Kaori JOGO¹, Motoo ITO², Shigeru WAKITA³, Sachio KOBAYASHI², and Jong Ik LEE¹
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13272]
¹Division of Earth-System Sciences, Korea Polar Research Institute, 26 Songdomirae-ro, Yeonsu-gu, Incheon 21990,South Korea
²Kochi Institute for Core Sample Research, JAMSTEC B200 Monobe, Nankoku, Kochi 783-8502, Japan
³Center for Computational Astrophysics, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka,Tokyo 181-8588, Japan
Published by arrangement with John Wiley & Sons

We observed metamorphosed clasts in the CV3 chondrite breccias Graves Nunataks 06101, Vigarano, Roberts Massif 04143, and Yamato‐86009. These clasts are coarse‐grained polymineralic rocks composed of Ca‐bearing ferroan olivine (Fa_24–40, up to 0.6 wt% CaO), diopside (Fs_7–12Wo_44–50), plagioclase (An_52–75), Cr‐spinel (Cr/[Cr + Al] = 0.4, Fe/[Fe + Mg] = 0.7), sulfide and rare grains of Fe‐Ni metal, phosphate, and Ca‐poor pyroxene (Fs₂₄Wo₄). Most clasts have triple junctions between silicate grains. The rare earth element (REE) abundances are high in diopside (REE ~3.80–13.83 × CI) and plagioclase (Eu ~12.31–14.67 × CI) but are low in olivine (REE ~0.01–1.44 × CI) and spinel (REE ~0.25–0.49 × CI). These REE abundances are different from those of metamorphosed chondrites, primitive achondrites, and achondrites, suggesting that the clasts are not fragments of these meteorites. Similar mineralogical characteristics of the clasts with those in the Mokoia and Yamato‐86009 breccias (Jogo et al. 2012) suggest that the clasts observed in this study would also form inside the CV3 chondrite parent body. Thermal modeling suggests that in order to reach the metamorphosed temperatures of the clasts of >800 °C, the clast parent body should have accreted by ~2.5–2.6 Ma after CAIs formation. The consistency of the accretion age of the clast parent body and the CV3 chondrule formation age suggests that the clasts and CV3 chondrites could be originated from the same parent body with a peak temperature of 800–1100 °C. If the body has a peak temperature of >1100 °C, the accretion age of the body becomes older than the CV3 chondrule formation age and multiple CV3 parent bodies are likely.

Christian KOEBERL^1,2 and Ludovic FERRIERE¹
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13250]
¹Natural History Museum, Burgring 7, A-1010 Vienna, Austria
²Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
Published by arrangement with John Wiley & Sons

ibyan Desert Glass (LDG) is an enigmatic natural glass, about 28.5 million years old, which occurs on the floor of corridors between sand dunes of the southwestern corner of the Great Sand Sea in western Egypt, near the Libyan border. The glass occurs as centimeter‐ to decimeter‐sized, irregularly shaped, and strongly wind‐eroded pieces. The origin of the LDG has been the subject of much debate since its discovery, and a variety of exotic processes were suggested, including a hydrothermal sol‐gel process or a lunar volcanic source. However, evidence of an impact origin of these glasses included the presence of schlieren and partly or completely digested minerals, such as lechatelierite, baddeleyite (a high‐T breakdown product of zircon), and the presence of a meteoritic component in some of the glass samples. The source material of the glass remains an open question. Geochemical data indicate that neither the local sands nor sandstones from various sources in the region are good candidates to be the sole precursors of the LDG. No detailed studies of all local rocks exist, though. There are some chemical and isotopic similarity to rocks from the BP and Oasis impact structures in Libya, but no further evidence for a link between these structures and LDG was found so far. These complications and the lack of a crater structure in the area of the LDG strewn field have rendered an origin by airburst‐induced melting of surface rocks as a much‐discussed alternative. About 20 years ago, a few shocked quartz‐bearing breccias (float samples) were found in the LDG strewn field. To study this question further, several basement rock outcrops in the LDG area were sampled during three expeditions in the area. Here we report on the discovery of shock‐produced planar microdeformation features, namely planar fractures (PFs), planar deformation features (PDFs), and feather features (FFs), in quartz grains from bedrock samples. Our observations show that the investigated samples were shocked to moderate pressure, of at least 16 GPa. We interpret these observations to indicate that there was a physical impact event, not just an airburst, and that the crater has been almost completely eroded since its formation.

Stephen M.Elardo^a,b, Anat Shahar^a, Timothy D. Mock^c, Corliss K. Sio^a,d
Earth and Planetary Science Letters 511, 12-24 Link to Article [https://doi.org/10.1016/j.epsl.2019.02.025]
^aGeophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA
^bDepartment of Geological Sciences, University of Florida, Gainesville, FL 32611, USA
^cDepartment of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA
^dNuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
Copyright Elsevier

We have conducted high-pressure, high-temperature isotope exchange experiments between molten silicate and molten Fe–Si–C-alloys to constrain the effect of Si on equilibrium Fe isotope fractionation during planetary core formation. The values of <span id="MathJax-Element-1-Frame" class="MathJax_SVG" style="box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; font-style: normal; font-weight: normal; line-height: normal; font-size: 16.200000762939453px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" role="presentation" data-mathml="Δ57″>Fe_{Metal-Silicate} at 1850 °C and 1 GPa determined by high-resolution MC-ICP-MS in this study range from <span id="MathJax-Element-2-Frame" class="MathJax_SVG" style="box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; font-style: normal; font-weight: normal; line-height: normal; font-size: 16.200000762939453px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" role="presentation" data-mathml="−0.013±0.054‰”>‰ (2SE) to 0.072 ± 0.085‰ with 1.34–8.14 atom % Si in the alloy, respectively. These results, although not definitive on their own, are consistent with previous experimental results from our group and a model in which elements that substitute for Fe atoms in the alloy structure (i.e., Ni, S, and Si) induce a fractionation of Fe isotopes between molten silicate and molten Fe-alloys during planetary differentiation. Using in situ synchrotron X-ray diffraction data for molten Fe-rich alloys from the literature, we propose a model to explain this fractionation behavior in which impurity elements in Fe-alloys cause the nearest neighbor atomic distances to shorten, thereby stiffening metallic bonds and increasing the preference of the alloy for heavy Fe isotopes relative to the silicate melt. This fractionation results in the bulk silicate mantles of the smaller terrestrial planets and asteroids becoming isotopically light relative to chondrites, with an enrichment of heavy Fe isotopes in their cores, consistent with magmatic iron meteorite compositions. Our model predicts a bulk silicate mantle <span id="MathJax-Element-3-Frame" class="MathJax_SVG" style="box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; font-style: normal; font-weight: normal; line-height: normal; font-size: 16.200000762939453px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" role="presentation" data-mathml="δ57″>Fe ranging from −0.01‰ to −0.12‰ for the Moon, −0.06‰ to −0.33‰ for Mars, and −0.08‰ to −0.33‰ for Vesta. Independent estimates of the <span id="MathJax-Element-4-Frame" class="MathJax_SVG" style="box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; font-style: normal; font-weight: normal; line-height: normal; font-size: 16.200000762939453px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" role="presentation" data-mathml="δ57″>Fe of primitive mantle source regions that account for Fe isotope fractionation during partial melting agree well with these ranges for all three planetary bodies and suggest that Mars and Vesta have cores with impurity (i.e., Ni, S, Si) abundances near the low end of published ranges. Therefore, we favor a model in which core formation results in isotopically light bulk silicate mantles for the Moon, Mars, and Vesta. The processes of magma ocean crystallization, mantle partial melting, and fractional crystallization of mantle-derived melts are all likely to result in heavy Fe isotope enrichment in the melt phase, which can explain why basaltic samples from these planetary bodies have variable <span id="MathJax-Element-5-Frame" class="MathJax_SVG" style="box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; font-style: normal; font-weight: normal; line-height: normal; font-size: 16.200000762939453px; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative;" tabindex="0" role="presentation" data-mathml="δ57″>Fe values consistently heavier than our bulk mantle estimates. Additionally, we find no clear evidence that Fe isotopes were fractionated to a detectable level by volatile depletion processes during or after planetary accretion, although it cannot be ruled out.

Clara Maurel^a, Benjamin P. Weiss^a, James F. J. Bryson^b
Earth and Planetary Science Letters 511, 12-24 Link to Article [https://doi.org/10.1016/j.epsl.2019.02.027]
^aDepartment of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA
^bDepartment of Earth Sciences, University of Cambridge, Cambridge, UK
Copyright Elsevier

Metallic microstructures in slowly-cooled iron-rich meteorites reflect the thermal and magnetic histories of their parent planetesimals. Of particular interest is the cloudy zone, a nanoscale intergrowth of Ni-rich islands within a Ni-poor matrix that forms below ∼350 °C by spinodal decomposition. The sizes of the islands have long been recognized as reflecting the low-temperature cooling rates of meteorite parent bodies. However, a model capable of providing quantitative cooling rate estimates from island sizes has been lacking. Moreover, these islands are also capable of preserving a record of the ambient magnetic field as they grew, but some of the key physical parameters required for recovering reliable paleointensity estimates from magnetic measurements of these islands have been poorly constrained. To address both of these issues, we present a numerical model of the structural and compositional evolution of the cloudy zone as a function of cooling rate and local composition. Our model produces island sizes that are consistent with present-day measured sizes. This model enables a substantial improvement in the calibration of paleointensity estimates and associated uncertainties. In particular, we can now accurately quantify the statistical uncertainty associated with the finite number of islands acquiring the magnetization and the uncertainty on their size at the time of the record. We use this new understanding to revisit paleointensities from previous pioneering paleomagnetic studies of cloudy zones. We show that these could have been overestimated by up to one order of magnitude but nevertheless still require substantial magnetic fields to have been present on their parent bodies. Our model also allows us to estimate absolute cooling rates for meteorites that cooled slower than <10,000 °C My⁻¹. We demonstrate how these cooling rate estimates can uniquely constrain the low-temperature thermal history of meteorite parent bodies. Using the main-group pallasites as an example, we show that our results are consistent with the previously-proposed unperturbed, conductive cooling at low temperature of a ∼200-km radius main-group pallasite parent body.

Paul L. G€OLLNER¹, Torben W€USTEMANN¹, Lisa BENDSCHNEIDER¹, Sebastian REIMERS¹, Martin D. CLARK^1,4, Lisa GIBSON², Peter C. LIGHTFOOT³, and Ulrich RILLER¹
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13268]
¹Institut für Geologie, Universität Hamburg, Bundesstraße 55, 20146 Hamburg, Germany
²Vale, North American Exploration, 337 Power St., Copper Cliff, Ontario, Canada
³Department of Earth Sciences, University of Western Ontario, 1151 Richmond Street N., London, Ontario N6A 5B7, Canada
⁴Present address: Department of Geology, University of the Free State, 205 Nelson Mandela Drive, Bloemfontein, South Africa
Published by arrangement with John Wiley & Sons

The 1.85 Ga Sudbury Igneous Complex (SIC) and its thermal aureole are unique on Earth with regard to unraveling the effects of a large impact melt sheet on adjacent target rocks. Notably, the formation of Footwall Breccia, lining the basal SIC, remains controversial and has been attributed to impact, cratering, and postcratering processes. Based on detailed field mapping and microstructural analysis of thermal aureole rocks, we identified three distinct zones characterized by static recrystallization, incipient melting, and crystallization textures. The temperature gradient in the thermal aureole increases toward the SIC and culminates in a zone of partial melting, which correlates spatially with the Footwall Breccia. We therefore conclude that assimilation of target rock into initially superheated impact melt and simultaneous deformation after cratering strongly contributed to breccia formation. Estimated melt fractions of the Footwall Breccia amount to 80 vol% and attest to an extreme loss in mechanical strength and, thus, high mobility of the Breccia during assimilation. Transport of highly mobile Footwall Breccia material into the overlying Sublayer Norite of the SIC and vice versa can be attributed to Raleigh–Taylor instability of both units, long‐term crater modification caused by viscous relaxation of crust underlying the Sudbury impact structure, or both.

Søren Grube PEDERSEN, Martin SCHILLER*, James N. CONNELLY, and Martin BIZZARRO
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13269]
Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7,DK-1350, Denmark
Published by arrangement with John Wiley & Sons

Planetary bodies a few hundred kilometers in radii are the precursors to larger planets but it is unclear whether these bodies themselves formed very rapidly or accreted slowly over several millions of years. Ordinary H chondrite meteorites provide an opportunity to investigate the accretion time scale of a small planetary body given that variable degrees of thermal metamorphism present in H chondrites provide a proxy for their stratigraphic depth and, therefore, relative accretion times. We exploit this feature to search for nucleosynthetic isotope variability of ⁵⁴Cr, which is a sensitive tracer of spatial and temporal variations in the protoplanetary disk’s solids, between 17 H chondrites covering all petrologic types to obtain clues about the parent body accretionary rate. We find no systematic variability in the mass‐biased corrected abundances of ⁵³Cr or ⁵⁴Cr outside of the analytical uncertainties, suggesting very rapid accretion of the H chondrite parent body consistent with turbulent accretion. By utilizing the μ⁵⁴Cr–planetary mass relationship observed between inner solar system planetary bodies, we calculate that the H chondrite accretion occurred at 1.1 ± 0.4 or 1.8 ± 0.2 Myr after the formation of calcium‐aluminum‐rich inclusions (CAIs), assuming either the initial ²⁶Al/²⁷Al abundance of inner solar system solids determined from angrite meteorites or CAIs from CV chondrites, respectively. Notably, these ages are in agreement with age estimates based on the parent bodies’ thermal evolution when correcting these calculations to the same initial ²⁶Al/²⁷Al abundance, reinforcing the idea of a secular evolution in the isotopic composition of inner disk solids.

Jan Render, Samuel Ebert, Christoph Burkhardt, Thorsten Kleine, Gregory A. Brennecka
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.03.011]
Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
Copyright Elsevier

Insights into the earliest stages of our Solar System can be derived from its oldest dated solids, calcium-aluminum-rich inclusions (CAIs). In particular, investigating isotopic anomalies of nucleosynthetic origin in CAIs offers potential clues to the genetic heritage of refractory inclusions and the reservoir(s) involved in their formation. To this point, however, nucleosynthetic anomalies in refractory inclusions have almost exclusively been recognized in (1) relatively large CAIs from CV3 chondrites, employing chemical purification and high-precision mass spectrometry, or (2) from sub-mm-sized hibonite-rich objects (e.g., PLACs, SHIBs) from the Murchison CM2 chondrite using much less precise in-situ techniques. Whereas the latter have been shown to be highly anomalous in their isotopic compositions, their genetic connection to ‘regular’ CAIs from carbonaceous chondrites remains poorly understood.

Here, we aim to address this issue by taking advantage of a new technique that allows for high-precision analysis of sub-mm-sized inclusions. Using this method, we report Ti isotope anomalies in a suite of twelve CAIs from five different CO carbonaceous chondrites, as well as ten refractory inclusions from the CM2 chondrite Jbilet Winselwan using MC-ICPMS. We find that these CO and CM CAIs exhibit Ti isotopic compositions very similar to those of previously investigated CV3 (and of two CK3) CAIs, suggesting a fundamental genetic relationship of CAIs found within these chondrite groups. As such, our data indicates that CAIs from various groups of carbonaceous chondrites formed from similar matter and in a single region of the solar nebula (i.e., derived from a single common CAI-formation region). Collectively, these data show evidence of large-scale transport of CAIs over a significant range of heliocentric distances, covering at least the accretion areas of the CV, CK, CO, and CM chondrites. In addition, we report two inclusions consisting of hibonite-rich crystal aggregates from Jbilet Winselwan that exhibit highly irregular nucleosynthetic Ti signatures, implying a distinct origin from the aforementioned CAIs. These inclusions may represent an earlier generation of refractory material, perhaps more akin to the previously mentioned PLACs and/or SHIBs.

V. Assis Fernandes^a,b,c,d, J. Hopp^c, W.H. Schwarz^c, J. P. Fritz^f,g, M. Trieloff^c and H. Povenmire^fGeochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.03.009]
^aMuseum für Naturkunde, Leibniz-Institute for Evolution and Biodiversity Research, Invalidenstraße 43, 10115 Berlin, Germany
^bSchool of Earth and Environmental Sciences, University of Manchester, Oxford Road, M13 9PL Manchester, United Kingdom
^cKlaus-Tschira-Labor für Kosmochemie, Institut für Geowissenschaften, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 234-236, 69120 Heidelberg, Germany
^dInstituto Dom Luiz, University of Lisbon, 1749-016 Lisbon, Portugal
^eSaalbau Weltraum Projekt, Liebigstrasse 6, 64646 Heppenheim, Germany
^fZentrum für Rieskrater und Impaktforschung (ZERIN), Nördlingen, Vordere Gerbergasse 3, 86720 Nördlingen, Germany
^gFlorida Institute of Technology (Retired), Melbourne, FL 32901, U.S.A
Copyright Elsevier

This study presents ⁴⁰Ar-³⁹Ar step heating ages of four North American tektites (three bediasites and one georgiaite) and two groundmass samples extracted at different depths from clast-rich impact melt rocks (CB-W61 and CB-W84) recovered by the USGS-ICDP Eyreville B drill-core about 9 km from the centre of the Chesapeake Bay impact structure. Radiometric age determination on both North American tektites and impact melt rocks from within Chesapeake Bay craters offers the first possibility to confirm the origin of these tektites. For this aim, argon data from 13 samples/aliquots of tektite rims, cores and bulk, and 4 samples/aliquots from two impact melt rocks were obtained over 15 to 26 step heating extractions. Age spectra of all tektite samples show plateaux comprising 62-98% of the ³⁹Ar release over consecutive intermediate and high temperature heating steps. Few low temperature extractions indicate excess ⁴⁰Ar. Inverse isochron ⁴⁰Ar/³⁶Ar intercepts of tektite samples are indistinguishable from air (295.5). However, impact melt rock spectra presented complex Ar-release affecting primarily the low temperature heating-steps. Inverse isochrones indicated excess argon from which the ⁴⁰Ar/³⁶Ar intercept was used to correct the age calculation. CB-W61 and CB-W61-2 ⁴⁰Ar/³⁶Ar intercepts are 354.5±2.5 and 327.2±6.3, respectively, and those for CB-W84 and CB-W84-2 are 332.0±7.3 and 329.6 ± 5.6, respectively. The inverse isochron weighted mean age (according to currently suggested K-decay constant revisions by Schwarz et al., 2011, Renne et al., 2011) for all four tektites is 34.86±0.25 Ma (MSWD=0.96, P=0.41; n=4) and for the two impact melt rocks is 37.16±3.65 Ma (MSWD=0.83, P=0.36). The combined tektite and impact melt rocks isochron mean age of 34.86±0.23 (0.32) Ma (MSWD=0.87, P=0.43) is slightly – though not significantly – higher than the plateau mean age of 34.55 ±0.27 (0.36) Ma (MSWD=0.66, P=0.62). Placing the age in the Global Stratotype Section and Point (GSSP) marine section exposed at Massignano, Italy, it falls below the Eocene/Oligocene (E/O) boundary overlapping with the 10.28 m Ir-anomaly. These results agree within errors with previously reported ages of 35.20±0.54 Ma mainly those derived from K-Ar and Ar-Ar total fusion analysis. An age of 34.86±0.32 Ma sets the Chesapeake Bay impact event close to the youngest of the three Ir anomalies at ∼35.0 Ma in the case the impactor was Ir-rich (e.g, a chondrite, primitive achondrite, stony-iron or iron meteorite). The concordance with the E/O boundary at ∼ 33.9 Ma seems only marginally possible, and only if the Ir contribution from the ejecta were, potentially, due either to its small amount becoming diluted in the geologic record or the impactor being Ir poor, e.g., of differentiated achondritic composition. This study also brings to front the need to re-establish the stratigraphic and palaeo-magnetic correlations across the globe for the Ir-anomalies and the magneto-stratigraphy during the mid- to late-Eocene and early-Oligocene, and the need to re-evaluate the markers for the Eocene-Oligocene boundary.

Yoshihiro HIDAKA^1,4, Naoki SHIRAI¹, Akira YAMAGUCHI^2,3, and Mitsuru EBIHARA^1,4
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13273]
¹Department of Chemistry, Tokyo Metropolitan University, Tokyo 192-0397, Japan
²National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan
³Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for AdvancedStudies), Tokyo 190-8518, Japan
⁴Present address: Department of Earth Sciences, Waseda University, Tokyo 169-8050, Japan
Published by arrangement with John Wiley & Sons

We have studied magnetic fractions of five acapulcoites, three lodranites, and two winonaites to investigate chemical compositions of their precursor materials and metallic partial melting processes occurring on their parent bodies. One winonaite metal is similar in composition to low Au, low Ni IAB iron subgroup, indicating genetic relationship between them. Magnetic fractions of chondrule‐bearing acapulcoite and winonaite have intermediate chemical compositions of metals between H chondrites and EL chondrites. This fact indicates that the precursor materials of acapulcoite–lodranites and winonaites were similar to H and/or EL chondrites in chemical compositions. Magnetic fractions in acapulcoite–lodranites have a large variety of chemical compositions. Most of them show enrichments of W, Re, Ir, Pt, Mo, and Rh, and one of them shows clear depletion in Re and Ir relative to those of chondrule‐bearing acapulcoite. Chemical compositional variations among acapulcoite–lodranite metals cannot be explained by a single Fe‐Ni‐S partial melting event, but a two‐step partial melting model can explain it.

C. A. LORENZ¹, A. A. SHIRYAEV^2,3, I. I. VLASOV⁴, and S. E. BORISOVSKY³
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13274]
¹1Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Kosygin St. 19, 119999 Moscow, Russia
²A. N. Frumkin Institute of Physical Chemistry and Electrochemistry RAS, Leninsky pr. 31 korp. 4, 119071, Moscow, Russia
³Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry RAS, Staromonetnyi per, 35, 119017,Moscow, Russia
⁴A.M. Prokhorov General Physics Institute RAS, Vavilova St., 38, 119991, Moscow, Russia
Published by arrangement with John Wiley & Sons

Four ureilites subjected to impact metamorphism in a pressure range of ~15–100 GPa were investigated for mineralogical and petrological features and optical luminescence of their diamonds with the aim to understand how properties of ureilitic diamonds are correlated with shock and thermal histories of the host meteorite. Petrological data show that all the investigated ureilites experienced multistage metamorphic histories. Some of them were shocked at least twice or/and underwent high‐temperature thermal metamorphism and fluid metasomatism in the parent body interior. Photoluminescence spectra of individual diamond grains reveal the presence of neutral and negatively charged nitrogen‐vacancy (NV⁰ and NV⁻, respectively) and H3 (two nitrogens and a vacancy) defects, indicating relatively high nitrogen contents of the diamonds and some degree of thermal annealing of the grains. The diamond grain size and morphology, a texture of graphite‐diamond aggregates, and spectroscopic properties of the diamond phase vary widely both within an individual meteorite and between the ureilites. Shock‐driven transformation of sp2‐C into diamond provides the most natural explanation of the observed spectroscopic diversity of the diamond grains if one takes into account strong dependence of the PT parameters and efficiency of the transformation on structure of the carbonaceous precursor.