Chondrule-forming Region Research Articles

Experimental constraints on formation of silica-rich igneous rims around chondrules in CR chondrites

Silica-rich igneous rims (SIRs) occur commonly as an outer rim layer on porphyritic chondrules in CR (Renazzo-like) chondrites, as well as more rarely in other chondrite groups. Formation conditions for SIRs can provide insight into chemical and physical conditions in the chondrule-forming region of the protoplanetary disk, as well as helping to understand temporal changes to successive thermal events within the CR and other chondrule formation regions. Both accretion and condensation models have been proposed as formation mechanisms of SIRs, but a lack of robust thermal constraints prevents a detailed interpretation. We have carried out an experimental study aimed to define the conditions of SIR formation by reproducing the texture, mineralogy, mineral chemistry, and silica polymorphs present in natural SIRs. Experiments conducted on an SIR bulk composition, at peak temperatures of 1310–1507 °C with linear cooling rates between 30 and 90 °C/hr successfully reproduce natural SIRs that contain cristobalite, low-Ca pyroxene, Ca-rich pyroxene, and glass. Microcrystalline mesostasis was formed in an experiment with lower cooling rates at low temperatures (6 °C/hr from <1200 °C). All silica in the experiments was cristobalite, including in experiments with maximum temperatures as low as 1310 °C. Since the cristobalite stability field in the silica phase diagram is >1470 °C, it is clear that silica polymorphs are not robust temperature indicators and that the presence of cristobalite in natural SIRs does not necessarily indicate high peak temperatures. We suggest that accretion of SIR precursors onto solidified chondrules, followed by melting, is the most likely scenario for their formation. Our constraints on SIR formation are similar to those that are usually discussed for chondrules. Therefore, in the CR chondrule-forming region, a repeatable heating mechanism is required that does not change significantly during the time in which condensing material evolved to highly silica-rich compositions. The need for recurring heating events is a general constraint for modeling chondrule formation.

Chromium Stable Isotope Panorama of Chondrites and Implications for Earth Early Accretion

Abstract We investigated the stable isotope fractionation of chromium (Cr) for a panorama of chondrites, including EH and EL enstatite chondrites and their chondrules and different phases (by acid leaching). We observed that chondrites have heterogeneous δ 53Cr values (per mil deviation of the 53Cr/52Cr from the NIST SRM 979 standard), which we suggest reflect different physical conditions in the different chondrite accretion regions. Chondrules from a primitive EH3 chondrite (SAH 97096) possess isotopically heavier Cr relative to their host bulk chondrite, which may be caused by Cr evaporation in a reduced chondrule-forming region of the protoplanetary disk. Enstatite chondrites show a range of bulk δ 53Cr values that likely result from variable mixing of isotopically different sulfide-silicate-metal phases. The bulk silicate Earth (δ 53Cr = –0.12 ± 0.02‰, 2SE) has a lighter Cr stable isotope composition compared to the average δ 53Cr value of enstatite chondrites (–0.05 ± 0.02‰, 2SE, when two samples out of 19 are excluded). If the bulk Earth originally had a Cr isotopic composition that was similar to the average enstatite chondrites, this Cr isotope difference may be caused by evaporation under equilibrium conditions from magma oceans on Earth or its planetesimal building blocks, as previously suggested to explain the magnesium and silicon isotope differences between Earth and enstatite chondrites. Alternatively, chemical differences between Earth and enstatite chondrite can result from thermal processes in the solar nebula and the enstatite chondrite-Earth, which would also have changed the Cr isotopic composition of Earth and enstatite chondrite parent body precursors.

Outward migration of chondrule fragments in the early Solar System: O-isotopic evidence for rocky material crossing the Jupiter Gap?

Determining the origins of chondrule precursors is key to constraining how material migrated in the early Solar System. Chondrules that were only partially melted during their formation retain portions of their solid precursors, termed relict grains. By measuring the chemical and O-isotopic compositions of relict grains in chondrules from an unequilibrated ordinary chondrite (UOC), and Renazzo-like carbonaceous (CR) and Mighei-like carbonaceous (CM) chondrites we constrain their origins and discuss implications for disk transport within the first 4 million years of the Solar System. For all three chondrite groups, the chemical and O-isotopic compositions of dusty olivine grains are sometimes consistent with the reduction of type I (FeO-poor) and/or type II (FeO-rich) chondrules from the same meteorite group. However, other dusty olivine grains from the CM chondrites and the UOC are found to be xenocrysts that require an origin from a source distinct from the host meteorite. This material plausibly originated as fragments of earlier-formed chondrules from another chondrite group or of partially or fully differentiated planetesimals that migrated into an active chondrule-forming region. Multiple CM chondrite dusty olivine chondrules have O-isotope compositions that match those of UOC chondrule olivine (Δ17O ∼ 0‰), suggesting an origin from an UOC source. This implies that UOC chondrules and/or chondrule fragments migrated from the inner Solar System outwards to CM chondrite chondrule-forming region, likely beyond the orbit of Jupiter. These UOC chondrules or chondrule fragments could have migrated outwards in the protoplanetary disk before the formation of the Jupiter Gap, or <300 μm diameter fragments could have migrated outwards after Gap formation as CM chondrite chondrule dusty olivine grains with Δ17O ∼ 0‰ were small enough to pass through Jupiter Gap. The identification of xenocrysts in each meteorite group studied here argues for widespread migration of material in the early Solar System, potentially crossing the Jupiter Gap.

Exploring the efficiency of stepwise dissolution in removal of stubborn non-radiogenic Pb in chondrule U-Pb dating

Chondrules in chondritic meteorites are unique witnesses of nebular and asteroidal processes that preceded large-scale planetary accretion. Together with refractory calcium-aluminium-rich inclusions (CAIs), they are the sources of our knowledge of the initial evolution of the early Solar System. We have investigated a single very large (>10 mm in longer dimension) chondrule, hereafter, the mega-chondrule A25-2, extracted from the Allende CV3 chondrite. We characterised texture, mineralogy and mineral chemistry of this chondrule, and studied its Al-Mg, U-Pb and U-isotope systematics. We also studied the distribution of U, Th and Pb, and measured Pb isotopic composition in individual minerals of A25-2 by secondary ion mass-spectrometry (SIMS). The main difficulty in absolute age determination was the presence of pervasive and resilient non-radiogenic Pb. In the search for the best way to separate radiogenic Pb from non-radiogenic Pb components of terrestrial and asteroidal origins, we used various protocols of multi-step leaching and assessed their efficiency in generating data suitable for the construction of an isochron. Testing the data filtering procedure led us to explore the behaviour of the stepwise leaching method in the presence of pervasive and resilient non-radiogenic Pb. The model age patterns observed in the final HF partial dissolution steps were probably induced by isotopic fractionation. Although step leaching did not yield fractions with highly radiogenic Pb, a Pb-Pb isochron age, corrected for measured 238U/235U was obtained by: (1) data filtering process based on strict analytical and geochemical criteria to include in the Pb-Pb isochron only leaching steps free from terrestrial contamination and (2) arithmetically recombined analyses to cancel the effects of leaching-induced isotopic fractionation.This extensive data processing yielded the age of 4568.5 ± 3.0 Ma, which we consider reliable within its uncertainty limits, although it is not as precise as, and more model dependent than, the age that could have been obtained if Pb isotopic compositions were more radiogenic. The 238U/235U ratio of the mega-chondrule is 137.764 ± 0.016, which is similar to the ratios obtained from single chondrules yet slightly different from small pooled Allende chondrules. The initial 27Al/26Al ratio inferred from internal isochron obtained from SIMS Al-Mg isotope measurements is (5.4 ± 6.5) × 10–6, which corresponds to 4565.0 + 0.8/−∞ Ma, assuming homogeneous distribution of 26Al throughout the protoplanetary disk at the canonical level (∼5.2 × 10−5). This age is 3.5 ± 3.1 Ma younger than the Pb-isotopic age. Calculation of 26Al-26Mg age assuming initial (27Al/26Al)0 of (1.36 ± 0.72) × 10–5 in the CV chondrule-forming region yields the age of 4566.4 + 0.8/−∞, which is consistent with the Pb-isotopic age.

Oxygen isotope systematics of chondrule olivine, pyroxene, and plagioclase in one of the most pristine CV3Red chondrites (Northwest Africa 8613).

We performed in situ oxygen three-isotope measurements of chondrule olivine, pyroxenes, and plagioclase from the newly described CVRed chondrite NWA 8613. Additionally, oxygen isotope ratios of plagioclase in chondrules from the Kaba CV3OxB chondrite were determined to enable comparisons of isotope ratios and degree of alteration of chondrules in both CV lithologies. NWA 8613 was affected by only mild thermal metamorphism. The majority of oxygen isotope ratios of olivine and pyroxenes plot along a slope-1 line in the oxygen three-isotope diagram, except for a type II and a remolten barred olivine chondrule. When isotopic relict olivine is excluded, olivine, low- and high-Ca pyroxenes are indistinguishable regarding Δ17O values. Conversely, plagioclase in chondrules from NWA 8613 and Kaba plot along mass-dependent fractionation lines. Oxygen isotopic disequilibrium between phenocrysts and plagioclase was caused probably by exchange of plagioclase with 16O-poor fluids on the CV parent body. Based on an existing oxygen isotope mass balance model, possible dust enrichment and ice enhancement factors were estimated. Type I chondrules from NWA 8613 possibly formed at moderately high dust enrichment factors (50× to 150× CI dust relative to Solar abundances); estimates for water ice in the chondrule precursors range from 0.2 to 0.6× the nominal amount of ice in dust of CI composition. Findings agree with results from an earlier study on oxygen isotopes in chondrules of the Kaba CV chondrite, providing further evidence for a relatively dry and only moderately high dust-enriched disk in the CV chondrule-forming region.

Oxygen isotope systematics of chondrules in the Murchison CM2 chondrite and implications for the CO-CM relationship

High-precision oxygen three-isotope measurements of olivine and pyroxene were performed on 29 chondrules in the Murchison CM2 chondrite by secondary ion mass spectrometry (SIMS). The oxygen isotope ratios of analyzed chondrules all plot very close to the primitive chondrule minerals (PCM) line. In each of 24 chondrules, the olivine and/or pyroxene grains analyzed show indistinguishable oxygen isotope ratios. Exceptions are minor occurrences of isotopically distinguished relict olivine grains, which were found in nine chondrules. The isotope homogeneity of these phenocrysts is consistent with a co-magmatic crystallization of olivine and pyroxene from the final chondrule melts and a significant oxygen isotope exchange between the ambient gas and the melts. Homogeneous type I chondrules with Mg#’s of 98.9–99.5 have host chondrule Δ17O values ranging from −6.0‰ to −4.1‰, with one exception (Δ17O: −1.2‰; Mg#: 99.6). Homogeneous chondrules with Mg#’s <96, including four type II chondrules (Mg# ∼65–70), have Δ17O values of around −2.5‰. Five type I chondrules (Mg# ≥99) have internally heterogeneous oxygen isotope ratios with Δ17O values ranging from −6.5‰ to −4.0‰, similar to those of host chondrule values. These heterogeneous chondrules have granular or porphyritic textures, convoluted outlines, and contain numerous metal grains dispersed within fine-grained silicates. This is consistent with a low degree of melting of the chondrule precursors, possibly because of a low temperature of the melting event and/or a shorter duration of melting. The Δ17O values of relict olivine grains in nine chondrules range from −17.9‰ to −3.4‰, while most of them overlap the range of the host chondrule values.Similar to those reported from multiple carbonaceous chondrites (Acfer 094, Y-82094, CO, and CV), the Δ17O ∼ −5‰ and high Mg# (≥99) chondrules, which might derive from a reduced reservoir with limited dust enrichments (∼50 × Solar System), dominate the population of chondrules in Murchison. Other chondrules in Murchison formed in more oxidizing environment (Mg# < 96) with higher Δ17O values of −2.5‰, in agreement with the low Mg# chondrules in Acfer 094 and CO chondrites and some chondrules in CV and CR chondrites. They might form in environments containing the same anhydrous precursors as for the Δ17O ∼ −5‰ and Mg# ∼99 chondrules, but enriched in 16O-poor H2O ice (∼0.3–0.4× the CI dust; Δ17O > 0‰) and at dust enrichments of ∼300–2000×.Regarding the Mg# and oxygen isotope ratios, the chondrule populations sampled by CM and CO chondrites are similar and indistinguishable. The similarity of these 16O-rich components in CO and CM chondrites is also supported by the common Fe/Mn ratio of olivine in type II chondrules. Although they accreted similar high-temperature silicates, CO chondrites are anhydrous compared to CM chondrites, suggesting they derived from different parent bodies formed inside and outside the snow line, respectively. If chondrules in CO and CM chondrites formed at the same disk locations but the CM parent body accreted later than the CO parent body, the snow line might have crossed the common chondrule-forming region towards the Sun between the time of the CO and CM parent bodies accretion.

Distribution of 26Al in the CR chondrite chondrule-forming region of the protoplanetary disk

We report on the mineralogy, petrography, and in situ measured oxygen- and magnesium-isotope compositions of eight porphyritic chondrules (seven FeO-poor and one FeO-rich) from the Renazzo-like carbonaceous (CR) chondrites Graves Nunataks 95229, Grosvenor Mountains 03116, Pecora Escarpment 91082, and Queen Alexandra Range 99177, which experienced minor aqueous alteration and very mild thermal metamorphism. We find no evidence that these processes modified the oxygen- or Al–Mg isotope systematics of chondrules in these meteorites. Olivine, low-Ca pyroxene, and plagioclase within an individual chondrule have similar O-isotope compositions, suggesting crystallization from isotopically uniform melts. The only exceptions are relict grains in two of the chondrules; these grains are 16O-enriched relative to phenocrysts of the host chondrules. Only the FeO-rich chondrule shows a resolvable excesses of 26Mg, corresponding to an inferred initial 26Al/27Al ratio [(26Al/27Al)0] of (2.5±1.6)×10−6 (±2SE). Combining these results with the previously reported Al–Mg isotope systematics of CR chondrules (Nagashima et al., 2014, Geochem. J. 48, 561), 7 of 22 chondrules (32%) measured show resolvable excesses of 26Mg; the presence of excess 26Mg does not correlate with the FeO content of chondrule silicates. In contrast, virtually all chondrules in weakly metamorphosed (petrologic type 3.0–3.1) unequilibrated ordinary chondrites (UOCs), Ornans-like carbonaceous (CO) chondrites, and the ungrouped carbonaceous chondrite Acfer 094 show resolvable excesses of 26Mg. The inferred (26Al/27Al)0 in CR chondrules with resolvable excesses of 26Mg range from (1.0±0.4)×10−6 to (6.3±0.9)×10−6, which is typically lower than (26Al/27Al)0 in the majority of chondrules from UOCs, COs, and Acfer 094. Based on the inferred (26Al/27Al)0, three populations of CR chondrules are recognized; the population characterized by low (26Al/27Al)0 (<3×10−6) is dominant. There are no noticeable trends with major and minor element or O-isotope compositions between these populations. The weighted mean (26Al/27Al)0 of 22 CR chondrules measured is (1.8±0.3)×10−6. An apparent agreement between the 26Al–26Mg ages (using weighted mean value) and the revised (using 238U/235U ratio for bulk CR chondrites of 137.7789±0.0085) 207Pb–206Pb age of a set of chondrules from CR chondrites (Amelin et al., 2002, Science297, 1678) is consistent with the initial 26Al/27Al ratio in the CR chondrite chondrule-forming region at the canonical level (∼5.2×10−5), allowing the use of 26Al–26Mg systematics as a chronometer for CR chondrules. To prove chronological significance of 26Al for CR chondrules, measurements of Al–Mg and U–Pb isotope systematics on individual chondrules are required. The presence of several generations among CR chondrules indicates some chondrules that accreted into the CR chondrite parent asteroid avoided melting by later chondrule-forming events, suggesting chondrule-forming processes may have occurred on relatively limited spatial scales. Accretion of the CR chondrite parent body occurred at >4.0-0.3+0.5Ma after the formation of CAIs with the canonical 26Al/27Al ratio, although rapid accretion after formation of the major population of CR chondrules is not required by our data.

A systematic for oxygen isotopic variation in meteoritic chondrules

Primitive meteorites are characteristically formed from an aggregation of sub-millimeter silicate spherules called chondrules. Chondrules are known to present large three-isotope oxygen variations, much larger than shown by any planetary body. We show here that the systematic of these oxygen isotopic variations results from open-system gas–melt exchanges during the formation of chondrules, a conclusion that has not been fully assessed up to now. We have considered Mg-rich porphyritic chondrules and have modeled the oxygen isotopic effects that would result from high-temperature interactions in the disk between precursor silicate dust and a gas enriched in SiO during the partial melting and evaporation of this dust. This formation process predicts: (i) a range of oxygen isotopic composition for bulk chondrules in agreement with that observed in Mg-rich porphyritic chondrules, and (ii) variable oxygen isotopic disequilibrium between chondrule pyroxene and olivine, which can be used as a proxy of the dust enrichment in the chondrule-forming region(s). Such enrichments are expected during shock waves that produce transient evaporation of dust concentrated in the mid-plane of the accretion disk or in the impact plumes generated during collisions between planetesimals. According to the present model, gas–melt interactions under high PSiO(gas) left strong imprints on the major petrographic, chemical and isotopic characteristics of Mg-rich porphyritic chondrules.

Terminal particle from Stardust track 130: Probable Al-rich chondrule fragment from comet Wild 2

A 4×6μm terminal particle from Stardust track 130, named Bidi, is composed of a refractory assemblage of Fo97 olivine, Al- and Ti-bearing clinopyroxene and anorthite feldspar (An97). Mineralogically, Bidi resembles a number of components found in primitive chondritic meteorites including Al-rich chondrules, plagioclase-bearing type I ferromagnesian chondrules and amoeboid olivine aggregates (AOAs). Measured widths of augite/pigeonite lamellae in the clinopyroxene indicate fast cooling rates suggesting that Bidi is more likely to be a chondrule fragment than an AOA. Bulk element concentrations, including an Al2O3 content of 10.2wt%, further suggests that Bidi is more akin to Al-rich rather than ferromagnesian chondrules. This is supported by high anorthite content of the plagioclase feldspar, overall bulk composition and petrogenetic analysis using a cosmochemical Al2O3–Ca2SiO4–Mg2SiO4 phase diagram. Measured minor element abundances of individual minerals in Bidi generally support an Al-rich chondrule origin but are not definitive between any of the object types. Oxygen isotope ratios obtained from olivine (+minor high-Ca pyroxene)fall between the TF and CCAM lines and overlap similar minerals from chondrules in primitive chondrites but are generally distinct from pristine AOA minerals. Oxygen isotope ratios are similar to some minerals from both Al-rich and type I ferromagnesian chondrules in unequilibrated carbonaceous, enstatite and ordinary chondrites. Although no single piece of evidence uniquely identifies Bidi as a particular object type, the preponderance of data, including mineral assemblage, bulk composition, mineral chemistry, inferred cooling rates and oxygen isotope ratios, suggest that Bidi is more closely matched to Al-rich chondrules than AOAs or plagioclase-bearing type I ferromagnesian chondrules and likely originated in a chondrule-forming region in the inner solar system.

Variations in the O-isotope composition of gas during the formation of chondrules from the CR chondrites

To better understand the environment of chondrule formation and constrain the O-isotope composition of the ambient gas in the Renazzo-like carbonaceous (CR) chondrite chondrule-forming region, we studied the mineralogy, petrology, and in situ O-isotope compositions of olivine in 11 barred olivine (BO) chondrules and pyroxene and silica in three type I porphyritic chondrules from the CR chondrites Gao-Guenie (b), Graves Nunataks (GRA) 95229, Pecora Escarpment (PCA) 91082, and Shişr 033. BO chondrules experienced a higher degree of melting than porphyritic chondrules, and therefore, it has been hypothesized that they more accurately recorded the O-isotope composition of the gas in chondrule-forming regions. We studied the O-isotope composition of silica as it has been hypothesized to have formed via direct condensation from the gas.BO chondrules constitute ∼4% of the total CR chondrule population by volume. On a three-isotope oxygen diagram (δ17O vs. δ18O), olivine phenocrysts in type I and type II BO chondrules plot along ∼slope-1 line; with the exception of a type II BO chondrule that plots along ∼slope-0.5 line. Olivine phenocrysts in type I and type II BO chondrules have similar but more restricted ranges of Δ17O values (∼−3.8‰ to ∼−1.3‰ and ∼−0.8‰ to ∼+1.4‰, respectively) than those in type I and type II porphyritic chondrules (∼−4.6‰ to ∼−0.3‰ and ∼−1.8‰ to ∼+0.9‰, respectively). The observation that olivine grains in type I BO chondrules have similar chemical and O-isotope compositions to those of olivine in their porphyritic counterparts argues against the hypothesis that olivine grains in type I porphyritic chondrules are xenocrysts and represent relict fragments of early formed planetesimals.The compositional and O-isotope data suggest that BO chondrules experienced more extensive, but incomplete exchange with the ambient gas than porphyritic chondrules. We suggest that CR chondrules formed from relatively 16O-enriched solids in the presence of relatively 16O-depleted gaseous H2O. The O-isotope compositions of chondrule olivine likely result from differences in the O-isotope composition of both the chondrule precursors and the ambient gas during chondrule formation. The inferred O-isotope composition of this gas (Δ17O ranges from ∼−3‰ to +3‰) is inconsistent with a high abundance of water from the outer Solar System, which has been predicted to be isotopically heavy.

Petrology and oxygen isotope compositions of chondrules in E3 chondrites

Chondrules in E3 chondrites differ from those in other chondrite groups. Many contain near-pure endmember enstatite (Fs<1). Some contain Si-bearing FeNi metal, Cr-bearing troilite, and, in some cases Mg, Mn- and Ca-sulfides. Olivine and more FeO-rich pyroxene grains are present but much less common than in ordinary or carbonaceous chondrite chondrules. In some cases, the FeO-rich grains contain dusty inclusions of metal. The oxygen three-isotope ratios (δ18O, δ17O) of olivine and pyroxene in chondrules from E3 chondrites, which are measured using a multi-collection SIMS, show a wide range of values. Most enstatite data plots on the terrestrial fractionation (TF) line near whole rock values and some plot near the ordinary chondrite region on the 3-isotope diagram. Pyroxene with higher FeO contents (∼2–10wt.% FeO) generally plots on the TF line similar to enstatite, suggesting it formed locally in the EC (enstatite chondrite) region and that oxidation/reduction conditions varied within the E3 chondrite chondrule-forming region. Olivine shows a wide range of correlated δ18O and δ17O values and data from two olivine-bearing chondrules form a slope ∼1 mixing line, which is approximately parallel to but distinct from the CCAM (carbonaceous chondrite anhydrous mixing) line. We refer to this as the ECM (enstatite chondrite mixing) line but it also may coincide with a line defined by chondrules from Acfer 094 referred to as the PCM (Primitive Chondrite Mineral) line (Ushikubo et al., 2011). The range of O isotope compositions and mixing behavior in E3 chondrules is similar to that in O and C chondrite groups, indicating similar chondrule-forming processes, solid–gas mixing and possibly similar 16O-rich precursors solids. However, E3 chondrules formed in a distinct oxygen reservoir.Internal oxygen isotope heterogeneity was found among minerals from some of the chondrules in E3 chondrites suggesting incomplete melting of the chondrules, survival of minerals from previous generations of chondrules, and chondrule recycling. Olivine, possibly a relict grain, in one chondrule has an R chondrite-like oxygen isotope composition and may indicate limited mixing of materials from other reservoirs. Calcium–aluminum-rich inclusions (CAIs) in E3 chondrites have petrologic characteristics and oxygen isotope ratios similar to those in other chondrite groups. However, chondrules from E3 chondrites differ markedly from those in other chondrite groups. From this we conclude that chondrule formation was a local event but CAIs may have all formed in one distinct place and time and were later redistributed to the various chondrule-forming and parent body accretion regions. This also implies that transport mechanisms were less active at the time of and following chondrule formation.

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.

Chemical and oxygen isotopic compositions of accretionary rim and matrix olivine in CV chondrites: Constraints on the evolution of nebular dust

A correlation of petrography, mineral chemistry and in situ oxygen isotopic compositions of fine-grained olivine from the matrix and of fine- and coarse-grained olivine from accretionary rims around Ca–Al-rich inclusions (CAIs) and chondrules in CV chondrites is used here to constrain the processes that occurred in the solar nebula and on the CV parent asteroid. The accretionary rims around Leoville, Vigarano, and Allende CAIs exhibit a layered structure: the inner layer consists of coarse-grained, forsteritic and 16O-rich olivine (Fa 1–40 and Δ 17O = −24‰ to −5‰; the higher values are always found in the outer part of the layer and only in the most porous meteorites), whereas the middle and the outer layers contain finer-grained olivines that are more fayalitic and 16O-depleted (Fa 15–50 and Δ 17O = −18‰ to +1‰). The CV matrices and accretionary rims around chondrules have olivine grains of textures, chemical and isotopic compositions similar to those in the outer layers of accretionary rims around CAIs. There is a correlation between local sample porosity and olivine chemical and isotopic compositions: the more compact regions (the inner accretionary rim layer) have the most MgO- and 16O-rich compositions, whereas the more porous regions (outer rim layers around CAIs, accretionary rims around chondrules, and matrices) have the most MgO- and 16O-poor compositions. In addition, there is a negative correlation of olivine grain size with fayalite contents and Δ 17O values. However, not all fine-grained olivines are FeO-rich and 16O-poor; some small (<1 μm in Leoville and 5–10 μm in Vigarano and Allende) ferrous (Fa >20) olivine grains in the outer layers of the CAI accretionary rims and in the matrix show significant enrichments in 16O (Δ 17O = −20‰ to −10‰). We infer that the inner layer of the accretionary rims around CAIs and, at least, some olivine grains in the finer portions of accretionary rims and CV matrices formed in an 16O-rich gaseous reservoir, probably in the CAI-forming region. Grains in the outer layers of the CAI accretionary rims and in the rims around chondrules as well as matrix may have also originated as 16O-rich olivine. However, these olivines must have exchanged O isotopes to variable extents in the presence of an 16O-poor reservoir, possibly the nebular gas in the chondrule-forming region(s) and/or fluids in the parent body. The observed trend in isotopic compositions may arise from mixtures of 16O-rich forsterites with grain overgrowths or newly formed grains of 16O-poor fayalitic olivines formed during parent body metamorphism. However, the observed correlations of chemical and isotopic compositions of olivine with grain size and local porosity of the host meteorite suggest that olivine accreted as a single population of 16O-rich forsterite and subsequently exchanged Fe–Mg and O isotopes in situ in the presence of aqueous solutions (i.e., fluid-assisted thermal metamorphism).

Remelting of refractory inclusions in the chondrule‐forming regions: Evidence from chondrule‐bearing type C calcium‐aluminum‐rich inclusions from Allende

Abstract— We describe the mineralogy, petrology, oxygen, and magnesium isotope compositions of three coarse‐grained, igneous, anorthite‐rich (type C) Ca‐Al‐rich inclusions (CAIs) (ABC, TS26, and 93) that are associated with ferromagnesian chondrule‐like silicate materials from the CV carbonaceous chondrite Allende. The CAIs consist of lath‐shaped anorthite (An99), Cr‐bearing Al‐Ti‐diopside (Al and Ti contents are highly variable), spinel, and highly åkermanitic and Na‐rich melilite (Åk63–74, 0.4–0.6 wt% Na2O). TS26 and 93 lack Wark‐Lovering rim layers; ABC is a CAI fragment missing the outermost part. The peripheral portions of TS26 and ABC are enriched in SiO2 and depleted in TiO2 and Al2O3 compared to their cores and contain relict ferromagnesian chondrule fragments composed of forsteritic olivine (Fa6–8) and low‐Ca pyroxene/pigeonite (Fs1Wo1–9). The relict grains are corroded by Al‐Ti‐diopside of the host CAIs and surrounded by haloes of augite (Fs0.5Wo30–42). The outer portion of CAI 93 enriched in spinel is overgrown by coarse‐grained pigeonite (Fs0.5–2Wo5–17), augite (Fs0.5Wo38–42), and anorthitic plagioclase (An84). Relict olivine and low‐Ca pyroxene/pigeonite in ABC and TS26, and the pigeonite‐augite rim around 93 are 16O‐poor (Δ17O ∼ −1‰ to −8‰). Spinel and Al‐Ti‐diopside in cores of CAIs ABC, TS26, and 93 are 16O‐enriched (Δ17O down to −20‰), whereas Al‐Ti‐diopside in the outer zones, as well as melilite and anorthite, are 16O‐depleted to various degrees (Δ17O = −11‰ to 2‰). In contrast to typical Allende CAIs that have the canonical initial 26Al/27Al ratio of ∼5 × 10−5 ABC, 93, and TS26 are 26Al‐poor with (26Al/27Al)0 ratios of (4.7 ± 1.4) × 10−6 (1.5 ± 1.8) × 10−6 &lt;1.2 × 10−6 respectively. We conclude that ABC, TS26, and 93 experienced remelting with addition of ferromagnesian chondrule silicates and incomplete oxygen isotopic exchange in an 16O‐poor gaseous reservoir, probably in the chondrule‐forming region. This melting episode could have reset the 26Al‐26Mg systematics of the host CAIs, suggesting it occurred ∼2 Myr after formation of most CAIs. These observations and the common presence of relict CAIs inside chondrules suggest that CAIs predated formation of chondrules.

Oxygen isotopic compositions of chondrules: Implications for evolution of oxygen isotopic reservoirs in the inner solar nebula

We review the oxygen isotopic compositions of minerals in chondrules and compound objects composed of a chondrule and a refractory inclusion, and bulk oxygen isotopic compositions of chondrules in unequilibrated ordinary, carbonaceous, enstatite, and Kakangari-like chondrites, focusing on data acquired using secondary ion mass-spectrometry and laser fluorination coupled with mass-spectrometry over the last decade. Most ferromagnesian chondrules from primitive (unmetamorphosed) chondrites are isotopically uniform (within 3–4‰ in Δ 17O) and depleted in 16O (Δ 17O>−7‰) relative to amoeboid olivine aggregates (AOAs) and most calcium–aluminum-rich inclusions (CAIs) (Δ 17O<−20‰), suggesting that these classes of objects formed in isotopically distinct gaseous reservoirs, 16O-poor and 16O-rich, respectively. Chondrules uniformly enriched in 16O (Δ 17O<−15‰) are exceptionally rare and have been reported only in CH chondrites. Oxygen isotopic heterogeneity in chondrules is mainly due to the presence of relict grains. These appear to consist of chondrules of earlier generations and rare refractory inclusions; with rare exceptions, the relict grains are 16O-enriched relative to chondrule phenocrysts and mesostasis. Within a chondrite group, the magnesium-rich (Type I) chondrules tend to be 16O-enriched relative to the ferrous (Type II) chondrules. Aluminum-rich chondrules in ordinary, enstatite, CR, and CV chondrites are generally 16O-enriched relative to ferromagnesian chondrules. No systematic differences in oxygen isotopic compositions have been found among these chondrule types in CB chondrites. Aluminum-rich chondrules in carbonaceous chondrites often contain relict refractory inclusions. Aluminum-rich chondrules with relict CAIs have heterogeneous oxygen isotopic compositions (Δ 17O ranges from −20‰ to 0‰). Aluminum-rich chondrules without relict CAIs are isotopically uniform and have oxygen isotopic compositions similar to, or approaching, those of ferromagnesian chondrules. Phenocrysts and mesostases of the CAI-bearing chondrules show no clear evidence for 16O-enrichment compared to the CAI-free chondrules. Spinel, hibonite, and forsterite of the relict refractory inclusions largely retained their original oxygen isotopic compositions. In contrast, plagioclase and melilite of the relict CAIs experienced melting and 16O-depletion to various degrees, probably due to isotopic exchange with an 16O-poor nebular gas. Several igneous CAIs experienced isotopic exchange with an 16O-poor nebular gas during late-stage remelting in the chondrule-forming region. On a three-isotope diagram, bulk oxygen isotopic compositions of most chondrules in ordinary, enstatite, and carbonaceous chondrites plot above, along, and below the terrestrial fractionation line, respectively. Bulk oxygen isotopic compositions of chondrules in altered and/or metamorphosed chondrites show evidence for mass-dependent fractionation, reflecting either interaction with a gaseous/fluid reservoir on parent asteroids or open-system thermal metamorphism. Bulk oxygen isotopic compositions of chondrules and oxygen isotopic compositions of individual minerals in chondrules and refractory inclusions from primitive chondrites plot along a common line of slope of ∼1, suggesting that only two major reservoirs (gas and solids) are needed to explain the observed variations. However, there is no requirement that each had a permanently fixed isotopic composition. The absolute ( 207Pb– 206Pb) and relative ( 27Al– 26Mg) chronologies of CAIs and chondrules and the differences in oxygen isotopic compositions of most chondrules ( 16O-poor) and most refractory inclusions ( 16O-rich) can be interpreted in terms of isotopic self-shielding during UV photolysis of CO in the initially 16O-rich (Δ 17O∼−25‰) parent molecular cloud or protoplanetary disk. According to these models, the UV photolysis preferentially dissociates C 17O and C 18O in the parent molecular cloud and in the peripheral zones of the protoplanetary disk. If this process occurs in the stability field of water ice, the released atomic 17O and 18O are incorporated into water ice, while the residual CO gas becomes enriched in 16O. During the earliest stages of evolution of the protoplanetary disk, the inner solar nebula had a solar H 2O/CO ratio and was 16O-rich. During this time, AOAs and the 16O-rich CAIs and chondrules formed. Subsequently, the inner solar nebula became H 2O- and 16O-depleted, because ice-rich dust particles, which were depleted in 16O, agglomerated outside the snowline (∼5 AU), drifted rapidly towards the Sun and evaporated. During this time, which may have lasted for ∼3 Myr, most chondrules and the 16O-depleted igneous CAIs formed. We infer that most chondrules formed from isotopically heterogeneous, but 16O-depleted precursors, and experienced isotopic exchange with an 16O-poor nebular gas during melting. Although the relative roles of the chondrule precursor materials and gas–melt isotopic exchange in establishing oxygen isotopic compositions of chondrules have not been quantified yet, mineralogical, chemical, and isotopic evidence indicate that Type I chondrules may have formed in chemical and isotopic equilibrium with nebular gas of variable isotopic composition. Whether these variations were spatial or temporal are not known yet.

Origin of low-Ca pyroxene in amoeboid olivine aggregates: Evidence from oxygen isotopic compositions

Amoeboid olivine aggregates (AOAs) in primitive carbonaceous chondrites consist of forsterite (Fa <2), Fe,Ni-metal, spinel, Al-diopside, anorthite, and rare gehlenitic melilite (Åk <15). ∼10% of AOAs contain low-Ca pyroxene (Fs 1–3Wo 1–5) that is in corrosion relationship with forsterite and is found in three major textural occurrences: ( i) thin (<15 μm) discontinuous layers around forsterite grains or along forsterite grain boundaries in AOA peripheries; ( ii) 5–10-μm-thick haloes and subhedral grains around Fe,Ni-metal nodules in AOA peripheries, and ( iii) shells of variable thickness (up to 70 μm), commonly with abundant tiny (3–5 μm) inclusions of Fe,Ni-metal grains, around AOAs. AOAs with the low-Ca pyroxene shells are compact and contain euhedral grains of Al-diopside surrounded by anorthite, suggesting small (10%–20%) degree of melting. AOAs with other textural occurrences of low-Ca pyroxene are rather porous. Forsterite grains in AOAs with low-Ca pyroxene have generally 16O-rich isotopic compositions (Δ 17O < −20‰). Low-Ca pyroxenes of the textural occurrences ( i) and ( ii) are 16O-enriched (Δ 17O < −20‰), whereas those of ( iii) are 16O-depleted (Δ 17O = −6‰ to −4‰). One of the extensively melted (>50%) objects is texturally and mineralogically intermediate between AOAs and Al-rich chondrules. It consists of euhedral forsterite grains, pigeonite, augite, anorthitic mesostasis, abundant anhedral spinel grains, and minor Fe,Ni-metal; it is surrounded by a coarse-grained igneous rim largely composed of low-Ca pyroxene with abundant Fe,Ni-metal-sulfide nodules. The mineralogical observations suggest that only spinel grains in this igneous object were not melted. The spinel is 16O-rich (Δ 17O ∼ −22‰), whereas the neighboring plagioclase mesostasis is 16O-depleted (Δ 17O ∼ −11‰). We conclude that AOAs are aggregates of solar nebular condensates (forsterite, Fe,Ni-metal, and CAIs composed of Al-diopside, anorthite, spinel, and ±melilite) formed in an 16O-rich gaseous reservoir, probably CAI-forming region(s). Solid or incipiently melted forsterite in some AOAs reacted with gaseous SiO in the same nebular region to form low-Ca pyroxene. Some other AOAs appear to have accreted 16O-poor pyroxene-normative dust and experienced varying degrees of melting, most likely in chondrule-forming region(s). The most extensively melted AOAs experienced oxygen isotope exchange with 16O-poor nebular gas and may have been transformed into chondrules. The original 16O-rich signature of the precursor materials of such chondrules is preserved only in incompletely melted grains.

Ca,Al-rich inclusions, amoeboid olivine aggregates, and Al-rich chondrules from the unique carbonaceous chondrite Acfer 094: I. mineralogy and petrology

Based on their mineralogy and petrography, ∼200 refractory inclusions studied in the unique carbonaceous chondrite, Acfer 094, can be divided into corundum-rich (0.5%), hibonite-rich (1.1%), grossite-rich (8.5%), compact and fluffy Type A (spinel-melilite-rich, 50.3%), pyroxene-anorthite-rich (7.4%), and Type C (pyroxene-anorthite-rich with igneous textures, 1.6%) Ca,Al-rich inclusions (CAIs), pyroxene-hibonite spherules (0.5%), and amoeboid olivine aggregates (AOAs, 30.2%). Melilite in some CAIs is replaced by spinel and Al-diopside and/or by anorthite, whereas spinel-pyroxene assemblages in CAIs and AOAs appear to be replaced by anorthite. Forsterite grains in several AOAs are replaced by low-Ca pyroxene. None of the CAIs or AOAs show evidence for Fe-alkali metasomatic or aqueous alteration. The mineralogy, textures, and bulk chemistry of most Acfer 094 refractory inclusions are consistent with their origin by gas-solid condensation and may reflect continuous interaction with SiO and Mg of the cooling nebula gas. It appears that only a few CAIs experienced subsequent melting. The Al-rich chondrules (ARCs; >10 wt% bulk Al 2O 3) consist of forsteritic olivine and low-Ca pyroxene phenocrysts, pigeonite, augite, anorthitic plagioclase, ± spinel, FeNi-metal, and crystalline mesostasis composed of plagioclase, augite and a silica phase. Most ARCs are spherical and mineralogically uniform, but some are irregular in shape and heterogeneous in mineralogy, with distinct ferromagnesian and aluminous domains. The ferromagnesian domains tend to form chondrule mantles, and are dominated by low-Ca pyroxene and forsteritic olivine, anorthitic mesostasis, and Fe,Ni-metal nodules. The aluminous domains are dominated by anorthite, high-Ca pyroxene and spinel, occasionally with inclusions of perovskite; have no or little FeNi-metal; and tend to form cores of the heterogeneous chondrules. The cores are enriched in bulk Ca and Al, and apparently formed from melting of CAI-like precursor material that did not mix completely with adjacent ferromagnesian melt. The inferred presence of CAI-like material among precursors for Al-rich chondrules is in apparent conflict with lack of evidence for melting of CAIs that occur outside chondrules, suggesting that these CAIs were largely absent from chondrule-forming region(s) at the time of chondrule formation. This may imply that there are several populations of CAIs in Acfer 094 and that mixing of “normal” CAIs that occur outside chondrules and chondrules that accreted into the Acfer 094 parent asteroid took place after chondrule formation. Alternatively, there may have been an overlap in the CAI- and chondrule-forming regions, where the least refractory CAIs were mixed with Fe-Mg chondrule precursors. This hypothesis is difficult to reconcile with the lack of evidence of melting of AOAs which represent aggregates of the least refractory CAIs and forsterite grains.

Igneous rims on low-FeO and high-FeO chondrules in ordinary chondrites

Many ordinary chondrite (OC) chondrules are surrounded by rims that show evidence of appreciable melting and can be called igneous; these rims formed after the melting and solidification of the host chondrule. We studied thirteen igneous rims on low-FeO chondrules (Fa or Fs <10 mol%) and nine rims on high-FeO chondrules (Fa or Fs >10 mol%) in ordinary chondrites of petrographic type ≤3.5 by electron-microprobe analysis and scanning-electron microscopy. In both sets, olivine and pyroxene compositions of the rims are similar to or moderately higher in FeO than those in the host. Igneous rims around low-FeO chondrules show evidence of large degree (>80%) melting including: (1) resorbed chondrule surfaces, (2) rounded metal/troilite nodules and feldspathic mesostasis, (3) euhedral and subhedral morphology of olivine and pyroxene and limited ranges in grain sizes, and (4) a rarity of relict grains. Most low-FeO chondrules have olivine as their major phase, but pyroxene is commonly the predominant phase in their rims. Rims on high-FeO chondrules generally have smaller grain sizes and exhibit lower degrees of melting than rims around low-FeO chondrules; Mg-rich relict grains are observed in high-FeO rims. Most low-FeO chondrules have matrix-like rims around the igneous rims, but matrix-like rims are rare around rims on high-FeO chondrules. There is a continuum between igneous rims and enveloping secondaries of compound chondrules; the existence of both sets of objects supports the view that, in the OC region, most mm-size grain assemblages experienced more than one melting event. The precursors melted to produce the igneous rims on low-FeO chondrules differ from those that comprise the rims on high-FeO chondrules. A sizeable fraction of the precursors of low-FeO rims are from the host chondrule. Most high-FeO rims appear to consist largely of melted matrix-like materials. If the rim precursors are mainly independent, the similar Fa and Fs contents in mafic minerals of rims and host chondrules indicate that (1) OC nebular subregions (bounded in space and/or in time) were dominated by well-mixed solids, (2) many chondrules experienced multiple heating events during a short period, and were then withdrawn from the chondrule-forming region, and (3) chondrules formed at different times were mixed before agglomeration to form chondrites. If the host chondrules were the main source of the low-FeO rim materials, the fact that pyroxene is generally more abundant than in the host suggests that SiO 2-normative matrix was also an important ingredient and/or that kinetics favored pyroxene crystallization in the rim. Rims around high-FeO chondrules were heated to lower temperatures than rims around low-FeO chondrules. The rarity of matrix-like rims around high-FeO igneous rims may indicate that the dust-chondrule ratio was lower where they formed.

On the relationship between isolated and chondrule olivine grains in the carbonaceous chondrite ALHA77307

The origin of isolated olivine grains in carbonaceous chondrites has been suggested to be either by direct condensation or by fragmentation of chondrules. In an attempt to resolve this debate, isolated olivine grains in the carbonaceous chondrite ALHA77307 have been studied in detail. Zoning characteristics and minor element compositions of olivines from all isolated and chondrule occurrences are described. The data show that a strong relationship exists between isolated olivines and the chondrule population. Chondrule olivine compositions and zoning properties are consistent with olivine crystallization from the molten chondrules. Because properties of isolated grains are very similar to chondrule olivines, an igneous origin for most, if not all, isolated grains is proposed. Large isolated forsterite grains may be derived from fragmentation of “macroporphyritic” type I chondrules which contain one or very few large olivine grains. Relic forsterite grains in type II chondrules, and complex isolated olivine grains with forsteritic cores and fayalite-rich rims, provide evidence that fragmentation and recycling of chondrules and their components was a common and efficient process in the chondrule-forming region.