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”.     

Evidence for relict grains in chondrules of Qingzhen,an E3 type enstatite chondrite

Petrographic and chemical studies of the Qingzhen chondrite strongly suggest that it is the most highly unequilibrated (type 3) enstatite chondrite recognized so far. Qingzhen contains abundant, well-defined chondrules, some of which were incompletely molten during the chondrule formation process. The relict olivine grains within these chondrules contain dusty inclusions of almost pure metallic Fe, which appear to be the in-situ reduction product of the fayalitic component of the olivine. The reduction process presumably took place at the time of chondrule formation and the chondrule precursor material must have been more oxidized than average enstatite chondrite material. We believe that this oxidized material may have formed at the enstatite chondrite formation location in the solar nebula, provided fluctuations in the degree of oxidation of the nebular gas existed at such locations. Reheating of this material under more reducing conditions would lead to the observed reduction of the olivine. Igneous olivines within chondrules always contain detectable amounts of CaO, while relict olivines are essentially CaO-free. This seems to suggest that the relict olivines did not originate during a previous igneous process of chondrule formation and might represent condensation products from the early solar nebula.     

Granoblastic olivine aggregates in magnesian chondrules: Planetesimal fragments or thermally annealed solar nebula condensates?

Granoblastic olivine aggregates (GOA) have been discovered in some Type I magnesian chondrules within carbonaceous chondrites by Libourel and Krot [Libourel, G., Krot, A.N., 2007. Evidence for the presence of planetesimal material among the precursors of magnesian chondrules of nebular origin. Earth Planet. Sci. Lett. 254, 1–8], who proposed an origin from pre-existing planetesimals. Amoeboid olivine aggregates (AOA), generally considered as aggregates of solar nebula condensates and found within similar carbonaceous chondrites, display similar equilibrium texture, though on a finer scale. For these reasons, we conducted experiments to determine if annealing of olivine required time scales appropriate to planetesimal or nebular heating. Pressed < 43 µm and < 63 µm San Carlos olivine powder (Fo_88.4) was isothermally heated at temperatures ranging from 1350–1550 °C for 1–100 h. The 100 h runs yield olivine aggregates with well-developed granoblastic texture at all temperatures, manifest as a network of randomly-oriented and sutured olivine grains with 120° triple junctions. Individual olivine grains are 4–6 sided and polygonal by 1450 °C and equigranular texture is developed at high temperature (1500–1550 °C). Melting of olivine commences at 1450–1500 °C and aids in ‘ripening’ and suturing (grain coarsening and grain boundary migration). Textural equilibrium is clearly met at 1550 °C. A planetesimal origin cannot be ruled out; however, the experimental evidence reveals that granoblastic texture can be reproduced in an interval not inconsistent with heating times for nebular objects. GOA may have experienced higher degrees of thermal processing than the finer-grained AOA. If the precursors were the same, grain coarsening would have to be accompanied by modification to bulk and isotopic compositions. However, the precursors could have been olivine condensates formed later than AOA. Annealing may have been a widespread process operating in the primordial solar nebula responsible for thermal processing and formation of GOA prior to their incorporation into chondrules.     

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.     

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.     

ALH85085: a unique volatile-poor carbonaceous chondrite with possible implications for nebular fractionation processes

Allan Hills 85085 is a unique chondrite with affinities to the Al Rais-Renazzo clan of carbonaceous chondrites. Its constituents are less than 50 μm in mean size. Chondrules and microchondrules of all textures are present; nonporphyritic chondrules are unusually abundant. The mean compositions of porphyritic, nonporphyritic and barred olivine chondrules resemble those in ordinary chondrites except that they are depleted in volatile elements. Ca-, Al-rich inclusions are abundant and largely free of nebular alteration; they comprise types similar to those in CM and CO chondrites, as well as unique types. Calcium dialuminate occurs in several inclusions. Metal, silicate and sulfide compositions are close to those in CM-CO chondrites and Al Rais and Renazzo. C1-chondrite clasts and metal-rich “reduced” clasts are present, but opaque matrix is absent. Siderophile abundances in ALH85085 are extremely high (e.g., Fe/Si= 1.7 × solar), and volatiles are depleted (e.g., Na/Si= 0.25 × solar, S/Si= 0.03 × solar). Nonvolatile lithophile abundances are similar to those in Al Rais, Renazzo, and CM and CO chondrites.ALH85085 agglomerated when temperatures in the nebula were near 1000 K, in the same region where Renazzo, Al Rais and the CI chondrites formed. Agglomeration of high-temperature material may thus be a mechanism by which the fractionation of refractory lithophiles occurred in the nebula. Chondrule formation must have occurred at high temperatures when clumps of precursors were small. After agglomeration, ALH85085 was annealed and lightly shocked. C1 and other clasts were subsequently incorporated during late-stage brecciation.     

Nitrogen abundances and isotopic compositions in stony meteorites

Nitrogen contents range from a few parts per million in ordinary chondrites and achondrites to several hundred parts per million in enstatite chondrites and carbonaceous chondrites. Four major isotopic groups are recognized: (1) C1 and C2 carbonaceous chondrites have δ¹⁵N of+30to+50%.; (2) enstatite chondrites have δ¹⁵N of?30to?40‰; (3) C3 chondrites have low δ¹⁵N with large internal variations; (4) ordinary chondrites have δ¹⁵N of?10to+20‰. The major variations are primary, representing isotopic abundances established at the time of condensation and accretion. Secondary processes, such as spallation reactions, solar wind implantation and metamorphic loss may cause small but observable isotopic variations in particular cases. The large isotopic difference between enstatite chondrites and carbonaceous chondrites cannot be accounted for by equilibrium condensation from a homogeneous nebular gas, and requires either unusually large kinetic effects, or a temporal or spatial variation of isotopic composition of the nebula. Nitrogen isotopic heterogeneity in the nebula due to nuclear processes has not been firmly established, but may be required to account for the large variations found within the Allende and Leoville meteorites. The unique carbonaceous chondrite, Renazzo, has δ¹⁵N of+170%., which is well beyond the range of all other data, and also requires a special source. It is not yet possible, from the meteoritic data, to establish the mode of accretion of nitrogen onto the primitive Earth.     

Origin of magnetite in oxidized CV chondrites: in situ measurement of oxygen isotope compositions of Allende magnetite and olivine

Magnetite in the oxidized CV chondrite Allende mainly occurs as spherical nodules in porphyritic-olivine (PO) chondrules, where it is associated with Ni-rich metal and/or sulfides. To help constrain the origin of the magnetite, we measured oxygen isotopic compositions of magnetite and coexisting olivine grains in PO chondrules of Allende by an in situ ion microprobe technique. Five magnetite nodules form a relatively tight cluster in oxygen isotopic composition with delta 18O values from -4.8 to -7.1% and delta 17O values from -2.9 to -6.3%. Seven coexisting olivine grains have oxygen isotopic compositions from -0.9 to -6.3% in delta 18O and from -4.6 to -7.9% in delta 17O. The delta 17O values of the magnetite and coexisting olivine do not overlap; they range from -0.4 to -2.6%, and from -4.0 to -5.7%, respectively. Thus, the magnetite is not in isotopic equilibrium with the olivine in PO chondrules, implying that it formed after the chondrule formation. The delta 17O of the magnetite is somewhat more negative than estimates for the ambient solar nebula gas. We infer that the magnetite formed on the parent asteroid by oxidation of metal by H2O which had previously experienced minor O isotope exchange with fine-grained silicates.     

Meteorite paleomagnetism - From magnetic domains to planetary fields and core dynamos

Meteorites represent the earliest records of the evolution of the solar system, providing information on the conditions, processes and chronology for formation of first solids, planetesimals and differentiated bodies. Evidence on the nature of magnetic fields in the early solar system has been derived from chondritic meteorites. Chondrules, which are millimeter sized silicate spherules formed by rapid melting and cooling, have been shown to retain remanent magnetization records dating from the time of chondrule formation and accretion of planetesimals. Studies on different meteorite classes, including ordinary and carbonaceous chondrites, have however provided contrasting results with wide ranges for protoplanetary disk magnetic fields. Developments on instrumentation and techniques for rock magnetic and paleointensity analyses are allowing increased precision. Micromagnetic and an array of geochemical, petrographic and electronic microscopy analyses provide unprecedented resolution, characterizing rock magnetic properties at magnetic domain scales. We review studies on chondrules from the Allende meteorite that reveal relationships among hysteresis parameters and physical properties. Coercivity, remanent and saturation remanence parameters correlate with chondrule size and density; in turn related to internal chondrule structure, mineralogy and morphology. Compound, fragmented and rimmed chondrules show distinct hysteresis properties, related to mineral composition and microstructures. The remanent magnetization record and paleointensity estimates derived from the Allende and other chondrites support remanent acquisition under influence of internal magnetic fields within parent planetesimals. Results support that rapid differentiation following formation of calcium-aluminum inclusions and chondrules gave rise to differentiated planetesimals with iron cores, capable of generating and sustaining dynamo action for million year periods. The Allende chondrite may have derived from a partly differentiated planetesimal which sustained an internal magnetic field.     

Magnesium isotopes constraints on the origin of Mg-rich olivines from the Allende chondrite: Nebular versus planetary?

High precision Mg isotope measurements by multi-collector ion microprobe show that refractory olivines from the Allende chondrite, either olivines isolated in the matrix (2 samples studied) or olivines in type I chondrules (6 samples studied), have variable δ²⁶Mg* enrichments and deficits (calculated in permil as the ²⁶Mg deviation from the instrumental mass fractionation line) relative to the Earth. Most average δ²⁶Mg* (noted δ²⁶Mg*_av) values (between 10 and 20 analyses per chondrule) are negative but the total range is from ?0.029 (± 0.010) ‰ (2 sigma errors) to + 0.011 (± 0.011) ‰ with an exception of one olivine at + 0.043 (± 0.023) ‰. These variations in δ²⁶Mg*_av reflect the formation of the olivines from reservoirs enriched in various amounts of ²⁶Mg by the decay of short-lived ²⁶Al (T_1/2 = 0.73 Ma). Similarly, 30 analyses of olivines from the Eagle Station pallasite show a δ²⁶Mg*_av value of ?0.033 ± 0.008‰, as negative as some olivines from Allende chondrules and the Solar system initial δ²⁶Mg* value of ?0.038 ± 0.004‰ (defined at the time of formation of type B Ca–Al-rich inclusions – CAIs – when ²⁶Al/²⁷Al = 5.23 × 10^?5, Jacobsen et al., 2008).Because olivines are Al-poor and because their Mg isotopic compositions are not reset during the chondrule forming events, their δ²⁶Mg*_av can be used to calculate model crystallization ages relative to various theoretical Mg isotope growth curves. The two end-member scenarios considered are (i) a “nebular” growth in which the Al/Mg ratio remains chondritic and (ii) a “planetary” growth in which a significant increase of the Al/Mg ratio can be due to, for instance, olivine magmatic fractionation. The low δ²⁶Mg*_av value of olivines from the Eagle Station pallasite demonstrate that metal-silicate differentiation occurred as early as ~ 0. 15_- 0. 23^+ 0. 29 Ma after CAIs in either of the growth scenarios. Similarly the variable δ²⁶Mg*_av values of refractory olivines can be understood if they were formed in planetesimals which started to differentiate as early as the Eagle Station parent body. Accretion of these planetesimals must have been coeval to the formation of CAIs and their disruption could explain why their fragments (Mg-rich olivines) were distributed in the chondrule forming regions of the disk.     

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.     

Stable calcium isotopic composition of meteorites and rocky planets

New measurements of mass-dependent calcium isotope effects in meteorites, lunar and terrestrial samples show that Earth, Moon, Mars, and differentiated asteroids (e.g., 4-Vesta and the angrite and aubrite parent bodies) are indistinguishable from primitive ordinary chondritic meteorites at our current analytical resolution (± 0.07‰ SD for the ⁴⁴Ca/⁴⁰Ca ratio). In contrast, enstatite chondritic meteorites are slightly enriched in heavier calcium isotopes (ca. + 0.5‰) and primitive carbonaceous chondritic meteorites are depleted in heavier calcium isotopes (ca. ? 0.5‰). The calcium isotope effects cannot be easily ascribed to evaporation or intraplanetary differentiation processes. The isotopic variations probably survive from the earliest stages of nebular condensation, and indicate that condensation occurred under non-equilibrium (undercooled nebular gas) conditions. Some of this early high-temperature calcium isotope heterogeneity is recorded by refractory inclusions (Niederer and Papanastassiou, 1984) and survived in planetesimals, but virtually none of it survived through terrestrial planet accretion. The new calcium isotope data suggest that ordinary chondrites are representative of the bulk of the refractory materials that formed the terrestrial planets; enstatite and carbonaceous chondrites are not. The enrichment of light calcium isotopes in bulk carbonaceous chondrites implies that their compositions are not fully representative of the solar nebula condensable fraction.     

Petrogenesis of opaque assemblages in the Ningqiang carbonaceous chondrite

Numerous round to oblate opaque assemblages (OAs) are found in chondrules and matrix of the Ningqiang carbonaceous chondrite. They are mainly composed of Ni-rich metal,magnetite,Fe,Ni-sulfides,with minor amounts of phosphate,phosphoran-olivine,pyroxene and trace amounts of nano-sized platinum-group metal alloys. The mineralogy of Ningqiang OAs is very similar to that of OAs previously reported in Ca,Al-rich inclusions of CV chondrites. Being a rare mineral phase in nature,phosphoran-olivine is thought to form by nonequilibrium reactions between P-bearing molten metal and olivine crystals during rapid cooling. Its occurrence in Ningqiang OAs indicates that the precursor of OAs was locally produced during chondrule formation,rather than directly condensed from the solar nebula as previously thought. The petrographic and mineralogical characteristics of Ningqiang OAs reveal that OAs formed by low temperature alterations of pre-existing homogeneous alloys within chondrules on a planetary body.     

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.     

The Niger (I) carbonaceous chondrite and implications for the origin of aggregates and isolated olivine grains in C2 chondrites

The origin of olivine grains in C2 carbonaceous chondrites is a controversial topic: directly condensed material or detrital remnants of preexisting chondrules? This study shows that the Niger C2 meteorite is similar to Murchison but reveals several interesting features in relation to the origin of the olivine. Microprobe analysis of olivine (Si, Fe, Mg, Ca, Mn, Cr), glass and nickel-iron inclusions within the grains, and Fe-S-O phase as well as the relationships between the olivine grains in the aggregates, between the grains and the interstitial phyllosilicate matrix, between the inclusions and their host olivine grains, and the morphology of some aggregates all show that two populations of olivine coexist, probably crystallized from chondrule melts rather than by direct condensation from a solar nebula gas. The characteristics of the nickel-iron inclusions within the olivine suggest a magmatic chondrule-making stage from previously condensed materials.     

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.     

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.     

Zirconium isotope evidence for incomplete admixing of r-process components in the solar nebula

Isotopic anomalies in Mo and Zr have recently been reported for bulk chondrites and iron meteorites and have been interpreted in terms of a primordial nucleosynthetic heterogeneity in the solar nebula. We report precise Zr isotopic measurements of carbonaceous, ordinary and enstatite chondrites, eucrites, mesosiderites and lunar rocks. All bulk rock samples yield isotopic compositions that are identical to the terrestrial standard within the analytical uncertainty. No anomalies in ⁹²Zr are found in any samples including high Nb/Zr eucrites and high and low Nb/Zr calcium-aluminum-rich inclusions (CAIs). These data are consistent with the most recent estimates of <10⁻⁴ for the initial ⁹²Nb/⁹³Nb of the solar system. There exists a trace of isotopic heterogeneity in the form of a small excess of r-process ⁹⁶Zr in some refractory CAIs and some metal-rich phases of Renazzo. A more striking enrichment in ⁹⁶Zr is found in acetic acid leachates of the Allende CV carbonaceous chondrite. These data indicate that the r- and s-process Zr components found in presolar grains were well mixed on a large scale prior to planetary accretion. However, some CAIs formed before mixing was complete, such that they were able to sample a population of r-process-enriched material. The maximum amount of additional r-process component that was added to the otherwise well-mixed Zr in the molecular cloud or disk corresponds to ∼0.01%.     

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.     

Lithium isotope composition of ordinary and carbonaceous chondrites, and differentiated planetary bodies: Bulk solar system and solar reservoirs

In order to better constrain the Li isotope composition of the bulk solar system and Li isotope fractionation during accretion and parent body processes, Li isotope compositions and concentrations were determined on a number of meteorite falls and finds. This is the first comprehensive study that systematically investigates a representative set of samples from carbonaceous chondrites (CI, CM2, CO3, CV3, CK4 and one ungrouped member), enstatite chondrites (EH, EL), ordinary chondrites (H, L, LL), and achondrites (one eucrite, diogenites, one pallasite, and a silicate inclusion from a IAB iron).

Carbonaceous chondrites have an average isotope composition of δ⁷Li = + 3.2‰ ± 1.9 (2σ) which agrees with the average composition of relatively pristine olivines (representative for the bulk composition) from the Earth primitive upper mantle (PUM). This is lighter than the average δ⁷Li of the basaltic differentiates of the Earth, Moon and Mars and the achondrites. It is an important observation, however, that the lighter end of the isotopic range of the differentiates always coincides with the averages of the mantle olivines and the carbonaceous chondrites. From this we conclude that the bulk of the inner solar system consists mostly of material from carbonaceous chondrites and that the variation seen in the differentiates is due to planetary body processes. Ordinary chondrites are significantly lighter than carbonaceous chondrites. No significant differences in δ⁷Li exist between enstatite chondrites (n = 3) and carbonaceous or ordinary chondrites. The difference between carbonaceous and ordinary chondrites and the variability within the chondrites could indicate the existence of distinct Li isotope reservoirs in the early solar nebula.