A liquid-supported condensation of major minerals in the solar nebula: Evidence from glasses in the Kaba (CV3) chondrite

Glasses, in the Kaba CV3 chondrite, occur as mesostasis in chondrules and aggregates and as inclusions in olivines, both confined or open and connected to the mesostasis. The inclusions in olivine and the glassy mesostasis of aggregates seem to have formed contemporaneously. The confined glass inclusions and open inclusions in olivine were formed during olivine growth and the mesostasis glass during olivine aggregation. All glasses have high trace element contents (10-20×CI) with unfractionated CI-normalized abundances of refractory trace elements. In contrast, V, Mn, Li, and Cr are depleted in all glasses with respect to the refractory trace elements, as is Rb in the glass inclusions in olivine but not in the mesostasis glass. This abundance pattern indicates vapor fractionation and a common condensation origin for both glasses. Glasses of confined glass inclusions in olivine have a SiAlCa-rich composition with a chondritic Ca/Al ratio. Glasses of open glass inclusions and mesostasis are poor in Ca and enriched in alkalis. However, Ca contents of olivines indicate crystallization from a Ca-rich melt of a composition similar to that of the glass inclusions. In addition, trace element abundances indicate that these glasses (liquids) probably had an original composition similar to that of the inclusion glass. They apparently lost Ca in exchange for alkalis in a metasomatic exchange reaction, presumably with the vapor. There is now growing evidence that liquids can indeed condense from a solar nebula gas, provided the gas/dust ratio is sufficiently low. In these regions with enhanced oxygen fugacity as compared to that of a nebula of solar composition, liquids (the glass precursor) probably played an important role in growing crystals from the vapor by liquid-phase epitaxy. The glasses appear to be the remnants of this thin liquid layer interface that supported the growth of olivine from the vapor following the Vapor-Liquid-Solid process. This liquid will have a refractory composition and will have trace element contents which are in equilibrium with the vapor, and, therefore, will not change much during the time of olivine growth. The composition of the liquid seems to be unconstrained by the phases it is in contact with. Samples of this liquid will be retained as glass inclusions in olivine. The glassy mesostasis could also be a sample of this liquid that got trapped in inter-crystal spaces. The mesostasis glass subsequently behaved as an open system and its Ca was exchanged—presumably with the vapor—for the alkali elements Na, K, and Rb. In contrast, glass inclusions in olivine were protected by the host, could not react, and thus preserved the original composition of this liquid.     

The origin of non-porphyritic pyroxene chondrules in UOCs: Liquid solar nebula condensates?

A total of 56 non-porphyritic pyroxene and pyroxene/olivine micro-objects from different unequilibrated ordinary chondrites were selected for detailed studies to test the existing formation models. Our studies imply that the non-porphyritic objects represent quickly quenched liquids with each object reflecting a very complex and unique evolutionary history. Bulk major element analyses, obtained with EMPA and ASEM, as well as bulk lithophile trace element analyses, determined by LA-ICP-MS, resulted in unfractionated (solar-like) ratios of CaO/Al₂O₃, Yb/Ce as well as Sc/Yb in many of the studied objects and mostly unfractionated refractory lithophile trace element (RLTE) abundance patterns. These features support an origin by direct condensation from a gas of solar nebula composition. Full equilibrium condensation calculations show that it is theoretically possible that pyroxene-dominated non-porphyritic chondrules with flat REE patterns could have been formed as droplet liquid condensates directly from a nebular gas strongly depleted in olivine. Thus, it is possible to have enstatite as the stable liquidus phase in a 800 × Cl dust-enriched nebular gas at a ptot of 10⁻³ atm, if about 72% of the original Mg is removed (as forsterite?) from the system. Condensation of liquids from vapor (primary liquid condensation) could be considered as a possible formation process of the pyroxene-dominated non-porphyritic objects. This process can produce a large spectrum of chemical compositions, which always have unfractionated RLTE abundances. Late stage and subsolidus metasomatic events appear to have furthered the compositional diversity of chondrules and related objects by addition of moderately volatile and volatile elements to these objects by exchange reactions with the chondritic reservoir (e.g., V, Cr, Mn, FeO as well as K and Na). The strong fractionation displayed by the volatile lithophile elements could be indicative of a variable efficiency of metasomatic processes occurring during and/or after chondrule formation. Histories of individual objects differ in detail from each other and clearly indicate individual formation and subsequent processing.     

The formation conditions of enstatite chondrites: Insights from trace element geochemistry of olivine‐bearing chondrules in Sahara 97096 (EH3)

We report in situ LA‐ICP‐MS trace element analyses of silicate phases in olivine‐bearing chondrules in the Sahara 97096 (EH3) enstatite chondrite. Most olivine and enstatite present rare earth element (REE) patterns comparable to their counterparts in type I chondrules in ordinary chondrites. They thus likely share a similar igneous origin, likely under similar redox conditions. The mesostasis however frequently shows negative Eu and/or Yb (and more rarely Sm) anomalies, evidently out of equilibrium with olivine and enstatite. We suggest that this reflects crystallization of oldhamite during a sulfidation event, already inferred by others, during which the mesostasis was molten, where the complementary positive Eu and Yb anomalies exhibited by oldhamite would have possibly arisen due to a divalent state of these elements. Much of this igneous oldhamite would have been expelled from the chondrules, presumably by inertial acceleration or surface tension effects, and would have contributed to the high abundance of opaque nodules found outside them in EH chondrites. In two chondrules, olivine and enstatite exhibit negatively sloped REE patterns, which may be an extreme manifestation of a general phenomenon (possibly linked to near‐liquidus partitioning) underlying the overabundance of light REE observed in most chondrule silicates relative to equilibrium predictions. The silicate phases in one of these two chondrules show complementary Eu, Yb, and Sm anomalies providing direct evidence for the postulated occurrence of the divalent state for these elements at some stage in the formation reservoir of enstatite chondrites. Our work supports the idea that the peculiarities of enstatite chondrites may not require a condensation sequence at high C/O ratios as has long been believed.     

ELEMENTAL COMPOSITIONS OF MAJOR SILICIC PHASES IN CHONDRULES OF UNEQUILIBRATED CHONDRITIC METEORITES

Elemental compositions of olivine, low-Ca pyroxene and mesostasis in chondrules from type-3 ordinary chondrites (OC), CV3, CO3, CM2 and EH3 chondrites were compiled in a search for mineral compositional differences among chondrules of different chondrite groups. Such differences are demonstrated. A few elements occur in silicic phases in amounts proportional to their bulk chondrule concentrations: e.g., Mn in OC chondrules, Ti in CV chondrules, Cr in EH chondrules. However, OC chondrules have higher bulk Cr than CM-CO chondrules, higher Cr in mesostasis, but lower Cr in olivine and low-Ca pyroxene. The higher oxidation state of OC chondrules implies that Cr is more likely to be trivalent, and thus, less likely to enter the olivine crystal structure and more likely to concentrate in pyroxene and mesostasis. CV and OC chondrules have similar high bulk Fe and mesostasis Fe, but OC chondrules have much more FeO in olivine and low-Ca pyroxene. The remaining Fe in CV chondrules is reduced and occurs as metal blebs in the mesostasis. Relative to OC chondrules, EH chondrules have lower bulk Ca, lower Ca in pyroxene and mesostasis, but higher (by a factor of 2) Ca in olivine. EH chondrules may have been incompletely melted, preserving relict refractory lithophile-rich olivine nuclei. OC chondrules are richer than EH chondrules in FeO; they have a lower melting temperature and may have been more completely melted during chondrule formation.     

Barred olivine chondrules in ordinary chondrites: Constraints on chondrule formation

In general, barred olivine (BO) chondrules formed from completely melted precursors. Among BO chondrules in unequilibrated ordinary chondrites, there are significant positive correlations among chondrule diameter, bar thickness, and rim thickness. In the nebula, smaller BO precursor droplets cooled faster than larger droplets (due to their higher surface area/volume ratios) and grew thinner bars and rims. There is a bimodal distribution in the olivine FeO content in BO chondrules, with a hiatus between 11 and 19 wt% FeO. The ratio of (FeO rich)/(FeO poor) BO chondrules decreases from 12.0 in H to 1.6 in L to 1.3 in LL. This is the opposite of the case for porphyritic chondrules: the mean (FeO rich)/(FeO poor) modal ratio increases from 0.8 in H to 1.8 in L to 2.8 in LL. During H chondrite agglomeration, most precursor dustballs were small with low bulk FeO/(FeO + MgO) ratios and moderately high melting temperatures. The energy available for chondrule melting from flash heating was relatively low, capable of completely melting many ferroan dusty precursors (to form FeO-rich BO chondrules), but incapable of completely melting many magnesian dusty precursors (to form FeO-poor BO chondrules). When L and LL chondrites agglomerated somewhat later, significant proportions of precursor dustballs were relatively large and had moderately high bulk FeO/(FeO + MgO) ratios. The energy available from flash heating was higher, capable of completely melting higher proportions of magnesian dusty precursors to form FeO-poor BO chondrules. These differences may have resulted from an increase in the amplitude of lightning discharges in the nebula caused by enhanced charge separation.     

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

Abstract— We measured the sizes and textural types of 719 intact chondrules and 1322 chondrule fragments in thin sections of Semarkona (LL3.0), Bishunpur (LL3.1), Krymka (LL3.1), Piancaldoli (LL3.4) and Lewis Cliff 88175 (LL3.8). The mean apparent diameter of chondrules in these LL3 chondrites is 0.80 φ units or 570 μm, much smaller than the previous rough estimate of ～900 μm. Chondrule fragments in the five LL3 chondrites have a mean apparent cross‐section of 1.60 φ units or 330 μm. The smallest fragments are isolated olivine and pyroxene grains; these are probably phenocrysts liberated from disrupted porphyritic chondrules. All five LL3 chondrites have fragment/ chondrule number ratios exceeding unity, suggesting that substantial numbers of the chondrules in these rocks were shattered. Most fragmentation probably occurred on the parent asteroid. Porphyritic chondrules (porphyritic olivine + porphyritic pyroxene + porphyritic olivine‐pyroxene) are more readily broken than droplet chondrules (barred olivine + radial pyroxene + cryptocrystalline). The porphyritic fragment/chondrule number ratio (2.0) appreciably exceeds that of droplet‐textured objects (0.9). Intact droplet chondrules have a larger mean size than intact porphyritic chondrules, implying that large porphyritic chondrules are fragmented preferentially. This is consistent with the relatively low percentage of porphyritic chondrules within the set of the largest chondrules (57%) compared to that within the set of the smallest chondrules (81%). Differences in mean size among chondrule textural types may be due mainly to parent‐body chondrule‐fragmentation events and not to chondrule‐formation processes in the solar nebula.     

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

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

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

Abstract— The outer portions of many type I chondrules (Fa and Fs <5 mol%) in CR chondrites (except Renazzo and Al Rais) consist of silica‐rich igneous rims (SIRs). The host chondrules are often layered and have a porphyritic core surrounded by a coarse‐grained igneous rim rich in low‐Ca pyroxene. The SIRs are sulfide‐free and consist of igneously‐zoned low‐Ca and high‐Ca pyroxenes, glassy mesostasis, Fe, Ni‐metal nodules, and a nearly pure SiO₂ phase. The high‐Ca pyroxenes in these rims are enriched in Cr (up to 3.5 wt% Cr₂O₃) and Mn (up to 4.4 wt% MnO) and depleted in Al and Ti relative to those in the host chondrules, and contain detectable Na (up to 0.2 wt% Na₂O). Mesostases show systematic compositional variations: Si, Na, K, and Mn contents increase, whereas Ca, Mg, Al, and Cr contents decrease from chondrule core, through pyroxene‐rich igneous rim (PIR), and to SIR; FeO content remains nearly constant. Glass melt inclusions in olivine phenocrysts in the chondrule cores have high Ca and Al, and low Si, with Na, K, and Mn contents that are below electron microprobe detection limits. Fe, Ni‐metal grains in SIRs are depleted in Ni and Co relative to those in the host chondrules. The presence of sulfide‐free, SIRs around sulfide‐free type I chondrules in CR chondrites may indicate that these chondrules formed at high (>800 K) ambient nebular temperatures and escaped remelting at lower ambient temperatures. We suggest that these rims formed either by gas‐solid condensation of silica‐normative materials onto chondrule surfaces and subsequent incomplete melting, or by direct SiO_(gas) condensation into chondrule melts. In either case, the condensation occurred from a fractionated, nebular gas enriched in Si, Na, K, Mn, and Cr relative to Mg. The fractionation of these lithophile elements could be due to isolation (in the chondrules) of the higher temperature condensates from reaction with the nebular gas or to evaporation‐recondensation of these elements during chondrule formation. These mechanisms and the observed increase in pyroxene/olivine ratio toward the peripheries of most type I chondrules in CR, CV, and ordinary chondrites may explain the origin of olivine‐rich and pyroxene‐rich chondrules in general.     

The Allende multicompound chondrule (ACC)—Chondrule formation in a local super‐dense region of the early solar system

In Allende, a very complex compound chondrule (Allende compound chondrule; ACC) was found consisting of at least 16 subchondrules (14 siblings and 2 independents). Its overall texture can roughly be described as a barred olivine object (BO). The BO texture is similar in all siblings, but does not exist in the two independents, which appear as relatively compact olivine‐rich units. Because of secondary alteration of pristine Allende components and the ACC in particular, only limited predictions can be made concerning the original compositions of the colliding melt droplets. Based on textural and mineralogical characteristics, the siblings must have been formed on a very short time scale in a dense, local environment. This is also supported by oxygen isotope systematics showing similar compositions for all 16 subchondrules. Furthermore, the ACC subchondrules are isotopically distinct from typical Allende chondrules, indicating formation in or reaction with a more ¹⁶O‐poor reservoir. We modeled constraints on the particle density required at the ACC formation location, using textural, mineral‐chemical, and isotopic observations on this multicompound chondrule to define melt droplet collision conditions. In this context, we discuss the possible relationship between the formation of complex chondrules and the formation of macrochondrules and cluster chondrites. While macrochondrules may have formed under similar or related conditions as complex chondrules, cluster chondrites certainly require different formation conditions. Cluster chondrites represent a mixture of viscously deformed, seemingly young chondrules of different chemical and textural types and a population of older chondrules. Concerning the formation of ACC calculations suggest the existence of very local, kilometer‐sized, and super‐dense chondrule‐forming regions with extremely high solid‐to‐gas mass ratios of 1000 or more.     

The Bencubbin chondrite breccia and its relationship to CR chondrites and the ALH85085 chondrite

Abstract— Bencubbin is an unclassified meteorite breccia which consists mainly of host silicate (～40 vol.%) and host metal (～60%) components. Rare (< 1%) ordinary chondrite clasts and a dark xenolith (formerly called a carbonaceous chondrite clast) are also found. A petrologic study of the host silicates shows that they have textures, modes, mineralogy and bulk compositions that are essentially the same as that of barred olivine (BO) chondrules, and they are considered to be BO chondritic material. Bulk compositions of individual host silicate clasts are identical and differ only in their textures which are a continuum from coarsely barred, to finely barred, to feathery microcrystalline; these result from differing cooling rates. The host silicates differ from average BO chondrules only in being angular clasts rather than fluid droplet-shaped objects, and in being larger in size (up to 1 cm) than most chondrules; but large angular to droplet-shaped chondrules occur in many chondrites. Bencubbin host metallic FeNi clasts have a positive Ni-Co trend, which coincides with that of a calculated equilibrium nebular condensation path. This appears to indicate a chondritic, rather than impact, origin for this component as well. The rare ordinary chondrite clast and dark xenolith also contain FeNi metal with compositions similar to that of the host metal. Two scenarios are offered for the origin of the Bencubbin breccia. One is that the Bencubbin components are chondritic and were produced in the solar nebula. Later brecciation, reaggregation and minor melting of the chondritic material resulted in it becoming a monomict chondritic breccia. The alternative scenario is that the Bencubbin components formed as a result of major impact melting on a chondritic parent body; the silicate fragments were formed from an impact-induced lava flow and are analogous to the spinifex-textured rocks characteristic of terrestrial komatiites. Both scenarios have difficulties, but the petrologic, chemical and isotopic data are more consistent with Bencubbin being a brecciated chondrite. Bencubbin has a number of important chemical and isotopic characteristics in common with the major components in the CR (Renazzo-type) chondrites and the unique ALH85085 chondrite, which suggests that their major components may be related. These include: (1) Mafic silicates that are similarly Mg-rich and formed in similar reducing environments. (2) Similarly low volatiles; TiO₂, Al₂O₃ and Cr₂O₃ contents are also similar. (3) Similar metallic FeNi compositions that sharply differ from those in other chondrites. (4) Remarkable enrichments in ¹⁵N. (5) Similar oxygen isotopic compositions that lie on the same mixing line. Thus, the major components of the Bencubbin breccia are highly similar to those of the ALH85085 and CR chondrites and they may have all formed in the same isotopic reservoir, under similar conditions, in the CR region of the solar nebula.     

The Ningqiang Meteorite: Classification and Petrology of an Anomalous CV Chondrite

Abstract— Ningqiang is an anomalous CV chondrite (oxidized subgroup) containing a high abundance of aggregational inclusions (13.7 vol.%) and low abundances of refractory inclusions (1.0^+1.0_–0.5 vol.%) and bulk refractory lithophiles (～0.82 × CV). Ningqiang may have agglomerated after most refractory inclusions at the nebular midplane had already been incorporated into other objects. Coarse-grained rims surround only ～5% of Ningqiang chondrules, compared to ～50% in normal CV chondrites. Aggregational inclusions appear to have formed by incipient melting of fine-grained aggregates at relatively low temperatures in the solar nebula, possibly by the mechanism responsible for chondrule formation. Granoblastic porphyritic chondrules, which contain olivines forming 120° triple junctures and no mesostasis, probably formed in the solar nebula by incomplete melting of precursor materials that were olivine normative and had relatively low concentrations of Si, Ca, Al, Fe and Na.     

Refractory inclusions and aluminum‐rich chondrules in Sayh al Uhaymir 290 CH chondrite: Petrography and mineralogy

Abstract— Here we report the petrography, mineralogy, and bulk compositions of Ca,Al‐rich inclusions (CAIs), amoeboid olivine aggregate (AOA), and Al‐rich chondrules (ARCs) in Sayh al Uhaymir (SaU) 290 CH chondrite. Eighty‐two CAIs (0.1% of the section surface area) were found. They are hibonite‐rich (9%), grossite‐rich (18%), melilite ± spinel‐rich (48%), fassaite ± spinel‐rich (15%), and fassaite‐anorthite‐rich (10%) refractory inclusions. Most CAIs are rounded in shape and small in size (average = 40 μm). They are more refractory than those of other groups of chondrites. CAIs in SaU 290 might have experienced higher peak heating temperatures, which could be due to the formation region closer to the center of protoplanetary disk or have formed earlier than those of other groups of chondrites. In SaU 290, refractory inclusions with a layered texture could have formed by gas‐solid condensation from the solar nebula and those with an igneous texture could have crystallized from melt droplets or experienced subsequent melting of pre‐existing condensates from the solar nebula. One refractory inclusion represents an evaporation product of pre‐existing refractory solid on the basis of its layered texture and melting temperature of constituting minerals. Only one AOA is observed (75 μm across). It consists of olivine, Al‐diopside, anorthite, and minor spinel with a layered texture. CAIs and AOA show no significant low‐temperature aqueous alteration. ARCs in SaU 290 consist of diopside, forsterite, anorthite, Al‐enstatite, spinel, and mesostasis or glass. They can be divided into diopside‐rich, Al‐enstatite‐rich, glass‐rich, and anorthite‐rich chondrules. Bulk compositions of most ARCs are consistent with a mixture origin of CAIs and ferromagnesian chondrules. Anorthite and Al‐enstatite do not coexist in a given ARC, implying a kinetic effect on their formation.     

The oxygen isotopic properties of olivines in the Semarkona ordinary chondrite

Abstract— In order to explore the origin of chondrules and the chondrites, the O isotopic compositions of nine olivine grains in seven chondrules from the primitive Semarkona LL3.0 chondrite have been determined by ion microprobe. The data plot in the same general region of the three-isotope plot as whole-chondrule samples from ordinary chondrites previously measured by other techniques. There are no significant differences between the O isotopic properties of olivine in the various chondrule groups in the present study, but there is a slight indication that the data plot at the ¹⁶O-rich end of the ordinary chondrite field. This might suggest that the mesostasis contains isotopically heavy O. The olivines in the present study have O isotopic compositions unlike the ¹⁶O-rich olivine grains from the Julesburg ordinary chondrite. Even though olivines in group A chondrules have several properties in common with them, the ¹⁶O-rich Julesburg olivines previously reported are not simply olivines from group A chondrules.     

Chondrules: Precursors and interactions with the nebular gas

Abstract– Chondrule compositions suggest either ferroan precursors and evaporation, or magnesian precursors and condensation. Type I chondrule precursors include granoblastic olivine aggregates (planetary or nebular) and fine‐grained (dustball) precursors. In carbonaceous chondrites, type I chondrule precursors were S‐free, while type II chondrules have higher Fe/Mn than in ordinary chondrites. Many type II chondrules contain diverse forsteritic relicts, consistent with polymict dustball precursors. The relationship between finer and coarser grained type I chondrules in ordinary chondrites suggests more evaporation from more highly melted chondrules. Fe metal in type I, and Na and S in type II chondrules indicate high partial pressures in ambient gas, as they are rapidly evaporated at canonical conditions. The occurrence of metal, sulfide, or low‐Ca pyroxene on chondrule rims suggests (re)condensation. In Semarkona type II chondrules, Na‐rich olivine cores, Na‐poor melt inclusions, and Na‐rich mesostases suggest evaporation followed by recondensation. Type II chondrules have correlated FeO and MnO, consistent with condensation onto forsteritic precursors, but with different ratios in carbonaceous chondrites and ordinary chondrites, indicating different redox history. The high partial pressures of lithophile elements require large dense clouds, either clumps in the protoplanetary disk, impact plumes, or bow shocks around protoplanets. In ordinary chondrites, clusters of type I and type II chondrules indicate high number densities and their similar oxygen isotopic compositions suggest recycling together. In carbonaceous chondrites, the much less abundant type II chondrules were probably added late to batches of type I chondrules from different O isotopic reservoirs.     

Multiple melting in a four‐layered barred‐olivine chondrule with compositionally heterogeneous glass from LL3.0 Semarkona

Chondrule K7p from LL3.0 Semarkona consists of four nested barred‐olivine (BO) chondrules. The innermost BO chondrule (chondrule 1) formed by complete melting of an olivine‐rich dustball. After formation, the chondrule was incorporated into another olivine‐rich dustball. A second heating event caused this second dustball to melt; the mesostasis and some of the olivine in chondrule 1 were probably also melted at this time, but the chondrule 1 structure remained largely intact. At this stage, the object was an enveloping compound BO chondrule. This two‐step process of melting and dustball enshrouding repeated two more times. The different proportions of olivine and glass in chondrules 1–4 suggest that the individual precursor dustballs differed in the amounts of chondrule fragments they contained and the mineral proportions in those fragments. The final dustball (which ultimately formed chondrule 4) was somewhat more ferroan; after melting, crystallizing, and quenching, chondrule 4 contained olivine and glass with higher FeO and MnO contents than those of the earlier formed chondrules. Subsequent aqueous alteration on the LL parent body transformed the abundant metal blebs and stringers at the chondrule surface into carbide, iron oxide, and minor Ni‐rich metal. Portions of the mesostasis underwent dissolution, producing holes and adjacent blades of more resistant material. Much of the glass in the chondrule remained isotropic, even after minor hydration and leaching. The sharp, moderately lobate boundary between the extensively altered mesostasis and the isotropic glass represents the reaction front beyond which there was little or no glass dissolution.     

Multi-technique investigation reveals new mineral, chemical, and textural heterogeneity in the Tagish Lake C2 chondrite

The Tagish Lake meteorite, an ungrouped C2 chondrite that is related to CI and CM chondrites, is a heterogeneous accretionary breccia with several distinct lithologies that, in bulk, are thought to represent the first known sample of a primitive carbonaceous D-type asteroid. Textural and chemical zoning of clasts and matrix have been little studied and promise additional insight into early solar system processes in both the solar nebula and on the Tagish Lake parent asteroid. We have examined an intact 2.9 g fragment and two polished thin sections from the spring 2000 (non-pristine) Tagish Lake collection to ascertain the major mineralogy and textures of notable features such as chondrules, amoeboid olivine aggregates (AOAs), inclusions, clasts, matrix, and fusion crust. We designed three stages of analysis for this friable meteorite: an initial, non-destructive in situ reconnaissance by μXRD to document meteorite mineralogy and textures and to identify features of interest, followed by spatially correlated μXRD, SEM-EDX and colour SEM-CL analysis of polished thin sections to fully understand mineralogy and the record of texture development, and finally higher resolution SEM-BSE mapping to document smaller scale relationships.Our analyses reveal several previously unreported or poorly characterized features: (1) distinctive colour cathodoluminescence (CL) zoning in relict CAI spinel, in chondrule and AOA forsterite, and in calcite nodules occurring throughout the Tagish Lake matrix. Forsterite frequently shows CL colour and intensity zonation that does not correspond with major or minor element differences resolvable with EPMA, indicating a trace element and/or structural CL-activation mechanism for the zonation that is likely of secondary origin; (2) an irregular inclusion dominated by magnesioaluminate spinel, dolomite, and phyllosilicates with traces of a Ca, Ti oxide phase (likely perovskite) interpreted to be a relict CAI; (3) variable preservation of mesostasis glass in porphyritic olivine chondrules. We anticipate that our multi-technique methodology, particularly non-destructive μXRD, can be successfully applied to other rare and friable materials such as the pristine Tagish Lake fragments.     

The onset of metamorphism in ordinary and carbonaceous chondrites

Abstract— Ordinary and carbonaceous chondrites of the lowest petrologic types were surveyed by X‐ray mapping techniques. A variety of metamorphic effects were noted and subjected to detailed analysis using electron microprobe, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and cathodoluminescence (CL) methods. The distribution of Cr in FeO‐rich olivine systematically changes as metamorphism increases between type 3.0 and type 3.2. Igneous zoning patterns are replaced by complex ones and Cr‐rich coatings develop on all grains. Cr distributions in olivine are controlled by the exsolution of a Cr‐rich phase, probably chromite. Cr in olivine may have been partly present as tetrahedrally coordinated Cr³⁺. Separation of chromite is nearly complete by petrologic type 3.2. The abundance of chondrules showing an inhomogeneous distribution of alkalis in mesostasis also increases with petrologic type. TEM shows this to be the result of crystallization of albite. Residual glass compositions systematically change during metamorphism, becoming increasingly rich in K. Glass in type I chondrules also gains alkalis during metamorphism. Both types of chondrules were open to an exchange of alkalis with opaque matrix and other chondrules. The matrix in the least metamorphosed chondrites is rich in S and Na. The S is lost from the matrix at the earliest stages of metamorphism due to coalescence of minute grains. Progressive heating also results in the loss of sulfides from chondrule rims and increases sulfide abundances in coarse matrix assemblages as well as inside chondrules. Alkalis initially leave the matrix and enter chondrules during early metamorphism. Feldspar subsequently nucleates in the matrix and Na re‐enters from chondrules. These metamorphic trends can be used to refine classification schemes for chondrites. Cr distributions in olivine are a highly effective tool for assigning petrologic types to the most primitive meteorites and can be used to subdivide types 3.0 and 3.1 into types 3.00 through 3.15. On this basis, the most primitive ordinary chondrite known is Semarkona, although even this meteorite has experienced a small amount of metamorphism. Allan Hills (ALH) A77307 is the least metamorphosed CO chondrite and shares many properties with the ungrouped carbonaceous chondrite Acfer 094. Analytical problems are significant for glasses in type II chondrules, as Na is easily lost during microprobe analysis. As a result, existing schemes for chondrule classification that are based on the alkali content of glasses need to be revised.     

Size-frequency distributions of chondrules in CO3 chondrites

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

Isolated olivines in the Yamato 82042 CM2 chondrite: The tracing of major condensation events in the solar nebula

Abstract— Yamato 82042 is an unusual CM2 chondrite consisting mainly of phyllosilicates, a few olivines and carbonates, very minor sulphides and trace metal. Olivine occurs: (1) as isolated grains dispersed in the phyllosilicate matrix, (2) as constituents of mineral aggregates or accretionary fragments associated with abundant phyllosilicates and minor sulphides, and (3) as objects which resemble barred olivine chondrules also associated with phyllosilicates. Olivine, from all occurrences, ranges in composition from 0.26 to 22.6 weight % FeO, but generally contains less than 1.25 wt.% FeO. Minor element contents, particularly Ca, Al, and Cr, are relatively high and are generally correlated, as reported for olivines in other carbonaceous chondrites. However, we report here uncorrected trends for the same minor elements which occur in distinct areas (volumes) within the same olivines. These compositional trends may be due to condensation of olivine from a vapor of non-solar composition and partial mobilization of Ca during later annealing. If this is the case, the data may be used to trace changes in the Ca/Al ratio of the parent medium during the formation of these olivines, provided that it is possible to distinguish the effects of any post-formation annealing which could have redistributed the minor elements. Some isolated olivines show distinctive minor element zoning which severely limits the possibility of any post-formation redistribution of these elements. Accordingly, these isolated olivines indeed retain evidence of early condensation processes in the solar nebula, though non-classic conditions are implied for their formation.     

Properties of chondrules in EL3 chondrites,comparison with EH3 chondrites,and the implications for the formation of enstatite chondrites

Abstract— The study of chondrules provides information about processes occurring in the early solar system. In order to ascertain to what extent these processes played a role in determining the properties of the enstatite chondrites, the physical and chemical properties of chondrules from three EL3 chondrites and three EH3 chondrites have been examined by optical, cathodoluminescence (CL), and electron microprobe techniques. Properties examined include size, texture, CL, and composition of both individual phases and bulk chondrules. The textures, distribution of textures, and composition of silicates of the EL3 chondrules resemble those of EH3 chondrules. However, the chondrules from the two classes differ in that (1) the size distribution of the EL chondrules is skewed to larger values than EH chondrules, (2) the enstatite in EL chondrules displays varying shades of red CL due to the presence of fine‐grained sulfides and metal in the silicates, and (3) the mesostasis of EH chondrules is enriched in Na relative to that of EL chondrules. The similarities between the chondrules of the two classes suggest similar precursor materials, while the differences suggest that there was not a single reservoir of meteoritic chondrules, but that their origin was fairly local. The differences in the size distribution of chondrules in EH and EL chondrites may be explained by aerodynamic and gravitational sorting during accumulation of the meteoric material, while differences in CL and mesostasis properties may reflect differences in formation conditions and cooling rate following chondrule formation. We argue that our observations are consistent with the formation of enstatite chondrites in a thick dynamic regolith on their parent body.