Petrographic comparison of refractory inclusions from different chemical groups of chondrites

Abstract— Twenty‐four refractory inclusions (40–230 μm, with average of 86 ± 40 μm) were found by X‐ray mapping of 18 ordinary chondrites. All inclusions are heavily altered, consisting of finegrained feldspathoids, spinel, and Ca‐pyroxene with minor ilmenite. The presence of feldspathoids and lack of melilite are due to alteration that took place under oxidizing conditions as indicated by FeO‐ZnO‐rich spinel and ilmenite. The pre‐altered mineral assemblages are dominated by two types: one rich in melilite, referred to as type A‐like, and the other rich in spinel, referred to as spinelpyroxene inclusions. This study and previous data show similar type and size distributions of refractory inclusions in ordinary and enstatite chondrites. A survey of refractory inclusions was also conducted on Allende and Murchison in order to make unbiased comparison with their counterparts in other chondrites. The predominant inclusions are type A and spinel‐pyroxene, with average sizes of 170 ± 130 μm (except for two mm‐sized inclusions) in Allende and 150 ± 100 μm in Murchison. The relatively larger sizes are partially due to common conglomerating of smaller nodules in both chondrites. The survey reveals closely similar type and size distributions of refractory inclusions in various chondrites, consistent with our previous data of other carbonaceous chondrites. The petrographic observations suggest that refractory inclusions in various groups of chondrites had primarily formed under similar processes and conditions, and were transported to different chondrite‐accreting regions. Heterogeneous abundance and distinct alteration assemblages of refractory inclusions from various chondrites could be contributed to transporting processes and secondary reactions under different conditions.     

The grain size distribution of matrix in primitive chondrites

The matrix of primitive chondrites is composed of submicron crystals embedded in amorphous silicates. These grains are thought to be the remains of relatively unprocessed dust from the inner regions of the protoplanetary disk. The matrix of primitive meteorites is often compared to chondritic porous interplanetary dust particles (CP-IDPs) which are believed to be of cometary origin, having accreted in the outermost regions of the solar nebula. Crystalline grains in CP-IDPs show evidence of a size–density relationship between the silicates and sulfides suggesting that these components experienced sorting prior to accretion. Here, we investigate whether such evidence of sorting is also present in the matrix constituents of primitive chondrites. We report findings from our study of grain size distributions of discrete silicate and opaque (sulfide and metal) grains within the matrix of the primitive meteorites Acfer 094 (C2-ung.), ALHA77307 (CO3), MIL 07687 (C3-ung.), and QUE 99177 (CR2). Mean radii of matrix silicate grains range from 103 nm in QUE 99177 to 2018 nm in MIL 07687. The opaque grains show a wider variation, with average radii ranging from 15 nm in QUE 99177 to 219 nm in MIL07687. Our results indicate that, in contrast to CP-IDPs, the size distribution of matrix components of these primitive meteorites cannot be explained by aerodynamic sorting that took place prior to accretion. We conclude that any evidence of sorting is likely to have been lost due to a greater variety and degree of processing experienced on these primitive chondrites than on cometary parent bodies.     

Mid‐infrared spectroscopy of refractory inclusions (CAIs) in CV and CO chondrites

Abstract— We present laboratory mid‐infrared absorption spectra (2.5 urn to 16.0 μm) of powdered calcium‐aluminum‐rich inclusions (CAIs) and matrix separated from the carbonaceous chondrites Allende (CV3.2), Vigarano (CV3.3), and Ornans (C03.3). Two groups of spectra with different features were found for the CAI: in the first group spectra are dominated by spinel, pyroxene, and sodalite ± nepheline, where main features occur at 9.3 μm, 10.3 μm, and 11.3 μm. In the second group, characteristic minerals are spinel and melilite with typical band maxima at 11.0 μm and 12.3 μm, and a broad feature between 14.0 μn and 15.0 μn. The position of the broad spinel feature probably depends on its iron content. Comparison of band positions in spectra from the CAI components to observed circumstellar emission spectra indicates the potential occurrence of CAI‐like material. Pyroxene‐ and spinel‐rich features could occur in spectra of dust around the Herbig Ae star HD104237, the T Tauri star Hen3‐600 and the post‐AGB star R Sge. Melilite‐ and spinel‐rich components possibly appear in the spectrum of HD 104237, Hen3‐600, 04187_1927, R Sge, and the planetary nebula Hb 12. There is also indication for a spinel component in dust from the Herbig Ae/Be star HD 179218. The spectra of the AGB stars R Cas and θ Aps show no features of CAl‐type spinel.     

The most primitive mesosiderite Northwest Africa 1878, subgroup 0

All mesosiderites previously reported were subjected to thermal metamorphism and/or partial melting on the parent body. Therefore, their primordial features have been mostly lost. Here, we report detailed petrological and mineralogical features on a mesosiderite, Northwest Africa (NWA) 1878. This meteorite comprises silicate lithology and aggregates of small spheroidal Fe‐Ni metal grains. Silicate lithology typically shows igneous texture without recrystallization features, and mainly consists of low‐Ca pyroxene and plagioclase. Pyroxenes often show normal zoning. Exsolution lamella of augite is rarely noticed and very thin in width, compared with other mesosiderites. A few magnesian olivine grains are encountered without typical corona texture around them. They are not equilibrated with pyroxene on a large scale. Plagioclase shows a wide compositional range. These results show that NWA 1878 hardly experienced thermal metamorphism, distinguished from mesosiderites of subgroups 1–4. Therefore, we propose that this is classified as subgroup 0 mesosiderite. Nevertheless, NWA 1878 was locally subjected to secondary reactions, such as weak reduction of pyroxene and Fe‐Mg diffusion between olivine and pyroxene, on the parent body.     

Petrography of refractory inclusions in CM2.6 QUE 97990 and the origin of melilite‐free spinel inclusions in CM chondrites

Abstract— Queen Alexandra Range (QUE) 97990 (CM2.6) is among the least‐altered CM chondrites known. It contains 1.8 vol% refractory inclusions; 40 were studied from a single thin section. Inclusion varieties include simple, banded and nodular structures as well as simple and complex distended objects. The inclusions range in mean size from 30 to 530 μm and average 130 ± 90 μm. Many inclusions contain 25 ± 15 vol% phyllosilicate (predominantly Mg‐Fe serpentine); several contain small grains of perovskite. In addition to phyllosilicate, the most abundant inclusions in QUE 97990 consist mainly of spinel‐pyroxene (35%), followed by spinel (20%), spinel‐pyroxene‐olivine (18%), pyroxene (12%), pyroxene‐olivine (8%) and hibonite ± spinel (8%). Four pyroxene phases occur: diopside, Al‐rich diopside (with ≥ 8.0 wt% Al₂O₃), Al‐Ti diopside (i.e., fassaite), and (in two inclusions) enstatite. No inclusions contain melilite. Aqueous alteration of refractory inclusions transforms some phases (particularly melilite) into phyllosilicate; some inclusions broke apart during alteration. Melilite‐free, phyllosilicate‐bearing, spinel inclusions probably formed from pristine, phyllosilicate‐free inclusions containing both melilite and spinel. Sixty‐five percent of the refractory inclusions in QUE 97990 appear to be largely intact; the major exception is the group of spinel inclusions, all of which are fragments. Whereas QUE 97990 contains about 50 largely intact refractory inclusions/cm², estimates from literature data imply that more‐altered CM chondrites have lower modal abundances (and lower number densities) of refractory inclusions: Mighei (CM ? 2.3) contains roughly 0.3–0.6 vol% inclusions (?10 largely intact inclusions/cm²); Cold Bokkeveld (CM2.2) contains ?0.01 vol% inclusions (on the order of 6 largely intact inclusions/cm²).     

Forsterite‐bearing type B refractory inclusions from CV3 chondrites: From aggregates to volatilized melt droplets

Abstract– Detailed petrologic and oxygen isotopic analysis of six forsterite‐bearing Type B calcium‐aluminum‐rich inclusions (FoBs) from CV3 chondrites indicates that they formed by varying degrees of melting of primitive precursor material that resembled amoeboid olivine aggregates. A continuous evolutionary sequence exists between those objects that experienced only slight partial melting or sintering through objects that underwent prolonged melting episodes. In most cases, melting was accompanied by surface evaporative loss of magnesium and silicon. This loss resulted in outer margins that are very different in composition from the cores, so much so that in some cases, the mantles contain mineral assemblages that are petrologically incompatible with those in the cores. The precursor objects for these FoBs had a range of bulk compositions and must therefore have formed under varying conditions if they condensed from a solar composition gas. Five of the six objects show small degrees of mass‐dependent oxygen isotopic fractionation in pyroxene, spinel, and olivine, consistent with the inferred melt evaporation, but there are no consistent differences among the three phases. Forsterite, spinel, and pyroxene are ¹⁶O‐rich with Δ¹⁷O ～ ?24‰ in all FoBs. Melilite and anorthite show a range of Δ¹⁷O from ?17‰ to ?1‰.     

Oxygen and Al‐Mg isotopic compositions of grossite‐bearing refractory inclusions from CO3 chondrites

The distribution of the short‐lived radionuclide ²⁶Al in the early solar system remains a major topic of investigation in planetary science. Thousands of analyses are now available but grossite‐bearing Ca‐, Al‐rich inclusions (CAIs) are underrepresented in the database. Recently found grossite‐bearing inclusions in CO3 chondrites provide an opportunity to address this matter. We determined the oxygen and magnesium isotopic compositions of individual phases of 10 grossite‐bearing CAIs in the Dominion Range (DOM) 08006 (CO3.0) and DOM 08004 (CO3.1) chondrites. All minerals in DOM 08006 CAIs as well as hibonite, spinel, and pyroxene in DOM 08004 are uniformly ¹⁶O‐rich (Δ¹⁷O = ?25 to ?20‰) but grossite and melilite in DOM 08004 CAIs are not; Δ¹⁷O of grossite and melilite range from ~ ?11 to ~0‰ and from ~ ?23 up to ~0‰, respectively. Even within this small suite, in the two chondrites a bimodal distribution of the inferred initial ²⁶Al/²⁷Al ratios (²⁶Al/²⁷Al)₀ is seen, with four having (²⁶Al/²⁷Al)₀ ≤1.1 × 10^?5 and six having (²⁶Al/²⁷Al)₀ ≥3.7 × 10^?5. Five of the ²⁶Al‐rich CAIs have (²⁶Al/²⁷Al)₀ within error of 4.5 × 10^?5; these values can probably be considered indistinguishable from the “canonical” value of 5.2 × 10^?5 given the uncertainty in the relative sensitivity factor for grossite measured by secondary ion mass spectrometry. We infer that the ²⁶Al‐poor CAIs probably formed before the radionuclide was fully mixed into the solar nebula. All minerals in the DOM 08006 CAIs, as well as spinel, hibonite, and Al‐diopside in the DOM 08004 CAIs retained their initial oxygen isotopic compositions, indicating homogeneity of oxygen isotopic compositions in the nebular region where the CO grossite‐bearing CAIs originated. Oxygen isotopic heterogeneity in CAIs from DOM 08004 resulted from exchange between the initially ¹⁶O‐rich (Δ¹⁷O ~?24‰) melilite and grossite and ¹⁶O‐poor (Δ¹⁷O ~0‰) fluid during hydrothermal alteration on the CO chondrite parent body; hibonite, spinel, and Al‐diopside avoided oxygen isotopic exchange during the alteration. Grossite and melilite that underwent oxygen isotopic exchange avoided redistribution of radiogenic ²⁶Mg and preserved undisturbed internal Al‐Mg isochrons. The Δ¹⁷O of the fluid can be inferred from O‐isotopic compositions of aqueously formed fayalite and magnetite that precipitated from the fluid on the CO parent asteroid. This and previous studies suggest that O‐isotope exchange during fluid–rock interaction affected most CAIs in CO ≥3.1 chondrites.     

Fe‐Ni metal in primitive chondrites: Indicators of classification and metamorphic conditions for ordinary and CO chondrites

Abstract— We report the results of our petrological and mineralogical study of Fe‐Ni metal in type 3 ordinary and CO chondrites, and the ungrouped carbonaceous chondrite Acfer 094. Fe‐Ni metal in ordinary and CO chondrites occurs in chondrule interiors, on chondrule surfaces, and as isolated grains in the matrix. Isolated Ni‐rich metal in chondrites of petrologic type lower than type 3.10 is enriched in Co relative to the kamacite in chondrules. However, Ni‐rich metal in type 3.15–3.9 chondrites always contains less Co than does kamacite. Fe‐Ni metal grains in chondrules in Semarkona typically show plessitic intergrowths consisting of submicrometer kamacite and Ni‐rich regions. Metal in other type 3 chondrites is composed of fine‐ to coarse‐grained aggregates of kamacite and Ni‐rich metal, resulting from metamorphism in the parent body. We found that the number density of Ni‐rich grains in metal (number of Ni‐rich grains per unit area of metal) in chondrules systematically decreases with increasing petrologic type. Thus, Fe‐Ni metal is a highly sensitive recorder of metamorphism in ordinary and carbonaceous chondrites, and can be used to distinguish petrologic type and identify the least thermally metamorphosed chondrites. Among the known ordinary and CO chondrites, Semarkona is the most primitive. The range of metamorphic temperatures were similar for type 3 ordinary and CO chondrites, despite them having different parent bodies. Most Fe‐Ni metal in Acfer 094 is martensite, and it preserves primary features. The degree of metamorphism is lower in Acfer 094, a true type 3.00 chondrite, than in Semarkona, which should be reclassified as type 3.01.     

Chemical studies of H chondrites 9: Volatile trace element composition and petrographic classification of equilibrated H chondrites

Abstract— To examine the thermal history of the parent body/bodies of equilibrated H chondrites, we treated data for 11 volatile trace elements (Co, Rb, Ag, Se, Cs, Te, Zn, Cd, Bi, Tl, and In in order of putative volatility) in 90 falls: 15 H4; 46 H5, and 29 H6. Using univariate statistical tests, contents of few of these elements differ significantly between any two of these suites. One element, Cs, differs systematically between all three pairs of suites; Co and Tl differ between two pairs of suites. For Co and Cs, contents varied as H4 > H5 > H6; while for Tl, contents varied as H4 < H5 < H6. Using multivariate statistical tests, all three suites can be distinguished compositionally, with trace element contents in the H5 suite being intermediate to those of H4 and H6. Surprisingly, the multivariate distinguishability reflects contents of less volatile Co, Rb, Ag, Se and Cs, and not of highly volatile Te, Zn, Cd, Bi, Tl and In. The compositional trends apparently reflect heterogeneous accretion >600 K, with the suites deriving from a stratified parent body/bodies.     

Mineralogical characterization of primitive,type‐3 lithologies in Rumuruti chondrites

Abstract— Rumuruti (R) chondrites constitute a new, well‐established chondrite group different from the carbonaceous, ordinary, and enstatite chondrites. Many of these samples are gas‐rich regolith breccias showing the typical light‐dark structure and consist of abundant fragments of various parent‐body lithologies embedded in a fine‐grained olivine‐rich matrix. Unequilibrated type‐3 lithologies among these fragments have frequently been mentioned in various publications. In this study, detailed mineralogical data on seven primitive fragments from the R‐chondrites Dar al Gani 013 and Hughes 030 are presented. The fragments range from ～300 μ in size up to several millimeters. Generally, the main characteristics can be summarized as follows: (1) Unequilibrated type‐3 fragments have a well‐preserved chondritic texture with a chondrule‐to‐matrix ratio of ～1:1. Chondrules and chondrule fragments are embedded in a fine‐grained olivine‐rich matrix. Thus, the texture is quite similar to that of type‐3 carbonaceous chondrites. (2) In all cases, matrix olivines in type‐3 fragments have a significantly higher Fa content (44–57 mol%) than olivines in other (equilibrated) lithologies (38–40 mol% Fa). (3) Olivines and pyroxenes occurring within chondrules or as fragments are highly variable in composition (Fa_0–65 and Fs_0–33, respectively) and, generally, more magnesian than those found in equilibrated R chondrites. Agglomerated material of the R‐chondrite parent body (or bodies) was highly unequilibrated. It is suggested that the material that accreted to form the parent body consisted of chondrules and chondrule fragments, mainly having Mg‐rich silicate constituents, and Fe‐rich highly oxidized fine‐grained materials. The dominating phase of this fine‐grained material may have been Fa‐rich olivine from the beginning. The brecciated whole rocks, the R‐chondrite regolith breccias, were not significantly reheated subsequent to brecciation or during lithification, as indicated by negligible degree of equilibration between matrix components and Mg‐rich olivines and pyroxenes in primitive type‐3 fragments.     

Petrology and oxygen isotopic compositions of calcium‐aluminum‐rich inclusions in primitive CO3.0‐3.1 chondrites

The petrologic and oxygen isotopic characteristics of calcium‐aluminum‐rich inclusions (CAIs) in CO chondrites were further constrained by studying CAIs from six primitive CO3.0‐3.1 chondrites, including two Antarctic meteorites (DOM 08006 and MIL 090010), three hot desert meteorites (NWA 10493, NWA 10498, and NWA 7892), and the Colony meteorite. The CAIs can be divided into hibonite‐bearing inclusions (spinel‐hibonite spherules, monomineralic grains, hibonite‐pyroxene microspherules, and irregular/nodular objects), grossite‐bearing inclusions (monomineralic grains, grossite‐melilite microspherules, and irregular/nodular objects), melilite‐rich inclusions (fluffy Type A, compact type A, monomineralic grains, and igneous fragments), spinel‐pyroxene inclusions (fluffy objects resembling fine‐grained spinel‐rich inclusions in CV chondrites and nodular/banded objects resembling those in CM chondrites), and pyroxene‐anorthite inclusions. They are typically small (98.4 ± 54.4 µm, 1SD) and comprise 1.54 ± 0.43 (1SD) area% of the host chondrites. Melilite in the hot desert and Colony meteorites was extensively replaced by a hydrated Ca‐Al‐silicate during terrestrial weathering and converted melilite‐rich inclusions into spinel‐pyroxene inclusions. The CAI populations of the weathered COs are very similar to those in CM chondrites, suggesting that complete replacement of melilite by terrestrial weathering, and possibly parent body aqueous alteration, would make the CO CAIs CM‐like, supporting the hypothesis that CO and CM chondrites derive from similar nebular materials. Within the CO3.0‐3.1 chondrites, asteroidal alteration significantly resets oxygen isotopic compositions of CAIs in CO3.1 chondrites (?¹⁷O: ?25 to ?2‰) but left those in CO3.0‐3.05 chondrites mostly unchanged (?¹⁷O: ?25 to ?20‰), further supporting the model whereby thermal metamorphism became evident in CO chondrites of petrologic type ≥3.1. The resistance of CAI minerals to oxygen isotope exchange during thermal metamorphism follows in the order: melilite + grossite < hibonite + anorthite < spinel + diopside + forsterite. Meanwhile, terrestrial weathering destroys melilite without changing the chemical and isotopic compositions of melilite and other CAI minerals.     

A carbon and nitrogen isotope study of diamond from primitive chondrites

Abstract Diamonds isolated from primitive chondrites of the carbonaceous, ordinary and enstatite groups have been analysed by high-resolution stepped combustion, followed by measurement of their C and N isotopes using a newly adapted technique that allows quantitative measurements of C/N ratios. The δ¹³C of the diamond is shown to vary between meteorite groups from ?32 to ?38%0, and the measured C/N ratios suggest that the N concentration of diamond ranges over a factor of 7 from 1800 ppm (Tieschitz) to 13,000 ppm (Adrar 003). The δ¹⁵N of N released from diamond is constrained to ?348 ± 7%. The complexity of the C release pattern and C/N ratio during combustion implies the presence of more than one component, which suggests that either more than one type of diamond is present in the samples, or unidentified additional phases are located in the acid-resistant residue. The components are present in varying proportions between meteorite groups. The data are compatible with a model of a mix of different diamond populations (some probably presolar and some possibly solar) existing in the early solar nebula, where each population originally contributed a roughly equal amount to chondrites of every class. Subsequent metamorphism has resulted in overall variations in δ¹³C and C/N ratios in diamond isolated from meteorites of differing petrologic grade without significantly altering the N isotopic composition. Possible ways for this to be achieved are explored.     

NWA 1152 and Sahara 00182: New primitive carbonaceous chondrites with affinities to the CR and CV groups

Abstract— We have investigated the mineralogy, petrography, bulk chemistry, and light element isotope composition of the ungrouped chondrites North West Africa (NWA) 1152 and Sahara 00182. NWA 1152 contains predominantly type 1 porphyritic olivine (PO) and porphyritic olivinepyroxene (POP) chondrules. Chondrule silicates are magnesium‐rich (Fo_{98.8 ± 1.2}, n = 36; Fs_{2.3 ± 2.1} Wo_{1.2 ± 0.3}, n = 23). Matrix comprises ?40 vol% of the sample and is composed of a micron sized silicate groundmass with larger silicate, sulfide, magnetite, and Fe‐Ni metal (Ni ?50 wt%) grains. Phyllosilicates were not observed in the matrix. Refractory inclusions are rare (0.3 vol%) and are spinel pyroxene aggregates or amoeboid olivine aggregates; melilite is absent from the refractory inclusions. Sahara 00182 contains predominantly type 1 PO chondrules, POP chondrules are less common. Most chondrules contain blebs of, and are often rimmed with, Fe‐Ni metal and sulfide. Chondrule phenocrysts are magnesium‐rich (Fo_{92.2 ± 0.6}, n = 129; Fs_{4.4 ± 1.8} Wo_{1.3 ± 1.1}, n = 16). Matrix comprises ?30 vol% of the meteorite and is predominantly sub‐micron silicates, with rare larger silicate gains. Matrix Fe‐Ni metal (mean Ni = 5.8 wt%) and sulfide grains are up to mm scale. No phyllosilicates were observed in the matrix. Refractory inclusions are rare (1.1 vol%) and melilite is absent. The oxygen isotope composition of NWA 1152 falls within the range of the CV chondrites with δ¹⁷O = ?3.43%0 δ¹⁸O = 0.70%0 and is similar to Sahara 00182, δ¹⁷O = ?3.89%0, δ¹⁸O = ?0.19%0 (Grossman and Zipfel 2001). Based on mineralogical and petrographic characteristics, we suggest NWA 1152 and Sahara 00182 show many similarities with the CR chondrites, however, oxygen isotopes suggest affinity with the CVs. Thus, neither sample can be assigned to any of the currently known carbonaceous chondrite groups based on traditionally recognized characteristics. Both samples demonstrate the complexity of inter‐ and intra‐group relationships of the carbonaceous chondrites. Whatever their classification, N WA 1152 and Sahara 00182 represent a source of relatively pristine solar system material.     

Refractory inclusions in carbonaceous chondrites: Records of early solar system processes

Chondrites consist of three major components: refractory inclusions (Ca,Al‐rich inclusions [CAIs] and amoeboid olivine aggregates), chondrules, and matrix. Here, I summarize recent results on the mineralogy, petrology, oxygen, and aluminum‐magnesium isotope systematics of the chondritic components (mainly CAIs in carbonaceous chondrites) and their significance for understanding processes in the protoplanetary disk (PPD) and on chondrite parent asteroids. CAIs are the oldest solids originated in the solar system: their U‐corrected Pb‐Pb absolute age of 4567.3 ± 0.16 Ma is considered to represent time 0 of its evolution. CAIs formed by evaporation, condensation, and aggregation in a gas of approximately solar composition in a hot (ambient temperature >1300 K) disk region exposed to irradiation by solar energetic particles, probably near the protoSun; subsequently, some CAIs were melted in and outside their formation region during transient heating events of still unknown nature. In unmetamorphosed, type 2–3.0 chondrites, CAIs show large variations in the initial ²⁶Al/²⁷Al ratios, from <5 × 10^–6 to ~5.25 × 10^–5. These variations and the inferred low initial abundance of ⁶⁰Fe in the PPD suggest late injection of ²⁶Al by a wind from a nearby Wolf–Rayet star into the protosolar molecular cloud core prior to or during its collapse. Although there are multiple generations of CAIs characterized by distinct mineralogies, textures, and isotopic (O, Mg, Ca, Ti, Mo, etc.) compositions, the ²⁶Al heterogeneity in the CAI‐forming region(s) precludes determining the duration of CAIs formation using ²⁶Al‐²⁶Mg systematics. The existence of multiple generations of CAIs and the observed differences in CAI abundances in carbonaceous and noncarbonaceous chondrites may indicate that CAIs were episodically formed and ejected by a disk wind from near the Sun to the outer solar system and then spiraled inward due to gas drag. In type 2–3.0 chondrites, most CAIs surrounded by Wark–Lovering rims have uniform Δ¹⁷O (= δ¹⁷O?0.52 × δ¹⁸O) of ~ ?24‰; however, there is a large range of Δ¹⁷O (from ~?40 to ~ ?5‰) among them, suggesting the coexistence of ¹⁶O‐rich (low Δ¹⁷O) and ¹⁶O‐poor (high Δ¹⁷O) gaseous reservoirs at the earliest stages of the PPD evolution. The observed variations in Δ¹⁷O of CAIs may be explained if three major O‐bearing species in the solar system (CO, H₂O, and silicate dust) had different O‐isotope compositions, with H₂O and possibly silicate dust being ¹⁶O‐depleted relative to both the Genesis solar wind Δ¹⁷O of ?28.4 ± 3.6‰ and even more ¹⁶O‐enriched CO. Oxygen isotopic compositions of CO and H₂O could have resulted from CO self‐shielding in the protosolar molecular cloud (PMC) and the outer PPD. The nature of ¹⁶O‐depleted dust at the earliest stages of PPD evolution remains unclear: it could have either been inherited from the PMC or the initially ¹⁶O‐rich (solar‐like) MC dust experienced O‐isotope exchange during thermal processing in the PPD. To understand the chemical and isotopic composition of the protosolar MC material and the degree of its thermal processing in PPD, samples of the primordial silicates and ices, which may have survived in the outer solar system, are required. In metamorphosed CO3 and CV3 chondrites, most CAIs exhibit O‐isotope heterogeneity that often appears to be mineralogically controlled: anorthite, melilite, grossite, krotite, perovskite, and Zr‐ and Sc‐rich oxides and silicates are ¹⁶O‐depleted relative to corundum, hibonite, spinel, Al,Ti‐diopside, forsterite, and enstatite. In texturally fine‐grained CAIs with grain sizes of ~10–20 μm, this O‐isotope heterogeneity is most likely due to O‐isotope exchange with ¹⁶O‐poor (Δ¹⁷O ~0‰) aqueous fluids on the CO and CV chondrite parent asteroids. In CO3.1 and CV3.1 chondrites, this process did not affect Al‐Mg isotope systematics of CAIs. In some coarse‐grained igneous CV CAIs, O‐isotope heterogeneity of anorthite, melilite, and igneously zoned Al,Ti‐diopside appears to be consistent with their crystallization from melts of isotopically evolving O‐isotope compositions. These CAIs could have recorded O‐isotope exchange during incomplete melting in nebular gaseous reservoir(s) with different O‐isotope compositions and during aqueous fluid–rock interaction on the CV asteroid.     

Ca,Al‐rich inclusions in Rumuruti (R) chondrites

Abstract— Rumuruti chondrites (R chondrites) constitute a well‐characterized chondrite group different from carbonaceous, ordinary, and enstatite chondrites. Many of these meteorites are breccias containing primitive type 3 fragments as well as fragments of higher petrologic type. Ca,Al‐rich inclusions (CAIs) occur within all lithologies. Here, we present the results of our search for and analysis of Al‐rich objects in Rumuruti chondrites. We studied 20 R chondrites and found 126 Ca,Al‐rich objects (101 CAIs, 19 Al‐rich chondrules, and 6 spinel‐rich fragments). Based on mineralogical characterization and analysis by SEM and electron microprobe, the inclusions can be grouped into six different types: (1) simple concentric spinel‐rich inclusions (42), (2) fassaite‐rich spherules, (3) complex spinel‐rich CAIs (53), (4) complex diopside‐rich inclusions, (5) Al‐rich chondrules, and (6) Al‐rich (spinel‐rich) fragments. The simple concentric and complex spinel‐rich CAIs have abundant spinel and, based on the presence or absence of different major phases (fassaite, hibonite, Na,Al‐(Cl)‐rich alteration products), can be subdivided into several subgroups. Although there are some similarities between CAIs from R chondrites and inclusions from other chondrite groups with respect to their mineral assemblages, abundance, and size, the overall assemblage of CAIs is distinct to the R‐chondrite group. Some Ca,Al‐rich inclusions appear to be primitive (e.g., low FeO‐contents in spinel, low abundances of Na,Al‐(Cl)‐rich alteration products; abundant perovskite), whereas others were highly altered by nebular and/or parent body processes (e.g., high concentrations of FeO and ZnO in spinel, ilmenite instead of perovskite, abundant Na,Al‐(Cl)‐rich alteration products). There is complete absence of grossite and melilite, which are common in CAIs from most other groups. CAIs from equilibrated R‐chondrite lithologies have abundant secondary Ab‐rich plagioclase (oligoclase) and differ from those in unequilibrated type 3 lithologies which have nepheline and sodalite instead.     

The pyroxene pallasites,Vermillion and Yamato 8451: Not quite a couple

Abstract— Two pallasites, Vermillion and Yamato (Y)‐8451, have been studied to obtain petrologic, trace element, and O‐isotopic data. Both meteorites contain low‐Ca and high‐Ca pyroxenes (<2% by volume) and have been dubbed “pyroxene pallasites.” Pyroxene occurs as large individual grains, as inclusions in olivine and in other pyroxene, and as grains along the edges of olivine. Symplectic overgrowths, sometimes found in Main Group and Eagle Station pallasites, are not seen in the pyroxene pallasites. Olivine compositions are Fa_10–12, similar to those of Main Group pallasites. Siderophile trace element data show that metal in the two meteorites have significantly differing compositions that are, for many elements, outside the range of the Main Group and Eagle Station pallasites. These compositions also differ from those of IAB and IIIAB iron meteorites. Rare earth element (REE) patterns in merrillite are similar to those seen in other pallasites, indicating formation by subsolidus reaction between metal and silicate, with the merrillite inheriting its pattern from the surrounding silicates. The O‐isotopic compositions of Vermillion and Y‐8451 are similar but differ from Main Group or Eagle Station pallasites, as well as other achondrite and primitive achondrite groups. Although Vermillion and Y‐8451 have similar mineralogy, pyroxene compositions, REE patterns, and O‐isotopic compositions, there is sufficient evidence to resist formally grouping these two meteorites. This evidence includes the texture of Vermillion, siderophile trace element data, and the presence of cohenite in Vermillion.     

Comparison of the trace element composition of Tagish Lake with other primitive carbonaceous chondrites

Abstract— A meteorite fall on 2000 January 18 was detected by U.S. Defense Department satellites which established its pre‐impact orbit. Fresh samples were collected from frozen Tagish Lake in British Columbia a week later and some properties of these samples reveal it to be a unique meteorite. We characterized Tagish Lake and 8 other samples using inductively‐coupled plasma mass spectrometry and radiochemical neutron activation analysis: data for 47 elements reveal that each of 9 carbonaceous chondrites of different type exhibit the Orgueil‐normalized plateaus expected for members of such types. Trends evident in Tagish Lake differ from all other carbonaceous chondrites, including CI and CM. Samples of Tagish Lake collected later show similar patterns affected by weathering.     

Compositional diversity of ordinary chondrites inferred from petrology,bulk chemical,and oxygen isotopic compositions of the lowest FeO ordinary chondrite,Yamato 982717

We performed a petrologic, geochemical, and oxygen isotopic study of the lowest FeO ordinary chondrite (OC), Yamato (Y) 982717. Y 982717 shows a chondritic texture composed of chondrules and chondrule fragments, and mineral fragments set in a finer grained, clastic matrix, similar to H4 chondrites. The composition of olivine (Fa_{11.17 ± 0.48 (1σ)}) and low‐Ca pyroxene (Fs_{11.07 ± 0.98 (1σ)}Wo_{0.90 ± 0.71(1σ)}) is significantly more magnesian than those of typical H chondrites (Fa_16.0‐20, Fs_14.5‐18.0), as well as other known low‐FeO OCs (Fa_12.8‐16.7; Fs_13‐16). However, the bulk chemical composition of Y 982717, in particular lithophile and moderately volatile elements, is within the range of OCs. The bulk siderophile element composition (Ni, Co) is within the range of H chondrites and distinguishable from L chondrites. The O‐isotopic composition is also within the range of H chondrites. The lack of reduction textures indicates that the low olivine Fa content and low‐Ca pyroxene Fs content are characteristics of the precursor materials, rather than the result of reduction during thermal metamorphism. We suggest that the H chondrites are more compositionally diverse than has been previously recognized.     

Thermoluminescence sensitivity and thermal history of type 3 ordinary chondrites: Eleven new type 3.0–3.1 chondrites and possible explanations for differences among H,L, and LL chondrites

Abstract— We review induced thermoluminescence (TL) data for 102 unequilibrated ordinary chondrites (UOCs), many data just published in abstracts, in order to identify particularly primitive UOCs and further explore TL systematics that may have implications for the history of the chondrites and their parent body. We have identified 11 UOCs of petrologic types 3.0–3.1: Adrar 003, Elephant Moraine (EET) 90066, EET 90161, Grosvenor Mountains (GRO) 95502, Lewis Cliff (LEW) 88477, Meteorite Hills (MET) 96503, Yamato (Y)‐790787, Y‐791324, Y‐791558, Y‐793565, and Y‐793596. These samples represent an important new resource for researchers interested in the nature of primitive solar system materials. Previously reported trends in which TL sensitivity increases with TL peak temperature and TL peak width, which we interpret in terms of crystallization of feldspar in the ordered or disordered forms during metamorphism, are confirmed by the new data. Importantly, the present data strengthen the trend described earlier in which the mean level of metamorphism experienced by UOCs increases along the series LL, L and H. This suggests either different burial depths for the UOCs from each class, or formation at similar depths in regoliths of different thickness.     

Refractory inclusions from the ungrouped carbonaceous chondrites MacAlpine Hills 87300 and 88107

Abstract— MacAlpine Hills (MAC) 87300 and 88107 are two unusual carbonaceous chondrites that are intermediate in chemical composition between the CO3 and CM2 meteorite groups. Calcium‐aluminum‐rich inclusions (CAIs) from these two meteorites are mostly spinel‐pyroxene and melilite‐rich (Type A) varieties. Spinel‐pyroxene inclusions have either a banded or nodular texture, with aluminous diopside rimming Fe‐poor spinel. Melilite‐rich inclusions (Åk_4–42) are irregular in shape and contain minor spinel (FeO <1 wt%), perovskite and, more rarely, hibonite. The CAIs in MAC 88107 and 87300 are similar in primary mineralogy to CAIs from low petrologic grade CO3 meteorites but differ in that they commonly contain phyllosilicates. The two meteorites also differ somewhat from each other: melilite is more abundant and slightly more Al‐rich in inclusions from MAC 88107 than in those from MAC 87300, and phyllosilicate is more abundant and Mg‐poor in MAC 87300 CAIs relative to that in MAC 88107. These differences suggest that the two meteorites are not paired. The CAI sizes and the abundance of melilite‐rich CAIs in MAC 88107 and 87300 suggests a genetic relationship to CO3 meteorites, but the CAIs in both have suffered a greater degree of aqueous alteration than is observed in CO meteorites. Aluminum‐rich melilite in CAIs from both meteorites generally contains excess ²⁶Mg, presumably from the in situ decay of ²⁶Al. Although well‐defined isochrons are not observed, the ²⁶Mg excesses are consistent with initial ²⁶Al/²⁷Al ratios of approximately 3–5 times 10^?5. An unusual hibonite‐bearing inclusion is isotopically heterogeneous, with two large and abutting hibonite crystals showing significant differences in their degrees of mass‐dependent fractionation of ²⁵Mg/²⁴Mg. The two crystals also show differences in their inferred initial ²⁶Al/²⁷Al ratios, 1 × 10^?5 vs. ≤3 × 10^?6.