Distinct Parent Bodies Research Articles

Isotopic evidence for a common parent body of IIG and IIAB iron meteorites

Magmatic iron meteorites are thought to sample the metallic cores of differentiated planetesimals and are subdivided into several chemical groups, each representing a distinct parent body. The only exceptions are the groups IIAB and IIG, which have been proposed to sample two immiscible melts from the same core. To test this model, we report the first Fe, Ni, O, and Cr isotope data for IIG iron meteorites and the first high-precision O isotope data for IIAB iron meteorites. The new data demonstrate that IIG iron meteorites belong to the non-carbonaceous (NC) meteorites. This is evident from the isotope anomaly of each of the four elements investigated, where the IIG irons always overlap with the compositions of NC meteorites but are distinct from those of carbonaceous (CC) meteorites. Moreover, among the NC meteorites and in particular, the NC irons, the isotopic composition of the IIG irons overlaps only with that of the IIAB irons. The combined Fe-Ni-O-Cr isotope data for IIAB and IIG iron meteorites, therefore, reveal formation from a single isotopic reservoir, indicating a strong genetic link between the two groups. The indistinguishable isotopic composition of the IIAB and IIG irons, combined with chemical evidence for the formation of IIG irons as late-stage liquids of the IIAB core, strongly suggests that both groups originate from the same core. The results underscore the strength of utilizing multiple elements and their isotopic compositions to establish genetic links among meteorites, rather than using a single element. They also highlight the significance of integrating multiple geochemical tracers and petrologic observations to accurately determine genetic relationships and the formation of meteorites within the same parent body.

Unusual Abundances from Planetary System Material Polluting the White Dwarf G238-44

Ultraviolet and optical spectra of the hydrogen-dominated atmosphere white dwarf star G238-44 obtained with FUSE, Keck/HIRES, HST/COS, and HST/STIS reveal 10 elements heavier than helium: C, N, O, Mg, Al, Si, P, S, Ca, and Fe. G238-44 is only the third white dwarf with nitrogen detected in its atmosphere from polluting planetary system material. Keck/HIRES data taken on 11 nights over 24 yr show no evidence for variation in the equivalent width of measured absorption lines, suggesting stable and continuous accretion from a circumstellar reservoir. From measured abundances and limits on other elements, we find an anomalous abundance pattern and evidence for the presence of metallic iron. If the pollution is from a single parent body, then it would have no known counterpart within the solar system. If we allow for two distinct parent bodies, then we can reproduce the observed abundances with a mix of iron-rich Mercury-like material and an analog of an icy Kuiper Belt object with a respective mass ratio of 1.7:1. Such compositionally disparate objects would provide chemical evidence for both rocky and icy bodies in an exoplanetary system and would be indicative of a planetary system so strongly perturbed that G238-44 is able to capture both asteroid and Kuiper Belt–analog bodies near-simultaneously within its <100 Myr cooling age.

Nickel-rich, volatile depleted iron meteorites: Relationships and formation processes

Ungrouped iron meteorites Tishomingo, Willow Grove, and Chinga, and group IVB iron meteorites, are Ni-rich. Similarities include enrichments of 10–100 × CI for some refractory siderophile elements, and equivalent depletions in more volatile siderophile elements. Superimposed on the overall enrichment/depletion trend, certain siderophile elements (P, W, Fe, Mo) are depleted relative to elements of similar volatility. All three ungrouped irons derive from parent bodies formed in the early Solar System. Willow Grove and Chinga are characterized by cosmic ray exposure corrected 182W/184W consistent with metal-silicate segregation on their parent bodies within 1–3 Myr of Solar System formation, within the age range determined for segregation of magmatic iron meteorite parent bodies, including group IVB irons. Tishomingo is characterized by a younger model age 4–5 Myr subsequent to Solar System formation, reflecting either late stage melting resulting from 26Al decay, or an impact resetting. The discovery of stishovite in Tishomingo, indicating exposure to a minimum shock pressure of 8–9 GPa, is consistent with the latter.Stishovite in Tishomingo and chromite included in troilite-daubréelite in IVB irons allows oxygen isotopic composition comparison between these meteorites. Different mass independent oxygen isotopic compositions of IVB irons and Tishomingo indicate genetically distinct parent bodies. By contrast, mass independent Mo isotopic compositions overlap within analytical uncertainties, indicating a similar, carbonaceous chondrite (CC) type genetic heritage. Molybdenum and 183W isotopic data for Chinga and Willow Grove indicate derivation from CC type parent bodies. Willow Grove shares Mo and 183W isotopic compositions with the Ni-rich South Byron Trio (SBT) grouplet and the Milton pallasite. These Ni-rich meteorites likely formed in the same general nebular environment as other CC planetesimals, likely the outer Solar System.Highly siderophile element (HSE) abundances of Willow Grove and Tishomingo are similar to some IVB meteorites, consistent with formation by moderate degrees of fractional crystallization from initial metallic melts with low S and P, and modestly fractionated HSE. The comparable HSE abundances of Tishomingo and Willow Grove to some IVB irons, yet substantially higher Ni concentrations, indicate formation on parent bodies with lower bulk HSE abundances or HSE concentration in proportionally smaller volumes of metal. HSE abundances in Chinga are considerably lower than in IVB irons, highly fractionated, and processes responsible for these remain elusive.For IVB irons and these ungrouped irons, high temperature condensation likely dominated the enrichment and depletion of the refractory and volatile siderophile elements, respectively. Parent body degassing may have also played a role. Relative depletion of volatile siderophile elements is not, however, a universal feature of high-Ni meteorites. The SBT and Milton pallasite are Ni-rich, but less depleted in the more volatile siderophile elements. Nickel enrichment was likely driven by oxidation of Fe metal during parent body accretion or core segregation. Oxidation of the Tishomingo and Willow Grove parent bodies may have occurred at ∼ IW + 1, indicated by relative Mo and W depletions due to metal/water reaction during differentiation. Late-stage reduction, indicated by the presence of Cr-bearing sulfides in Tishomingo and IVB irons, may have resulted from exhaustion of the oxidant.

Asteroids accretion, differentiation, and break-up in the Vesta source region: Evidence from cosmochemistry of mesosiderites

The cosmochemistry of meteorites provides unique clues on asteroids accretion, differentiation, collisional break-up and reassembly - processes of critical importance for understanding planet formation in the early solar system. Mesosiderites are a complex group of achondrites whose nearly 50:50 metal-silicate composition is interpreted in the literature as resulting from the mixing of core and crustal materials derived from differentiated asteroids. Because of their complex nature and contrasting geochemical, isotopic, and spectroscopic data, the formation mechanism of mesosiderites is still poorly understood and is open to a large variety of planetary differentiation scenarios and collisional histories. In this study, based on new petrographic and geochemical data of 16 mesosiderites, we investigate in detail the proposal that mesosiderites are related to the howardite-eucrite-diogenite (HED) meteorite group, whose parent body is widely considered to be asteroid 4 Vesta (∼500 km diameter), the target of the recent NASA’s Dawn mission. We present the first high precision oxygen isotope analyses on the matrix of a set of mesosiderite samples, coupled with new chemical and petrographic analyses of mesosiderites Um Hadid, Estherville, and Mount Padbury. Concordant Δ17O values between mesosiderites (–0.241 ± 0.015 (2σ)) and howardite-eucrite-diogenites (−0.241 ± 0.017‰ (2σ)) indicate that they derived from the same oxygen isotope reservoir, but petrological evidence, in particular the distinctly lower Fe/Mn ratios and the larger lithological diversity in mesosiderites, indicates that they formed within different parent bodies. This suggests that mesosiderites and howardite-eucrite-diogenites originated in distinct parent bodies that accreted in the 4 Vesta source region but experienced different geologic evolution in terms of crustal differentiation and impact history, which were more complex and catastrophic in the mesosiderite parent body.

The relationship between CM and CO chondrites: Insights from combined analyses of titanium, chromium, and oxygen isotopes in CM, CO, and ungrouped chondrites

A close relationship between CM and CO chondrites has been suggested by previous petrologic and isotopic studies, leading to the suggestion that they may originate from similar precursor materials or even a common parent body. In this study, we evaluate the genetic relationship between CM and CO chondrites using Ti, Cr, and O isotopes. We first provide additional constraints on the ranges of ε50Ti and ε54Cr values of bulk CM and CO chondrites by reporting the isotopic compositions of CM2 chondrites Murchison, Murray, and Aguas Zarcas and the CO3.8 chondrite Isna. We then report the ε50Ti and ε54Cr values for several ungrouped and anomalous carbonaceous chondrites that have been previously reported to exhibit similarities to the CM and/or CO chondrite groups, including Elephant Moraine (EET) 83226, EET 83355, Grosvenor Mountains (GRO) 95566, MacAlpine Hills (MAC) 87300, MAC 87301, MAC 88107, and Northwest Africa (NWA) 5958, and the O-isotope compositions of a subset of these samples. We additionally report the Ti, Cr, and O isotopic compositions of additional ungrouped chondrites LaPaz Ice Field (LAP) 04757, LAP 04773, Lewis Cliff (LEW) 85332, and Coolidge to assess their potential relationships with known carbonaceous and ordinary chondrite groups. LAP 04757 and LAP 04773 exhibit isotopic compositions indicating they are low-FeO ordinary chondrites. The isotopic compositions of Murchison, Murray, Aguas Zarcas, and Isna extend the compositional ranges defined by the CM and CO chondrites in ε50Ti versus ε54Cr space. The majority of the ungrouped carbonaceous chondrites with documented similarities to the CM and/or CO chondrites plot outside the CM and CO group fields in plots of ε50Ti versus ε54Cr, Δ17O versus ε50Ti, and Δ17O versus ε54Cr. Therefore, based on differences in their Ti, Cr, and O isotopic compositions, we conclude that the CM, CO, and ungrouped carbonaceous chondrites likely represent samples of multiple distinct parent bodies. We also infer that these parent bodies formed from precursor materials that shared similar isotopic compositions, which may indicate formation in regions of the protoplanetary disk that were in close proximity to each other.

CV chondrites: More than one parent body

CV chondrites are one of the most studied group of carbonaceous chondrites. Based on a number of mineralogical features, they have been divided into three sub-groups: CVOxA, CVOxB, and CVRed. These sub-groups are classically interpreted as coming from a single parent body, with a common protolith affected by significant parent body fluid-assisted metasomatism occurring at different temperatures and/or redox conditions. In this work, we studied a set of 53 CV chondrites. We classified them into the three sub-groups, measured their apparent chondrule sizes and their matrix modal abundance. We measured the triple oxygen isotopic composition for 17 of them. The distributions of chondrule size and matrix abundances in CVOxA and CVOxB cannot be statistically distinguished. Conversely, CVRed and CVOx have distinct distributions. These two robust and simple petrographic indicators combined with the previous knowledge of the peak metamorphic temperatures experienced by these meteorites show that CVOx and CVRed originate from two distinct parent bodies. On the other hand, CVOxA and CVOxB likely originate from the same parent body, with CVOxA representing deeper, more metamorphosed levels. For clarification of the chondrite classification scheme, in which one group should ultimately represent a single parent body, we propose to divide the CV group into two proper groups (and not subgroups as in the current scheme), keeping the names CVRed and CVOx. These two groups can be readily separated by estimating the average nickel content of their sulfides.

Water and heat: New constraints on the evolution of the CV chondrite parent body

This paper focuses on the post-accretion history of CV3 chondrites, through a combination of petrographic and mineralogical characterization, magnetic measurements, spectral (Raman and Infrared) and thermo-gravimetric analysis of 31 meteorites (including 7 falls, 21 Antarctic and 3 non-Antarctic finds) spanning a wide metamorphic range.We classify the 21 Antarctic chondrites and the Bukhara fall into the CVRed, CVOxA, and CVOxB subgroups. We establish quantitative parameters relevant for this sub-classification. In comparison to CVOx, CVRed chondrites are characterized by (i) a lower abundance of matrix, (ii) a higher abundance of metal, (iii) the presence of Ni-poor sulfides. In comparison to CVOxB, CVOxA are characterized by (i) similar matrix abundance, (ii) a higher abundance of metal, (iii) the presence of metal almost exclusively under the form of awaruite, (iv) lower Ni content of sulfides, (v) lower magnetic susceptibility and saturation remanence.Both CVOx (CVOxA and CVOxB) and CVRed experienced aqueous alteration, and contain oxyhydroxides and phyllosilicates. We show that the abundance of these hydrated secondary minerals observed today in individual CV chondrites decreases with their peak metamorphic temperature. This is interpreted either as partial dehydration of these secondary minerals or limited hydration due to the rapid exhaustion of the water reservoir during parent body thermal metamorphism. Moreover, the lower abundance of oxyhydroxides (that have a lower thermal stability than phyllosilicates and may in large part postdate the peak of thermal metamorphism) in more metamorphosed CV chondrites is interpreted as lower availability of aqueous fluids during retrograde metamorphism in these meteorites.Lastly, we show that in comparison to CVOxB, CVOxA are systematically (i) more metamorphosed, (ii) less hydrated, (iii) depleted in ferromagnetic minerals, (iv) but enriched in metal in the form of secondary awaruite. CVOxA may be thermally metamorphosed CVOxB. CVRed are significantly different from CVOx (matrix abundances, alteration products, opaque minerals), but span the same wide metamorphic range. This could be indicative of a laterally heterogeneous CV parent body, or suggest the existence of distinct parent bodies for CVOx and CVRed chondrites.

3-μm reflectance spectroscopy of carbonaceous chondrites under asteroid-like conditions

We measured 3-μm reflectance spectra of 21 meteorites that represent all carbonaceous chondrite types available in terrestrial meteorite collections. The measurements were conducted at the Laboratory for Spectroscopy under Planetary Environmental Conditions (LabSPEC) at the Johns Hopkins University Applied Physics Laboratory (JHU APL) under vacuum and thermally-desiccated conditions (asteroid-like conditions). This is the most comprehensive 3-μm dataset of carbonaceous chondrites ever acquired in environments similar to the ones experienced by asteroids. The 3-μm reflectance spectra are extremely important for direct comparisons with and appropriate interpretations of reflectance data from ground-based telescopic and spacecraft observations of asteroids. We found good agreement between 3-μm spectral characteristics of carbonaceous chondrites and carbonaceous chondrite classifications. The 3-μm band is diverse, indicative of varying composition, thus suggesting that these carbonaceous chondrites experienced distinct parent body aqueous alteration and metamorphism environments. The spectra of CI chondrites, from which significant amount of water adsorbed under ambient conditions was removed, are consistent with Mg-serpentine and clay minerals. The high abundances of organics in CI chondrites is also associated with the mineralogy of these chondrites, oxyhydroxides- and complex clay minerals-rich. CM chondrites, which are cronstedtite-rich, have shallower 3-μm band than CI chondrites, suggesting they experienced less aqueous alteration. CR chondrites showed moderate aqueous alteration relative to CI and CM chondrites. CV chondrites, except for Efremovka, have a very shallow 3-μm band, consistent with their lower phyllosilicate proportions. CO chondrites, like most CVs, have a very shallow 3-μm band that suggest they experienced minor aqueous alteration. The 3-μm band in CH/CBb is deep and broad centered ~3.11 μm, possibly due to the high abundance of FeNi metal and presence of heavily hydrated clasts in these chondrites. The 3-μm spectra of Essebi (C2-ung) and EET 83226 are more consistent with CM chondrites' spectra. The 3-μm spectra of Tagish lake (C2-ung), on the other hand, are consistent with CI chondrites. None of these spectral details could have been resolved without removing the adsorbed water before acquiring spectra.

Ferrous oxide-rich asteroid achondrites

Ferrous oxide (FeO)-rich asteroid achondrites can be defined as asteroid-derived samples that experienced incipient partial melting processes in the early Solar System (>4.5 Ga) leading to melt-residues and cumulate and melt rocks that have high FeO in silicate grains (molar Mg/[Mg + Fe] < 80), implying relatively oxidative conditions (fO2 of IW +1 to +3). These achondrites include olivine-dominated brachinite and brachinite-like achondrite meteorites, ungrouped meteorites including Lewis Cliff 88763, Northwest Africa (NWA) 6693 and NWA 6704, Tafassasset, NWA 011/1296, and the oligoclase-rich meteorites Graves Nunataks (GRA) 06128 and GRA 06129. Ferrous oxide-rich asteroidal achondrites differ from other partially-melted achondrites, including ureilites and acapulcoite-lodranites in that the latter have higher molar Mg/ (Mg + Fe) in silicate grains, and lower fO2 (IW 0 to −2). New mineral chemical, whole-rock major- and trace-element and highly siderophile element (HSE: Re, Os, Ir, Ru, Rh, Pt, Pd, Au) abundance data, and O and Os isotope data are presented for FeO-rich achondrite meteorites Allan Hills 84025 (brachinite), Miller Range (MIL) 090206 and MIL 090405 (brachinite-like achondrites), and NWA 6693 (ungrouped). These results, combined with available data for FeO-rich asteroidal achondrites, reveal that these rocks include nearly-pure residues after partial melting, to samples formed by melt-rock reaction and as cumulates, requiring variable to extensive Fe-Ni-S partial melting, and between 1 and 20% silicate partial melting. The FeO-rich asteroidal achondrites originate from at least four distinct parent bodies, based on O-Cr-Ti isotope systematics, and occur for both carbonaceous and non-carbonaceous chondrite precursor sources. The initial water and volatile contents of FeO-rich asteroid achondrites were similar to carbonaceous chondrite groups, implying both carbonaceous and non-carbonaceous precursor materials generated water-rich partially-melted asteroidal bodies. The existence and recognition of FeO-rich asteroid achondrites explains the otherwise enigmatic nature of some iron meteorite groups (e.g., IVA, IVB) that require segregation from an oxidized asteroid parent body. The internal structure of some asteroid parent bodies was likely to be complex, reflecting early differentiation processes of nascent core formation, Fe-Ni-S melt pooling, variable silicate partial melting, igneous differentiation and the important role of melt-rock reaction, melt refertilization and late-stage C- (reduced bodies) or S-rich (oxidized bodies) fluid and vapor reactions.

Reassessing the origin and chronology of the unique achondrite Asuka 881394: Implications for distribution of 26Al in the early Solar System

The achondrite Asuka 881394 has mineralogy broadly similar to that of eucrites but is isotopically, chemically and texturally distinct from them. Previous U-Pb chronology shows that it is very old; forming within the first 0.8 Ma of the formation of calcium-aluminum rich inclusions (CAIs). However, the age difference between Asuka 881394 and other very old Solar System materials (CAIs, quenched angrites) measured with the 26Al-26Mg and 53Mn-53Cr extinct radionuclide chronometers, and 207Pb/206Pb chronometer, is not the same. This could be interpreted to reflect heterogeneity in the distribution of 26Al and 53Mn in the early Solar System. Given the significant implications for the early Solar System chronology if 26Al and 53Mn are indeed heterogeneously distributed, in this study we further investigate the origin of Asuka 881394 and the apparent age discordance between short-lived and absolute chronometers, by focusing on measurement of its ε54Cr composition, renewed measurements of the absolute Pb-Pb age and new, high precision measurements of its 26Al-26Mg systematics.New Cr isotope data places additional constraints on the origin of Asuka 881394; its ε 54Cr value of −0.37 ± 0.10ε is resolvable outside of uncertainty from HED meteorites (−0.72 ± 0.10ε), reinforcing evidence from oxygen isotopic analyses that suggest it originated from a distinct parent body, unlikely to be 4 Vesta. New Pb-Pb analyses, combined with using a directly measured 238U/235U ratio for age calculation, result in a recalculated Pb-Pb age of 4564.95 ± 0.53 Ma, ∼1.5 Ma younger than the previously reported age. With this age Asuka 881394 remains one of the oldest known achondrites in our Solar System. New high precision 26Al-26Mg data produce an initial 26Al/27Al ratio of 1.48 ± 0.12 × 10−6, within error of previous data. This ratio corresponds to an Al-Mg age of 4563.69 ± 0.36 Ma or 4564.83 ± 0.21 Ma relative to CAIs and the D’Orbigny angrite, respectively. Thus, depending on which age anchor is used, the 26Al-26Mg age is either 1.26 Ma or 0.12 Ma younger than the new Pb-Pb age, the latter being unresolved within analytical uncertainty.Though a potential age discrepancy between 26Al-26Mg and Pb-Pb could be a result of heterogeneous distribution of 26Al, we demonstrate with our new high precision Mg isotope data, in conjunction with petrographic evidence, that the Mg isotope system has been disturbed in Asuka 881394. We suggest that the 26Al-26Mg system closed to diffusion after the U-Pb system, either due to slow cooling on the parent body or low-grade metamorphic re-equilibration of Mg. Thus, we can satisfactorily explain the observed age discrepancy between 26Al-26Mg and U-Pb systems in Asuka 881394 without invoking heterogeneous distribution of 26Al/27Al ratio in the early Solar System.Comparison of the Asuka 881394 data with that of other anomalous achondrites from distinct parent bodies suggests that these could also have evolved from a source region with a canonical 26Al/27Al ratio. Because these achondrites have significant differences in their ε54Cr-Δ17O systematics, which could be indicative of location within the early protoplanetary disk, it is consistent with homogeneous distribution of 26Al in the early Solar System. Angrites remain an outlier; either because they evolved from a source with a lower 26Al/27Al ratio or because their 26Al-26Mg or Pb-Pb data are problematic. In either case, this suggests that basaltic angrites are questionable as the age anchor for the entire Solar System as a whole, and other very old, well preserved achondrites should be considered for that role.

Characterization of the organic matter and hydration state of Antarctic micrometeorites: A reservoir distinct from carbonaceous chondrites

This work presents a multi-analysis on 35 Antarctic micrometeorites (AMMs) (Concordia collection 2006) by coupled Raman and Infrared (IR) spectroscopies, in comparison with samples from type 1 and 2 carbonaceous CM, CR and CI chondrites. We identified the Raman G- and D-bands revealing the presence of polyaromatic carbonaceous material on raw particles in a subset of 16 particles. Thirteen AMMs (10 Fg + 1 Fg-Sc + 1 Sc) were selected from this first subset, and analyzed by infrared microscopy along with 4 AMMs (2 Fg + 1 Fg-Sc + 1 Sc) from a previous study by Dobrica et al. (2011). These analyses showed that scoriaceous, fine-grained scoriaceous and part of the fine-grained AMMs are not hydrated, with a weak abundance of carbonaceous matter. According to the Raman criterion defined by Dobrica et al. (2011), hydrous AMMs do not show structural modifications induced by heating through the atmospheric entry. In several hydrous AMMs, the carbonaceous matter abundance is found larger than in Orgueil (CI), Murchison (CM) and QUE 99177 (CR) chondrites and their mineral content exhibit differences reflected by the structure of the silicate 10 µm band. These observations suggest that part of the AMMs originates from one, or several, distinct parent bodies with respect to primitive carbonaceous chondrites. Each hydrous Fg-AMMS displays higher CH2/CH3 ratio and a smaller carbonyl abundance than chondrites, which point toward a mild processing during atmospheric entry, possibly oxidation, which did not modify the carbon backbone and therefore do not induce differences in Raman spectroscopy.

Relict chondrules in primitive achondrites: Remnants from their precursor parent bodies

We studied the petrography, analyzed the chemical compositions, constrained the closure temperatures (via geothermometry), and determined the oxidation states of relict chondrules in Campo del Cielo (IAB iron meteorite), Graves Nunataks (GRA) 98028 (acapulcoite), and Netschaëvo (IIE iron meteorite) to constrain their formation conditions and investigate links to known meteorite groups. Despite having been thermally metamorphosed, mineral phases within relict chondrules retain information about their precursor compositions. The sizes and textures of relict chondrules, and silicate and chromite compositions indicate that Campo del Cielo, GRA 98028, and Netschaëvo had distinct parent bodies that were similar to, but different from, known chondrite groups. To determine the utility of relict chondrule sizes in thermally metamorphosed meteorites, we determined the chondrule size distributions in the LL chondrites Semarkona (LL3.00), Soko-Banja (LL4), Siena (LL5), and Saint-Séverin (LL6), and the H chondrites Clovis (No. 1) (H3.6), Kesen (H4), Arbol Solo (H5), and Estacado (H6). As expected, mean chondrule diameters increase with degree of thermal metamorphism.We find that Campo del Cielo and GRA 98028 were reduced during thermal metamorphism, consistent with previous studies, indicating that their precursors were initially more FeO-rich than their current compositions. In contrast to previous studies, we find no evidence for reduction of silicates in Netschaëvo. Normal zoning of olivine in Netschaëvo is consistent with crystallization and suggests its silicates are near their primary FeO-contents. The presence of elongated chromite grains along olivine grain boundaries in Netschaëvo indicates formation during thermal metamorphism under oxidizing conditions. Due to the absence of reduction and the composition of chromite being distinct from that of metamorphosed H chondrites, we conclude that Netschaëvo, and by extension the IIE iron meteorites, are not from the H chondrite parent body.

The origin of aubrites: Evidence from lithophile trace element abundances and oxygen isotope compositions

We report the abundances of a selected set of “lithophile” trace elements (including lanthanides, actinides and high field strength elements) and high-precision oxygen isotope analyses of a comprehensive suite of aubrites. Two distinct groups of aubrites can be distinguished: (a) the main-group aubrites display flat or light-REE depleted REE patterns with variable Eu and Y anomalies; their pyroxenes are light-REE depleted and show marked negative Eu anomalies; (b) the Mount Egerton enstatites and the silicate fraction from Larned display distinctive light-REE enrichments, and high Th/Sm ratios; Mount Egerton pyroxenes have much less pronounced negative Eu anomalies than pyroxenes from the main-group aubrites.Leaching experiments were undertaken to investigate the contribution of sulfides to the whole rock budget of the main-group aubrites. Sulfides contain in most cases at least 50% of the REEs and of the actinides. Among the elements we have analyzed, those displaying the strongest lithophile behaviors are Rb, Ba, Sr and Sc.The homogeneity of the Δ17O values obtained for main-group aubrite falls [Δ17O=+0.009±0.010‰ (2σ)] suggests that they originated from a single parent body whose differentiation involved an early phase of large-scale melting that may have led to the development of a magma ocean. This interpretation is at first glance in agreement with the limited variability of the shapes of the REE patterns of these aubrites. However, the trace element concentrations of their phases cannot be used to discuss this hypothesis, because their igneous trace-element signatures have been modified by subsolidus exchange. Finally, despite similar O isotopic compositions, the marked light-REE enrichments displayed by Mount Egerton and Larned suggest that they are unrelated to the main-group aubrites and probably originated from a distinct parent body.

Nature and degree of aqueous alteration in CM and CI carbonaceous chondrites

AbstractWe investigated the petrologic, geochemical, and spectral parameters that relate to the type and degree of aqueous alteration in nine CM chondrites and one CI (Ivuna) carbonaceous chondrite. Our underlying hypothesis is that the position and shape of the 3 μm band is diagnostic of phyllosilicate mineralogy. We measured reflectance spectra of the chondrites under dry conditions (elevated temperatures) and vacuum (10−8 to 10−7 torr) to minimize adsorbed water and mimic the space environment, for subsequent comparison with reflectance spectra of asteroids. We have identified three spectral CM groups in addition to Ivuna. “Group 1,” the least altered group as determined from various alteration indices, is characterized by 3 μm band centers at longer wavelengths, and is consistent with cronstedtite (Fe‐serpentine). “Group 3,” the most altered group, is characterized by 3 μm band centers at shorter wavelengths and is consistent with antigorite (serpentine). “Group 2” is an intermediate group between group 1 and 3. Ivuna exhibits a unique spectrum that is distinct from the CM meteorites and is consistent with lizardite and chrysotile (serpentine). The petrologic and geochemical parameters, which were determined using electron microprobe analyses and microscopic observations, are found to be consistent with the three spectral groups. These results indicate that the distinct parent body aqueous alteration environments experienced by these carbonaceous chondrites can be distinguished using reflectance spectroscopy. High‐quality ground‐based telescopic observations of Main Belt asteroids can be expected to reveal not just whether an asteroid is hydrated, but also details of the alteration state.

Zinc isotopes in HEDs: Clues to the formation of 4-Vesta, and the unique composition of Pecora Escarpment 82502

The δ66Zn (permil deviation of the 66Zn/64Zn ratio from a terrestrial standard) values for a suite of 20 non-Antarctic HED (howardite–eucrite–diogenite) meteorites and one mesosiderite, and for eight Antarctic eucrites and diogenites, were measured in order to determine the role of volatization in the formation of their presumed parent body, the asteroid 4-Vesta. The 20 non-Antarctic HEDs had δ66Zn values that ranged from −2.0‰ to +1.67‰, with a mean value of −0.01±0.39‰ (2se); this range likely represents a small-scale heterogeneity due to brecciation induced by multiple impacts. The non-Antarctic eucrites (δ66Zn=+0.00±0.58‰ (2se), n=12) were isotopically the same as the diogenites (δ66Zn=−0.31±0.80‰ (2se), n=4), and the howardites (δ66Zn=+0.26±0.37‰ (2se), n=4). On average, non-Antarctic eucrite falls were isotopically heavier (+0.50‰) than non-Antarctic finds (−1.00‰). The Antarctic finds studied were all unbrecciated samples, and they were significantly heavier than the non-Antarctic samples with a δ66Zn range of +1.63‰ to +6.22‰ for four eucrites (mean, +4.32‰) and +0.94‰ to +1.60‰ for three diogenites (mean+1.23‰), excluding one anomalous sample, while their Zn concentration is significantly lower than the brecciated samples. These data suggest that the unbrecciated eucrites probably represent the eucritic crust shortly after differentiation and cooling of the parent asteroid, at which time volatization of lighter zinc isotopes led to an isotopically heavy crust. Early impact events caused the ejection of these unbrecciated meteorites, which were subsequently spared from brecciation caused by multiple additional impacts on the much larger Vesta. The range of δ66Zn values and Zn concentration for the brecciated HEDs in this study supports a major contribution to the Vestan surface by chondritic impactors (−1.30<δ66Zn<+0.76‰ for ordinary and carbonaceous chondrites). The anomalous eucrite PCA 82502 (δ66Zn=−7.75‰) is significantly isotopically lighter than the other HEDs and is the natural sample with the lightest Zn isotopic composition reported in the solar system to date. This meteorite most likely originated from a distinct parent body.

New constraints on early Solar System chronology from Al–Mg and U–Pb isotope systematics in the unique basaltic achondrite Northwest Africa 2976

We report elemental abundances and the isotopic systematics of the short-lived 26Al– 26Mg (half-life of ∼0.73 Ma) and long-lived U–Pb radiochronometers in the ungrouped basaltic meteorite Northwest Africa (NWA) 2976. The bulk geochemical composition of NWA 2976 is clearly distinct from that of the eucrites and angrites, but shows broad similarities to that of the paired NWA 001 and 2400 ungrouped achondrites indicating that it is likely to also be paired with these two samples. The major and trace element abundances in NWA 2976 further indicate that it formed by extensive melting and magmatic fractionation processes on its parent body. The Al–Mg and Pb–Pb isotope systematics indicate that this meteorite represents the earliest stages of crust formation on a differentiated parent body in the early Solar System. The absolute Pb–Pb internal isochron age of NWA 2976, obtained from acid leaching residues of three whole-rock samples and two pyroxene separates, is 4562.89 ± 0.59 Ma (MSWD = 0.02). This Pb–Pb age is calculated using the measured 238U/ 235U ratio of a NWA 2976 whole-rock of 137.751 ± 0.018 (2 σ) which was determined relative to the recently revised value of 137.840 ± 0.008 for the SRM 950a U isotope standard. The Al–Mg systematics reveal the presence of 26Mg isotopic anomalies produced by the decay of 26Al with an ( 26Al/ 27Al) 0 of (3.94 ± 0.16) × 10 −7, and indicate a time of formation of 0.26 ± 0.18 Ma after the D’Orbigny angrite. Using the revised Pb–Pb age of 4563.36 ± 0.34 Ma for the D’Orbigny anchor (corrected for its U isotopic composition), we deduce an Al–Mg model age of 4563.10 ± 0.38 Ma for NWA 2976, which is consistent with its Pb–Pb internal isochron age. The concordance of the Pb–Pb and Al–Mg chronometers, when taking into account the differences in the U isotopic compositions of the D’Orbigny and NWA 2976 achondrites (whose parent bodies likely formed in distinct regions of early Solar System as indicated by their different oxygen isotopic compositions), implies that 26Al was homogeneously distributed in the early Solar System. It also suggests that igneous processes on planetesimals, as represented by the formation of various basaltic meteorite groups that likely originated on distinct parent bodies (e.g., eucrites and angrites, as well as ungrouped achondrites), were widespread throughout the protoplanetary disk within the first ∼5 Ma of the history of the Solar System.

Nature of volatile depletion and genetic relationships in enstatite chondrites and aubrites inferred from Zn isotopes

Enstatite meteorites include the undifferentiated enstatite chondrites and the differentiated enstatite achondrites (aubrites). They are the most reduced group of all meteorites. The oxygen isotope compositions of both enstatite chondrites and aubrites plot along the terrestrial mass fractionation line, which suggests some genetic links between these meteorites and the Earth as well. For this study, we measured the Zn isotopic composition of 25 samples from the following groups: aubrites (main group and Shallowater), EL chondrites, EH chondrites and Happy Canyon (impact-melt breccia). We also analyzed the Zn isotopic composition and elemental abundance in separated phases (metal, silicates, and sulfides) of the EH4, EL3, and EL6 chondrites. The different groups of meteorites are isotopically distinct and give the following values (‰): aubrite main group (−7.08 < δ 66Zn < −0.37); EH3 chondrites (0.15 < δ 66Zn < 0.31); EH4 chondrites (0.15 < δ 66Zn < 0.27); EH5 chondrites (δ 66Zn = 0.27 ± 0.09; n = 1); EL3 chondrites (0.01 < δ 66Zn < 0.63); the Shallowater aubrite (1.48 < δ 66Zn < 2.36); EL6 chondrites (2.26 < δ 66Zn < 7.35); and the impact-melt enstatite chondrite Happy Canyon (δ 66Zn = 0.37). The aubrite Peña Blanca Spring (δ 66Zn = −7.04‰) and the EL6 North West Forrest (δ 66Zn = 7.35‰) are the isotopically lightest and heaviest samples, respectively, known so far in the Solar System. In comparison, the range of Zn isotopic composition of chondrites and terrestrial samples (−1.5 < δ 66Zn < 1‰) is much smaller ( Luck et al., 2005; Herzog et al., 2009). EH and EL3 chondrites have the same Zn isotopic composition as the Earth, which is another example of the isotopic similarity between Earth and enstatite chondrites. The Zn isotopic composition and abundance strongly support that the origin of the volatile element depletion between EL3 and EL6 chondrites is due to volatilization, probably during thermal metamorphism. Aubrites show strong elemental depletion in Zn compared to both EH and EL chondrites and they are enriched in light isotopes (δ 66Zn down to −7.04‰). This is the opposite of what would be expected if Zn elemental depletion was due to evaporation, assuming the aubrites started with an enstatite chondrite-like Zn isotopic composition. Evaporation is therefore not responsible for volatile loss from aubrites. On Earth, Zn isotopes fractionate very little during igneous processes, while differentiated meteorites show only minimal Zn isotopic variability. It is therefore very unlikely that igneous processes can account for the large isotopic fractionation of Zn in aubrites. Condensation of an isotopically light vapor best explains Zn depletion and isotopically light Zn in these puzzling rocks. Mass balance suggests that this isotopically light vapor carries Zn lost by the EL6 parent body during thermal metamorphism and that aubrites evolved from an EL6-like parent body. Finally, Zn isotopes suggest that Shallowater and aubrites originate from distinct parent bodies.

Differentiation of Vesta and the parent bodies of other achondrites

Numerical simulations have been performed to understand the planetary differentiation of Vesta and other differentiated asteroids. The short‐lived nuclides 26Al and 60Fe were used as the heat sources for the differentiation of asteroids of sizes 20–270 km. The essential aim was to simulate the two proposed differentiation scenarios associated with the formation of the basaltic achondrites. One of these scenarios deals with their origin by partial melting of silicates. The other scenario involves the origin of basalts from the residual melt left subsequent to crystallization in a convective and cooling magma ocean. The core‐mantle differentiation in both the scenarios was commenced subsequent to 40% silicate melting. The partial melt scenario seems to contradict the general trend in the chronological sequence of core‐mantle and mantle‐crust differentiations observed in various differentiated meteorites unless the irons meteorite parent bodies differentiated rapidly compared to prolonged accretion and differentiation of distinct parent bodies of achondrites. Even though the geochemical records of siderophile elements in eucrites favor the residual melt scenario, based on our simulations it is difficult to rapidly and globally cool the magma oceans to produce residual basaltic melts subsequent to crystallization within initial ∼10 million years. This is in contradiction with the chronological records of the basalts that suggest their early origin. In order to make this scenario viable by invoking rapid and global cooling of the magma ocean it would be essential to substantially remove the insulating crust of the planetesimal by continuous bombardment of small planetesimals in the early solar system.

Iron meteorites: Crystallization, thermal history, parent bodies, and origin

We review the crystallization of the iron meteorite chemical groups, the thermal history of the irons as revealed by the metallographic cooling rates, the ages of the iron meteorites and their relationships with other meteorite types, and the formation of the iron meteorite parent bodies. Within most iron meteorite groups, chemical trends are broadly consistent with fractional crystallization, implying that each group formed from a single molten metallic pool or core. However, these pools or cores differed considerably in their S concentrations, which affect partition coefficients and crystallization conditions significantly. The silicate-bearing iron meteorite groups, IAB and IIE, have textures and poorly defined elemental trends suggesting that impacts mixed molten metal and silicates and that neither group formed from a single isolated metallic melt. Advances in the understanding of the generation of the Widmanstätten pattern, and especially the importance of P during the nucleation and growth of kamacite, have led to improved measurements of the cooling rates of iron meteorites. Typical cooling rates from fractionally crystallized iron meteorite groups at 500–700 °C are about 100–10,000 °C/Myr, with total cooling times of 10 Myr or less. The measured cooling rates vary from 60 to 300 °C/Myr for the IIIAB group and 100–6600 °C/Myr for the IVA group. The wide range of cooling rates for IVA irons and their inverse correlation with bulk Ni concentration show that they crystallized and cooled not in a mantled core but in a large metallic body of radius 150±50 km with scarcely any silicate insulation. This body may have formed in a grazing protoplanetary impact. The fractionally crystallized groups, according to Hf–W isotopic systematics, are derived originally from bodies that accreted and melted to form cores early in the history of the solar system, <1 Myr after CAI formation. The ungrouped irons likely come from at least 50 distinct parent bodies that formed in analogous ways to the fractionally crystallized groups. Contrary to traditional views about their origin, iron meteorites may have been derived originally from bodies as large as 1000 km or more in size. Most iron meteorites come directly or indirectly from bodies that accreted before the chondrites, possibly at 1–2 AU rather than in the asteroid belt. Many of these bodies may have been disrupted by impacts soon after they formed and their fragments were scattered into the asteroid belt by protoplanets.

Amino acid analyses of Antarctic CM2 meteorites using liquid chromatography-time of flight-mass spectrometry

Abstract— Amino acid analyses of the Antarctic CM2 chondrites Allan Hills (ALH) 83100 and Lewis Cliff (LEW) 90500 using liquid chromatography-time of flight-mass spectrometry (LC-ToF-MS) coupled with UV fluorescence detection revealed that these carbonaceous meteorites contain a suite of indigenous amino acids not present in Antarctic ice. Several amino acids were detected in ALH 83100, including glycine, alanine, β-alanine, γ-amino-n-butyric acid (γ-ABA), and α-aminoisobutyric acid (AIB) with concentrations ranging from 250 to 340 parts per billion (ppb). In contrast to ALH 83100, the CM2 meteorites LEW 90500 and Murchison had a much higher total abundance of these amino acids (440–3200 ppb). In addition, ALH 83100 was found to have lower abundances of the α-dialkyl amino acids AIB and isovaline than LEW 90500 and Murchison. There are three possible explanations for the depleted amino acid content in ALH 83100: 1) amino acid leaching from ALH 83100 during exposure to Antarctic ice meltwater, 2) a higher degree of aqueous alteration on the ALH 83100 parent body, or 3) ALH 83100 originated on a chemically distinct parent body from the other two CM2 meteorites. The high relative abundance of ɛ-amino-n-caproic acid (EACA) in the ALH 83100 meteorite as well as the Antarctic ice indicates that Nylon-6 contamination from the Antarctic sample storage bags may have occurred during collection.