Magma ascent in planetesimals: Control by grain size

1Tim Lichtenberg, 2Tobias Keller,3Richard F.Katz,4Gregor J.Golabek,1Taras V.Gerya
Earth and Planetary Science Letters 507, 154-165 Link to Article [https://doi.org/10.1016/j.epsl.2018.11.034]
1Institute of Geophysics, ETH Zürich, Sonneggstrasse 5, 8092 Zürich, Switzerland
2Department of Geophysics, Stanford University, Stanford, CA 94305, United States
3Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, United Kingdom
4Bayerisches Geoinstitut, University of Bayreuth, Universitätsstrasse 30, 95440 Bayreuth, Germany
Copyright Elsevier

Rocky planetesimals in the early solar system melted internally and evolved chemically due to radiogenic heating from 26Al. Here we quantify the parametric controls on magma genesis and transport using a coupled petrological and fluid mechanical model of reactive two-phase flow. We find the mean grain size of silicate minerals to be a key control on magma ascent. For grain sizes ≳1 mm, melt segregation produces distinct radial structure and chemical stratification. This stratification is most pronounced for bodies formed at around 1 Myr after formation of Ca, Al-rich inclusions. These findings suggest a link between the time and orbital location of planetesimal formation and their subsequent structural and chemical evolution. According to our models, the evolution of partially molten planetesimal interiors falls into two categories. In the magma ocean scenario, the whole interior of a planetesimal experiences nearly complete melting, which would result in turbulent convection and core–mantle differentiation by the rainfall mechanism. In the magma sill scenario, segregating melts gradually deplete the deep interior of the radiogenic heat source. In this case, magma may form melt-rich layers beneath a cool and stable lid, while core formation would proceed by percolation. Our findings suggest that grain sizes prevalent during the internal heating stage governed magma ascent in planetesimals. Regardless of whether evolution progresses toward a magma ocean or magma sill structure, our models predict that temperature inversions due to rapid 26Al redistribution are limited to bodies formed earlier than ≈1 Myr after CAIs. We find that if grain size was ≲1 mm during peak internal melting, only elevated solid–melt density contrasts (such as found for the reducing conditions in enstatite chondrite compositions) would allow substantial melt segregation to occur.

Zinc isotopic variations in ureilites

1Yann-Aurélien Brugier, 1Jean Alix Barrat,2Bleuenn Gueguen, 1Arnaud Agranier, 3,4Akira Yamaguchi, 5Addi Bischoff
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.12.009]
1Laboratoire Géosciences Océan (UMR CNRS 6538), Université de Bretagne Occidentale et Institut Universitaire Européen de la Mer, Place Nicolas Copernic, 29280 Plouzané, France
2UMS CNRS 3113, Université de Bretagne Occidentale et Institut Universitaire Européen de la Mer, Place Nicolas Copernic, 29280 Plouzané, France
3National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan
4Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (the Graduate University for Advanced Studies), Tokyo 190-8518, Japan
5Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
Copyright Elsevier

The Ureilite Parent Body (UPB) was a C-rich planetary embryo disrupted by impact. Ureilites are fragments of the UPB mantle and among the most numerous achondrites. Zinc isotopic data are presented for 26 unbrecciated ureilites and a trachyandesite (ALM-A) from the same parent body. The δ66Zn values of ureilites range from 0.40 to 2.71 ‰ including literature results. Zinc isotopic compositions do not correlate with the compositions of olivine cores, with C and O isotopic compositions, with Zn abundances, nor with shock grades. The wide range of δ66Zn displayed by the ureilites is chiefly explained by evaporation processes that took place during the catastrophic breakup of the UPB. During breakup, the high temperatures of the UPB mantle allowed Zn to evaporate, regardless of the intensity the shock suffered by the ureilitic debris. For the most shocked of them, post-shock heating permitted greater evaporation, and heavier Zn isotopic compositions. The surface of the UPB was certainly much colder than the mantle before the breakup. Therefore, crustal rocks were probably less prone to Zn evaporation. ALM-A, the sole crustal rock analyzed at present, has a δ66Zn value (0.67 ‰) significantly higher than those of regular chondrites. This result indicates that its mantle source displayed already non-chondritic Zn isotopic compositions before the breakup of the UPB.

Amino acids on witness coupons collected from the ISAS/JAXA curation facility for the assessment and quality control of the Hayabusa2 sampling procedure

1,2Haruna Sugahara,1Yoshinori Takano,3Yuzuru Karouji,4Kazuya Kumagai,3Toru Yada,1Naohiko Ohkouchi,3 Masanao Abe and Hayabusa2 project team
Earth, Planets and Space 70, 194 Link to Article [https://doi.org/10.1186/s40623-018-0965-7]
1Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Natsushima, Yokosuka 237-0061, Japan
2Institut de Chimie de Nice, Université Côte d’Azur, CNRS, UMR 7272, 28 Avenue Valrose, 06108 Nice, France
3Japan Aerospace Exploration Agency (JAXA), Yohinodai, Sagamihara 252-5210, Japan
4Marine Works Japan Ltd., Oppamahigashi, Yokosuka 237-0063, Japan

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Shocked quartz grains in the early Cambrian Vakkejokk Breccia, Sweden—Evidence of a marine impact

1Carl Alwmark, 2Jens Ormö, 3Arne T. Nielsen
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13230]
1Department of Geology, Lund University,22362 Lund, Sweden
2Centro de Astrobiologia (INTA‐CSIC),28850 Torrejon de Ardoz, Spain
3Department of Geosciences & Natural Resource Management, University of Copenhagen, , 1350 Copenhagen, Denmark
Published by arrangement with John Wiley & Sons

Here we present a study of the abundance and orientation of planar deformation features (PDFs) in the Vakkejokk Breccia, a proposed lower Cambrian impact ejecta layer in the North‐Swedish Caledonides. The presence of PDFs is widely accepted as evidence for shock metamorphism associated with cosmic impact events and their presence confirms that the Vakkejokk Breccia is indeed the result of an impact. The breccia has previously been divided into four lithological subunits (from bottom to top), viz. lower polymict breccia (LPB), graded polymict breccia (GPB), top sandstone (TS), and top conglomerate (TC). Here we show that the LPB contains no shock metamorphic features, indicating that the material derives from just outside of the crater and represents low‐shock semi‐autochthonous bombarded strata. In the overlying, more fine‐grained GPB and TS, quartz grains with PDFs are relatively abundant (2–5% of the grain population), and with higher shock levels in the upper parts, suggesting that they have formed by reworking of more distal ejecta by resurge of water toward the crater in a marine setting. The absence of shocked quartz grains in the TC indicates that this unit represents later slumps associated with weathering and erosion of the protruding crater rim. Sparse shocked quartz grains (<0.2%) were also found in sandstone beds occurring at the same stratigraphic level as the Vakkejokk Breccia 15–20 km from the inferred crater site. It is currently unresolved whether the sandstone at these distal sites is related to the impact or just contains rare reworked quartz grains with PDFs.

Incipient devitrification of impact melt particles at Bosumtwi crater, Ghana: Implications for suevite cooling history and melt dispersion

1Rudolf Välja, 1Kalle Kirsimäe, 2,3Christian Köeberl, 4Daniel Boamah, 1Juho Kirs
Meteoritics & Plantary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13225]
1Department of Geology, University of Tartu, , 50411 Tartu, Estonia
2Department of Lithospheric Research, University of Vienna, , 1090 Vienna, Austria
3Natural History Museum, , 1010 Vienna, Austria
4Ghana Geological Survey Department, , Accra, Ghana
Published by arrangement with John Wiley & Sons

The petrographic, mineralogical, and geochemical compositions of the incipient devitrification products in impact melt fragments found in outer suevites at the Bosumtwi impact crater were studied to reconstruct the postimpact environmental constraints on the suevite formation and to refine its cooling history. Our study shows that devitrified melt/particles contain numerous microlitic crystals and crystal aggregates of different shapes derived from rapid cooling. The matrix of melt/particles in Bosumtwi suevites contains abundant Mg‐hercynite (pleonaste)‐type spinels with sizes rarely exceeding a few micrometers. High nucleation density of microlites suggests rapid crystallization under strong undercooling in the presence of abundant volatiles. Although the Bosumtwi impact event took place in a continental environment, the possible sources for elevated fluid/volatile content could have been the groundwater in the deeply weathered and fractured‐jointed Birimian basement, dewatering of abundant hydrous phases in weathered crust or hydrothermally altered basement, and the shale/phyllite–greywacke lithologies in the target rocks. Our results show that enough volatiles were present in the target rocks at the time of impact for the effective impact melt dispersion observed in Bosumtwi impactites.

Extending the paleogeographic range and our understanding of the Neoarchean Monteville impact spherule layer (Transvaal Supergroup, South Africa)

1Bruce M. Simonson, 2Nicolas J. Beukes, 3Sandra Biller
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13228]
1Geology Department, Oberlin College, , Oberlin, Ohio, 44074 USA
2DST‐NRF Centre of Excellence for Integrated Mineral and Energy Resource Analysis, Department of Geology, University of Johannesburg, , Auckland Park, 2006 South Africa
3SNAP‐Ed Program Manager, University of Wyoming Extension, , Laramie, Wyoming, 82071 USA
Published by arrangement with John Wiley & Sons

The Monteville spherule layer (MSL) was deposited in the Griqualand West Basin (GWB) on the Kaapvaal Craton approximately 2.63 Ga. The spherules were generated by a large impact and reworked by impact‐generated waves and/or currents. The MSL has been intersected in three previously undescribed cores. Two of the cores, GKF‐1 and GKP‐1, were drilled ~30 km west of the southernmost outcrop of the MSL. The third core, BH‐47, was drilled ~250 km south and east of the GWB. The MSL contains medium to coarse sand‐size spherules like those described previously in all three cores but each one was emplaced in a different way. In GKF‐1, the MSL is 90 cm thick and contains large rip‐up clasts of basinal carbonate and early diagenetic pyrite. In GKP‐1, the MSL is only 1.5 cm thick and consists largely of fine carbonate sand, yet it contains pyrite intraclasts up to ~1 cm long. In BH‐47, the MSL consists of a lower coarse sandy zone ~37 cm thick rich in spherules, carbonate peloids/ooids, pyrite intraclasts, and quartzose sand and an upper, finer sandy zone ~46 cm thick; neither zone contains any large intraclasts. The new occurrences triple the known extent of the MSL from ~15,000 to ~46,000 km2, support the oceanic impact model for the formation of the MSL, demonstrate that it is a persistent regional time‐stratigraphic marker, place new constraints on the Kaapvaal paleoshoreline at the time of impact, and support the existence of Vaalbara.