Nickel isotope fractionation during metal-silicate differentiation of planetesimals: experimental petrology and ab initio calculations

1,2J.Guignard,1G.Quitté,2M.Méheut,1M.J.Toplis,2F.Poitrasson,3D.Connetable,4M.Roskosz
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.10.028]
1IRAP, Université de Toulouse, CNRS, UPS, CNES, Toulouse, France
2GET, Université de Toulouse, CNRS, UPS, IRD, CNES, Toulouse, France
3CIRIMAT, CNRS, INP, ENSIACET, 4 allée Emile Monso, BP44362, 31030 Toulouse cedex 4, France
4IMPMC, CNRS, UMR 7590, Sorbonne Universités, Université Pierre et Marie Curie, Muséum National d’Histoire Naturelle, CP 52, 57 rue Cuvier, Paris F-75231, France
Copyright Elsevier

Metal-silicate fractionation of nickel isotopes has been experimentally quantified at 1623 K, with oxygen fugacities varying from 10-8.2 to 10-9.9 atm and for run durations from 0.5 to 1 h. Both kinetic and equilibrium fractionations have been studied. A wire loop set-up was used in which the metal reservoir is a pure nickel wire holding a silicate melt droplet of anorthite-diopside eutectic composition. During the course of the experiment, diffusion of nickel from the wire to the silicate occurred. The timescale to reach chemical equilibrium was fO2 dependent and decreased from 17 to 1 hour, as conditions became more reducing.

The isotopic composition of each reservoir was determined by Multicollector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICPMS) after Ni purification. The isotopic composition was found to be constant in the metallic wire, which therefore behaved as an infinite reservoir. On the contrary, strong kinetic fractionation was observed in the silicate melt (δNi down to -0.98 ‰.amu-1 relative to the standard). Isotopic equilibrium was typically reached after 24 hours. For equilibrated samples at 1623 K, no metal-silicate fractionation was observed within uncertainty (2SD), with ΔNiMetal-Silicate = 0.02 ± 0.04 ‰.amu-1.

Theoretical calculations of metal-silicate isotope fractionation at equilibrium were also performed on different metal-silicate systems. These calculations confirm (1) the absence of fractionation at high temperature and (2) a weak temperature dependence for Ni isotopic fractionation for the metal-olivine and metal-pyroxene pairs with the metal being slightly lighter isotopically.

Our experimental data were finally compared with natural samples. Some mesosiderites (stony-iron meteorites) show a ΔNiMetal-Silicate close to experimental values at equilibrium, whereas others exhibit positive metal-silicate fractionation that could reflect kinetic processes. Conversely, pallasites display a strong negative metal-silicate fractionation. This most likely results from kinetic processes with Ni diffusion from the silicate to the metal phase due to a change of Ni partition coefficient during cooling. In this respect we note that in these pallasites, iron isotopes show metal-silicate fractionation that is opposite direction to Ni, supporting the idea of kinetic isotope fractionation, associated with Fe-Ni interdiffusion.

Two generations of exsolution lamellae in pyroxene from Asuka 09545: Clues to the thermal evolution of silicates in mesosiderite

1,2Lidia Pittarello,1,3Seann McKibbin,4Akira Yamaguchi,5,6Gang Ji,5Dominique Schryvers,7Vinciane Debaille,7Philippe Claeys
American Mineralogist 104, 1663-1672 Link to Article [https://doi.org/10.2138/am-2019-7001]
1Analytical, Environmental, and Geo-Chemistry (AMGC), Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
2Department of Mineralogy and Petrography, Natural History Museum Vienna, Burgring 7, A-1010 Vienna, Austria.
3Geowissenschaftliches Zentrum, Georg-August Universität, Goldschmidtstraße 1, 37073 Göttingen, Germany.
4National Institute of Polar Research, Antarctic Meteorite Research Center, 10-3 Midoricho, Tachikawa, Japan
5Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
6University of Lille, CNRS, INRA, ENSCL, UMR 8207, UMET, Unité Matériaux et Transformations, F-59000 Lille, France.
7Laboratoire G-Time (Géochemie: Traçage isotopique, minéralogique et élémentaire), Université Libre de Bruxelles, Av. F.D. Roosevelt 50, 1050 Brussels, Belgium B-1050 Brussels, Belgium
Copyright: The Mineralogical Society of America

Mesosiderite meteorites consist of a mixture of crustal basaltic or gabbroic material and metal. Their formation process is still debated due to their unexpected combination of crust and core materials, possibly derived from the same planetesimal parent body, and lacking an intervening mantle component. Mesosiderites have experienced an extremely slow cooling rate from ca. 550 °C, as recorded in the metal (0.25–0.5 °C/Ma). Here we present a detailed investigation of exsolution features in pyroxene from the Antarctic mesosiderite Asuka (A) 09545. Geothermobarometry calculations, lattice parameters, lamellae orientation, and the presence of clinoenstatite as the host were used in an attempt to constrain the evolution of pyroxene from 1150 to 570 °C and the formation of two generations of exsolution lamellae. After pigeonite crystallization at ca. 1150 °C, the first exsolution process generated the thick augite lamellae along (100) in the temperature interval 1000–900 °C. By further cooling, a second order of exsolution lamellae formed within augite along (001), consisting of monoclinic low-Ca pyroxene, equilibrated in the temperature range 900–800 °C. The last process, occurring in the 600–500 °C temperature range, was likely the inversion of high to low pigeonite in the host crystal, lacking evidence for nucleation of orthopyroxene.

The formation of two generations of exsolution lamellae, as well as of likely metastable pigeonite, suggest non-equilibrium conditions. Cooling was sufficiently slow to allow the formation of the lamellae, their preservation, and the transition from high to low pigeonite. In addition, the preservation of such fine-grained lamellae limits long-lasting, impact reheating to a peak temperature lower than 570 °C. These features, including the presence of monoclinic low-Ca pyroxene as the host, are reported in only a few mesosiderites. This suggests a possibly different origin and thermal history from most mesosiderites and that the crystallography (i.e., space group) of low-Ca pyroxene could be used as parameter to distinguish mesosiderite populations based on their cooling history.

The Italian Solfatara as an analog for Mars fumarolic alteration

1Jessica Flahaut,2Janice L. Bishop,2,3Simone Silvestro,4,5Dario Tedesco,6Isabelle Daniel,7Damien Loizeau
American Mineralogist 104, 1565-1577 Link to Article [https://doi.org/10.2138/am-2019-6899]
1Centre de Recherches Pétrographiques et Géochimiques (CRPG), UMR7358 CNRS-Université de Lorraine, 15 rue Notre-Dame des Pauvres, 54500 Vandœuvre-lès-Nancy, France. Orcid 0000-0002-0866-8086
2Carl Sagan Center, The SETI Institute, Mountain View, California 94043, U.S.A.
3INAF—Osservatorio Astronomico di Capodimonte, Napoli, Italy
4Campania University—Luigi Vanvitelli, Caserta, Italy
5Osservatorio Vesuviano—Istituto Nazionale di Geochimica e Vulcanologia, Napoli, Italy
6Université de Lyon, Université Lyon 1, Ens de Lyon, CNRS, UMR 5276, Lab. de Géologie de Lyon, Villeurbanne F-69622, France. Orcid 0000-0002-1448-7919
7IAS, CNRS/Université Paris Sud, 91400 Orsay, France
Copyright: The Mineralogical Society of America

The first definitive evidence for continental vents on Mars is the in situ detection of amorphous silica-rich outcrops by the Mars Exploration Rover Spirit. These outcrops have been tentatively interpreted as the result of either acid sulfate leaching in fumarolic environments or direct precipitation from hot springs. Such environments represent prime targets for upcoming astrobiology missions but remain difficult to identify with certainty, especially from orbit. To contribute to the identification of fumaroles and hot spring deposits on Mars, we surveyed their characteristics at the analog site of the Solfatara volcanic crater in central Italy. Several techniques of mineral identification (VNIR spectroscopy, Raman spectroscopy, XRD) were used both in the field and in the laboratory on selected samples. The faulted crater walls showed evidence of acid leaching and alteration into the advanced argillic-alunitic facies, with colorful deposits containing alunite, jarosite, and/or hematite. Sublimates containing various Al and Fe hydroxyl-sulfates were observed around the active fumarole vents at 90 °C. One vent at 160 °C was characterized by different sublimates enriched in As and Hb sulfide species. Amorphous silica and alunite assemblages that are diagnostic of silicic alteration were also observed at the Fangaia mud pots inside the crater. A wide range of minerals was identified at the 665 m diameter Solfatara crater that is diagnostic of acid-steam heated alteration of a trachytic, porous bedrock. Importantly, this mineral diversity was captured at each site investigated with at least one of the techniques used, which lends confidence for the recognition of similar environments with the next-generation Mars rovers.

New lunar meteorite NWA 10986: A mingled impact melt breccia from the highlands—A complete cross section of the lunar crust

1S. E. Roberts,1M. C. McCanta,1,2M. M. Jean,1L. A. Taylor
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13406]
1Department of Barth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee 37996, USA
2Department of Geological Sciences, University of Alaska Anchorage, Anchorage, Alaska 99508, USA
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 10986 is a new mingled lunar meteorite found in 2015 in Western Sahara. This impact melt breccia contains abundant impact melt glass and clasts as large as 0.75 mm. Clasts are predominantly plagioclase and pyroxene‐rich and represent both highland and basalt lithologies. Highland lithologies include troctolites, gabbronorites, anorthositic norites, and troctolitic anorthosites. Basalt lithologies include crystalline clasts with large zoned pyroxenes representing very low titanium to low titanium basalts. In situ geochemical analysis of minerals within clasts indicates that they represent ferroan anorthosite, Mg‐suite, and gabbronorite lithologies as defined by the Apollo sample collection. Clasts representing magnesian anorthosite, or “gap” lithologies, are prevalent in this meteorite. Whole rock and in situ impact glass measurements indicate low incompatible trace element concentrations. Basalt clasts also have low incompatible trace element concentrations and lack evolved KREEP mineralogy although pyroxferroite grains are present. The juxtaposition of evolved, basaltic clasts without KREEP signatures and highland lithologies suggests that these basaltic clasts may represent cryptomare. The lithologies found in NWA 10986 offer a unique and possibly a complete cross section view of the Moon sourced outside of the Procellarum KREEP Terrane.

Accretion of differentiated achondritic and aqueously altered chondritic materials in the early solar system—Significance of an igneous fragment in the CM chondrite NWA 12651

1Samuel Ebert,1Markus Patzek,1Sarah Lentfort,1Addi Bischoff
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13407]
1Institut fur Planetologie, Westfälische Wilhelms-Universität Münster,  Wilhelm-Klemm-Str.10, 48149 Münster, Germany
Published by arrangement with John Wiley & Sons

One approach to decipher the dynamics of material transport and planetary accretion in the early solar system is to investigate xenolithic fragments in meteorites. In this work, we examined an igneous fragment from the NWA 12651 meteorite—the first igneous fragment found in any CM chondrite—by analyzing its mineralogy, rare earth elements (REEs), and O‐isotopes. The study shows that the exsolution lamellae of the igneous fragment consist of Fe‐rich and Ca‐rich pyroxene. Thus, the fragment was part of a progressive crystallization in a closed system, such as in a depleted magma reservoir or mantle. In this environment, the pyroxene co‐crystallized with plagioclase, resulting in a negative Eu anomaly and enrichment of the heavy REEs compared to the light REEs. The O‐isotopes of the fragment are more 16O‐enriched than the mafic minerals in the matrix or in other bulk CM chondrites; therefore, the fragment was formed in a different region than the NWA 12651 parent body. The iron meteorites Tucson and Deep Springs, the pallasite Milton, and the CB chondrites have similar O‐isotopes as the igneous fragment. However, no direct connection can be drawn and it is questionable if the fragment shares a same parent body with one of these meteorites. The close formation region to the CB chondrites may suggest a formation of the fragment in the carbonaceous chondrite region. Thus, a wide transport through the nebula of the early solar system may not have been necessary to move the fragment to the CM chondrite formation region.

MgAl2O4 spinels from Allende and NWA 763 carbonaceous chondrites: Structural refinement, cooling history, and trace element contents

1Davide Lenaz,2Vanni Lughi,3Diego Perugini,3,4Maurizio Petrelli,5Gianluca Turco,6Birger Schmitz
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13400]
1Department of Mathematics and Geosciences, University of Trieste, 34128 Trieste, Italy
2Department of Engineering and Architecture, University of Trieste, 34127 Trieste, Italy
3Department of Physics and Geology, University of Perugia,06123 Perugia, ltaly
4INFN, Sezione di Perugia, 06123 Perugia, Italy
5Department of Medical Sciences, University of Trieste,Piazza dell’Ospitale 1, 34125 Trieste, Italy
6Astrogeobiology Laboratory, Department of Physics, Lund University, 22100 Lund, Sweden
Published by arrangement with John Wiley & Sons

MgAl2O4 spinels from Allende and NWA 763 carbonaceous chondrites were studied by X‐ray single crystal diffraction, SEM, electron microprobe, LA‐ICP‐MS, and Raman spectroscopy. Those from Allende are almost pure, but, in one case, we found a strong FeOtot zonation. Spinels from NWA 763 show Mg‐Fe2+ substitutions. Almost pure MgAl2O4 spinels from both meteorites underwent slow cooling and reached their intracrystalline closure temperature (Tc) in the range 460–520 °C. The NWA 763 spinel with higher FeO content shows a Tc of about 720 °C. X‐ray single crystal diffraction and Raman spectroscopy suggest a slow cooling and an ordered structure with trivalent cations in M site and divalent in T site. Among the trace elements, Ti and Co are enriched with respect to the terrestrial analogs, while Mn, Ni, and Sn show intermediate values between different terrestrial occurrences. Vanadium cannot be used as a tracer of extraterrestrial origin as for Cr‐spinels, because its content is similar in extraterrestrial and terrestrial spinels. In the zoned crystal from Allende, Co show a strong zonation similar to that of FeO.

The spectroscopic properties of the Lixiaohua family, cradle of Main Belt Comets

1,2M.N.DePrá,3,4J.Licandro,1N.PinillaAlonso,3V.Lorenzie,2E.Rondón,2J.Carvano,2D.Morate,3,4J.De León
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.113473]
1Florida Space Institute, University of Central Florida, FL, USA
2Departamento de Astrofísica, Observatório Nacional, Rio de Janeiro, 20921-400, Brazil
3Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, 38205 La Laguna, Spain
4Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
5Fundación Galileo Galilei – INAF, Rambla José Ana Fernández Pérez, 7, 38712 Breña Baja, Santa Cruz de Tenerife, Spain
Copyright Elsevier

The Lixiaohua collisional family lies in the Outer Main Belt, close to the well characterized Themis primitive class family. It is one of the only three families that host two active asteroids that present cometary-like activity: 313P/ Gibbs and 358P/PANSTARRS (P/2012 T1). As a part of the PRIMitive Asteroid Spectoscopy Survey (PRIMASS), we present the results of a spectroscopic program where we acquired 36 objects in visible wavelengths, using the 4.1 m SOAR, and, 17 objects in the near-infrared, using the 3.58m Telescopio Nazionale Galileo, which provided the characterization of 43 out of the 756 identified Lixiaohua family members. We observed asteroids members of the Lixiaohua family with the aim of: (1) determining the spectral class and spectroscopic properties of the family, (2) estimating the presence of hydrated minerals on their surfaces by studying the 0.7 μm absorption band and the UV drop of reflectance below 0.5 μm, (3) analyzing if active asteroids 358P and 313P are probable family members. Our results show that the Lixiaohua family is consistently redder families than the Themis family and present a wide variety of slopes. We haven’t found an unambiguous trace of aqueous alteration in the spectra of the family members, at the observed wavelengths. Finally, we conclude that the Lixiaohua family is the probable source of the Main-Belt Comets 313P/ Gibbs and 358P/PANSTARRS.