¹Nicolas Dauphas, ²Franck Poitrasson, ¹Christoph Burkhardt, ³Hiroshi Kobayashi, ⁴Kosuke Kurosawa
¹Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60615, USA
²Laboratoire Géosciences Environnement Toulouse, CNRS UMR 5563 – UPS – IRD, 14-16, avenue Edouard Belin, 31400 Toulouse, France
³Department of Physics, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan
⁴Planetary Exploration Research Center, Chiba Institute of Technology, 2-17-1, Tsudanuma, Narashino, Chiba 275-0016, Japan

The bulk chemical compositions of planets are uncertain, even for major elements such as Mg and Si. This is due to the fact that the samples available for study all originate from relatively shallow depths. Comparison of the stable isotope compositions of planets and meteorites can help overcome this limitation. Specifically, the non-chondritic Si isotope composition of the Earth’s mantle was interpreted to reflect the presence of Si in the core, which can also explain its low density relative to pure Fe–Ni alloy. However, we have found that angrite meteorites display a heavy Si isotope composition similar to the lunar and terrestrial mantles. Because core formation in the angrite parent-body (APB) occurred under oxidizing conditions at relatively low pressure and temperature, significant incorporation of Si in the core is ruled out as an explanation for this heavy Si isotope signature. Instead, we show that equilibrium isotopic fractionation between gaseous SiO and solid forsterite at ∼1370 K in the solar nebula could have produced the observed Si isotope variations. Nebular fractionation of forsterite should be accompanied by correlated variations between the Si isotopic composition and Mg/Si ratio following a slope of ∼1, which is observed in meteorites. Consideration of this nebular process leads to a revised Si concentration in the Earth’s core of 3.6 (+6.0/−3.6) wt%(+6.0/−3.6) wt% and provides estimates of Mg/Si ratios of bulk planetary bodies.

Reference
Dauphas N, Poitrasson F, Burkhardt C, Kobayashi H, Kurosawa K (2015) Planetary and meteoritic Mg/Si and δ30Siδ30Si variations inherited from solar nebula chemistry. Earth and Planetary Science Letters 427, 236–248
Link to Article [doi:10.1016/j.epsl.2015.07.008]
Copyright Elsevier

¹Maximilian Matthes, ¹Mario Fischer-Gödde, ¹Thomas S. Kruijer, ²Ingo Leya, ¹Thorsten Kleine
¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
²Space Research and Planetology, University of Bern, Bern, Switzerland

The short-lived 107Pd-107Ag system is a versatile tool for dating iron meteorites, but neutron capture reactions during cosmic ray-exposure might have modified Ag isotope compositions. These cosmic ray-induced effects would vary depending on the exposure time of a sample and its location within the parent meteoroid and, therefore, could bias the age information inferred from Pd-Ag isotope systematics. Our new combined Pd-Ag and Pt isotope data for iron meteorites in conjunction with model calculations reveal large cosmic ray-induced downward shifts of 107Ag/109Ag, which preclude the determination of Pd-Ag isochrons based on measured Ag isotope compositions. For the strongly irradiated iron meteorites Ainsworth (IIAB) and Carbo (IID) these shifts are similar to or even larger than the effects from radiogenic ingrowth resulting from 107Pd-decay. For the less strongly irradiated IIIAB iron meteorites Boxhole, Grant and Henbury, the cosmic ray-induced shifts are smaller than the radiogenic 107Ag excesses, but are nevertheless significant. We have developed a method to quantify the cosmic ray-induced Ag isotope shifts using a neutron capture model and Pt isotope compositions as the neutron dose monitor. After correction, Pd-Ag isochrons are obtained for all investigated iron meteorites, even for the most strongly irradiated samples. The Pd-Ag ages inferred from the isochrons are in good agreement with other chronological data for iron meteorites, indicating that our neutron capture model provides a reliable correction method for quantifying cosmic ray-induced shifts on measured Ag isotope compositions. The Pd-Ag ages for iron meteorites obtained in this and previous studies indicate rapid crystallization and cooling of the parental metal cores within a few Ma after core formation and solar system formation. Such rapid cooling can be attributed to either small parent body sizes or collisional erosion of the insulating silicate mantle from larger bodies. The collisions would have facilitated rapid cooling below Pd-Ag isotopic closure and so in this case the Pd-Ag ages would effectively date the time of the collisions.
Reference
Matthes M, Fischer-Gödde M, Kruijer TS, Leya I, Kleine T (2015) Pd-Ag chronometry of iron meteorites: correction of neutron capture-effects and application to the cooling history of differentiated protoplanets. Geochimica et Cosmochimica Acta (in Press).
Link to Article [doi:10.1016/j.gca.2015.07.027]
Copyright Elsevier

¹A. M. Jellinek, ²M. G. Jackson
¹Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, British Columbia, V6T 1Z4 Canada
²Department of Earth Science, University of California Santa Barbara, Santa Barbara, California 93106-9630, USA

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

Reference
Jellinek AM, Jackson MG (2015) Connections between the bulk composition, geodynamics and habitability of Earth. Nature Geoscience (in Press)
Link to Article [doi:10.1038/ngeo2488]

^1,2,3Michael K. Weisberg, ^2,3Denton S. Ebel, ^4,5Daisuke Nakashima, ⁴Noriko T. Kita, ⁶Munir Humayun
¹Department of Physical Sciences, Kingsborough Community College, City University New York, Brooklyn, NY 11235
²Department of Earth and Environmental Sciences, Graduate Center, City University New York, New York, NY 10016
³Department of Earth & Planetary Sciences, American Museum of Natural History, NY, NY 10024
⁴WiscSIMS, Department of Geosciences, University of Wisconsin-Madison, WI 53706
⁵Department of Earth and Planetary Material Sciences, Faculty of Science, Tohoku University, Aoba, Sendai, Miyagi 980-8578, Japan
⁶Department of Earth, Ocean & Atmospheric Science, and National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310

Northwest Africa (NWA) 5492 and Grosvenor Mountains (GRO) 95551 are metal-rich chondrites having silicate (olivine and pyroxene) compositions that are more reduced than those in other metal-rich chondrites, such as the CH and CB chondrites. Additionally, sulfides in NWA 5492 and GRO 95551 are more abundant and not related to the metal, as in the CB chondrites. Average metal compositions in NWA 5492 and GRO 95551 are close to H chondrite metal. Oxygen isotope ratios of NWA 5492 and GRO 95551 components (chondrules and fragments) show a range of compositions with most having Δ17O values > 0 ‰. Since there is no matrix component, their average chondrule + fragment oxygen isotopic compositions are considered to be representative of whole rock and are sandwiched between the values for enstatite (E) and ordinary (O) chondrites. These data argue for a close relationship between NWA 5492 and GRO 95551 and suggest that they are the first examples of a new type of metal-rich chondrite.
Oxygen isotope ratios of chondrules in NWA 5492 and GRO 95551 show considerable overlap with chondrules in O, E and R chondrites, with average compositions indistinguishable from LL3 chondrules, suggesting considerable mixing between these Solar System materials during chondrule formation and/or that their precursors experienced similar formation environments and/or processes. Another characteristic shared between NWA 5492 and GRO 95551 and O, E and R chondrites is that they are all relatively dry (low abundances of hydrated minerals), compared to many C chondrites and have fewer, smaller CAIs than many C chondrites. (No CAIs were found in NWA 5492 or GRO 95551 but they contain rare Al-rich chondrules.) We suggest that O, E, R and the NWA 5492 and GRO 95551 chondrites are closely related Solar System materials.

Reference
Weisberg MK, Ebel DS, Nakashima D, Kita NT, Humayun M (2015) Petrology and geochemistry of chondrules and metal in nwa 5492 and gro 95551: a new type of metal-rich chondrite. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.07.021]

Copyright Elsevier

¹J.A. Barrat, ²R.C. Greenwood, ²A.B. Verchovsky, ³Ph. Gillet, ⁴C. Bollinger, ⁴J.A. Langlade, ¹C. Liorzou, ²I.A. Franchi
¹Université de Bretagne Occidentale, Institut Universitaire Européen de la Mer, CNRS UMR 6538, Place Nicolas Copernic, 29280 Plouzané, France
²Planetary and Space Sciences, Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA,United Kingdom
³EPSL, Institute of Condensed Matter Physics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Station 3, CH-1015 Lausanne, Switzerland
⁴CNRS UMS 3113, I.U.E.M., Place Nicolas Copernic, 29280 Plouzané Cedex, France

The unique achondrite NWA 7325 is an unusual olivine gabbro composed chiefly of calcic plagioclase (An85-93), diopsidic pyroxene (En50.1-54.0 Wo44.8-49.3 Fs0.6-1.3), and forsteritic olivine (Fo97). It is Al and Mg-rich and Fe and Na-poor and displays very low concentrations of incompatible trace elements, much below 0.3 x CI abundances for many of them. It is also characterized by huge Eu and Sr anomalies (Eu/Eu∗=65, Srn/Cen=240). Although the O isotopic composition of NWA 7325 and some ureilites (those with olivine cores in the range Fo75 to Fo88) are similar, a genetic relationship between them is unlikely due to the Fe-poor composition of NWA 7325. It is almost certainly derived from a distinct planetesimal, not previously sampled by other achondrites. The low Na/Al, Ga/Al, Zn/Al ratios as well as the low K, Rb and Cs shown by NWA 7325, suggest a volatile-depleted parent body. This unique gabbro is demonstrably a cumulate, but the composition of its parental melt cannot be precisely assessed. However, the liquid from which NWA 7325 crystallized would have been very poor in incompatible trace elements (Yb in the range of 0.25 to 1.5 x CI abundance) with a very large positive Eu anomaly. Such a melt cannot be the product of the early magmatic activity on a small parent body. Instead, we propose that the parental melt to NWA 7325 formed as a consequence of the total melting of an ancient gabbroic lithology, possibly upon impact, in agreement with the systematics of 26Al-26Mg. Based on recent dating, the crustal material that was parental to NWA 7325 must have been older than 4562.8 Ma, and formed possibly ≈4566 Ma ago. If this scenario is correct, NWA 7325 provides evidence of one of the earliest crusts on a differentiated body so far studied.

Reference
Barrat JA, Greenwood RC, Verchovsky AB, Gillet P, Bollinger C, Langlade JA, Liorzou C, Franchi IA (2015)
Crustal differentiation in the early solar system: clues from the unique achondrite Northwest Africa 7325 (NWA 7325). Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.07.020]

Copyright Elsevier

^1,2Bashar Baghdadi, ²Albert Jambon, ³Jean-Alix Barrat
¹Damascus University, Faculty of Sciences, Department of Geology, Damascus, Syria
²Sorbonne Universités, UPMC Univ Paris 06, CNRS, Institut des Sciences de la Terre de Paris (iSTeP), 4 place Jussieu 75005 Paris, France
³Université de Brest, CNRS UMR 6538 (Domaines Océaniques), I.U.E.M., Place Nicolas Copernic, 29280 Plouzané Cedex, France

Northwest Africa (NWA) 3164 and 5167 are two angrites with a granulitic texture unlike that of other angrites, with a variable, up to millimeter grain size. Besides mineralogical and chemical similarities to other angrites, NWA 3164 and 5167 exhibit unique characteristics. Ca-rich olivine dominates (in NWA 3164: ∼49 vol%, Fo57; NWA 5167: 40 vol%, Fo59). Fassaitic clinopyroxene is the second major phase (in NWA 3164: 29 vol%; in NWA 5167: 36 vol%) with a significant Tschermak component. In addition, two Al-rich phases are present: plagioclase An99 and hercynitic spinel (∼6 and ∼7 vol% respectively for NWA 3164; 17 and 4 vol% respectively for NWA 5167). Heavily weathered iron sulfide and kamacite, (9 wt% in NWA 3164; 4 wt% in NWA 5167) are the remaining minor phases, a unique feature among angrites. All mineral phases are homogeneous. Like other angrites, NWA 3164 and 5167 exhibit a superchondritic Ca/Al ratio, with negligible amounts of alkalis and very low silica content. The presence of metal results from the incorporation of exogenous iron following impact. Subsequent annealing resulted in the observed granulitic texture. Major element composition indicates that both NWA 3164 and 5167 are derived from a picritic angrite precursor after incorporation of metal and annealing.

After correction for iron and Fo90 olivine incorporation, bulk rock REE abundances of both NWA 3164 and NWA 5167 appear lower than those of quenched angrites, showing the lowest absolute abundances among angrites. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) analyses of clinopyroxene, olivine and anorthite indicate that difference of Rare Earth Element (REE) abundances between NWA 3164 and NWA 5167 can be explained by adding traces of phosphate to NWA 3164. NWA 3164/5167 trace elements abundances are similar, showing depletion in volatile elements and enrichment in refractory lithophile elements such as Ca, Ti, and Al. The most incompatible elements are depleted as well, unlike other angrites. This indicates that the source of these younger angrites was more depleted in incompatible elements when compared to the older magmatic angrites. The low Hf/W is understood as the result of exogenous iron incorporation and therefore the Hf/W and W isotopic heterogeneity of the Angrite Parent Body (APB) mantle is secondary. Comparison with other angrites suggests that iron incorporation may be necessary to explain their low Hf/W and W isotopic compositions.

Reflectance
Baghdadi B, Jambon A, Barrat J-A (2015) Metamorphic Angrite Northwest Africa 3164/5167 Compared to Magmatic Angrites. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.07.022]

Copyright Elsevier

¹Richard C. Greenwood, ²Jean-Alix Barrat, ³Edward R.D. Scott, ⁴Henning Haack, ⁵Paul C. Buchanan, ¹Ian.A. Franchi, ^6,7Akira Yamaguchi, ¹Diane Johnson, ⁸Alex W.R. Bevan, ⁹Thomas H. Burbine
¹Planetary and Space Sciences, Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
²CNRS UMR 6538 (Domaines Océaniques), U.B.O.-I.U.E.M., Place Nicolas Copernic, 29280 Plouzané Cedex, France
³Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, HI 96822, USA
⁴Centre for Star and Planet Formation, Natural History Museum of Denmark, Øster Voldgade 5-7, DK-1350 Copenhagen K, Denmark
⁵Department of Chemistry and Geology, Kilgore College, 1100 Broadway, Kilgore, TX 75662, USA
⁶National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan
⁷Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies), Tachikawa, Tokyo 190-8518, Japan
⁸Department of Earth and Planetary Sciences, Western Australian Museum, Locked Bag 49 Welshpool DC, WA 6986, Australia
⁹Astronomy Department, Mount Holyoke College, South Hadley, MA 01075, USA

Evidence from iron meteorites indicates that a large number of differentiated planetesimals formed early in Solar System history. These bodies should have had well-developed olivine-rich mantles and consequentially such materials ought to be abundant both as asteroids and meteorites, which they are not. To investigate this “Great Dunite Shortage” we have undertaken a geochemical and oxygen isotope study of main-group pallasites and dunitic rocks from mesosiderites.

Oxygen isotope analysis of 24 main-group pallasites (103 replicates) yielded a mean Δ17O value of -0.187±0.016‰ (2σ), which is fully resolved from the HED Δ17O value of -0.246 ± 0.014 (2σ) obtained in our earlier study and demonstrates that both groups represent distinct populations and were derived from separate parent bodies. Our results show no evidence for Δ17O bimodality within the main-group pallasites, as suggested by a number of previous studies.
Olivine-rich materials from the Vaca Muerta, Mount Padbury and Lamont mesosiderites, and from two related dunites (NWA 2968 and NWA 3329), have Δ17O values within error of the mesosiderite average. This indicates that these olivine-rich materials are co-genetic with other mesosiderite clasts and are not fragments from an isotopically distinct pallasite-like impactor. Despite its extreme lithologic diversity the mesosiderite parent body was essentially homogeneous with respect to Δ17O, a feature best explained by an early phase of large-scale melting (magma ocean), followed by prolonged igneous differentiation.
Based on the results of magma ocean modeling studies, we infer that Mg-rich olivines in mesosiderites formed as cumulates in high-level chambers and do not represent samples of the underlying mantle. By analogy, recently documented Mg-rich olivines in howardites may have a similar origin.
Although the Dawn mission did not detect mesosiderite-like material on Vesta, evidence linking the mesosiderites and HEDs includes: i) their nearly identical oxygen isotope compositions; ii) the presence in both of coarse-grained Mg-rich olivines; iii) both have synchronous Lu-Hf and Mn-Cr ages; iv) there are compositional similarities between the metal in both; and v) mesosiderite-like material has been identified in a howardite breccia. The source of the mesosiderites remains an outstanding question in meteorite science.
The underrepresentation of olivine-rich materials amongst both asteroids and meteorites results from a range of factors. However, evidence from pallasites and mesosiderites indicates that the most important reason for this olivine shortage lies in the early, catastrophic destruction of planetesimals in the terrestrial planet-forming region and the subsequent preferential loss of their olivine-rich mantles.

Reference
Greenwood RC, Barrat J-A, Scott ERD, Haack H, Buchanan PC, Franchi IA, Yamaguchi A, Johnson D, Bevan AWR, Burbine TH (2015) Geochemistry and oxygen isotope composition of main-group pallasites and olivine-rich clasts in mesosiderites: Implications for the “Great Dunite Shortage” and HED-mesosiderite connection. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.07.023]

Copyright Elsevier

¹Alan E. Rubin
¹Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA

Paris is the least aqueously altered CM chondrite identified to date, classified as subtype 2.7; however, literature data indicate that some regions of this apparently brecciated meteorite may be subtype 2.9. The suite of CAIs in Paris includes 19% spinel–pyroxene inclusions, 19% spinel inclusions, 8% spinel–pyroxene–olivine inclusions, 43% pyroxene inclusions, 8% pyroxene–olivine inclusions, and 3% hibonite-bearing inclusions. Both simple and complex inclusions are present; some have nodular, banded, or distended structures. No melilite was identified in any of the inclusions in the present suite, but other recent studies have found a few rare occurrences of melilite in Paris CAIs. Because melilite is highly susceptible to aqueous alteration, it is likely that it was mostly destroyed during early-stage parent-body alteration. Two of the CAIs in this study are part of compound CAI–chondrule objects. Their presence suggests that there were transient heating events (probably associated with chondrule formation) in the nebula after chondrules and CAIs were admixed. Also present in Paris are a few amoeboid olivine inclusions (AOI) consisting of relatively coarse forsterite rims surrounding fine-grained, porous zones containing diopside and anorthite. The interior regions of the AOIs may represent fine-grained rimless CAIs that were incorporated into highly porous forsterite-rich dustballs. These assemblages were heated by an energy pulse that collapsed and coarsened their rims, but failed to melt their interiors.

Reference
Rubin AE (2015) An American on Paris: Extent of aqueous alteration of a CM chondrite and the petrography of its refractory and amoeboid olivine inclusions. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12482]
Published by arrangement with John Wiley&Sons

¹Charles K. Shearer, ¹Paul V. Burger, ¹Aaron S. Bell, ²Yunbin Guan, ³Clive R. Neal
¹Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA
²Division of Earth and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
³Department of Civil & Environmental Engineering & Earth Sciences, University of Notre Dame, Notre Dame, Indiana, USA

Remotely sensed observations from recent missions (e.g., GRAIL, Kaguya, Chandrayaan-1) have been interpreted as indicating that the deep crust and upper mantle are close to or at the lunar surface in many large impact basins (e.g., Crisium, Apollo, Moscoviense). If this is correct, the capability of either impact or volcanic processes to transport mantle lithologies to the lunar surface should be enhanced in these regions. Somewhat problematic to these observations and interpretations is that examples of mantle lithologies in the lunar sample collection (Apollo Program, Luna Program, lunar meteorites) are at best ambiguous. Dunite xenoliths in high-Ti mare basalt 74275 are one of these ambiguous examples. In this high-Ti mare basalt, olivine occurs in three generations: olivine associated with dunite xenoliths, olivine megacrysts, and olivine microphenocrysts. The dunite xenoliths are anhedral in shape and are generally greater than 800 μm in diameter. The interior of the xenoliths are fairly homogeneous with regard to many divalent cations. For example, the Mg# (Mg/Mg + Fe × 100) ranges from 82 to 83 in their interiors and decreases from 82 to 68 over the 10–30 μm wide outer rim. Titanium and phosphorus X-ray maps of the xenolith illustrate that these slow diffusing elements preserve primary cumulate zoning textures. These textures indicate that the xenoliths consist of many individual olivine grains approximately 150–200 μm in diameter with low Ti, Al, and P cores. These highly incompatible elements are enriched in the outer Fe-rich rims of the xenoliths and slightly enriched in the rims of the individual olivine grains. Highly compatible elements in olivine such as Ni exhibit a decrease in the rim surrounding the xenolith, an increase in the incompatible element depleted cores of the individual olivine grains, and a slight decrease in the “interior rims” of the individual olivine grains. Inferred melt composition, liquid lines of descent, and zoning profiles enable the reconstruction of the petrogenesis of the dunite xenoliths. Preservation of primary magmatic zoning (Ti, P, Al) and lack of textures similar to high-pressure mineral assemblages exhibited by the Mg-suite (Shearer et al. 2015) indicate that these xenoliths do not represent deep crustal or shallow mantle lithologies. Further, they are chemically and mineralogically distinct from Mg-suite dunites identified from the Apollo 17 site. More likely, they represent olivine cumulates that crystallized from a low-Ti mare basalt at intermediate to shallow crustal levels. The parent basalt to the dunite xenolith lithology was more primitive than low-Ti basalts thus far returned from the Moon. Furthermore, this parental magma and its more evolved daughter magmas are not represented in the basalt sample suite returned from the Taurus-Littrow Valley by the Apollo 17 mission. The dunite xenolith records several episodes of crystallization and re-equilibration. During the last episode of re-equilibration, the dunite cumulate was sampled by the 74275 high-Ti basalt and transported over a period of 30–70 days to the lunar surface.

Reference
Shearer CK, Burger PV, Bell AS, Guan Y, Neal CR (2015) Exploring the Moon’s surface for remnants of the lunar mantle 1. Dunite xenoliths in mare basalts. A crustal or mantle origin? Meteoritics&Planetary Science
Link to Article [DOI: 10.1111/maps.12480]

Published by arrangement with John Wiley&Sons

^1,2Wei Jia Ong, ¹Christine Floss
¹Laboratory for Space Sciences and Physics Department, Washington University, St. Louis, Missouri, USA
²National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan, USA

We carried out Fe isotopic analyses on 21 O-rich presolar grains from the Acfer 094 ungrouped carbonaceous chondrite. Presolar grains were identified on the basis of oxygen isotopic ratios, and elemental compositions were measured by Auger spectroscopy. The Fe isotopic measurements were carried out by analyzing the Fe isotopes as negative secondary oxides with the NanoSIMS to take advantage of the higher spatial resolution of the Cs+ primary ion beam. Our results demonstrate the effectiveness of this approach for measuring both 54Fe/56Fe and 57Fe/56Fe. The ion yield for FeO– is significantly lower than for Fe+, but this is not a serious limitation for presolar silicate grains with Fe as a major element. Most of the grains analyzed are ferromagnesian silicates, but we also measured four oxide grains. Iron contents are high in all of the grains, ranging from 10 to 40 atom%. Three of the grains belong to oxygen isotope Group 4. All of them have 54Fe/56Fe and 57Fe/56Fe ratios that are solar within errors, consistent with an origin in the outer zones of a Type II supernova, as indicated by their oxygen isotopic compositions. The remaining grains belong to oxygen isotope Group 1, with origins in low-mass AGB stars. The majority of these also have solar 54Fe/56Fe and 57Fe/56Fe ratios. However, four grains are depleted in 57Fe; one is also slightly depleted in 54Fe. Current AGB models predict excesses in 57Fe with 54Fe/56Fe ratios that largely reflect the metallicity of the parent star. While the solar 57Fe/56Fe ratios are consistent with formation of the grains in early third dredge-up episodes, these models cannot account for the grains with 57Fe depletions. Comparison with galactic evolution models suggests formation of these grains from stars with significantly subsolar metallicity; however, these models also predict large depletions in 54Fe, which are not observed in the grains. Thus, the isotopic compositions of these grains remain unexplained.

Reference
Ong WJ, Floss C (2015) Iron isotopic measurements in presolar silicate and oxide grains from the Acfer 094 ungrouped carbonaceous chondrite. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12475]

Publsihed by arrangement with John Wiley&Sons