¹R. T. Pidgeon, ¹R. E. Merle, ¹M. L. Grange, ^1,2A. A. Nemchin,²M. J. Whitehouse
¹Department of Applied Geology, Curtin University, Perth, Western Australia, Australia
²Swedish Museum of Natural History, Stockholm, Sweden

Impact breccia 14311, was collected from the Apollo 14 landing site as a potential sample of the underlying Fra Mauro Formation. Published zircon U-Pb ages of >4000 Ma date the source material of the breccia and the apatite U-Pb age of ~3940 Ma is interpreted as dating thermal resetting of the apatite U-Pb systems. In this contribution we present new age information on the late stage thermal history of the breccia based on the annealing of radiation damage in the zircons. From Raman spectroscopic determination of the radiation damage within SIMS analytical spots on the zircons and the U and Th concentrations determined on these spots, we demonstrate that the radiation damage in the zircons has been annealed and we estimate the age of annealing at 3410 ± 80 Ma. This age is interpreted as a cooling age following heating of the breccia to above the annealing temperature of ~230 °C for stage 1 radiation damage in zircon, but below the temperature needed to reset the U-Pb system of apatite (~500 °C). It is proposed that this thermal event was associated with the prolonged period of Mare volcanism, from 3150 to 3750 Ma, that generated massive basalt flows in the vicinity of the sample location.

Reference
Pidgeon RT, Merle RE, Grange ML, Nemchin AA, Whitehouse MJ (2015) Annealing of radiation damage in zircons from Apollo 14 impact breccia 14311: Implications for the thermal history of the breccia.
Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12572]
Published by arrangement with John Wiley & Sons

^1,2Hiroshi Nagaoka, ³Yuzuru Karouji, ⁴Hiroshi Takeda, ⁵Timothy J. Fagan, ⁶Mitsuru Ebihara, ^1,2Nobuyuki Hasebe
¹Research Institute for Science and Engineering, Waseda University, Shinjuku 169-8555, Tokyo, Japan
²School of Advanced Science and Engineering, Waseda University, Shinjuku 169-8555, Tokyo, Japan
³Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), Sagamihara 252-5210, Kanagawa, Japan
⁴Department of Earth and Planetary Science, University of Tokyo, Hongo 113-0033, Tokyo, Japan
⁵Department of Earth Science School of Education, Waseda University, Shinjuku 169-8050, Tokyo, Japan
⁶Department of Chemistry, Tokyo Metropolitan University, Hachioji 192-0397, Tokyo, Japan

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Reference
Nagaoka H, Karouji Y, Takeda H, Fagan TJ, Ebihara M, Hasebe N (2015)
Mineralogy and petrology of lunar meteorite Northwest Africa 2977 consisting of olivine cumulate gabbro including inverted pigeonite. Earth, Planets and Space 67, 200

Link to Article [doi:10.1186/s40623-015-0368-y]

¹Lars E. Borg, ^1,2Gregory A. Brennecka, ³Steven J.K. Symes
¹Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue L-231, Livermore CA 94550 USA
²Institut für Planetologie, Westfälische Wilhelms-Universität Munster, Wilkhekm-Klemm-Str. 10, 48149 Münster Germany
³Department of Chemistry, University of Tennessee-Chattanooga, Chattanooga, TN 37403, USA

High precision Sm−Nd isotopic analyses have been completed on a suite of 11 martian basaltic meteorites in order to better constrain the age of silicate differentiation on Mars associated with the formation of their mantle sources. These data are used to evaluate the merits and disadvantages of various mathematical approaches that have been employed in previous work on this topic. Ages determined from the Sm−Nd isotopic systematics of individual samples are strongly dependent on the assumed Nd isotopic composition of the bulk planet. This assumption is problematic given differences observed between the Nd isotopic composition of Earth and chondritic meteorites and the fact that these materials are both commonly used to represent bulk planetary Nd isotopic compositions. Ages determined from the slope of 146Sm−142Nd whole rock isochrons are not dependent on the assumed 142Nd/144Nd ratio of the planet, but require the sample suite to be derived from complementary, contemporaneously-formed reservoirs. In this work, we present a mathematical expression that defines the age of formation of the source regions of such a suite of samples that is based solely on the slope of a 143Nd−142Nd whole rock isochron and is also is independent of any a priori assumptions regarding the bulk isotopic composition of the planet. This expression is also applicable to mineral isochrons and has been used to successfully calculate 143Nd−142Nd model crystallization ages of early refractory solids as well as lunar samples. This permits ages to be obtained using only Nd isotopic measurements without the need for 147Sm/144Nd isotope dilution determinations. When used in conjunction with high-precision Nd isotopic measurements completed on martian meteorites this expression yields an age of formation of the martian basaltic meteorite source regions of 4504 ± 6 Ma. Because the Sm−Nd model ages for the formation of martian source regions are commonly interpreted to record the age at which large scale mantle reservoirs formed during planetary differentiation associated with magma ocean solidification, the age determined here implies that magma ocean solidification occurred several tens of millions of years after the beginning of the Solar System. Recent thermal models, however, suggest that Mars-sized bodies cool rapidly in less than ∼5 Ma after accretion ceases, even in the presence of a thick atmosphere. Assuming these models are correct, an extended period of accretion is necessary to provide a mechanism to keep portions of the martian mantle partially molten until 4504 Ma. Late accretional heating of Mars could either be associated with protracted accretion occurring at a quasi-steady state or alternatively be associated with a late giant impact. If this scenario is correct, then accretion of Mars-sized bodies takes up to 60 Ma and is likely to be contemporaneous with the core formation and possibly the onset of silicate differentiation. This further challenges the concept that isotopic equilibrium is attained during primordial evolution of planets, and may help to account for geochemical evidence implying addition of material into planetary interiors after core formation was completed.

Reflectance
Borg LE, Brennecka GA, Symes SJK (2015) Accretion Timescale and Impact History of Mars Deduced from the Isotopic Systematics of Martian Meteorites. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.12.002]
Copyright Elsevier

¹A. Nathues et al. (>10)*
¹Institute for Solar System Research, Goettingen, Germany
*Find the extensive, full author and affiliation list on the publishers Website

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Reference
Nathues A et al. (2015) Sublimation in bright spots on (1) Ceres. Nature 528, 237–240
Link to Article [doi:10.1038/nature15754]

^1,2Pierre Bonnand, ^1,3Ian J. Parkinson, ^4,5Mahesh Anand
¹Department of Environment, Earth and Ecosystems, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
²Department of Earth Sciences, University of Oxford, South Parks Roads, Oxford, OX1 3AN, United Kingdom
³School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Clifton BS8 1RJ, Bristol, United Kingdom
⁴Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
⁵Department of Earth Sciences, The Natural History Museum, London SW7 5DB, United Kingdom

We present the first stable chromium isotopic data from mare basalts in order to investigate the similarity between the Moon and the Earth’s mantle. A double spike technique coupled with MC-ICP-MS measurements was used to analyse 19 mare basalts, comprising high-Ti, low-Ti and KREEP-rich varieties. Chromium isotope ratios (δ53Cr) for mare basalts are positively correlated with indices of magmatic differentiation such as Mg# and Cr concentration which suggests that Cr isotopes were fractionated during magmatic differentiation. Modelling of the results provides evidence that spinel and pyroxene are the main phases controlling the Cr isotopic composition during fractional crystallisation. The most evolved samples have the lightest isotopic compositions, complemented by cumulates that are isotopically heavy. Two hypotheses are proposed to explain this fractionation: (i) equilibrium fractionation where heavy isotopes are preferentially incorporated into the spinel lattice and (ii) a difference in isotopic composition between Cr2+ and Cr3+ in the melt. However, both processes require magmatic temperatures below 1200˚C for appreciable Cr3+ to be present at the low oxygen fugacities found in the Moon (IW -1 to -2 log units). There is no isotopic difference between the most primitive high-Ti, low-Ti and KREEP basalts, which suggest that the sources of these basalts were homogeneous in terms of stable Cr isotopes. The least differentiated sample in our sample set is the low-Ti basalt 12016, characterised by a Cr isotopic composition of -0.222 ± 0.025 ‰, which is within error of the current BSE value (-0.124 ± 0.101 ‰). The similarity between the mantles of the Moon and Earth is consistent with a terrestrial origin for a major fraction of the lunar Cr. This similarity also suggests that Cr isotopes were not fractionated by core formation on the Moon.

Reference
Bonnand P, Parkinson IJ, Anand M (2015) Mass dependent fractionation of stable chromium isotopes in mare basalts: implications for the formation and the differentiation of the Moon. Geochimica et Cosmochicmica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.11.041]
Copyright Elsevier

¹S. Yamamoto et al. (>10)*
¹Center for Environmental Measurement and Analysis, National Institute for Environmental Studies, Tsukuba, Japan
*Find the extensive, full author and affiliation list on the publishers website

We report the global distribution of areas exhibiting no absorption features (featureless or FL) on the lunar surface, based on the reflectance spectral data set obtained by the Spectral Profiler onboard Kaguya/SELENE. We found that FL sites are located in impact basins and large impact craters in the Feldspathic Highlands Terrane (FHT), while there are no FL sites in the Procellarum regions nor the South Pole–Aitken basin. FL sites in each impact basin/crater are mainly found at the peak rings or rims, where the purest anorthosite (PAN) sites are also found. At the local scale, most of the FL and PAN points are associated with impact craters and peaks. Most of the FL spectra show a steeper (redder) continuum than the PAN spectra, suggesting the occurrence of space weathering effects. We propose that most of the material exhibiting a FL spectrum originate from space weathered PAN. Taking into account all the occurrence trends of FL sites on the Moon, we propose that both the FL and PAN materials were excavated from the primordial lunar crust during ancient basin formations below the megaregolith in the highlands. Since the FL and PAN sites are widely distributed over the lunar surface, our new data may support the existence of a massive PAN layer below the lunar surface.

Reference
Yamamoto S et al.(2015) Featureless spectra on the Moon as evidence of residual lunar primordial crust. Journal of Geophysical Research, Planets (in Press)
Link to Article [doi: 10.1002/2015JE004935]
Published by arrangement with John Wiley&Sons

^1,2J. Labidi, ¹A. Shahar, ¹C. Le Losq, ¹V.J. Hillgren, ¹B.O. Mysen, ²J. Farquhar
¹Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20015, USA.
²Department of Geology, University of Maryland, College Park MD, 20740, USA

The Earth’s mantle displays a subchondritic 34S/32S ratio. Sulfur is a moderately siderophile element (i.e. iron-loving), and its partitioning into the Earth’s core may have left such a distinctive isotope composition on the terrestrial mantle. In order to constrain the sulfur isotope fractionation occurring during core-mantle differentiation, high-pressure and temperature experiments were conducted with synthetic mixtures of metal and silicate melts. With the purpose to identify the mechanism(s) responsible for the S isotope fractionations, we performed our experiments in different capsules – namely, graphite and boron nitride capsules – and thus at different fO2, with varying major element chemistry of the silicate and metal fractions.

The S isotope fractionations Δ34Smetal-silicate of equilibrated metal alloys versus silicate melts is +0.2±0.1‰ in a boron-free and aluminum-poor system quenched at 1-1.5 GPa and 1650 ˚C. The isotope fractionation increases linearly with increasing boron and aluminum content, up to +1.4±0.2‰, and is observed to be independent of the silicon abundance as well as of the fO2 over ∼ 3.5 log units of variations explored here. The isotope fractionations are also independent of the graphite or nitride saturation of the metal. Only the melt structural changes associated with aluminum and boron concentration in silicate melts have been observed to affect the strength of sulfur bonding. These results establish that the structure of silicate melts has a direct influence on the S2- average bonding strengths.

These results can be interpreted in the context of planetary differentiation. Indeed, the structural environments of silicate evolve strongly with pressure. For example, the aluminum, iron or silicon coordination numbers increase under the effect of pressure. Consequently, based on our observations, the sulfur-bonding environment is likely to be affected. In this scheme, we tentatively hypothesize that S isotope fractionations between the silicate mantle and metallic core of terrestrial planetary bodies would depend on the average pressure at which their core-mantle differentiation occurred.

Reference
Labidi J, Shahara A, Le Losq C, Hillgren VJ, Mysen BO, Farquhar J (2015) Experimentally determined sulfur isotope fractionation between metal and silicate and implications for planetary Differentiation. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.12.001]
Copyright Elsevier

^1,2J. de León et al. (>10)*
¹Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, 38205, La Laguna, Spain
²Department of Astrophysics, University of La Laguna, 38205, Tenerife, Spain
*Find the extensive, full author and affiliation list on the publishers website

The Polana-Eulalia family complex is located in the inner part of the asteroid belt, bounded by the ν6ν6 and the 3:1 resonances, where we can find another three collisional families of primitive asteroids (Erigone, Clarissa, and Sulamitis), and a low-albedo population of background objects. This region of the belt is believed to be the most likely origin of the two primitive near-Earth asteroids that are the current targets of two sample return missions: NASA’s OSIRIS-REx and JAXA’s Hayabusa 2 to asteroids (101955) Bennu and (162173) Ryugu (also known as 1999 JU3), respectively. Therefore, understanding these families will enhance the scientific return of these missions.

We present the results of a spectroscopic survey of asteroids in the region of the Polana-Eulalia family complex, and also asteroids from the background population of low-albedo, low-inclination objects. We obtained visible spectra of a total of 65 asteroids, using the 10.4m Gran Telescopio Canarias (GTC) and the 3.6m Telescopio Nazionale Galileo (TNG), both located at the El Roque de Los Muchachos Observatory, in the island of La Palma (Spain), and the 3.6m New Technology Telescope (NTT), located at the European Southern Observatory of La Silla, in Chile. From the spectral analysis of our sample we found that, in spite of the presence of distinct dynamical groups, the asteroids in this region present spectral homogeneity at visible wavelengths, showing a continuum of spectral slopes, from blue to moderately red, typical of primitive asteroids classified as B- and C-types. We conclude that visible spectra can not be used to distinguish between members of the Polana and the Eulalia families, or members of the background population.

The visible spectra of the two targets of sample return missions, asteroids Bennu and Ryugu, are compatible with the spectra of the asteroids in this region, supporting previous studies that suggested either the Polana family or the background population as the most likely origins of these NEAs.

Reference
de León J et al. (2015) Visible Spectroscopy of the Polana-Eulalia Family Complex: Spectral Homogeneity. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.11.014]

Copyright Elsevier

¹Caroline Fitoussi, ¹Bernard Bourdon,¹Xueying Wang
¹Laboratoire de Géologie de Lyon (Ecole Normale Supérieure de Lyon, CNRS and Université Claude Bernard de Lyon), ENS Lyon, 46 allée d’Italie, 69364 Lyon Cedex 07, France

The Earth formed in a swarm of Moon- to Mars-sized objects that collided together to build our planet. A large body of work has been dedicated to understanding the Earth’s composition as being made of single groups or mixtures of chondrites, however, these models cannot account for the isotopic and elemental characteristics of the Earth. Here, we test mixtures of meteorites, including achondrites, analyzed for seven isotope systems (O, Cr, Ni, Ti, Mo, Ca and Sr), to reproduce the isotope compositions of the Earth and Mars. Our Monte Carlo inversion (a numerical method based on generation of random numbers used to invert multiparameter models) yields a new compositional model where Earth and Mars come almost entirely from the same source material. This finding is in striking agreement with recent planetary formation models in which Earth and Mars formed in a common narrow zone of the protoplanetary disk with Mars being ejected to its current position which prevented further accretion. An important outcome of the model is that a significant mass fraction of the Earth could have been made of volatile depleted and refractory enriched planetary bodies such as angrites (among the oldest known achondrites). This conclusion is also in agreement with new Si isotope data in angrites which suggest that a component of angrites would help explain the difference in δ30Siδ30Si between the bulk silicate Earth and its building blocks. Our model matches all isotope compositions for both planets, reproduces the volatile element budget of Mars, and accounts for the enrichment in refractory elements of the Earth and Mars compared to chondrites.

Reference
Fitoussi C, Bourdon B, Wang X (2015) The building blocks of Earth and Mars: A close genetic link. Earth and Planetary Science Letters (in Press)
Link to Article [doi:10.1016/j.epsl.2015.11.036]

Copyright Elsevier

^1,2,3Vincenzo Stagno, ⁴Luca Bindi, ⁵Changyong Park, ⁶Sergey Tkachev, ⁶Vitali B. Prakapenka, ^1,7H.-K. Mao, ¹Russell J. Hemley, ⁸Paul J. Steinhardt,¹Yingwei Fei
¹Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20015, U.S.A.
²Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan
³Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo 152-8550, Japan
⁴Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, I-50121 Florence, Italy
⁵HPCAT, Geophysical Laboratory, Carnegie Institution of Washington, Argonne, Illinois 60439, U.S.A.
⁶Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, U.S.A.
⁷Center for High Pressure Science and Technology Advanced Research, Shanghai 201203, P.R. China
⁸Department of Physics and Princeton Center for Theoretical Science, Princeton University, Princeton, New Jersey 08544, U.S.A.

Icosahedrite, the first natural quasicrystal with composition Al63Cu24Fe13, was discovered in several grains of the Khatyrka meteorite, a CV3 carbonaceous chondrite. The presence of icosahedrite associated with high-pressure phases like ahrensite and stishovite indicates formation at high pressures and temperatures due to an impact-induced shock. Previous experimental studies on the stability of synthetic icosahedral AlCuFe have either been limited to ambient pressure, for which they indicate incongruent melting at ~1123 K, or limited to room-temperature, for which they indicate structural stability up to about 35 GPa. These data are insufficient to experimentally constrain the formation and stability of icosahedrite under the conditions of high pressure and temperature that formed the Khatyrka meteorite. Here we present the results of room-temperature, high-pressure diamond-anvil cells measurements of the compressional behavior of synthetic icosahedrite up to ~50 GPa. High P-T experiments were also carried out using both laser-heated diamond-anvil cells combined with in situ synchrotron X-ray diffraction (at ~42 GPa) and multi-anvil apparatus (at 21 GPa) to investigate the structural evolution and crystallization of possible coexisting phases. The results demonstrate that the quasiperiodic order of icosahedrite is retained over the P-T range explored. We find that pressure acts to stabilize the icosahedral symmetry at temperatures much higher than previously reported. Direct solidification of AlCuFe quasicrystals from an unusual Al-Cu-rich melt is possible but it is limited to a narrow temperature range. Alternatively, quasicrystals may form after crystallization through solid-solid reactions of Al-rich phases. In either case, our results show that quasicrystals can preserve their structure even after hypervelocity impacts spanning a broad range of pressures and temperatures.

Reference
Stagno V, Bindi L, Park C, Tkachev S, Prakapenka VB, Mao H-K, Hemley RJ, Steinhardt PJ,Fei Y (2015) Quasicrystals at extreme conditions: The role of pressure in stabilizing icosahedral Al63Cu24Fe13 at high temperature. American Mineralogist 100, 2412-2418
Link to Article [doi: 10.2138/am-2015-5412]

Copyright: The Mineralogical Society of America