Petrogenesis of complex veins in the Chantonnay (L6f) chondrite

Two cross-cutting veins in the Chantonnay (L6f) chondrite illustrate different patterns of fractionation of total chondritic shock melts. The earlier vein, which is dark-colored and bears abundant host rock xenoliths, is strongly reduced and sodium-poor relative to the bulk meteorite. It resembles and may be cogenetic with melt pockets in Chantonnay. The later vein, which is lighter-colored and somewhat vesicular, lacks evidence of either Na loss or reduction but shows modest internal differentiation. Its metal and total iron contents (26.5 wt.%) are higher than normal for L-group chondrites.The trend of chemical fractionation recorded in the earlier Chantonnay vein resembles that reported for chondrules in ordinary chondrites, suggesting that chemical variations among chondrules in part reflect variations among their parental shock melts.     

Incipient melting in and shock classification of L-group chondrites

Thirty-three of 52 type L4 to L6 chondrites that we have examined in thin section contain closed bodies of crystal-laden glass or devitrified glass (melt pockets) that testify to in-situ melting. A close correlation between the distribution of melt pockets and shock intensity as inferred from the characteristics of olivine and plagioclase indicates that the pockets reflect shock melting. The appearance of pockets coincides with a sharp decrease of⁴⁰Ar in L-group chondrites, suggesting that shock melting was responsible for loss of argon and raising the possibility that this process redistributed other volatile elements as well. The use of three criteria for shock intensity — olivine and plagioclase characteristics, and the presence or absence of melt pockets — leads to a refined shock classification for equilibrated chondrites that is based entirely on petrographic observations.     

Fine-grained aggregates in L3 chondrites

The textures and chemical compositions of the constituent minerals of the fine-grained aggregates (FGA's) of L3 chondrites were studied by the backscattered electron image technique, electron probe microanalysis, and transmission electron microscopy. Plagioclase and glass in the interstices between fine grains of olivine and pyroxene indicate that the FGA's once partly melted. Compositional zoning and decomposition texture of pyroxenes are similar to those observed in chondrules, indicating a common cooling history of the FGA's and chondrules. Therefore, the mechanism that caused melting of the FGA's is considered to be the same as for chondrules. Bulk compositions of the FGA's are within the range of those of chondrules, so some chondrules probably were produced by complete melting of the same precursor materials as those of the FGA's. The precursor materials must have included fine olivine and other grains that probably are condensates.     

Characteristics in naturally and experimentally shocked chondrites: A clue to P-T conditions of impacted asteroids

The aim of this study is to compare the experimentally shock-induced features with those in naturally shocked chondrites and to test the feasibility of experimentally calibrating naturally induced features in shocked H- and L-chondrites. Samples of the Jilin chondrite (H5) were experimentally shock-loaded at the following peak pressures: 12, 27, 39, 53, 78, 83, 93 and 133 GPa respectively. Chondritic melts were first obtained at P>78 GPa and more than 60% melting was achieved at P~133 GPa. No high-pressure phases were observed in any of the shocked samples, neither in the deformed nor in the molten regions. Textural relations and mineral assemblages of the shocked samples are comparable to those encountered in the heavily shocked H-chondrite Yanzhuang but differ considerably from those found in heavily shocked L6 chondrites. Shock melt veins in L6 chondrites contain high-pressure polymorphs of olivine and pyroxene and high pressure liquidus phases. Scaling from shock experiments on millimeter-sized samples to natural shock features on kilometer-sized asteroids poses considerable problems in quantifying the P-T conditions during natural shock events on asteroids.     

Compositions of droplet chondrules in the Manych (L-3) chondrite and the origin of chondrules

Expanded beam microprobe analyses of 18 drop-formed chondrules and 5 irregular masses of devitrified glass in the Manych chondrite show trends and ranges of chemical variation similar to those reported previously for large microporphyritic chondrules in this meteorite. These variations are inconsistent with differentiation of chondrules by crystal-liquid fractionation or separation of immiscible silicate and Fe-Ni-S liquids at various oxygen fugacities. They appear to reflect non-representative sampling of microporphyritic precursor rocks texturally and mineralogically similar to, but in some cases coarser than, the microporphyritic chondrules in Manych. About half of the droplet chondrules and devitrified glasses also bear evidence of more or less vapor-liquid fractionation.The chemical and petrographic properties of Manych chondrules are best explained by a genetic model which entails: (1) melting of extended masses of chondritic material (≥10 cm across); (2) extraction of immiscible Fe-Ni-S liquids; (3) crystallization of the remaining silicate liquids to form microporphyritic rocks; and (4) fragmentation of these rocks to produce microporphyritic chondrules or, with remelting, droplet chondrules. The initial melting may have been caused by either impact or solar heating, but fragmentation and remelting of the microporphyritic precursor rocks were most likely caused by impact.     

Constraints for chondrule formation from Ca–Al distribution in carbonaceous chondrites

Chondritic meteorites and their components formed in the protoplanetary disk surrounding the nascent sun. We show here that the two volumetrically dominating components of carbonaceous chondrites, chondrules and matrix did not form independently. They must have been derived from a single, common source. We analyzed Ca and Al in chondrules and matrix of the CV type carbonaceous chondrites Allende and Y-86751. The Ca/Al-ratios of chondrules and matrix of both chondrites are complementary, but in case of Allende chondrules have sub-chondritic and matrix super-chondritic Ca/Al-ratios and in case of Y-86751 chondrules have super-chondritic and matrix sub-chondritic Ca/Al-ratios. This rules out the redistribution of Ca between chondrules and matrix during parent body alteration. Tiny spinel grains in the matrix produce the high Al in the matrix of Y-86751. In Allende these spinels were most probably included in chondrules. The most plausible explanation for this Ca- and Al-distribution in the same type of chondrite is that both chondrules and matrix formed from the same chemical reservoir. Tiny differences in nebular conditions during formation of these two meteorites must have led to the observed differences. These are severe constraints for all models of chondrule formation. Any model involving separate formation of chondrules and matrix, such as the X-wind model can be excluded.     

Remarques sur la matrice des chondrites ordinaires d'apres l'observation de quelques chondrites a bronzite peu ou pas choquees

Microscopic investigations have been done on the chondrites Sena and Nadiabondi (H5, not shocked), Ste. Marguerite en Comines (H4, very slightly shocked), Allegan (H5, slightly shocked). Only in such cases can the matrix be easily observed and compared to those of type 3 chondrites. The <100 μm debris found in types 4 and 5 that we have observed are not the result of the metamorphism of type 3 fines.The abundance of tiny debris is in direct relation with the intensity of the shock though this shock was insufficient to provoke either the induration of the stones or a significant loss of rare gases. The bulk of the fines are the result of local disaggregation of the most brittle parts from chondrules and fragments.A low-temperature matrix has not been observed in these meteorites but only in H3 chondrites, as a coating around the chondrules. The accretion modelists should take into account the absence or the scarcity of fine particles in their calculations.     

A constraint on impact theories of chondrule formation

The association between agglutinates and chondrule-like spherules, which characterizes the assemblage of impact-derived melt products in lunar regolith samples and some gas-rich achondrites, is not found in primitive chondrites. This observation suggests that impacts into a parent-body regolith are unlikely to have produced the chondrules. We believe that if chondrules were formed from impact melt, it was probably generated by jetting during particle-to-particle collisions, presumably in the nebula.     

A new kind of primitive chondrite, Allan Hills 85085

Allan Hills 85085 is a chemically and mineralogically unique chondrite whose components have suffered little metamorphism or alteration. This chondrite is unique because it has fewer and smaller chondrules (4 wt. %; mean diameter 16 μm) than any other chondrite, more metallic Fe,Ni (36%) and lithic and mineral silicate fragments (56%), and a lower abundance of troilite (2%) and volatiles. Most chondrules are cryptocrystalline or glassy and are depleted in volatiles, some small chondrules are also very depleted in refractory lithophiles. Matrix lumps (4%) partly resemble CI and CM matrices and may be foreign to the parental asteroid. Despite these differences, the components of ALH 85085 have some features common to most type 2 and the least metamorphosed type 3 chondrites: metallic Fe,Ni grains that contain 0.1–1 wt.% Cr, Si and P; Fe/(Fe + Mg) values of olivines, pyroxenes and chondrules are concentrated in the range 1–6 at.% with a few percent in the range 7–30%; porphyritic chondrules are chondritic in composition (except for their low volatile abundances). Thus the components of ALH 85085 probably have similar origins to those of components in other chondrites, and their properties largely reflect nebular, not asteroidal, processes.The bulk composition of ALH 85085 fits none of the nine groups of chondrites: it is richer in Fe (1.4 × CI levels when normalized to Si) and poorer in Na and S (0.1–0.2 × CI) than other chondrites. Low volatile concentrations are due to a low matrix abundance and loss of volatiles during or prior to chondrule formation, not to volatile loss during metamorphism. Chondrule textures imply extensive heating of chondrule melts above the liquidus, consistent with loss of volatiles from small volumes of melt during chondrule formation. The small size of chondrules is partly due to extensive fragmentation by impacts, which may have occurred on the parent asteroid or in the solar nebula. Collisions between chondrule precursor aggregates in the nebula could also be responsible for the small sizes of chondrules.Assuming that ALH 85085 is a representative sample of an asteroid, its properties lend support to models for the origins of the Earth, eucrite parent body and volatile-poor iron meteorites that invoke chondritic planetesimals depleted in volatiles. The existence of ALH 85085 and Kakangari suggests that the nine chondrite groups may provide a remarkably poor sample of the primitive chondritic material from which the asteroids formed. Certain similarities between ALH 85085 and Bencubbin and Weatherford suggest that the latter two primitive meteorites may actually be chondrites with even higher metal abundances (50–60 wt.%) and very large, partly fragmented chondrules.     

Some clues to the history of the H-group chondrites

SEM, optical and chemical observations have been performed on 12 H3-6 chondrites, 9 of them being also studied by other groups. Morphological features of chondrules and crystals (growth steps) are shown; the significance of the finely crystallised troilite in Menow and Ambapur Nagla is discussed in the light of the discovery that the NiFe blebs associated with it are Ni-rich (50–60% Ni). Sulphur should have been mobilized without shock evidence possibly as a result of solar heating. Pre-chondritic relict material is recognized by anomalous or variable mineral compositions, and in some cases, by the presence of overgrowths on relict cores. After short notes on individual chondrites, a tentative history of H chondrites is proposed. The chondrule-forming episode is considered as a remelting of pre-existing material. The accretion would immediately follow this event for type 6 (around 1000°C), and would occur at progressively lower temperature for types 5 and 4. Type 3 would represent material coming from an extended source region, an hypothesis consistent with the broader range composition of the particles and with their cooling before accretion to much lower temperatures (below 350°C).     

Structural deformation of the Leoville chondrite

Foliations defined by alignment of elongated chondrules have been noted previously in chondrites, but none displays this effect so well as Leoville (CV3). The shapes of Leoville chondrules were produced by deformation in situ, as indicated by inclusions and clasts with similar shapes and preferred orientations to those of chondrules. Similarities in the aspect ratios of apparent strain ellipses measured for chondrules alone (1.9 and 2.0 by several methods) and for the whole meteorite (2.0) indicate either that Leoville deformed homogeneously or that it deformed as a framework of touching chondrules. This amount of strain corresponds to approximately 33% uniaxial shortening, assuming constant volume. Because the strain ellipse was measured in only one orientation, this strain value is a minimum estimate for the meteorite. Lack of correlation between foliation and either shock or thermal effects argues that impact or metamorphism are unlikely to have produced this deformation. Compaction due to overburden from progressive accretion on the chondrite parent body is suggested to have been its cause.Estimates of maximum deviatoric stresses in the interiors of asteroid-sized bodies and constraints on maximum temperatures for CV3 chondrites are consistent with diffusional flow as the deformation mechanism for olivine in these chondrules. Diffusional flow is also suggested by the scarcity of observed lattice dislocations. Deformation of Leoville olivines by this mechanism at geologically reasonable strain rates appears to require higher temperatures than those believed to have been experienced by this meteorite (< 600°C). However, differences in olivine grain size, the presence of water, or a more complex deformation history might explain this discrepancy.     

The composition and origin of large microporphyritic chondrules in the Manych (L-3) chondrite

The majority (26/37) of the largest chondrules (d ≥ 1400 μm) exposed in a thin section of the Manych chondrite are more or less rounded fragments of microporphyry, most of which contain from 50 to 80 vol.% olivine. Modal and phase analyses were used to calculate the approximate bulk compositions of nine such chondrules. Six vary modestly around the mean composition of L-group chondrites less most of their metal and troilite and are thought to have formed by bulk melting of L-group material with loss of an immiscible Fe-Ni-S liquid. Two other chondrules, which are olivine-rich and Na- and Si-poor, formed in the same way but with some loss of volatile constituents to a vapor phase. The ninth chondrule, an olivine-poor microporphyry, may be a non-representative sample of a coarser microporphyritic rock.Comparison of these microporphyritic chondrules with the products of controlled cooling experiments and with chemically similar olivine microporphyry in the St. Mesmin chondrite (LL-breccia) suggests that the microporphyritic chondrules are fragments of magmatic rocks which crystallized from masses of liquid no less than 10 cm across.     

Lithic fragments,glasses and chondrules from Luna 16 fines

Bulk compositions of igneous and microbreccia lithic fragments, glasses, and chondrules from Luna 16 fines as well as compositions of minerals in basaltic lithic fragments were determined with the electron microprobe. Igneous lithic fragments and glasses are divided into two groups, the anorthositic-noritic-troctolitic (hereafter referred to as ANT) and basaltic groups. Chondrules are always of ANT composition and microbreccia lithic fragments are divided into groups 1 and 2. The conclusions reached may be summarized as follows: (1) Luna 16 fines are more similar in composition to Apollo 11 than to Apollo 12 and 14 materials (e.g. Apollo 11 igneous lithic fragments and glasses fall into similar ANT and basaltic groups; abundant norites in Luna 16 and Apollo 11 are not KREEP as in Apollo 12 and 14; Luna 16 basaltic lithic fragments may represent high-K and low-K suites as is the case for Apollo 11; rare colorless to greenish, FeO-rich and TiO₂-poor glasses were found in both Apollo 11 and Luna 16; Luna 16 spinels are similar to Apollo 11 spinels but unlike those from Apollo 12). (2) No difference was noted in the composition of lithic fragments, glasses and chondrules from Luna 16 core tube layers A and D. (3) Microbreccia lithic fragments of group 1 originated locally by mixing of high proportions of basaltic with small proportions of ANT materials. (4) Glasses are the compositional analogs to the lithic fragments and not to the microbreccias; most glasses were produced directly from igneous rocks. (5) Glasses show partial loss of Na and K due to vaporization in the vitrification process. (6) Luna 16 chondrules have ANT but not basaltic composition. It is suggested that either liquid droplets of ANT composition are more apt to nucleate from the supercooled state; or basaltic droplets have largely been formed in small and ANT droplets in large impact events (in the latter case, probability for homogeneous and inhomogeneous nucleation is larger. (7) No evidence for ferric iron and water-bearing minerals was found. (8) Occurrence of a great variety of igneous rocks in Luna 16 samples (anorthosite, noritic anorthosite, anorthositic norite, olivine norite, troctolite, and basalt) confirm our earlier conclusion that large-scale melting or partial melting to considerable depth and extensive igneous differentiation must have occurred on the moon.     

On the origin of isolated olivine grains in type 2 carbonaceous chondrites

The origin of olivine grains isolated in the matrix of C2 carbonaceous chondrites is an important problem. If these grains are condensates from a solar nebular gas, they contain compositional, isotopic and physical features that further elucidate that process. If, however, they are grains released by the breakup of chondrules, then many important condensation features have been lost during the melting that took place to form chondrules.In evaluating these two possibilities, care must be taken to determine which inclusions in C2 meteorites are actual chondrules and which are aggregates of grains that have never undergone melting. The two main types of aggregates, pyroxene-rich and pyroxene-poor, are forty to fifty times more abundant than chondrules. Four scenarios are presented to account for the kinds of aggregates and isolated grains seen in the Murchison C2 meteorite. An analysis of these scenarios is made in light of olivine crystal morphology, comparison of composition of glass inclusions inside olivine grains with interstitial glass in true chondrules and size distributions of olivines, isolated, in aggregates and in chondrules.It is concluded that no scenario that includes a chondrule-making step can account for the observed population of isolated olivine grains. An origin by direct condensation, partial comminution, aggregation and accretion best accounts for the sizes and morphological features observed.     

Textural evidence bearing on the origin of isolated olivine crystals in C2 carbonaceous chondrites

In some cases the mechanical competence of chondrules in carbonaceous chondrites has been reduced by alteration of their mesostasis glass to friable phyllosilicate, providing a mechanism by which euhedral olivines can be separated from chondrules. Morphological features of isolated olivine grains found in carbonaceous chondrites are similar to those of olivine phenocrysts in chondrules. These observations suggest that the isolated olivine grains formed in chondrules, by crystallization from a liquid, rather than by condensation from a vapor.     

New kind of type 3 chondrite with a graphite-magnetite matrix

We have discovered four clasts in three ordinary-chondrite regolith breccias which are a new kind of type 3 chondrite. Like ordinary and carbonaceous type 3 chondrites, they have distinct chondrules, some of which contain glass, highly heterogeneous olivines and pyroxenes, and predominantly monoclinic low-Ca pyroxenes. But instead of the usual fine-grained, Fe-rich silicate matrix, the clasts have a matrix composed largely of aggregates of micron- and submicron-sized graphite and magnetite. The bulk compositions of the clasts as well as the types of chondrules (largely porphyritic) are typical of type 3 ordinary chondrites, although chondrules in the clasts are somewhat smaller (0.1–0.5 mm). A close relationship with ordinary chondrites is also indicated by the presence of similar graphite-magnetite aggregates in seven type 3 ordinary chondrites. This new kind of chondrite is probably the source of the abundant graphite-magnetite inclusions in ordinary-chondrite regolith breccias, and may be more common than indicated by the absence of whole meteorites made of chondrules and graphite-magnetite.     

Chondrite thermal histories: Clues from electron microscopy of orthopyroxene

Optically “striated” orthopyroxenes in two ordinary chondrites, Allegan (H5) and Quenggouk (H4), are compared with shock-affected orthopyroxenes in Saint-Sévérin (LL6) and Ambapur Nagla (H5) by high-voltage transmission electron microscopy. The striated orthopyroxenes have very many, thin, evenly distributed lamellae of clinopyroxene. They are undeformed and also lack evidence of partial inversion from clinopyroxene to orthopyroxene. Striated orthopyroxene does not seem to be a reliable indicator of prograde metamorphism. Instead, it is interpreted as inverted protopyroxene, produced during the cooling of chondrules at slower rates than the rapid quenching of Type 3 chondrules. The conclusions are consistent with retrograde models for the evolution of H-group chondrites, in which the higher Petrologic Types are attributed to retarded cooling due to accretionary processes leading to the growth of the parent body. The thermal histories of ordinary chondrites could be greatly clarified by further experimental work on inversions in bronzitic pyroxenes.     

Oxygen isotopic heterogeneities,their petrological correlations,and implications for melt origins of chondrules in unequilibrated ordinary chondrites

Ten whole chondrules separated from the Dhajala (H3, 4), Hallingeberg (L3), and Semarkona (LL3) chondrites were individually analyzed for bulk element composition by instrumental neutron activation with half of each chondrule subsequently sacrificed for oxygen isotopic analysis and half retained for petrographic and electron microprobe analysis. On a three-isotope plot (δ¹⁷O vs. δ¹⁸O), the chondrules neither cluster near their respective chondrite hosts nor in the vicinities of previously recognized chondrite group averages. Instead, they define a trend resolvable into mixing and fractionation components but dominated by mixing in a manner similar to that previously observed for clasts from the LL3 chondrite ALHA76004. Covariations of chondrule isotopic mixing and fractionation parameters with petrological parameters were sought by two-variable linear least-squares regression analyses. However, the only two isotopic/petrological correlations significant at the 95% confidence level were δ¹⁷O vs. total bulk Fe (r = ?0.68) and mixing parameter,m¹⁸, vs. bulk weight ratio (CaO + Al₂O₃)/MgO (r = +0.67). Other correlations of apparent statistical significance were found by treating the chondrules as separate porphyritic (3 porphyritic olivine-pyroxene, 1 porphyritic olivine, 1 barred olivine) and non-porphyritic (4 radial pyroxene, 1 granular pyroxene/cryptocrystalline) textural subgroups. The reliability of the trends, based on so few samples, is not clear but the results at least indicate that possible existence of distinct isotopic/petrological subgroups of chondrules should be further investigated. Absence of certain isotopic/petrological trends expected as condensation effects argues against direct nebular condensation as the dominant process of chondrule formation. Instead, a model involving melting of heterogeneous solids, followed by various degrees of liquid/gas exchange, is favored. In any case, chondrule oxygen isotopic evolution was dominated by two-component mixing; fractional vaporization was, at most, a second-order effect. In addition to chondrules, parent bodies of unequilibrated ordinary chondrites must have also incorporated a¹⁶O-rich component which might have been fine-grained “matrix”.     

Elemental abundances in chondrules from unequilibrated chondrites: Evidence for chondrule origin by melting of pre-existing materials

Bulk abundances of Na, Mg, Al, Ca, Sc, V, Cr, Mn, Fe, Co, Ni, Zn, La, Sm, Eu, Yb, Lu, Ir, and Au were determined by neutron activation analysis of chondrules separated from unequilibrated H-, L-, and LL-chondrites (Tieschitz, Hallingeberg, Chainpur, Semarkona) and correlated with chondrule petrographic properties. Despite wellknown compositional differences among the whole-rock chondrites, the geometric mean compositions of their respective chondrule suites are nearly indistinguishable from each other for many elements. Relative to the condensible bulk solar system (approximated by the Cl chondrite Orgueil), chondrules are enriched in lithophile and depleted in siderophile elements in a pattern consistent with chondrule formation by melting of pre-existing materials, preceded or attended by silicate/metal fractionation. Relative to nonporphyritic chondrules, porphyritic chondrules are enriched in refractory and siderophile elements, suggesting that these two chondrule groups may have formed from different precursor materials.     

High temperature rims around chondrules in primitive chondrites: evidence for fluctuating conditions in the solar nebula

Rims or rim sequences surrouding chondrules have been identified in carbonaceous and unequilibrated ordinary chondrites. These chondrule rims include three chemical subtypes: Fe,Ca-rich and Fe,Ni-metal-rich rims, which occur predominantly in Kainsaz (CO3), and ferromagnesian rims which occur in Kainsaz (CO3), Allende (CV3), Renazzo (CR2), Chainpur (LL3), Semarkona (LL3), Krymaka (L3), and Tieschitz (H3). The compositions of minerals in these rims are often drastically different from those in the underlying chondrule cores, indicating that the solar nebula was chemically heterogeneous. In many cases the compositions of the rims require an environment that was much more oxidizing than a solar composition gas. Particularly interesting is that some of the Fe,Ca-rich chondrule rims are remarkably similar to some of the rims around refractory inclusions, suggesting that chondrules and refractory inclusions experienced late, coeval processing. The textures of the chondrule rims suggest they formed at high temperatures and that they accreted onto chondrules that had already solidified. The lengthscale of the thermal heterogeneities necessary to make available hot material that could accrete to cold chondrules has been calculated to be less than 10 km, implying there were localized heat sources in the solar nebula.