Joseph I. Goldstein^a, Jijin Yang^b and Edward R.D. Scott^c

^aDepartment of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, MA, USA
^bCarl Zeiss Microscopy, LLC. One Zeiss Drive, Thornwood, NY, USA
^cHIGP, University of Hawaii, Honolulu, HI, USA

Analyses and modeling of Ni zoning in taenite in differentiated meteorites provide metallographic cooling rates at ∼500 °C that are inconsistent with conventional formation models. Group IVA iron meteorites have very diverse cooling rates of 100-6600 °C/Myr indicating that they cooled inside a large metallic body with little or no silicate mantle (Yang et al., 2007). Wasson and Hoppe (2012) have questioned these diverse cooling rates on the basis of their ion probe measurements of Ni/Co ratios at the kamacite-taenite interface in two group IVA and in two group IIIAB iron meteorites. To investigate their claims and to assess methods for determining relative cooling rates from kamacite-taenite interface compositions, we have analyzed 38 meteorites—13 IVA, 14 IIIAB irons, 4 IAB complex irons, 6 pallasites and a mesosiderite—using the electron probe microanalyzer (EPMA). Ni concentrations in taenite (Niγ) and kamacite (Niα) at kamacite-taenite interfaces are well correlated with metallographic cooling rates: Niγ values increase from 30 to 52 wt.% while Niα decreases from 7 to 4 wt.% as cooling rates decrease. EPMA measurements of Niγ, Niα, and Niα/ Niγ, can therefore be used to provide order-of-magnitude estimates of relative cooling rates. Concentrations of Co in kamacite and taenite at their interface (Coα, Coγ) are controlled by bulk Ni and Co composition, as well as cooling rate. The ratios Coα/Coγ and (Co/Ni)α/(Co/Ni)γ are correlated with cooling rate, but because of significant scatter, these parameters should not be used to estimate cooling rates. Our analyses of 13 group IVA irons provide robust support for diverse cooling rates that decrease with increasing bulk Ni, consistent with measurements of cloudy zone size and tetrataenite width. Apparent equilibration temperatures, which are inferred from Niγ values and the Fe-Ni-P phase diagram and Ni diffusion rates in taenite, show that cooling rates of IVA irons vary by a factor of ≈100, in excellent agreement with the metallographic cooling rates. Similar calculations using NiγNiα and Coα/Coγ ratios and phase diagram data give factors that are an order of magnitude lower but have larger uncertainties. Thus we strongly disagree with the conclusion ofWasson and Hoppe (2012) that interface concentrations of Ni and Co are in any way in conflict with the cooling rates of Yang et al. (2008). Our measurements confirm that the IVA irons could not have cooled in an asteroidal core surrounded by a silicate mantle, and also that main-group pallasites cooled slower than IIIAB irons and did not cool at the boundary between the mantle and core from which the IIIAB irons originated. Our data provide additional evidence that mesosiderites, which formed by impact mixing of Fe-Ni melt and crustal rocks, cooled at uniquely slow rates.

Reference
Goldstein JI, Yang J and Scott ERD (in press) Determining cooling rates of iron and stony-iron meteorites from measurements of Ni and Co at kamacite-taenite interfaces. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.05.025]
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C. Pirim^a, M.A. Pasek^b, D.A. Sokolov^a, A.N. Sidorov^a, R. Gann^a and T.M. Orlando^a

^aSchool of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332, USA
^bDepartment of Geology, University of South Florida, Tampa, FL 33620, USA

This work presents a comprehensive investigation of schreibersite inclusions within iron-poor and iron-rich meteorites, and of the associated intrinsic low-temperature oxidation products observed after exposure to terrestrial weathering. First, a thermodynamic equilibrium modeling of the oxidation of schreibersite was carried out and showed that oxidation is mostly limited to the surface in the absence of other ions and/or water. This oxidation occurs rapidly (less than a few weeks) and is mediated by the atmosphere, forming primarily iron oxides and iron phosphates. Second, detailed analyses of meteorite schreibersite inclusions and synthetic schreibersite surrogates (Fe₃P and Fe₂NiP) were performed using surface characterization techniques such as micro-Raman spectroscopy, atomic force microscopy (AFM), electrostatic force microscopy (EFM), electron microprobe analysis (EPMA), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Such thorough analyses are required as all prebiotic reactivity studies performed nowadays use meteoritic samples that have been somewhat exposed to Earth weathering. We find that, while the meteorite samples have not been introduced directly into water, they all bear significant oxidation signatures that appear to be similar for both studied short-term and long-term natural weathering corrosion processes. In addition, we find that synthetic schreibersite samples have similar surface and sub-surface chemistry and are reasonable chemical proxies for natural schreibersite. The thorough analytical studies detailed in this paper provide a chemical model for schreibersite oxidation products.

Reference
Pirim C, Pasek MA, Sokolov DA, Sidorov AN, Ganna R and Orlando TM (in press) Investigation of schreibersite and intrinsic oxidation products from Sikhote-Alin, Seymchan, and Odessa meteorites and Fe3P and Fe2NiP synthetic surrogates. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.05.027]
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Amy M. Gaffney and Lars E. Borg

Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue L-231, Livermore, CA 94550

The time at which the moon solidified can be determined from the Lu-Hf isotope systematics of lunar rocks derived from magma sources that formed during crystallization of the lunar magma ocean. The final magma ocean crystallization product, termed urKREEP, is enriched in incompatible trace elements including K, REE and P. We have determined the initial Hf isotopic compositions of four samples, two KREEP basalts and two Mg-suite norites. The incompatible trace element compositions of these samples are controlled by an urKREEP component, and therefore the initial Hf isotopic compositions of these samples represent the Hf isotopic evolution of urKREEP. In order to correct the effects of neutron irradiation on the Hf isotopic compositions of these samples, we have developed a model that uses the stable Hf and Sm isotopic compositions measured on an irradiated sample to determine and correct for the thermal and epithermal neutron fluence that has modified the Hf isotopic composition of the sample. We use our corrected results to calculate a ¹⁷⁶Lu-¹⁷⁶Hf urKREEP model age of 4353 ± 37 Ma and the ¹⁷⁶Lu/¹⁷⁷Hf of urKREEP to be 0.0153 ± 0.0033. The Lu-Hf model age is concordant with the re-calculated Sm-Nd urKREEP model age of 4389 ± 45 Ma, and we take the average of these ages, 4368 ± 29 Ma, to represent the time at which urKREEP formed. This age is concordant with the age of the most reliably dated ferroan noritic anorthosite as well as ¹⁴²Nd model ages for the formation or re-equilibration of mare basalt sources. Taken together, these ages indicate that the Moon experienced a widespread, large-scale magmatic event around 4370 Ma, most plausibly attributed to solidification of the lunar magma ocean.

Reference
Gaffney AM and Borg LE (in press) A young solidification age for the lunar magma ocean. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.05.028]
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Winfried H. Schwarz^1,2 andHans J. Lippolt²

¹Institut für Geowissenschaften, Universität Heidelberg, Heidelberg, Germany
²Laboratorium für Geochronologie, Universität Heidelberg, Heidelberg, Germany

Seven impact melts from various places in the Nördlinger Ries were dated by ⁴⁰Ar-³⁹Ar step-heating. The aim of these measurements was to increase the age data base for Ries impact glasses directly from the Ries crater, because there is only one Ar-Ar step-heating spectrum available in the literature. Almost all samples display saddle-shaped age spectra, indicating the presence of excess argon in most Ries glass samples, most probably inherited argon from incompletely degassed melt and possibly also excess argon incorporated during cooling from adjacent phases. In contrast, moldavites usually contain no inherited argon, probably due to their different formation process implying solidification during ballistic transport. The plateau age of the only flat spectrum is 14.60 ± 0.16 (0.20) Ma (2σ), while the total age of this sample is 14.86 ± 0.20 (0.22) Ma (isochron age: 14.72 ± 0.18 [0.22] Ma [2σ]), proofing the chronological relationship of the Ries impact and moldavites. The total ages of the other samples range between 15.77 ± 0.52 and 20.4 ± 1.0 Ma (2σ), implying approximately 2–40% excess⁴⁰Ar (compared to the nominal age of the Ries crater) in respective samples. Thus, the age of 14.60 ± 0.16 (0.20) (2σ) (14.75 ± 0.16 [0.20 Ma] [2σ], calculated using the most recent suggestions for the K decay constants) can be considered as reliable and is within uncertainties indistinguishable from the most recent compilation for the age of the moldavite tektites.

Reference
Schwarz WH and Lippolt HJ  (in press) 40Ar-39Ar step-heating of impact glasses from the Nördlinger Ries impact crater—Implications on excess argon in impact melts and tektites. Meteoritics & Planetary Science
[doi:10.1111/maps.12309]
Published by arrangement with John Wiley & Sons

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C. Alwmark¹, S. Alwmark-Holm¹, J. Ormö² and E. Sturkell³

¹Department of Geology, Lund University, Lund, Sweden
²Centro de Astrobiología (CSIC/INTA), Instituto Nacional de Técnica Aeroespacial, Madrid, Spain
³Department of Earth Sciences, University of Gothenburg, Göteborg, Sweden

The Målingen structure in Sweden has for a long time been suspected to be the result of an impact; however, no hard evidence, i.e., shock metamorphic features or traces of the impactor, has so far been presented. Here we show that quartz grains displaying planar deformation features (PDFs) oriented along crystallographic planes typical for shock metamorphism are present in drill core samples from the structure. The shocked material was recovered from basement breccias, below the sediment infill, and the distribution of the orientation of the shock-produced PDFs indicates that the studied material experienced low shock pressures. Based on our findings, we can exclude that the material is transported from the nearby Lockne impact structure, which means that the Målingen structure is a separate impact structure, the seventh confirmed impact structure in Sweden. Furthermore, sedimentological and biostratigraphic aspects of the deposits that fill the depression at Målingen are very similar to features at the Lockne impact structure. This implies a coeval formation age and thus also the confirmation of the first known marine target doublet impact craters on Earth (i.e., the Lockne–Målingen pair).

Reference
Alwmark C, Alwmark-Holm S, Ormö J and Sturkell E  (in press) Shocked quartz grains from the Målingen structure, Sweden—Evidence for a twin crater of the Lockne impact structure. Meteoritics & Planetary Science
[doi:10.1111/maps.12314]
Published by arrangement with John Wiley & Sons

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Tomasz Brachaniec, Krzysztof Szopa and Łukasz Karwowski

Department of Geochemistry, Mineralogy and Petrology, Faculty of Earth Sciences, University of Silesia, Sosnowiec, Poland

We report the first occurrence of moldavites in Poland. This discovery confirms the hypothesis that moldavites could have been distributed up to 500 km from the Ries crater in Germany. The tektites were reworked from Middle Miocene sediments and redeposited in Late Miocene (Pannonian) fluvial deposits of the Gozdnicka Formation in Lower Silesia. The Polish moldavites are represented by nine (<8 mm) fragments with a total of 0.471 g. The lack of the autochthonous tektites indicates that tektites investigated here had to be redeposited in a fluvial environment, probably from the Lusatian area. The chemical composition of the Polish moldavites plots in the same area with those from other localities.

Reference
Brachaniec T, Szopa K and Karwowski Ł  (in press) Discovery of the most distal Ries tektites found in Lower Silesia, southwestern Poland. Meteoritics & Planetary Science
[doi:10.1111/maps.12311]
Published by arrangement with John Wiley & Sons

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Ulli Raschke¹, Patrice Tristan Zaag¹, Ralf Thomas Schmitt¹ and Wolf Uwe Reimold^1,2

¹Museum für Naturkunde Berlin, Berlin, Germany
²Humboldt-Universität zu Berlin, Berlin, Germany

El’gygytgyn is a 3.6 Ma, 18 km diameter, impact crater formed in an approximately 88 Ma old volcanic target in Northeast Siberia. The structure has been the subject of a recent ICDP drilling project. In parallel to those efforts, a Russian-German expedition was undertaken in summer 2011 to investigate the permafrost soil, lake terraces, and the volcanic rocks of the southern and eastern crater rim. This provided the unique opportunity for mapping and sampling of the volcanic target rocks around a large part of this complex impact structure. Samples from 43 outcrops were collected and analyzed petrographically and geochemically. The results were combined with earlier mapping outcomes to create a new geological map of this impact structure and its immediate environs, at the scale of 1:50,000. Compositions of our rock suites are compared with the lithologies of the 2009 ICDP drill core. The ignimbrite described as lower bedrock in the ICDP drill core shows petrographically and chemically strong similarities to the rhyolitic and rhyodacitic ignimbrites observed on surface. The suevite sequence exposed in the ICDP drill core is a mixture of all observed target rocks at their respective proportions in the area. In contrast to previous studies, the calculated average target composition of El’gygytgyn takes the contribution of the basic target rocks into consideration: mafic and intermediate rocks approximately 7.5%, and felsic rocks approximately 92.5%.

Reference
Raschke U, Zaag PT, Schmitt RT and Reimold WU  (in press) The 2011 expedition to the El’gygytgyn impact structure, Northeast Russia: Toward a new geological map for the crater area. Meteoritics & Planetary Science
[doi:10.1111/maps.12306]
Published by arrangement with John Wiley & Sons

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Maitrayee Bose^a, Thomas J. Zega^b and Peter Williams^a

^aArizona State University, Department of Chemistry and Biochemistry, Physical Sciences Building, Room D-57, PO Box 871604, Tempe, AZ 85287, United States
^bLunar and Planetary Laboratory, University of Arizona, 1629 East University Blvd., Tucson, AZ 85721, United States

Insight into the presolar and interstellar grain inventory of the CO3 chondrite Queen Alexandra Range (QUE) 97416 is gained through correlated secondary ion mass spectrometry (SIMS), transmission electron microscopy (TEM), and synchrotron-based X-ray absorption near-edge structure spectroscopy (XANES). Only one presolar silicate grain [; ] that may have formed in a low-mass Red Giant or Asymptotic Giant Branch star occurs in the coarse-grained matrix of QUE 97416. No other presolar grains were identified. Although presolar grains are rare in QUE 97416, numerous (898±259 ppm) ¹⁵N-rich domains ( to +3069‰) occur in the thin section. Based on TEM of an extracted section, two ¹⁵N-rich domains are amorphous, C-bearing, and texturally uniform, and they are embedded in a ferromagnesian silicate matrix with varied grain sizes. The individual ¹⁵N-rich organic regions with high  (+2942±107‰ and +2341±140‰) exhibit diverse carbon functional groups, such as aromatic, vinyl-keto, amidyl, and carboxylic functionality, while the nitrogen XANES reveals traces of nitrile functionality. QUE 97416 appears to have escaped aqueous alteration based on the absence of hydrated minerals but is thermally altered, which could have resulted in the destruction of presolar grains. However, this process at >400 °C metamorphic temperatures was inefficient in destroying the carriers of N isotope anomalies, which may indicate the resistant nature of the organic carriers and/or the limited extent of thermal metamorphism on the QUE 97416 parent body.

Reference
Bose M, Zega TJ and Williams P  (2014) Assessment of alteration processes on circumstellar and interstellar grains in Queen Alexandra Range 97416. Earth and Planetary Science Letters 399:128.
[doi:10.1016/j.epsl.2014.05.007]
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Futoshi Takahashi, Hideo Tsunakawa, Masaki Matsushima & Hisayoshi Shimizu

Department of Earth and Planetary Sciences, Kyushu University, Fukuoka 812-8581, Japan
Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152-8551, Japan
Earthquake Research Institute, University of Tokyo, Tokyo 113-0032, Japan
Department of Earth and Environmental Sciences, Kumamoto University, Kumamoto 860-8555, Japan

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

Reference
Takahashi et al. (2014) Reorientation of the early lunar pole.  Nature Geoscience 7:401.
[doi:10.1038/ngeo2173]

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Katharine L. Robinson & G. Jeffrey Taylor

Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, Hawaii 96822, USA

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

Reference
Robinson KL & Taylor GJ (2014) Heterogeneous distribution of water in the Moon.  Nature Geoscience 7:401.
[doi:10.1038/ngeo2173]

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