplanetary disk and therefore needs to be understood. Identifying possible precursors and the conditions of their transformation into chondrules is an essential step. Here we investigate whether refractory inclusions (RI) can be converted into Type IA chondrule analogs by isothermal heating and dynamic crystallization experiments, and report a new constraint on chondrule peak temperatures. We prepared synthetic calcium-aluminum-rich inclusions (CAI) by sintering <20 µm An + Di + Sp powder at 1200 °C and synthetic AOA analogs from crushed <5 µm Fo gel or San Carlos olivine mixed with nuggets of synthetic CAI. We used the AOA analogs as starting materials in experiments and were able to reproduce the textures and mesostasis compositions of Type IA chondrules. However, in the charges, the olivine lacks asymmetric zonation and our mesostasis compositions show olivine fractionation trends, two differences from Type I chondrules indicating the requirement of condensation of Mg and SiO in the latter. Relict spinel is present in isothermal runs up to 1550 °C, but is totally resorbed by 1600 °C. We conclude that CAI and AOA were sintered essentially at their condensation temperatures and are appropriate precursors for chondrules. Chondrules with relict spinel must have formed at <1600 °C, much lower than their liquidus temperatures (∼1750 °C). Such peak temperatures are consistent with models of condensation during chondrule formation. In typical chondrules with no inclusions of AOA or CAI, spinel is an indicator of their near complete assimilation. Grains of spinel (sensu stricto) in chondrules are relicts of RI and constitute a largely untapped cosmochemical resource for the investigation of chondrule provenance.

A magmatic origin for silica-rich glass inclusions hosted in porphyritic magnesian olivines in chondrules: An experimental study

Rare silica-rich glass inclusions (69<SiO2<82wt.%) are described within magnesian olivines of porphyritic Type IA chondrules. These glass inclusion compositions are clearly out of equilibrium with their host Mg-olivines and their presence within the olivines is generally attributed to an unclear secondary process such as a late interaction with nebular gases. We performed dynamic crystallisation experiments that demonstrate that these Si-rich glass inclusions are actually magmatic in origin and were trapped inside olivines that crystallized slowly from a magma with a CI, i.e. solar, composition. Their silica-rich compositions are the consequence of the small volumes of inclusions, which inhibit the nucleation of secondary crystalline phase (Ca-poor pyroxene) but allow olivine to continue to crystallize metastably on the walls of the inclusions. We suggest that Si-rich glass inclusions could be the only reliable relicts of what were the first magmas of the solar system, exhibiting a CI, i.e. non-fractionated, composition.

Correlations and zoning patterns of phosphorus and chromium in olivine from H chondrites and the LL chondrite Semarkona

AbstractPhosphorus zoning is observed in olivines in high‐FeO (type IIA) chondrules in H chondrites over the entire range of petrologic grades: H3.1–H6. Features in P concentrations such as oscillatory and sector zoning, and high P cores are present in olivines that are otherwise unzoned in the divalent cations. Aluminum concentrations are low and not significantly associated with P zoning in chondrule olivines. In highly unequilibrated H chondrites, phosphorus zoning is generally positively correlated with Cr. Atomic Cr:P in olivine is roughly 1:1 (3:1 for one zone in one olivine in RC 075), consistent with Cr3+ charge‐balancing P5+ substituting for Si4+. Normal igneous zonation involving the dominant chrome species Cr2+ was observed only in the LL3.0 chondrite Semarkona. In more equilibrated chondrites (H3.5–H3.8), Cr spatially correlated with P is occasionally observed but it is diffuse relative to the P zones. In H4–H6 chondrites, P‐correlated Cr is absent. One signature of higher metamorphic grades (≥H3.8) is the presence of near matrix olivines that are devoid of P oscillatory zoning. The restriction to relatively high metamorphic grade and to grains near the chondrule–matrix interface suggests that this is a response to metasomatic processes. We also observed P‐enriched halos near the chondrule–matrix interface in H3.3–H3.8 chondrites, likely reflecting the loss of P and Ca from mesostasis and precipitation of Ca phosphate near the chondrule surface. These halos are absent in equilibrated chondrites due to coarsening of the phosphate and in unequilibrated chondrites due to low degrees of metasomatism. Olivines in type IA chondrules show none of the P‐zoning ubiquitous in type IIA chondrules or terrestrial igneous olivines, likely reflecting sequestration of P in reduced form within metallic alloys and sulfides during melting of type IA chondrules.

Weakly shocked and deformed CM microxenoliths in the Pułtusk H chondrite

AbstractThe Pułtusk meteorite is a brecciated H4–5 chondrite cut by darkened cataclastic zones. Within the breccia, relict type IA, IB, and IIA chondrules, and microxenoliths of carbonaceous CM chondrite lithology occur. This is the first description of foreign clasts in the Pułtusk meteorite. The matrix of the xenoliths was identified by usage of microprobe and Raman spectroscopic analyses. Raman spectra show distinct bands related to the presence of slightly ordered carbonaceous matter at approximately 1320 and 1580–1584 cm−1. Bands related to serpentine group minerals are also visible, especially a peak at 692 cm−1 and moreover other weak bands are interpreted as evidence for tochilinite. We decipher the metamorphic and deformational history of the xenoliths. They experienced aqueous alteration before being incorporated into the unaltered and well‐equilibrated parent rock of the Pułtusk chondrite. The xenoliths are weakly shocked as indicated by defects in the crystal structure of silicates and carbonates, but hydrated minerals (serpentine and tochilinite) are still present in the matrix. The carbonaceous matter within the clasts' matrix displays first order D and G Raman bands that suggests it is only slightly ordered as a result of mild thermal processing. Distinct shear bands are present in both the xenoliths and the surrounding rock, which testifies that the xenoliths were affected by a deformational event along with host rock. The host rock was brittly deformed, but the clasts experienced more ductile deformation revealed by semibrittle faulting of minerals, kinking of the tochilinite‐cronstedtite matrix, and injections of xenolithic material into the adjacent breccia. We argue that both processes, the high strain‐rate shear deformation and the incorporation of the xenoliths into the host Pułtusk breccia, could have been impact‐related. The Pułtusk xenoliths are, thus, rather spalled collisional fragments, than trapped fossil micrometeorites.

Origin of glass inclusions hosted in magnesian porphyritic olivines chondrules: Deciphering planetesimal compositions

Glass inclusions occur in the most primitive magnesian porphyritic olivines of chondrules formed very early in the solar system, and understanding their formation and preservation is crucial for explaining early solar system evolution. Several different and frequently opposed models have been proposed to explain the origin of these glass inclusions and their host olivines, ranging from condensation processes in the nebula to crystallization products of chondrule melts. However, none of these models fully satisfy all the petrographic, chemical and isotopic observations. In this experimental work, we show that the peculiar compositions of these inclusions that exhibit significant chemical disequilibrium between glass and host olivine result from the slow crystallization of metastable olivines from a magma. The inhibition of plagioclase nucleation resulting in metastable olivine crystallization instead of the equilibrium assemblage (anorthite+clinopyroxene) is a natural consequence of the high liquidus temperature of the magma. This result implies a restricted compositional range for the parental magmas of these melt inclusions. Olivine phenocrysts inside type IA chondrules are thus interpreted as fragments of magmatic cumulates formed in magma ocean-like environments on fully or partially molten planetesimals. In a similar manner as for terrestrial rocks, studies of melt inclusions, giving information about the earliest magma oceans, could open a new way of assessing the evolution of the earliest bodies of the Solar System.

Laboratory experiments bearing on the origin and evolution of olivine‐rich chondrules

Abstract– Evaporation rates of K2O, Na2O, and FeO from chondrule‐like liquids and the associated potassium isotopic fractionation of the evaporation residues were measured to help understand the processes and conditions that affected the chemical and isotopic compositions of olivine‐rich type IA and type IIA chondrules from Semarkona. Both types of chondrules show evidence of having been significantly or totally molten. However, these chondrules do not have large or systematic potassium isotopic fractionation of the sort found in the laboratory evaporation experiments. The experimental results reported here provide new data regarding the evaporation kinetics of sodium and potassium from a chondrule‐like melt and the potassium isotopic fractionation of evaporation residues run under various conditions ranging from high vacuum to pressures of one bar of H2+CO2, or H2, or helium. The lack of systematic isotopic fractionation of potassium in the type IIA and type IA chondrules compared with what is found in the vacuum and one‐bar evaporation residues is interpreted as indicating that they evolved in a partially closed system where the residence time of the surrounding gas was sufficiently long for it to have become saturated in the evaporating species and for isotopic equilibration between the gas and the melt. A diffusion couple experiment juxtaposing chondrule‐like melts with different potassium concentrations showed that the diffusivity of potassium is sufficiently fast at liquidus temperatures (DK &gt; 2 × 10−4cm2 s−1 at 1650 °C) that diffusion‐limited evaporation cannot explain why, despite their having been molten, the type IIA and type IA chondrules show no systematic potassium isotopic fractionation.

High precision SIMS oxygen three isotope study of chondrules in LL3 chondrites: Role of ambient gas during chondrule formation

We report high precision SIMS oxygen three isotope analyses of 36 chondrules from some of the least equilibrated LL3 chondrites, and find systematic variations in oxygen isotope ratios with chondrule types. FeO-poor (type I) chondrules generally plot along a mass dependent fractionation line (Δ 17O ∼ 0.7‰), with δ 18O values lower in olivine-rich (IA) than pyroxene-rich (IB) chondrules. Data from FeO-rich (type II) chondrules show a limited range of δ 18O and δ 17O values at δ 18O = 4.5‰, δ 17O = 2.9‰, and Δ 17O = 0.5‰, which is slightly 16O-enriched relative to bulk LL chondrites (Δ 17O ∼ 1.3‰). Data from four chondrules show 16O-rich oxygen isotope ratios that plot near the CCAM (Carbonaceous Chondrite Anhydrous Mineral) line. Glass analyses in selected chondrules are systematically higher than co-existing minerals in both δ 18O and Δ 17O values, whereas high-Ca pyroxene data in the same chondrule are similar to those in olivine and pyroxene phenocrysts. Our results suggest that the LL chondrite chondrule-forming region contained two kinds of solid precursors, (1) 16O-poor precursors with Δ 17O > 1.6‰ and (2) 16O-rich solid precursors derived from the same oxygen isotope reservoir as carbonaceous chondrites. Oxygen isotopes exhibited open system behavior during chondrule formation, and the interaction between the solid and ambient gas might occur as described in the following model. Significant evaporation and recondensation of solid precursors caused a large mass-dependent fractionation due to either kinetic or equilibrium isotope exchange between gas and solid to form type IA chondrules with higher bulk Mg/Si ratios. Type II chondrules formed under elevated dust/gas ratios and with water ice in the precursors, in which the ambient H 2O gas homogenized chondrule melts by isotope exchange. Low temperature oxygen isotope exchange may have occurred between chondrule glasses and aqueous fluids with high Δ 17O (∼5‰) in LL the parent body. According to our model, oxygen isotope ratios of chondrules were strongly influenced by the local solid precursors in the proto-planetary disk and the ambient gas during chondrule melting events.

Re-examining the role of chondrules in producing the elemental fractionations in chondrites

Abstract— The matrices of all primitive chondrites contain presolar materials (circumstellar grains and interstellar organics) in roughly CI abundances, suggesting that all chondrites accreted matrix that is dominated by a CI-like component. The matrix-normalized abundances of the more volatile elements (condensation temperatures <750–800 K) in carbonaceous and ordinary chondrites are also at or slightly above CI levels. The modest excesses may be due to low levels of these elements in chondrules and associated metal. Subtraction of a CI-like matrix component from a bulk ordinary chondrite composition closely matches the average composition of chondrules determined by instrumental neutron activation analysis (INAA) if some Fe-metal is added to the chondrule composition. Measured matrix compositions are not CI-like. Sampling bias and secondary redistribution of elements may have played a role, but the best explanation is that ˜10–30% of refractory-rich, volatile depleted material was added to matrix. If most of the more volatile elements are in a CI-dominated matrix, the major and volatile element fractionations must be largely carried by chondrules. There is both direct and indirect evidence for evaporation during chondrule formation. Type IIA and type B chondrules could have formed from a mixture of CI material and material evaporated from type IA chondrules. The Mg-Si-Fe fractionations in the ordinary chondrites can be reproduced with the loss of type IA chondrule material and associated metal. The loss of evaporated material from the chondrules could explain the volatile element fractionations. Mechanisms for how these fractionations occurred are necessarily speculative, but two possibilities are briefly explored.

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 (PH2 = 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 PH2 = 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.

The formation of chondrules by open-system melting of nebular condensates

Experiments were conducted under canonical nebular conditions to see whether the chemical compositions of the various chondrule types can be derived from a single CI-like starting material by open-system melting and evaporation. Experimental charges, produced at 1580 °C and PH2 of 1.31×10−5 atm over 1 to 18 hours, consisted of only two phases, porphyritic olivine crystals in glass. Sulfur, metallic-iron and alkalis were completely evaporated in the first minutes of the experiments and subsequently the main evaporating liquid oxides were FeO and SiO2. Olivines from short runs (2–4 hours) have compositions of Fo83-Fo89, as in Type IIA chondrules, while longer experimental runs (12–18 hours) produce ∼Fo99 olivine, similar to Type IA chondrules. The concentration of CaO in both olivine (up to 0.6 wt.%) and glass, and their Mg#, increased with increasing heating duration. Natural chondrules also show increasing CaO with decreasing S, alkalis, FeO and SiO2. The similarities in bulk chemistry, mineralogy and textures between Type IIA and IA chondrules and the experimental charges demonstrate that these chondrules could have formed by the evaporation of CI precursors. The formation of silica-rich chondrules (IIB and IB) by evaporation requires a more pyroxene-rich precursor.Based on the FeO evaporation rates measured here, Type IIA and IA chondrules, were heated for at least ∼0.5 and ∼3.5 h, respectively, if formed at 1580 °C and PH2 of 1.31×10−5 atm. Type II chondrules may have experienced higher cooling-rates and less evaporation than Type I.The experimental charges experienced free evaporation and exhibited heavy isotopic enrichments in silicon, as well as zero concentrations of S, Na and K, which are not observed in natural chondrules. However, experiments on potassium-rich melts at the same pressure but in closed capsules showed less evaporation of K, and less K isotopic mass fractionation, than expected as a function of decreasing cooling rate. Thus the environment in which chondrules formed is as important as the kinetic processes they experienced. If chondrule formation occurred under conditions in which evaporated gases remained in the vicinity of the residual melts, the extent of evaporation would be reduced and back reaction between the gas and the melt could contribute to the suppression of isotopic mass fractionation. Hence chondrule formation could have involved evaporative loss without Rayleigh fractionation. Volatile-rich Type II and volatile-poor Type I chondrules may have formed in domains with high and low chondrule concentrations, and high partial pressures of lithophile elements, respectively.

Condensation in dust-enriched systems

Full equilibrium calculations of the sequence of condensation of the elements from cosmic gases made by total vaporization of dust-enriched systems were performed in order to investigate the oxidation state of the resulting condensates. The computations included 23 elements and 374 gas species, and were done over a range of P tot from 10 −3 to 10 −6 bar and for enrichments up to 1000× in dust of Cl composition relative to a system of solar composition. Because liquids are stable condensates in dust-enriched systems, the MELTS nonideal solution model for silicate liquids (Ghiorso and Sack, 1995) was incorporated into the computer code. Condensation at 10 −3 bar and dust enrichments of 100×, 500×, and 1000× occur at oxygen fugacities of IW-3.1, IW-1.7, and IW-1.2, respectively, and, at the temperature of cessation of direct condensation of olivine from the vapor, yields X Fa of 0.019, 0.088, and 0.164, respectively. Silicate liquid is a stable condensate at dust enrichments >∼12.5× at 10 −3 bar and >∼425× at 10 −6 bar. At 500×, the liquid field is >1000 K wide and accounts for a maximum of 48% of the silicon at 10 −3 bar, and is 240 K wide and accounts for 25% of the silicon at 10 −6 bar. At the temperature of disappearance of liquid, X Fa of coexisting olivine is 0.025, 0.14, and 0.31 at 100×, 500×, and 1000×, respectively, almost independent of P tot. At 1000×, the Na 2O and K 2O contents of the last liquid reach 10.1 and 1.3 wt.%, respectively, at 10 −3 bar but are both negligible at 10 −6 bar. At 10 −3 bar, iron sulfide liquids are stable condensates at dust enrichments at least as low as 500× and coexist with silicate liquid at 1000×. No sulfide liquid is found at 10 −6 bar. At 10 −3 bar, the predicted distribution of Fe between metal, silicate and sulfide at 1310 K and a dust enrichment of 560× matches that found in H-group chondrites, and at 1330 K and 675× matches that of L-group chondrites prior to metal loss. Only at combinations of high P tot and high dust enrichment do the bulk chemical composition trends of condensates reach the FeO contents typical of type IIA chondrules at temperatures where dust and gas could be expected to equilibrate, ≥1200 K. Even under these conditions, however, the composition trajectories of predicted condensates pass through compositions with much more CaO + Al 2O 3 relative to MgO + SiO 2 than those of most type IA chondrules. Furthermore, on a plot of wt.% Na 2O vs. wt.% FeO, most chondrule compositions are too Na 2O-rich to lie along trends predicted for the bulk chemical compositions of the condensates at P tot ≤ 10 −3 bar and dust enrichments ≤1000×. Together, these chemical differences indicate that individual chondrules formed neither by quenching samples of the liquid + solid condensates that existed at various temperatures nor by quenching secondary liquids that formed from such samples. With the exception of very FeO-poor, Na 2O-rich glasses in type I chondrules and glasses with very high FeO and Na 2O in type II chondrules, however, many chondrule glass compositions fall along bulk composition trajectories for liquids in equilibrium with cosmic gases at 10 −3 bar and dust enrichments between 600× and 1000×. If these chondrules formed by secondary melting of mixtures of condensates that formed at different temperatures, nebular regions with characteristics such as these would have been necessary to prevent loss of Na 2O by evaporation and FeO by reduction from the liquid precursors of their glasses, assuming that the liquids were hot for a long enough time to have equilibrated with the gas.

Chondrules as products of dust collisions with totally molten droplets within a dust-rich nebular environment: An experimental investigation

Chondrules are usually interpreted as the products of incomplete melting of mineral aggregates in the solar nebula and have textures controlled by the number of residual nuclei present after melting. If barred olivine (BO) chondrules formed by nearly total melting at their respective liquidi, i.e., over a wide temperature range, it is surprising that there are so few totally glassy chondrules. The occurrence of fine-grained, accretionary rims on chondrules suggests mineral dust in the chondrule-forming environment which might have collided with totally molten droplets to produce chondrule textures. We have, therefore, conducted experiments in which mineral dusts encounter molten droplets during their cooling at 500°C/h and cause nucleation. Our experiments reproduce the majority of common chondrule textures. The production of textures in our experiments is a function of the temperature at which dust is encountered, the number of dust grains encountered and, to a lesser extent, the mineralogy of the encountered mineral grains. Crystal growth rates increase with lower dust encounter temperatures giving a textural range with increasingly skeletal or elongated crystals for lower temperature encounters. We have produced some less commonly discussed types of chondrule textures, e.g., barred-olivine-porphyritic-olivine pyroxene (BO/POP) by dust seeding that have not been reproduced by the incomplete melting of starting materials. The reproduction of BO/POP and excentroradial textures by dust collisions, and the scarcity of natural totally glassy chondrules, suggest that formation by seeding of total melts was common for chondrules with the lowest liquidus temperatures, e.g., FeO-rich chondrules. From these seeding experiments we suggest that the maximum melting temperature chondrules experienced is limited only to the temperature at which evaporation of chondrule liquid occurs. However, we have reproduced the very fine-grained microporphyritic textures characteristic of Type IA chondrules only by incomplete melting. Such chondrules could not have reached average internal temperatures above their liquidus temperatures. The conclusion, that at least some chondrules formed from total melts that collided with mineral dust during cooling, indicates that chondrule formation occurred within a region of the nebula rich in mineral dust.

Evaporation of olivine: Low pressure phase relations of the olivine system and its implication for the origin of chondritic components in the solar nebula

Low-pressure phase relations and vapor pressures of forsterite and fayalite were studied with the Knudsen method in the temperature range of 1400–1860°C for forsterite and 1100–1160°C for fayalite. The triple points are 5.2 × 10 −5 bar (1890°C) and 6.3 × 10 −8 bar (1217°C) for forsterite and fayalite, respectively. The enthalpy and entropy of evaporation of forsterite are ΔH v = 543 ± 33 kJ/mol and ΔS v = 169 ± 21 J/(mol·K), and those for fayalite are 502 ± 9 kJ/mol and 199 ± 6 J/(mol·K), respectively. By assuming that gas and olivine are ideal solutions, vaporous and solidus curves of the olivine solid solution system in the pressure range from 10 −10 to 10 −3 bar were calculated from the thermochemical data. At pressures above the triple point of forsterite, the phase diagram is the same as that at 1 bar, because the vapor field is at higher temperatures than those of liquid-bearing fields. At pressures between the triple points of forsterite and fayalite, liquid-bearing stability fields appear below the gas-solid stability field in the Fe-rich portion of the system. The compositional range of liquid-bearing fields expand with increasing pressure; at 10 −4 bar, the liquid-bearing fields cover most of the olivine system ( X Mg < 0.9). Liquid is absent at pressures below the triple point of fayalite, regardless of the composition. Large Mg Fe fractionation between gas and solid is more extensive, compared to the solid and liquid relationship. When heated in equilibrium, olivine melts before evaporation in the pressure range of 10 −7-10 −3 bar. In a solar nebula with total pressure of 10 −3 bar, olivine melts when the nebula is enriched in dust by more than an order of magnitude over the solar nebular value and when the (Mg + Si + Fe) H ratio increased. The origin of type IA and II chondrules and matrix olivine in ordinary and carbonaceous chondrites can be explained in terms of the pressure of olivine gas. Type IA chondrules were formed in the gas-solid phase field where volatiles (FeO, Na 2O, and others) were lost during heating events responsible for chondrule formation. The residual materials became enriched in refractory elements. Type II chondrules were formed in the temperature range of the liquid-solid phase field. Under these conditions, the original compositions were retained after heating of chondrule formation. Provided that type IA and II chondrules were formed at a similar temperature, type IA chondrules were formed at an olivine pressure one order of magnitude lower than that of type II chondrules. In this model, matrix olivine is a condensate from gas which formed by partial evaporation of chondritic material during type IA chondrule formation.

Roosevelt County 075: A petrologic, chemical and isotopic study of the most unequilibrated known H chondrite

Abstract— Roosevelt County (RC) 075 was recovered in 1990 as a single 258‐gram stone. Classification of this meteorite is complicated by its highly unequilibrated nature and its severe terrestrial weathering, but we favor H classification. This is supported by O isotopes and estimates of the original Fe, Ni metal content. The O isotopic composition is similar to that of a number of reduced ordinary chondrites (e.g., Cerro los Calvos, Willaroy), although RC 075 exhibits no evidence of reduced mineral compositions. Chondrule diameters are consistent with classification as an L chondrite, but large uncertainties in chondrule diameters of RC 075 and poorly constrained means of H, L and LL chondrites prevent use of this parameter for reliable classification. Other parameters are compromised by severe weathering (e.g., siderophile element abundances) or unsuitable for discrimination between unequilibrated H, L and LL chondrites (e.g., Co in kamacite, δ13C). Petrologic subtype 3.2± 0.1 is suggested by the degree of olivine heterogeneity, the compositions of chondrule olivines, the thermoluminescence sensitivity, the abundances and types of chondrules mapped on cathodoluminescence mosaics, and the amount of presolar SiC. The meteorite is very weakly shocked (S2), with some chondrules essentially unshocked and, thus, is classified as an H3.2(S2) chondrite. Weathering is evident by a LREE enrichment due to clay contamination, reduced levels of many siderophile elements, the almost total loss of Fe, Ni metal and troilite, and the reduced concentrations of noble gases. Some components of the meteorite (e.g., type IA chondrules, SiC) appear to preserve their nebular states, with little modification from thermal metamorphism. We conclude that RC 075 is the most unequilibrated H chondrite yet recovered and may provide additional insights into the origin of primitive materials in the solar nebula.

Composition of chondrule silicates in LL3-5 chondrites and implications for their nebular history and parent body metamorphism

Our petrologic studies of 75 type IA and type II porphyritic olivine chondrules in nine selected LL group chondrites of type 3.3 to type 5 and comparisons with published studies of chondrules in Semarkona (LL3.0) show that compositions of silicates and bulk chondrules, but not overall chondrule textures, vary systematically with the petrologic type of the chondrite. These compositional trends are due to diffusive exchange between chondrule silicates and other phases (e.g., matrix), such as those now preserved in Semarkona, during which olivines in both chondrule types gained Fe 2+ and Mn 2+ and lost Mg 2+, Cr 3+, and Ca 2+. In a given LL4-5 chondrite, the olivines from the two chondrule types are identical in composition. Enrichments of Fe 2+ in olivine are particularly noticeable in type IA chondrules from type 3.3–3.6 chondrites, especially near grain edges, chondrule rims, grain boundaries, and what appear to be annealed cracks. Compositional changes in low-Ca pyroxene lag behind those in coexisting olivine, consistent with its lower diffusion rates. With increasing petrologic type, low-Ca pyroxenes in type IA chondrules become enriched in Fe 2+ and Mn 2+ and depleted in Mg 2+, Cr 3+, and A1 3+. These compositional changes are entirely consistent with mineral equilibration in chondritic material during metamorphism. From these compositional data alone we cannot exclude the possibility that chondritic material was metamorphosed to some degree in the nebula, but we see no evidence favoring nebula over asteroidal metamorphism, nor evidence that the chondrule reacted with nebular gases after crystallization. Modelling of the equilibration of chondrule olivines suggests that heterogeneous FeO concentrations in olivine could be preserved after cooling from 600°C at rates of 1–10°C/Ma for at least tens to hundreds of millions of years. This is consistent with published estimates for the maximum metamorphic temperatures in type 3 chondrites, thermal histories derived from metallographic and fission-track cooling rates, and 4.4 Ga 40Ar- 39Ar ages for ordinary chondrites. Since the lifetime of the solar nebula was not more than 10 6 years and there is abundant evidence that meteorite parent bodies were heated, some even above their melting point, we confidently conclude that the formation of type 3.3–5 ordinary chondrites from type 3.0 material by metamorphism occurred in parental asteroids, not the solar nebula.

Disentangling nebular and asteroidal features of c03 carbonaceous chondrite meteorites

Analyses of olivines and low-Ca pyroxenes in a suite of porphyritic chondrules of types IA and II in each of ten CO3 chondrites and comparisons with similar chondrules in LL chondrites suggest that the CO3 chondrites may be divided into a metamorphic sequence of subtypes 3.0 to 3.7. Chondrules in Allan Hills A77307 and Colony, which we classify as type 3.0, show only igneous zoning, whereas in the other more metamorphosed type 3.1–3.7 CO chondrites, chondrule silicates are enriched in FeO, most markedly in olivine crystals near the rims of type IA chondrules and along cracks. The FeO enrichments in metamorphosed chondrules result largely from equilibration by solid-state diffusion between chondrules and FeO-rich matrices. Type 3.1–3.7 CO chondrites were formed from chondrites like Colony and ALH A77307 in heated asteroids or planetesimals. We find no evidence for nebular condensation of fayalite rims around these chondrules, as reported for CV3 chondrites, or nebular modification of chondrule silicates, as favored by some workers. Metallic Fe,Ni in ALH A77307 contains up to 0.1–1 wt% Cr, Si, and P, as in the least metamorphosed ordinary chondrites and CM2 chondrites, and grains of cohenite, Fe 3C, and haxonite, Fe 23C 6. These carbides, which are the first reported in carbonaceous chondrites, are associated with pentlandite and magnetite, as in certain LL3 chondrites. We assign the following petrologic subtypes to other CO chondrites: Kainsaz, 3.1; Felix, 3.2; ALH 82101 and Ornans, 3.3; Lancé and ALH A77029, 3.4; ALH A77003, 3.5; Warrenton, 3.6; and Isna, 3.7.

Petrology and mineralogy of Type II, FeO-rich chondrules in Semarkona (LL3.0): Origin by closed-system fractional crystallization, with evidence for supercooling

The petrology of type II porphyritic olivine chondrules in Semarkona (LL3.0) has been studied in detail. Olivines in these chondrules are euhedral, Fe-richand are strongly zoned from cores to rims of grains in FeO (Fa 10–30), Cr 2O 3 (0.2–0.6 wt%), MnO (0.2–0.7 wt%) and CaO (0.1–0.4 wt%). Interstitial mesostasis is rich in Si, Aland Ca and is glassy with abundant microcrystallites. Minor minerals include troilite, Fe,Ni metaland chromite. Some olivine grains contain euhedral, fayalite-rich cores that are probably produced during initial supercooling of the chondrule melt. Rare relict grains of forsteritic olivine have compositions very similar to olivines in type IA chondrules in Semarkona and may result from disaggregation of such chondrules. Apart from these relics, all properties of type II chondrules can be described by closed-system fractional crystallization of droplets which were essentially entirely molten. The chondrules did not originate as aggregates of large olivine grains and are unlikely to be fragments of precursor igneous rocks. Cooling rates of the order of 1000°C/h are indicated, from peak temperatures of around 1600°C. The chondrules show considerable evidence for supercooling and large departures from equilibrium during crystallization. There is no evidence for chondrule-nebula exchange either during or after crystallization, or any secondary processes such as aqueous alteration or thermal metamorphism. Bulk composition data suggest that precursor material included a refractory (Ca, Al, Ti-rich) component, similar to one of the precursor components of type IA chondrulesand a non-refractory (Fe and Si-rich) component. Type IA chondrules may have formed from type II chondrules by loss of Fe and volatiles. Alternatively, the two chondrule types may have formed in regions of considerable diversity in the solar nebula from precursor materials with different Fe Mg ratios.