IAB Iron Meteorites Research Articles

First widespread occurrence of rare phosphate chladniite in a meteorite, winonaite Graves Nunataks (GRA) 12510: Implications for phosphide–phosphate redox buffered genesis in meteorites

Abstract The first widespread occurrence of rare Na-, Ca-, and Mg, Mn, Fe-bearing phosphate chladniite was observed in meteorite Graves Nunataks (GRA) 12510, which is a primitive achondrite that sits within the winonaite class. Numerous 1–500 µm chladniite grains were found, often on the margins between silicate clasts and the kamacite portions of the large metal veins that permeated through the sample. The largest chladniite grains are associated with merrillite, kamacite, taenite, troilite, albite, forsterite, diopside, and enstatite, with a few tiny chladniite grains and an apatite grain enclosed within merrillite. GRA 12510s average chladniite composition is Na2.7Ca1.25(Mg10.02Mn0.69Fe0.20)Σ10.91(PO4)9. Electron backscattered diffraction (EBSD) patterns indicate varying degrees of nucleation and growth of chladniite grains. Additionally, the first pure Raman spectrum of chladniite is described here, revealing primary ν1 bands at 954, 974, and especially 984 cm–1. The co-occurrence and close association of merrillite, apatite, chladniite, and P-bearing metallic phases within GRA 12510 suggests that the fO2 of IW-2 to IW-4 is an intrinsic property of the precursor chondritic material, and the phosphate-phosphide reaction may have buffered the final winonaite and IAB iron meteorite phase assemblages. Altogether, chladniite appears to form alongside other phosphates, with their chemistries reflecting the diverse environment of their formation. Meteoritic chladniite likely formed through subsolidus oxidation of schreibersite, scavenging Na from albite, Ca from diopside, Mg from enstatite/forsterite, Fe from kamacite/taenite, and Mn from alabandite/chromite when available. A P0-P5+ redox-buffered environment also has implications for thermometry and fast cooling rates, although more experiments are needed to extrapolate powder reaction rates to those of larger crystals. Furthermore, phosphide-phosphate buffered experiments may aid in investigating equilibrium chemistry at fO2 values between IW-2 and IW-4, which have been challenging to explore experimentally due to the limited availability of solid metal-metal oxide buffers between IW (Fe-FeO) and IW-5 (Cr-Cr2O3) at temperatures and pressures relevant to planetary interiors. Future investigations of phosphide-phosphate redox-buffered genesis at fO2 values between IW-2 and IW-4 have important implications for primitive meteorite constituents (e.g., CAI values), partially differentiated planetesimals and planets, including Mercury and core formation on Earth.

Three new iron-phosphate minerals from the El Ali iron meteorite, Somalia: Elaliite, Fe2+8Fe3+ (PO4)O8; elkinstantonite, Fe4(PO4)2O; and olsenite, KFe4(PO4)3

Abstract Petrologic investigation of the El Ali IAB iron meteorite (Somalia) revealed three new minerals: elaliite (Fe2+8Fe3+ (PO4)O8, IMA 2022-087), elkinstantonite(Fe4(PO4)2O, IMA 2022-088), and olsenite (KFe4(PO4)3, IMA 2022-100). The name elaliite recognizes the occurrence of this mineral within the El Ali meteorite, originally located at 4° 17′ 17″N, 44° 53′ 54″E. Elkinstantonite is named after Linda (Lindy) Elkins-Tanton (b. 1965), a planetary scientist and professor in the School of Earth and Space Exploration at Arizona State University. The name olsenite is in honor of Edward J. Olsen (1927-2020), the former Curator of Mineralogy and Meteorites at the Field Museum of Natural History in Chicago (1960-1991). The new minerals and their names have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association. The holotype specimens of elaliite, elkinstantonite and olsenite consist of the polished block mount with accession number MET11814/2-1/EP1 deposited in the University of Alberta Meteorite Collection. Elaliite, elkinstantonite and olsenite occur along with wustite, troilite, sarcopside and Ca-bearing graftonite within inclusions in the iron-nickel metal (kamacite, 9.4 wt% Ni) that makes up the bulk of the El Ali sample. The empirical formulas determined by electron probe microanalysis for elaliite, elkinstantonite, and olsenite are: (Fe2+7.943Fe3+1.020Cr0.010Ni0.006Ca0.004Mn0.004)Σ8.987(P0.932Si0.077S0.005)Σ1.014O12, (Fe2+3.947Mn0.016Ni0.003Ca0.001Cr0.001)Σ3.968(P1.986Si0.014S0.013)Σ2.013O9, and (K0.820Na0.135Ca0.004)Σ0.959(Fe3.829Mn0.050)Σ3.879(P2.972S0.058Si0.017)Σ3.047O12. Electron backscatter diffraction was used to confirm the crystal structures of the three new minerals. Raman spectra for all three minerals are also presented.

An arrowhead made of meteoritic iron from the late Bronze Age settlement of Mörigen, Switzerland and its possible source

A search for artefacts made of meteoritic iron has been performed in archaeological collections in the greater area of the Lake of Biel, Switzerland. A single object made of meteoritic iron has been identified, an arrowhead with a mass of 2.9 g found in the 19th Century in the late Bronze Age (900–800 BCE) lake dwelling of Mörigen, Switzerland. The meteoritic origin is definitely proven by combining methods extended and newly applied to an archaeological artefact. Elemental composition (7.10–8.28 wt% Ni, 0.58–0.86 wt% Co, ∼300 ppm Ge), primary mineralogy consisting of the associated Ni-poor and Ni-rich iron phases kamacite (6.7 wt% Ni) and taenite (33.3 wt% Ni), and the presence of cosmogenic 26Al (1.7−0.4+0.5 dpm/kg). The Ni-rich composition below the oxidized crust and the marked difference to meteorites from the nearby (4–8 km) Twannberg iron meteorite strewn field is confirmed by muon induced X-ray emission spectrometry (8.28 wt% Ni). The Ni-Ge-concentrations are consistent with IAB iron meteorites, but not with the Twannberg meteorite (4.5 wt% Ni, 49 ppm Ge). The measured activity of 26Al indicates derivation from an iron meteorite with a large (2 t minimum) pre-atmospheric mass. The flat arrowhead shows a laminated texture most likely representing a deformed Widmanstätten pattern, grinding marks on the surface and remnants of wood-tar. Among just three large European IAB iron meteorites with fitting chemical composition, the Kaalijarv meteorite (Estonia) is the most likely source because this large crater-forming fall event happened at ∼1500 years BC during the Bronze Age and produced many small fragments. The discovery and subsequent transport/trade of such small iron fragments appears much more likely than in case of buried large meteorite masses. Additional artefacts of the same origin may be present in archaeological collections.

Edward R. D. Scott (1947–2021): A tribute to an exceptional scientist and wonderful man

Edward R. D. Scott (1947–2021): A tribute to an exceptional scientist and wonderful man

DO THE METALLIC METEORITES FOUND IN BOLIVIA HAVE THE SAME ORIGIN?

While reconstructing the history of meteorites related to Bolivia, we found that the only two metallic meteorites classified as “Found in Bolivia” are the Bolivia and Pooposo meteorites. They are classified within the group of MG IAB iron meteorites and have a similar history. Both were seriously mishandled before reaching the hands of meteorite experts and were commercialized by a priest. In light of these possible matches, some experts have recommended a comparative study of both meteorites to ascertain if they could be part of the same object. In this paper we analyze the data of abundant chemical elements found in both meteorites. The data was obtained from different analyses carried out on all known iron meteorites through methods such as: Radiochemical Neutron Activation Analysis (RNAA) and Instrumental Neutron Activation Analysis (INAA). These data have been published by different researchers: Buchwald (1975a); Scott (1978); Kracher et al. (1980); Wasson & Kallemeyn (2002). Our objective has been to determine if both specimens are part of a larger object that impacted on the surface of the Oruro plateau or if they are simply two independent iron meteorites.

Cooling rates and impact histories of group IAB and other IAB complex iron meteorites inferred from zoned taenite and the cloudy zone

AbstractMetallographic cooling rates of 10–20 K Myr−1 were obtained for four IAB iron meteorites from electron probe analyses of taenite and models simulating kamacite growth. Cooling rates were also determined for 21 irons in the IAB complex from the size of tetrataenite particles in the cloudy zone and a calibration curve using data for the four IAB irons and five other iron and stony iron groups. We find that the closely related IAB main group, low‐Au subgroups, Pitts grouplet irons, and San Cristobal cooled through 500 °C at 10–35 K Myr−1. The ungrouped IAB complex irons, Santa Catharina and Twin City, have bulk Ni contents of ~30 wt% and cooled much faster at ~200–1000 K Myr−1, most probably in a different parent body. Our cooling rates and observations allow conflicting interpretations of the nature of the low‐Ni phase in the cloudy zone to be reconciled. We infer that in fast cooled irons like Santa Catharina, the low‐Ni phase transforms to antitaenite, but in very slowly cooled meteorites like mesosiderites, martensite forms instead. In meteorites with intermediate cooling rates like IAB irons, the bulk of the cloudy zone probably contains antitaenite as the low‐Ni phase, but in the coarse region next to tetrataenite, martensite is present. Published isotopic ages show that metal–silicate mixing in group IAB irons occurred ~5–10 Myr after CAI formation, but the nature and timing of the impact are poorly constrained at present. Ar‐Ar ages testify to shock reheating of plagioclase during cooling over ~100 Myr. However, the cloudy zone is well preserved showing that metal was not heated significantly by shocks after slow cooling through 400 °C.

Neutron capture 128Xe and 129Xe in the San Juan mass of the Campo del Cielo IAB iron meteorite: Evidence for a high fluence of thermalized neutrons

AbstractThe petrographic study of the San Juan A2 polished section demonstrated textural and compositional similarities with the Campo del Cielo IAB iron meteorite, with trace element abundances in metal following the pattern of bulk Campo del Cielo. Xenon, neon, and helium isotopic compositions have been measured in radial graphite rims, massive graphite inclusions, fine‐grained graphite aggregates, cliftonite, and platy graphite. Two silicate inclusions and two areas of metal were also analyzed. 3He/4He versus 4He/21Ne data for San Juan metal plot next to the values reported for the El Taco fragment of Campo del Cielo, supporting San Juan being a part of the Campo del Cielo meteorite shower. Based on the Ne isotopic composition of its components, and the observed correlation between 128Xe and 129Xe, the San Juan A fragment of Campo del Cielo was well shielded from the primary galactic cosmic ray high‐energy irradiation. Its size allowed the secondary neutrons to be fairly well thermalized, receiving an equivalent (normalized to the research reactor with highly thermalized neutron spectrum) fluence of thermal and epithermal neutrons of 6.6 × 1017 n cm−2. Considering 1.8 × 108 years single‐stage and constant exposure geometry irradiation history for Campo del Cielo, and assuming the identical neutron flux spectra for the research reactor and Campo del Cielo, the average thermal equivalent neutron flux for San Juan is about 1.2 × 102 n cm−2 s−1. Xe isotopic composition in the radial graphite rims and platy graphite shows evidence of live 129I in San Juan and is consistent with a mixture of iodine‐derived and tellurium‐derived Xe.

Superconductivity found in meteorites

Meteorites can contain a wide range of material phases due to the extreme environments found in space and are ideal candidates to search for natural superconductivity. However, meteorites are chemically inhomogeneous, and superconducting phases in them could potentially be minute, rendering detection of these phases difficult. To alleviate this difficulty, we have studied meteorite samples with the ultrasensitive magnetic field modulated microwave spectroscopy (MFMMS) technique [J. G. Ramírez, A. C. Basaran, J. de la Venta, J. Pereiro, I. K. Schuller, Rep. Prog. Phys. 77, 093902 (2014)]. Here, we report the identification of superconducting phases in two meteorites, Mundrabilla, a group IAB iron meteorite [R. Wilson, A. Cooney, Nature 213, 274-275 (1967)] and GRA 95205, a ureilite [J. N. Grossman, Meteorit. Planet. Sci. 33, A221-A239 (1998)]. MFMMS measurements detected superconducting transitions in samples from each, above 5 K. By subdividing and remeasuring individual samples, grains containing the largest superconducting fraction were isolated. The superconducting grains were then characterized with a series of complementary techniques, including vibrating-sample magnetometry (VSM), energy-dispersive X-ray spectroscopy (EDX), and numerical methods. These measurements and analysis identified the likely phases as alloys of lead, indium, and tin.

Variations in the Magnetic Properties of Meteoritic Cloudy Zone

AbstractIron and stony‐iron meteorites form the Widmanstätten pattern during slow cooling. This pattern is composed of several microstructures whose length‐scale, composition and magnetic properties are dependent upon cooling rate. Here we focus on the cloudy zone: a region containing nanoscale tetrataenite islands with exceptional paleomagnetic recording properties. We present a systematic review of how cloudy zone properties vary with cooling rate and proximity to the adjacent tetrataenite rim. X‐ray photoemission electron microscopy is used to compare compositional and magnetization maps of the cloudy zone in the mesosiderites (slow cooling rates), the IAB iron meteorites and the pallasites (intermediate cooling rates), and the IVA iron meteorites (fast cooling rates). The proportions of magnetic phases within the cloudy zone are also characterized using Mössbauer spectroscopy. We present the first observations of the magnetic state of the cloudy zone in the mesosiderites, showing that, for such slow cooling rates, tetrataenite islands grow larger than the multidomain threshold, creating large‐scale regions of uniform magnetization across the cloudy zone that render it unsuitable for paleomagnetic analysis. For the most rapidly cooled IVA meteorites, the time available for Fe‐Ni ordering is insufficient to allow tetrataenite formation, again leading to behavior that is unsuitable for paleomagnetic analysis. The most reliable paleomagnetic remanence is recorded by meteorites with intermediate cooling rates ( 2–500 °C Myr ) which produces islands that are “just right” in both size and degree of Fe‐Ni order.

The layered structure model for winonaite parent asteroid implicated by textural and mineralogical diversity

The winonaites are primitive achondrites that are associated with IAB iron meteorites. They provide valuable insights into differentiation processes on asteroids in the early Solar System. However, there is still little understanding of the lithological diversity as well as the structure of the winonaite parent asteroid. In this work, we report the petrologic texture and mineralogy of a suite of winonaites (i.e., Northwest Africa (NWA) 725, NWA 6448, NWA 4024, Grove Mountains (GRV) 022890, GRV 021663, and Sahara (SAH) 02029) that exhibit a wide diversity of petrographic textures from primitive chondritic texture to coarse-grained equigranular texture. In particular, we recognized an unusual winonaite (SAH 02029) with a distinctive mineralogy and mineral chemistry (e.g., depleted in troilite, plagioclase contains melt inclusions, high plagioclase An values, and LREE-depleted clinopyroxene). The petrological and mineralogical features of SAH 02029 indicate that this meteorite has undergone silicate partial melting and may represent the residue of ~ 5–10 vol% partial melting. The textural and mineralogical diversity among winonaites suggests that the winonaite-IAB parent asteroid would have formed a four-layered structure during its evolution history: (1) surface layer consisting of precursor chondritic materials; (2) subsurface layer composed of diverse lithologies that experienced limited metamorphism and FeNi–FeS partial melting; (3) deep residues of silicate partial melting; and (4) interior layer consisting of incomplete differentiation metal pools. This conclusion enables us to establish constraints on the evolution history of winonaite-IAB parent body.

New insights into Mo and Ru isotope variation in the nebula and terrestrial planet accretionary genetics

When corrected for the effects of cosmic ray exposure, Mo and Ru nucleosynthetic isotope anomalies in iron meteorites from at least nine different parent bodies are strongly correlated in a manner consistent with variable depletion in s-process nucleosynthetic components. In contrast to prior studies, the new results show no significant deviations from a single correlation trend. In the refined Mo–Ru cosmic correlation, a distinction between the non-carbonaceous (NC) group and carbonaceous chondrite (CC) group is evident. Members of the NC group are characterized by isotope compositions reflective of variable s-process depletion. Members of the CC group analyzed here plot in a tight cluster and have the most s-process depleted Mo and Ru isotopic compositions, with Mo isotopes also slightly enriched in r- and possibly p-process contributions. This indicates that the nebular feeding zone of the NC group parent bodies was characterized by Mo and Ru with variable s-process contributions, but with the two elements always mixed in the same proportions. The CC parent bodies sampled here, by contrast, were derived from a nebular feeding zone that had been mixed to a uniform s-process depleted Mo–Ru isotopic composition.Six molybdenite samples, four glacial diamictites, and two ocean island basalts were analyzed to provide a preliminary constraint on the average Mo isotope composition of the bulk silicate Earth (BSE). Combined results yield an average μ97Mo value of +3 ± 6. This value, coupled with a previously reported μ100Ru value of +1 ± 7 for the BSE, indicates that the isotopic composition of the BSE falls precisely on the refined Mo–Ru cosmic correlation. The overlap of the BSE with the correlation implies that there was homogeneous accretion of siderophile elements for the final accretion of 10 to 20 wt% of Earth's mass. The only known cosmochemical materials with an isotopic match to the BSE, with regard to Mo and Ru, are some members of the IAB iron meteorite complex and enstatite chondrites.

Multiple sulfur isotopic composition of main group pallasites support genetic links to IIIAB iron meteorites

This study reports the quadruple sulfur isotope composition of troilite nodules from Main Group Pallasites. Values range from −0.23‰ to 0.34‰ (average = 0.03 ± 0.17‰ S.D.) in δ34S and 0.008‰ to 0.025‰ (average = 0.018 ± 0.006‰ S.D.) in Δ33S and −0.38 to −0.01 (average = −0.17 ± 0.11‰ S.D.) in Δ36S. The variance of these analyses is comparable to estimates of analytical uncertainty (±0.3‰, ±0.008‰, and ±0.3‰, for δ34S, Δ33S, and Δ36S, respectively) and the average of these values is taken as a constraint on the composition of sulfur in the MG Pallasite parent body. The different Δ33S value compared to CDT and IAB iron meteorites at a similar δ34S value is interpreted as a mass-independent signature. This signature is similar in magnitude and direction to previously published values observed in IIIAB iron meteorites, further supporting a genetic relationship between the two groups of meteorites.

Late metal–silicate separation on the IAB parent asteroid: Constraints from combined W and Pt isotopes and thermal modelling

The short-lived 182Hf–182W decay system is a powerful chronometer for constraining the timing of metal–silicate separation and core formation in planetesimals and planets. Neutron capture effects on W isotopes, however, significantly hamper the application of this tool. In order to correct for neutron capture effects, Pt isotopes have emerged as a reliable in-situ neutron dosimeter. This study applies this method to IAB iron meteorites, in order to constrain the timing of metal segregation on the IAB parent body.The ε182W values obtained for the IAB iron meteorites range from −3.61 ± 0.10 to −2.73 ± 0.09. Correlating εiPt with ε182W data yields a pre-neutron capture ε182W of −2.90 ± 0.06. This corresponds to a metal–silicate separation age of 6.0 ± 0.8 Ma after CAI for the IAB parent body, and is interpreted to represent a body-wide melting event. Later, between 10 and 14 Ma after CAI, an impact led to a catastrophic break-up and subsequent reassembly of the parent body. Thermal models of the interior evolution that are consistent with these estimates suggest that the IAB parent body underwent metal–silicate separation as a result of internal heating by short-lived radionuclides and accreted at around 1.4±0.1 Ma after CAIs with a radius of greater than 60 km.

Trace element inventory of meteoritic Ca-phosphates

Most extraterrestrial samples feature the two accessory Ca-phosphates (apatite-group minerals and merrillite), which are important carrier phases of the rare earth elements (REE). The trace-element concentrations (REE, Sc, Ti, V, Cr, Mn, Co, As, Rb, Sr, Y, Zr, Nb, Ba, Hf, Ta, Pb, Th, and U) of selected grains were analyzed by LA-ICP-MS and/or SIMS (REE only). This systematic investigation includes 99 apatite and 149 merrillite analyses from meteorites deriving from various asteroidal bodies including 1 carbonaceous chondrite, 8 ordinary chondrites, 3 acapulcoites, 1 winonaite, 2 eucrites, 5 shergottites, 1 ureilitic trachyandesite, 2 mesosiderites, and 1 silicate-bearing IAB iron meteorite. Although Ca-phosphates predominantly form in metamorphic and/or metasomatic reactions, some are of igneous origin. As late-stage phases that often incorporate the vast majority of their host’s bulk REE budget, the investigated Ca-phosphates have REE enrichments of up to two orders of magnitude compared to the host rock’s bulk concentrations. Within a single sample, each phosphate species displays a uniform REE-pattern, and variations are mainly restricted to their enrichment, therefore indicating similar formation conditions. Exceptions are brecciated samples, i.e., the Adzhi-Bogdo (LL3-6) ordinary chondrite. Despite this uniformity within single samples, distinct meteorite groups do not necessarily have unique REE-patterns. Four basic shapes dominate the REE patterns of meteoritic Ca-phosphates: (1) flat patterns, smoothly decreasing from La-Lu with prominent negative Eu anomalies (acapulcoites, eucrites, apatite from the winonaite and the ureilitic trachyandesite, merrillite from ordinary chondrites); (2) unfractionated patterns, with only minor or no anomalies (mesosiderites, enriched shergottites, IAB-iron meteorite); (3) LREE-enriched patterns, with either positive or slightly negative Eu anomalies (chondritic apatite); and (4) strongly LREE-depleted patterns, with negative Eu anomalies (depleted shergottites). The patterns do not correlate with the grade of metamorphism (petrologic type), specific adjacent mineral assemblages or with Ca-phosphate grain size. Neither the proportions of different REE, nor particular REE patterns themselves are universally correlated to a specific formation mechanism yet Eu (i.e., magnitude of the Eu anomaly) is a sensitive indicator to evaluate the timing of plagioclase and phosphate crystallization. Based on our data, U and Th abundances in apatite increase (almost linearly) with the grade of metamorphism, as well as with the differentiation of their host rock.

The ruthenium isotopic composition of the oceanic mantle

The approximately chondritic relative, and comparatively high absolute mantle abundances of the highly siderophile elements (HSE), suggest that their concentrations in the bulk silicate Earth were primarily established during a final ∼0.5 to 1% of “late accretion” to the mantle, following the cessation of core segregation. Consequently, the isotopic composition of the HSE Ru in the mantle reflects an amalgamation of the isotopic compositions of late accretionary contributions to the silicate portion of the Earth. Among cosmochemical materials, Ru is characterized by considerable mass-independent isotopic variability, making it a powerful genetic tracer of Earth's late accretionary building blocks. To define the Ru isotopic composition of the oceanic mantle, the largest portion of the accessible mantle, we report Ru isotopic data for materials from one Archean and seven Phanerozoic oceanic mantle domains. A sample from a continental lithospheric mantle domain is also examined. All samples have identical Ru isotopic compositions, within analytical uncertainties, indicating that Ru isotopes are well mixed in the oceanic mantle, defining a μ100Ru value of 1.2±7.2 (2SD). The only known meteorites with the same Ru isotopic composition are enstatite chondrites and, when corrected for the effects of cosmic ray exposure, members of the Main Group and sLL subgroup of the IAB iron meteorite complex which have a collective CRE corrected μ100Ru value of 0.9±3.0. This suggests that materials from the region(s) of the solar nebula sampled by these meteorites likely contributed the dominant portion of late accreted materials to Earth's mantle.

Platinum isotopes in iron meteorites: Galactic cosmic ray effects and nucleosynthetic homogeneity in the p-process isotope 190Pt and the other platinum isotopes

Platinum isotopes are sensitive to the effects of galactic cosmic rays (GCR), which can alter isotope ratios and mask nucleosynthetic isotope variations. Platinum also features one p-process isotope, 190Pt, which is very low abundance and therefore challenging to analyse. Platinum-190 is relevant for early solar-system chronology because of its decay to 186Os. Here, we present new Pt isotope data for five iron meteorite groups (IAB, IIAB, IID, IIIAB and IVA), including high-precision measurements of 190Pt for the IAB, IIAB and IIIAB irons, determined by multi-collector ICPMS. New data are in good agreement with previous studies and display correlations between different Pt isotopes. The slopes of these correlations are well-reproduced by the available GCR models. We report Pt isotope ratios for the IID meteorite Carbo that are consistently higher than the predicted effects from the GCR model. This suggests that the model predictions do not fully account for all the GCR effects on Pt isotopes, but also that the pre-atmospheric radii and exposure times calculated for Carbo may be incorrect. Despite this, the good agreement of relative effects in Pt isotopes with the predicted GCR trends confirms that Pt isotopes are a useful in-situ neutron dosimeter. Once GCR effects are accounted for, our new dataset reveals s- and r-process homogeneity between the iron meteorite groups studied here and the Earth. New 190Pt data for the IAB, IIAB and IIIAB iron meteorites indicate the absence of GCR effects and homogeneity in the p-process isotope between these groups and the Earth. This corresponds well with results from other heavy p-process isotopes and suggests their homogenous distribution in the inner solar system, although it does not exclude that potential p-process isotope variations are too diluted to be currently detectable.

Characterizing cosmochemical materials with genetic affinities to the Earth: Genetic and chronological diversity within the IAB iron meteorite complex

The IAB iron meteorite complex consists of a main group (MG) and five chemical subgroups (sLL, sLM, sLH, sHL, and sHH). Here, mass-independent Mo and radiogenic 182W isotope compositions are reported for IAB complex meteorites to evaluate the genetics and chronology, respectively, of the MG and subgroups. Osmium isotopes are used to correct for cosmic ray exposure effects on isotopes of Mo and W. The MG and three subgroups (i.e., sLL, sLM, and sLH), characterized by low Au abundances, have the same Mo isotopic compositions within analytical uncertainty, consistent with a common genetic origin. These meteorites, together with winonaites, are the only cosmochemical materials yet identified with Mo isotopic compositions that are identical to Earth. The Mo isotopic compositions of two subgroups characterized by higher Au abundances (sHL and sHH) are identical to one another within uncertainty, but differ from the low Au subgroups, indicating derivation from genetically distinct materials.The MG has a 182W, post calcium–aluminum inclusion (CAI) formation model age of 3.4±0.7 Ma. One of the low Au subgroups (sLM) is ∼1.7 Ma younger, whereas the high Au subgroups are ∼1.5–3 Ma older. The new Mo–W data, coupled with chemical data, indicate that the MG and the low Au subgroups formed in different impact-generated melts, some of which evidently formed on a chemically disparate, but genetically identical parent body. The high Au subgroups likely formed via core-formation processes on separate, internally-heated parent bodies from other IAB subgroups. The IAB complex meteorites fall on a linear trend defined by 94Mo/96Mo vs. 95Mo/96Mo, along with most other iron meteorite groups. Variation along this line was caused by mixing between at least two nebular components. One component was likely a pure s-process enriched nucleosynthetic carrier, and the other a homogenized nebular component. Sombrerete, currently classified as an sHL iron, has a Mo isotopic composition that is distinct from all IAB complex meteorites analyzed here. Along with group IVB iron meteorites and some ungrouped iron meteorites, it falls on a separate line from other meteorites which may reflect addition of an r-process-enriched component, and it should no longer be classified as a IAB iron.

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.

Geochemistry and growth morphology of alkali feldspar crystals from an IAB iron meteorite – insight into possible hypotheses of their crystallization

Alkali feldspar crystals have been recognized in the troilite-graphite nodules of the Morasko IAB iron meteorite. Their chemical, microtextural and structural properties were studied using electron microprobe analysis (EMPA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), transmission electron microscopy (TEM) and Raman spectroscopy. The feldspars occur as perthitic or antiperthitic intergrowths, whereas the albite lamellae are perfectly twinned. The structural properties reveal intergrown phases with fairly disordered patterns. The electron microprobe analyses demonstrate that the intergrown phases are mainly rich in sodium or potassium, resulting in compositions that are close to those of albite or orthoclase. The compositions, calculated on the basis of a segmented perthite-antiperthite image, showed that the Or-to-Ab proportions in the homogenized crystals were almost 0.3:0.7, thus indicating that the anorthoclase crystallized under high-temperature conditions. Two hypotheses of crystal formation could account for these characteristics: crystallization from a melt or from a metasomatic solution. Relics with evidence of metasomatic replacement of former minerals were not found. Accordingly, this work focuses on arguments that support the other hypothesis. Large ion lithophile elements (LILEs, e.g., Ba, Sr, Rb, LREE, Pb, and Ga) were used to track the origin of the crystals. Their concentrations indicate crystallization from a parent melt strongly depleted in LILEs. Alkali feldspar is commonly a product of a highly differentiated melt. However, highly differentiated melts are typically enriched in LILEs, which here is not the case. The melt that crystallized the feldspar cannot be related to impact-induced partial melting of the chondritic material alone. The derived melt probably was contaminated by silica-rich target material during interaction between the IAB projectile and the target material and was accompanied by metal and sulphide melts that were both immiscible with a silicate melt.

Distribution of p-process 174Hf in early solar system materials and the origin of nucleosynthetic Hf and W isotope anomalies in Ca–Al rich inclusions

Some nuclides that were produced in supernovae are heterogeneously distributed between different meteoritic materials. In some cases these heterogeneities have been interpreted as the result of interaction between ejecta from a nearby supernova and the nascent solar system. Particularly in the case of the oldest objects that formed in the solar system – Ca–Al rich inclusions (CAIs) – this view is confirm the hypothesis that a nearby supernova event facilitated or even triggered solar system formation. We present Hf isotope data for bulk meteorites, terrestrial materials and CAIs, for the first time including the low-abundance isotope 174Hf (∼0.16%). This rare isotope was likely produced during explosive O/Ne shell burning in massive stars (i.e., the classical “p-process”), and therefore its abundance potentially provides a sensitive tracer for putative heterogeneities within the solar system that were introduced by supernova ejecta. For CAIs and one LL chondrite, also complementary W isotope data are reported for the same sample cuts. Once corrected for small neutron capture effects, different chondrite groups, eucrites, a silicate inclusion of a IAB iron meteorite, and terrestrial materials display homogeneous Hf isotope compositions including 174Hf. Hafnium-174 was thus uniformly distributed in the inner solar system when planetesimals formed at the <50 ppm level. This finding is in good agreement with the evidently homogeneous distributions of p-process isotopes 180W, 184Os and possibly 190Pt between different iron meteorite groups. In contrast to bulk meteorite samples, CAIs show variable depletions in p-process 174Hf with respect to the inner solar system composition, and also variable r-process (or s-process) Hf and W contributions. Based on combined Hf and W isotope compositions, we show that CAIs sampled at least one component in which the proportion of r- and s-process derived Hf and W deviates from that of supernova ejecta. The Hf and W isotope anomalies in CAIs are therefore best explained by selective processing of presolar carrier phases prior to CAI formation, and not by a late injection of supernova materials. Likewise, other isotope anomalies in additional elements in CAIs relative to the bulk solar system may reflect the same process. The isotopic heterogeneities between the first refractory condensates may have been eradicated partially during CAI formation, because W isotope anomalies in CAIs appear to decrease with increasing W concentrations as inferred from time-integrated 182W/184W. Importantly, the 176Lu–176Hf and 182Hf–182W chronometers are not significantly affected by nucleosynthetic heterogeneity of Hf isotopes in bulk meteorites, but may be affected in CAIs.