¹R. Bastian Georg,²Anat Shahar
¹Trent University, Water Quality Centre, Trent University, 1600 West Bank Drive, Peterborough, K9J 7B8, Ontario, Canada
²Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, NW, Washington, D.C. 20015, U.S.A.

We present a new approach to model planetary accretion and continuous core formation, and discuss the implications if Earth accreted under conditions initially more oxidized than the modern day mantle. The modified model uses the same partitioning data that were previously used to model accretion under reducing conditions, however, changing the partitioning between accreting metal and silicate mantle means that reducing conditions fail to meet expected core/mantle values. Instead, the model requires conditions more oxidized than the modern day mantle to converge and to yield expected elemental core/mantle distribution values for moderately siderophile elements. The initial oxygen fugacity required to provide the crucial level of oxidation is approximately ΔIW ~ −1.2 to −1.7 and thus is in the range of carbonaceous and ordinary chondrites. The range of peak pressures for metal silicate partitioning is 60–6 GPa and oxygen fugacity must decrease to meet modern FeO mantle contents as accretion continues. Core formation under oxidizing conditions bears some interesting consequences for the terrestrial Si budget. Although the presented partitioning model can produce a Si content in the core of 5.2 wt%, oxidizing accretion may limit this to a maximum of ~3.0 to 2.2 wt%, depending on the initial fO2 in BSE, which places bulk earth Mg/Si ratio between 0.98–1.0. In addition, under oxidizing conditions, Si starts partitioning late during accretion, e.g., when model earth reached >60% of total mass. As a consequence, the high P-T regime reduces the accompanied isotope fractionation considerably, to 0.07‰ for 5.2 wt% Si in the core. The isotope fractionation is considerably less, when a maximum of 3.0 wt% in the core is applied. Under oxidizing conditions it becomes difficult to ascertain that the Si isotope composition of BSE is due to core-formation only. Bulk Earth’s Si isotope composition is then not chondritic and may have been inherited from Earth’s precursor material.

Reference
Georg RB, Shahar A (2015) The accretion and differentiation of Earth under oxidizing conditions. American Mineralogist 100, 2739-2748
Link to Article [doi: 10.2138/am-2015-5153]

Copyright: The Mineralogical Society of America

¹J.J. Bellucci, ^1,2A.A. Nemchin, ¹M.J. Whitehouse, ¹J.F. Snape, ²P.A. Bland, ²G.K. Benedix
¹Department of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
²Department of Applied Geology, Curtin University, Perth, WA 6845, Australia
¹Department of Geological Sciences, Stockholm University, SE-106 91 Stockholm, Sweden

The Pb isotopic compositions of maskelynite and pyroxene grains were measured in ALH84001 and three enriched Shergottites (Zagami, RBT04262, and LAR12011) by Secondary Ion Mass Spectrometry (SIMS). A maskelynite-pyroxene isochron for ALH84001 defines a crystallization age of 4089±73 Ma (2σ). The initial Pb isotopic composition of each meteorite was measured in multiple maskelynite grains. ALH84001 has the least radiogenic initial Pb isotopic composition of any Martian meteorite measured to date (i.e., 206Pb/204Pb=10.07±0.17, 2σ). Assuming an age of reservoir formation for ALH84001 and the enriched Shergottites of 4513 Ma (Borg et al., 2003, Lapen et al., 2010), a two stage Pb isotopic model has been constructed. This model links ALH84001 and the enriched Shergottites by their similar μ-value (238U/204Pb) of 4.1-4.6 from 4.51 Ga to 4.1 Ga and 0.17 Ga, respectively. The model employed here is dependent on a chondritic μ-value (~1.2) from 4567–4513 Ma, which implies core segregation had little to no effect on the μ-value(s) of the Martian mantle. The proposed Pb isotopic model here can be used to calculate ages that are in agreement with Rb-Sr, Lu-Hf and Sm-Nd ages previously determined in the meteorites and confirm the young (~170 Ma) ages of the enriched Shergottites and ancient, >4 Ga, age of ALH84001.

Reference
Bellucci JJ, Nemchin AA, Whitehouse MJ, Snape JF, Bland P, Benedix GK (2015) The Pb isotopic evolution of the Martian mantle constrained by initial Pb in Martian meteorites. Journal of Geophysical Research, Planets (in Press)
Link to Article [DOI: 10.1002/2015JE004809]
Published by arrangement with John Wiley & Sons

¹Bhatia, G. K., ¹Sahijpal, S.
¹Department of Physics, Panjab University, Chandigarh, India

Hf-W isotopic systematics of Martian meteorites have provided evidence for the early accretion and rapid core formation of Mars. We present the results of numerical simulations performed to study the early thermal evolution and planetary scale differentiation of Mars. The simulations are confined to the initial 50 Myr (Ma) of the formation of solar system. The accretion energy produced during the growth of Mars and the decay energy due to the short-lived radio-nuclides 26Al, 60Fe, and the long-lived nuclides, 40K, 235U, 238U, and 232Th are incorporated as the heat sources for the thermal evolution of Mars. During the core-mantle differentiation of Mars, the molten metallic blobs were numerically moved using Stoke’s law toward the center with descent velocity that depends on the local acceleration due to gravity. Apart from the accretion and the radioactive heat energies, the gravitational energy produced during the differentiation of Mars and the associated heat transfer is also parametrically incorporated in the present work to make an assessment of its contribution to the early thermal evolution of Mars. We conclude that the accretion energy alone cannot produce widespread melting and differentiation of Mars even with an efficient consumption of the accretion energy. This makes 26Al the prime source for the heating and planetary scale differentiation of Mars. We demonstrate a rapid accretion and core-mantle differentiation of Mars within the initial ~1.5 Myr. This is consistent with the chronological records of Martian meteorites.

Reference
Bhatia GK, Sahijpal S (2015) The early thermal evolution of Mars. Meteoritics & Planetary Science (in Press)
Link to Article [doi: 10.1111/maps.12573]
Published by arrangement with John Wiley & Sons

¹Jens Hopp, ^1,2Mario Trieloff,^3,4Ulrich Ott
¹Institut für Geowissenschaften, Universität Heidelberg, Im Neuenheimer Feld 234-236, D-69120 Heidelberg, Germany
²Klaus-Tschira-Labor für Kosmochemie, Universität Heidelberg, Im Neuenheimer Feld 234-236, D-69120 Heidelberg, Germany
³University of West Hungary, Károlyi Gáspár ter 4, H-9700 Szombathely, Hungary
⁴Max-Planck-Institut für Chemie, Hahn-Meitner-Weg 1, D-55128 Mainz, Germany

In order to elucidate the early thermal history of enstatite chondrite parent bodies we determined 129I-129Xe whole rock ages of enstatite chondrites (5 EH, 2 EL, one EH impact melt) relative to the Shallowater reference meteorite (4562.3±0.4 Ma, all errors are 1σ). I-Xe ages of both EL6 chondrites (LON 94100: -4.38±0.60 Ma and Neuschwanstein: -3.87±0.73 Ma – negative sign indicates ages younger than Shallowater) agree well with data of other EL6 chondrites. LON 94100 displayed a second isochron at lower temperatures equivalent to a younger age of -5.25±1.17 Ma, perhaps reflecting different retention temperatures of respective carrier phases during sequential cooling. The enstatite chondrites Abee (EH4), Indarch (EH4), EET 96135 (EH4/5) and St. Marks (EH5) encompass a I-Xe age range of +0.57±1.05 Ma (EET 96135 #1) to -0.45±0.72 Ma (Abee), again in agreement with previously reported ages of EH chondrites. Only the age of St. Marks differs strongly from previously reported younger ages, now being more in accordance with other members of the EH clan. The EH3 chondrite Sahara 97096 showed the youngest I-Xe age of -7.87±0.46 Ma distinctly younger than other I-Xe ages of EH chondrites, including other EH3s. Due to the apparent high retention temperature of the I-Xe system in enstatite (estimated >800°C) this young age implies a later resetting of the I-Xe system by a severe thermal, likely impact-induced, event. The EH impact melt LAP 02225 records a similarly young thermal event. Though no isochron relationship could be established, the data fall within an apparent I-Xe age range of +5 to +15 Ma, similar to Sahara 97096. Overall, EH chondrite parent body experienced a thermal history determined by a complex interplay between impact disturbances and parent body metamorphism.

Reference
Hopp J, Trieloff M, Ott U (2015) I-Xe ages of enstatite chondrites. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.11.014]
Copyright Elsevier

¹Jan Leitner, ²Christian Vollmer, ³Christine Floss, ⁴Jutta Zipfel, ¹Peter Hoppe
¹Max Planck Institute for Chemistry, Particle Chemistry Department, Hahn-Meitner-Weg 1, 55128 Mainz, Germany
²Institute for Mineralogy, Westfälische Wilhelms-Universität, Correnstrasse 24, 48149 Münster, Germany
³Laboratory for Space Sciences and Physics Department, Washington University, One Brookings Drive, St. Louis, MO 63130, USA
⁴Forschungsinstitut und Naturmuseum Senckenberg, Sektion Meteoritenforschung, Senckenberganlage 25, 60325 Frankfurt, Germany

Carbonaceous chondrites are fragments from primitive parent asteroids, which represent some of the most primitive meteorites accessible for laboratory analysis and offer therefore the best opportunity to explore the chemical and physical conditions in the early Solar System. Here, we report the identification of presolar grains, which are circumstellar condensates that date back from before the formation of our Solar System, in fine-grained dust rims around chondrules in carbonaceous chondrites. Average presolar grain abundances in the rims of aqueously altered chondrites (petrologic type 2) are three times higher than in the respective interchondrule matrices, while for the most pristine specimens (petrologic type 3), the opposite is observed. The presence of these grains implies a nebular origin of the rim material, and gives evidence for differing alteration pathways for different reservoirs of fine-grained material found in primitive meteorites. Moreover, our findings indicate formation of the fine-grained rims in the solar nebula prior to parent-body accretion, giving support to accretionary scenarios for parent-bodies in the presence of dust-rimmed chondrules.

Reference
Leitner J, Vollmer C, Floss C, Zipfel J, Peter Hoppe (2015) Ancient stardust in fine-grained chondrule dust rims from carbonaceous chondrites. Earth and Planetary Science Letters (in Press)
Link to Article [doi:10.1016/j.epsl.2015.11.028]

Copyright Elsevier

¹J.J. Bellucci, ^1,2A.A. Nemchin, ^1,3M.J. Whitehouse, ¹J.F. Snape, ^1,3R.B. Kielman, ²P.A. Bland, ²G.K. Benedix
¹Department of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
²Department of Applied Geology, Curtin University, Perth, WA 6845, Australia
³Department of Geological Sciences, Stockholm University, SE-106 91 Stockholm, Sweden

Determining the chronology and quantifying various geochemical reservoirs on planetary bodies is fundamental to understanding planetary accretion, differentiation, and global mass transfer. The Pb isotope compositions of individual minerals in the Martian meteorite Chassigny have been measured by Secondary Ion Mass Spectrometry (SIMS). These measurements indicate that Chassigny has mixed with a Martian reservoir that evolved with a long-term 238U/204Pb (μ ) value ∼ two times higher than those inferred from studies of all other Martian meteorites except 4.428 Ga clasts in NWA7533. Any significant mixing between this and an unradiogenic reservoir produces ambiguous trends in Pb isotope variation diagrams. The trend defined by our new Chassigny data can be used to calculate a crystallization age for Chassigny of 4.526±0.027 Ga4.526±0.027 Ga (2σ) that is clearly in error as it conflicts with all other isotope systems, which yield a widely accepted age of 1.39 Ga. Similar, trends have also been observed in the Shergottites and have been used to calculate a >4 Ga age or, alternatively, attributed to terrestrial contamination. Our new Chassigny data, however, argue that the radiogenic component is Martian, mixing occurred on the surface of Mars, and is therefore likely present in virtually every Martian meteorite. The presence of this radiogenic reservoir on Mars resolves the paradox between Pb isotope data and all other radiogenic isotope systems in Martian meteorites. Importantly, Chassigny and the Shergottites are likely derived from the northern hemisphere of Mars, while NWA 7533 originated from the Southern hemisphere, implying that the U-rich reservoir, which most likely represents some form of crust, must be widespread. The significant age difference between SNC meteorites and NWA 7533 is also consistent with an absence of tectonic recycling throughout Martian history.

Reference
Bellucci JJ, Nemchin AA, Whitehouse MJ, Snape JF, Kielman RB, Bland PA, Benedix GK (2015) A Pb isotopic resolution to the Martian meteorite age paradox. Earth and Planetary Science Letters (in Press)
Link to Article [doi:10.1016/j.epsl.2015.11.004]
Copyright Elsevier

¹Manuel Moreira, ²Sébastien Charnoz
¹Institut de Physique du Globe de Paris, Sorbonne Paris Cité, UMR CNRS 7154, Université Paris Diderot, France
²Laboratoire AIM (Astrophysique Instrumentation Modélisation), Sorbonne Paris Cité, Université Paris Diderot, CEA Irfu, UMR CNRS 7158, France

We discuss the origin of the neon isotopic signatures in chondrites and in the terrestrial mantle. There are two primary possible origins for neon in the Earth’s mantle. One origin is the dissolution of a dense primordial atmosphere with a solar composition of 20Ne/22Ne >13.4 into the mantle in a possible magma ocean stage during Earth’s accretion. The second origin, developed in this study, is that mantle neon was already in Earth’s parent bodies because of refractory grain irradiation by solar wind. We propose that solar wind implantation occurred early on dust within the accretion disk to allow such irradiation. Because solar wind implantation fractionates neon isotopes, the heavier isotopes are implanted deeper than the lighter ones because of different kinetic energies, and the process of implantation, if coupled with sputtering, leads to a steady state neon isotopic ratio (20Ne/22Ne ∼12.7) that is similar to what is observed in mantle-derived rocks (12.5–12.9), lunar soil grains (∼12.9) and certain gas-rich chondrites from all classes (enstatite, ordinary, rumuruti). Using a dust transport model in a turbulent and irradiated solar nebula, we estimated the equivalent irradiation age of a population of dust particles at three different distances from the sun (0.8, 1, 1.2 AU) and converted these ages into neon concentrations and isotopic ratios. The dust subsequently coagulated to form Earth’s parent bodies, which have the mean neon isotopic composition of the irradiated dust (non-irradiated dust is assumed to be free of neon). If this scenario of solar wind implantation coupled with sputtering in the precursors of Earth’s parent bodies is correct, it offers a simple alternative to the model of solar nebula gas incorporation by dissolution in a magma ocean.

Reference
Moreira M, Charnoz S (2015) The origin of the neon isotopes in chondrites and on Earth. Earth and Planetary Science Letters (in Press)
Link to Article [doi:10.1016/j.epsl.2015.11.002doi:10.1016/j.epsl.2015.11.002]
Copyright Elsevier

¹Fatemeh Sedaghatpour, ²Fang-Zhen Teng
¹Isotope Laboratory, Department of Geosciences and Arkansas
²Center for Space and Planetary Sciences, University of Arkansas, Fayetteville, AR 72701, USA

Magnesium isotopic compositions of 22 well-characterized differentiated meteorites including 7 types of achondrites and pallasite meteorites were measured to estimate the average Mg isotopic composition of their parent bodies and evaluate Mg isotopic heterogeneity of the solar system. The δ26Mg values are -0.236‰ and -0.190‰ for acapulcoite-lodranite and angrite meteorites, respectively and vary from -0.267‰ to -0.222‰ in the winonaite-IAB-iron silicate group, -0.369‰ to -0.292‰ in aubrites, -0.269‰ to -0.158‰ in HEDs, -0.299‰ to -0.209‰ in ureilites, -0.307‰ to -0.237‰ in mesosiderites, and -0.303‰ to -0.238‰ in pallasites. Magnesium isotopic compositions of most achondrites and pallasite meteorites analyzed here are similar and reveal no significant isotopic fractionation. However, Mg isotopic compositions of D′Orbigny (angrite) and some HEDs are slightly heavier than chondrites and the other achondrites studied here. The slightly heavier Mg isotopic compositions of angrites and some HEDs most likely resulted from either impact-induced evaporation or higher abundance of clinopyroxene with the Mg isotopic composition slightly heavier than olivine and orthopyroxene. The average Mg isotopic composition of achondrites (δ26Mg = -0.246 ± 0.082‰, 2SD, n = 22) estimated here is indistinguishable from those of the Earth (δ26Mg = -0.25 ± 0.07‰; 2SD, n = 139), chondrites (δ26Mg = -0.28 ± 0.06‰; 2SD, n = 38), and the Moon (δ26Mg = -0.26 ± 0.16‰) reported from the same laboratory. The chondritic Mg isotopic composition of achondrites, the Moon, and the Earth further reflects homogeneity of Mg isotopes in the solar system and the lack of Mg isotope fractionation during the planetary accretion process and impact events.

Reference
Sedaghatpour F, Teng F-Z (2015) Magnesium isotopic composition of achondrites. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.11.016]
Copyright Elsevier

^1,2Alex Ruzicka, ¹Ryan Brown, ^3,4Jon Friedrich, ^1,2 Melinda Hutson, ² Richard Hugo, ⁵Mark Rivers
¹Cascadia Meteorite Laboratory, Portland State University, 1721 SW Broadway, Portland, Oregon 97207, U.S.A.
²Department of Geology, Portland State University, 17 Cramer Hall, 1721 SW Broadway, Portland, Oregon 97207, U.S.A.
³Department of Chemistry, Fordham University, Bronx, New York 10458, U.S.A.
⁴Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York 10024, U.S.A.
⁵Consortium for Advanced Radiation Sources, University of Chicago, Argonne, Illinois 60439, U.S.A.

The conditions under which metal cores formed in silicate-metal planetary bodies in the early Solar System are poorly known. We studied the Buck Mountains 005 (L6) chondrite with serial sectioning, X-ray computed microtomography, and optical and electron microscopy to better understand how metal and troilite were redistributed as a result of a moderately strong (shock stage S4) shock event, as an example of how collisional processes could have contributed to differentiation. The chondrite was recovered on Earth in multiple small pieces, some of which have a prominent, 1.5–3 mm wide holocrystalline shock melt dike that forms a jointed, sheet-like structure, as well as an associated shock vein network. The data suggest that metal and troilite within the dike were melted, sheared, and transported as small parcels of melt, with metal moving out of the dike and along branching veins to become deposited as coarser nodules and veins within largely unmelted host. Troilite also mobilized but partly separated from metal to become embedded as finer-grained particles, vein networks, and emulsions intimately intergrown with silicates. Rock textures and metal compositions imply that shock melts cooled rapidly against relatively cool parent body materials, but that low-temperature annealing occurred by deep burial within the parent body. Our results demonstrate the ability of shock processes to create larger metal accumulations in substantially unmelted meteorite parent bodies, and they have implications for the formation of iron meteorites and for core formation within colliding planetesimals.

Reference
Ruzicka A, Brown R, Friedrich J, Hutson M, Hugo R, Rivers M(2015) Shock-induced mobilization of metal and sulfide in planetesimals: Evidence from the Buck Mountains 005 (L6 S4) dike-bearing chondrite. American Mineralogist 100, 2725-2738, Link to Article [doi:10.2138/am-2015-5225]
Copyright: The Mineralogical Society of America

¹Sandra Pizzarello, ¹Maitrayee Bose
¹Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, USA

Large isotopic anomalies are found in meteoritic insoluble organic materials (IOMs) and, for nitrogen, show 15N-excesses up to ${\delta }^{15}$N ~ 5000‰. These 15N-enrichments are commonly ascribed to presolar origins, but the attribution seems contradicted by available data on N-isotopes’ cosmic distribution. We report here that 15N hotspots in several IOMs are reduced by hydrothermal treatment and their loss correlates with 15N values of ammonia released upon treatment. Because released ammonia’s 15N-enrichments also relate with meteorites’ mineralogy, i.e., asteroidal processes, and no current models offer plausible explanations for the finding, we account for our data with a novel scenario whereby 15N-enriched ammonia produced in the solar nebula is incorporated by carbonaceous materials and delivered to early Earth by comets and meteorites. The proposal also implies that abundant reduced nitrogen, a required element in origins of life theories, could reach our nascent planet and other planetary systems affecting their habitability.

Reference
Pizzarello S, Bose M (2015) THE PATH OF REDUCED NITROGEN TOWARD EARLY EARTH: THE COSMIC TRAIL AND ITS SOLAR SHORTCUTS. The Astrophysical Journal 814, 2
Link to Article [http://dx.doi.org/10.1088/0004-637X/814/2/107]