¹Karin Maierhofer,^1,2,3Christian Koeberl,³Julie Brigham‐Grette
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13243]
¹Department of Lithospheric Research, University of Vienna, , A‐1090 Vienna, Austria
² Natural History Museum, , A‐1010 Vienna, Austria
³Department of Geosciences, University of Massachusetts, Amherst, Massachusetts, 01003 USA
Published by arrangement with John Wiley & Sons

The 3.6 Ma El’gygytgyn impact structure, located in northeast Chukotka in Arctic Russia, was largely formed in acidic volcanic rocks. The 18 km diameter circular depression is today filled with Lake El’gygytgyn (diameter of 12 km) that contains a continuous record of lacustrine sediments of the Arctic from the past 3.6 Myr. In 2009, El’gygytgyn became the focus of the International Continental Scientific Drilling Program (ICDP) in which a total of 642.4 m of drill core was recovered. Lithostratigraphically, the drill cores comprise lacustrine sediment sequences, impact breccias, and deformed target rocks. The impactite core was recovered from 316.08 to 517.30 meters below lake floor (mblf). Because of the rare, outstanding recovery, the transition zone, ranging from 311.47 to 317.38 m, between the postimpact lacustrine sediments and the impactite sequences, was studied petrographically and geochemically. The transition layer comprises a mixture of about 6 m of loose sedimentary and volcanic material containing isolated clasts of minerals and melt. Shock metamorphic effects, such as planar fractures (PFs) and planar deformation features (PDFs), were observed in a few quartz grains. The discoveries of silica diaplectic glass hosting coesite, kinked micas and amphibole, lechatelierite, numerous impact melt shards and clasts, and spherules are associated with the impact event. The occurrence of spherules, impact melt clasts, silica diaplectic glass, and lechatelierite, about 1 m below the onset of the transition, marks the beginning of the more coherent impact ejecta layer. The results of siderophile interelement ratios of the transition layer spherules give indications of the relative contribution of the meteoritical component.

Alan P. Boss
Astrophysical Journal 870, 3 Link to Article [DOI: 10.3847/1538-4357/aaf005 ]
Department of Terrestrial Magnetism, Carnegie Institution for Science, 5241 Broad Branch Road NW, Washington, DC 20015-1305, USA

Cosmochemical evaluations of the initial meteoritical abundance of the short-lived radioisotope (SLRI) ²⁶Al have remained fairly constant since 1976, while estimates for the initial abundance of the SLRI ⁶⁰Fe have varied widely recently. At the high end of this range, ⁶⁰Fe initial abundances have seemed to require ⁶⁰Fe nucleosynthesis in a core-collapse supernova, followed by incorporation into primitive meteoritical components within ~1 Myr. This paper continues the detailed exploration of this classical scenario, using models of the self-gravitational collapse of molecular cloud cores that have been struck by suitable shock fronts, leading to the injection of shock front gas into the collapsing cloud through Rayleigh–Taylor fingers formed at the shock–cloud interface. As before, these models are calculated using the FLASH three-dimensional, adaptive mesh refinement, gravitational hydrodynamical code. While the previous models used FLASH 2.5, the new models employ FLASH 4.3, which allows sink particles to be introduced to represent the newly formed protostellar object. Sink particles permit the models to be pushed forward farther in time to the phase where a ~1 M _⊙ protostar has formed, orbited by a rotating protoplanetary disk. These models are thus able to define what type of target cloud core is necessary for the supernova triggering scenario to produce a plausible scheme for the injection of SLRIs into the presolar cloud core: a ~3 M_⊙ cloud core rotating at a rate of ~3 × 10⁻¹⁴ rad s⁻¹ or higher.

Erika M. Holmbeck^1,2, Trevor M. Sprouse¹, Matthew R. Mumpower^2,3, Nicole Vassh¹, Rebecca Surman^1,2, Timothy C. Beers^1,2, and Toshihiko Kawano³
Astrophysical Journal 870, 23 Link to Article [DOI: 10.3847/1538-4357/aaefef ]
¹Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA
²JINA Center for the Evolution of the Elements, USA
³Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA

The rapid neutron-capture (“r-“) process is responsible for synthesizing many of the heavy elements observed in both the solar system and Galactic metal-poor halo stars. Simulations of r-process nucleosynthesis can reproduce abundances derived from observations with varying success, but so far they fail to account for the observed overenhancement of actinides, present in about 30% of r-process-enhanced stars. In this work, we investigate actinide production in the dynamical ejecta of a neutron star merger (NSM) and explore whether varying levels of neutron-richness can reproduce the actinide boost. We also investigate the sensitivity of actinide production on nuclear physics properties: fission distribution, β-decay, and mass model. For most cases, the actinides are overproduced in our models if the initial conditions are sufficiently neutron-rich for fission cycling. We find that actinide production can be so robust in the dynamical ejecta that an additional lanthanide-rich, actinide-poor component is necessary in order to match observations of actinide-boost stars. We present a simple actinide-dilution model that folds in estimated contributions from two nucleosynthetic sites within a merger event. Our study suggests that while the dynamical ejecta of an NSM are likely production sites for the formation of actinides, a significant contribution from another site or sites (e.g., the NSM accretion disk wind) is required to explain abundances of r-process-enhanced, metal-poor stars.

Ruobing Dong (董若冰)¹, Sheng-Yuan Liu (呂聖元)², and Jeffrey Fung (馮澤之)^3,4
Astrophysical Journal 870, 72 Link to Article [DOI: 10.3847/1538-4357/aaf38e ]
¹Department of Physics & Astronomy, University of Victoria, Victoria, BC, V8P 1A1, Canada
²Institute of Astronomy and Astrophysics, Academia Sinica, 11F of ASMAB, AS/NTU No.1, Sec. 4, Roosevelt Road, Taipei 10617, Republic of China
³Department of Astronomy, University of California at Berkeley, Campbell Hall, Berkeley, CA 94720-3411, USA
⁴NASA Sagan Fellow.

Protoplanets can produce structures in protoplanetary disks via gravitational disk–planet interactions. Once detected, such structures serve as signposts of planet formation. Here we investigate the kinematic signatures in disks produced by multi-Jupiter mass (M _J) planets using 3D hydrodynamics and radiative transfer simulations. Such a planet opens a deep gap, and drives transonic vertical motions inside. Such motions include both a bulk motion of the entire half-disk column, and turbulence on scales comparable to and smaller than the scale height. They significantly broaden molecular lines from the gap, producing double-peaked line profiles at certain locations, and a kinematic velocity dispersion comparable to thermal after azimuthal averaging. The same planet does not drive fast vertical motions outside the gap, except at the inner spiral arms and the disk surface. Searching for line broadening induced by multi-M _J planets inside gaps requires an angular resolution comparable to the gap width, an assessment of the gap gas temperature to within a factor of 2, and a high sensitivity needed to detect line emission from the gap.

^1,2Tanya V. Kizovski,^1,2 Kimberly T. Tait,¹Veronica E. Di Cecco,¹Lee F. White,²Desmond E. Moser
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13255]
¹Department of Natural History, Centre for Applied Planetary Mineralogy, Royal Ontario Museum, , Toronto, Ontario, M5S 2C6 Canada
²Department of Earth Sciences, University of Toronto, , Toronto, Ontario, M5S 3B1 Canada
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 6342 is an intermediate, poikilitic shergottite, found in Algeria in 2010. It is comprised of two distinct petrographic areas; poikilitic domains with rounded Mg‐rich olivine chadacrysts enclosed by large low‐Ca pyroxene oikocrysts, and a nonpoikilitic domain mainly comprised of subhedral olivine and vesicular recrystallized plagioclase. Oxygen fugacity conditions become more oxidizing during crystallization from the poikilitic to the nonpoikilitic domain (QFM−3.0 to QFM−2.2). As such, it is likely that NWA 6342 experienced a two‐stage (polybaric) crystallization history similar to that of the enriched poikilitic shergottites. NWA 6342 also experienced relatively high levels of shock metamorphism in comparison to most other poikilitic shergottites as evidenced by the fine‐grained recrystallization texture in olivine, as well as melting and subsequent crystallization of plagioclase. The recrystallization of plagioclase requires an extended period of postshock thermal metamorphism for NWA 6342 and similarly shocked intermediate poikilitic shergottites NWA 4797 and Grove Mountains 99027 most likely due to launch from Mars. The similarities in petrology, chemistry, and shock features between these three meteorites indicate that they have similar crystallization and shock histories; possibly originating from the same source area on Mars.

^1,2Naoya Imae,¹Makoto Kimura,^1,2Akira Yamaguchi,¹Hideyasu Kojima
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13257]
¹Antarctic Meteorite Research Center, National Institute of Polar Research, , Tachikawa‐shi, Tokyo, 190‐8518 Japan
²Department of Polar Science, School of Multidisciplinary Sciences, SOKENDAI (The Graduate University for Advanced Studies), , Tachikawa‐shi, Tokyo, 190‐8518 Japan
Published by arrangement with John Wiley & Sons

Using an X‐ray diffractometer, powder‐like diffraction patterns were acquired from in‐plane rotation of polished thin sections (PTSs) of 60 ordinary chondrites (23 H, 21 L, and 16 LL), in order to explore the thermal and shock metamorphism and its modifications of primordial features. The olivine (Ol) 130 peak position shown as Bragg indices clearly correlates with the chemical group for equilibrated ordinary chondrites (EOCs), while the peak is split or broad for unequilibrated ordinary chondrites (UOCs). The intensity ratio of kamacite may be useful for distinguishing the chemical group between H and L‐LL, but it is not definite because of heterogeneous terrestrial weathering of kamacite, especially in H chondrites. The summed intensities of the orthoenstatite (Oen) 511 and 421 peaks positively correlates with the metamorphic sequence from 3 to 6, while that of clinoenstatite (Cen) 22 is inversely correlated. The shock stage positively correlates with the summed full width of half maximum values of the Oen 511 and 421 peaks and the FWHM of Ol 130 peak for each class. Significant amount of Oen (Pbca) transformed through Cen (C2/c) finally to Cen (P2₁/c) is stable at high pressure for shock stage S6 (Tenham and NWA 4719). The shock melted LL chondrite is characterized by the occurrence of Cen and abundant homogeneous olivine. The effects of both thermal and shock metamorphism are thus incorporated into the bulk X‐ray diffraction (XRD) data. The bulk XRD method is useful for determining the bulk mineralogy, resulting in the classification of ordinary chondrites. The method is also applicable to samples other than PTS.

^1,2,3Sen Hu,^1,2,3Yangting Lin,¹Jianchao Zhang,¹Jialong Hao, ¹Weifan Xing,^1,2,3Ting Zhang,^1,2,3Wei Yang,¹Hitesh Changela
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13256]
¹Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029 China
²Institutes of Earth Science, Chinese Academy of Sciences, Beijing, 100029 China
³College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing, 100049 China
Published by arrangement with John Wiley & Sons

Zircons and apatites in clasts and matrix from the Martian breccia NWA 7034 are well documented, timing ancient geologic events on Mars. Furthermore, in this study, zircon trace elemental content, apatite volatile content, and apatite volatile isotopic compositions measured in situ could constrain the evolution of those geologic events. The U‐Pb dates of zircons in basalt, basaltic andesite, trachyandesite igneous clasts, and the matrix are similar (4.4 Ga) suggesting intense volcanism on ancient Mars. However, two metamict zircon grains found in the matrix have an upper intercept date of ~4465 Ma in crystalline, whereas amorphous areas have a lower intercept date of 1634 ± 93 Ma. The younger date is consistent with the date of apatites (1530 ± 65 Ma), suggesting a metamorphic event that completely reset the U‐Pb system in both the amorphous areas of zircon and all apatites. δD values in all apatites negatively correlate with water content in a two‐endmember mixing trend. The D (δD up to 2459‰) and ³⁷Cl heavy core (3.8‰) of a large apatite grain suggest a D‐, ³⁷Cl‐rich fluid during the metamorphic event ~1.6 Ga ago, consistent with the trace elements Y, Hf and Ti and P in zircons. The fluid was also therefore P‐rich. The D‐, ³⁷Cl‐poor H₂O‐rich rim (<313‰) suggests the degassing of water from the Martian Cl‐poor interior at a later time. This D‐, ³⁷Cl‐poor Martian mantle reservoir could have derived from volcanic intrusions postdating the younger metamorphic event recorded in NWA 7034.

¹Z.Gainsforth et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13265]
¹Space Sciences Laboratory, University of California, Berkeley, California, 94720 USA
Published by arrangement with John Wiley & Sons

In a consortium analysis of a large particle captured from the coma of comet 81P/Wild 2 by the Stardust spacecraft, we report the discovery of a field of fine‐grained material (FGM) in contact with a large sulfide particle. The FGM was partially located in an embayment in the sulfide. As a consequence, some of the FGM appears to have been protected from damage during hypervelocity capture in aerogel. Some of the FGM particles are indistinguishable in their characteristics from common components of chondritic‐porous interplanetary dust particles, including glass with embedded metals and sulfides and equilibrated aggregates. The sulfide exhibits surprising Ni‐rich lamellae, which may indicate that this particle experienced a long‐duration heating event after its formation but before incorporation into Wild 2.

¹Jiří Borovička,²Olga Popova,¹Pavel Spurný
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13259]
¹Astronomical Institute of the Czech Academy of Sciences, , CZ‐25165 Ondřejov, Czech Republic
²Institute for Dynamics of Geospheres, Russian Academy of Sciences, , 119334 Moscow, Russia
Published by arrangement with John Wiley & Sons

High entry speed (>25 km s⁻¹) and low density (<2500 kg m⁻³) are the two factors that lower the chance of a meteoroid to drop meteorites. The 26 g carbonaceous (CM2) meteorite Maribo recovered in Denmark in 2009 was delivered by a bright bolide observed by several instruments across northern and central Europe. By reanalyzing the available data, we confirmed the previously reported high entry speed of (28.3 ± 0.3) km s⁻¹ and trajectory with slope of 31° to the horizontal. In order to understand how such a fragile material survived, we applied three different models of meteoroid atmospheric fragmentation to the detailed bolide light curve obtained by radiometers located in Czech Republic. The Maribo meteoroid was found to be quite inhomogeneous with different parts fragmenting at different dynamic pressures. While 30–40% of the (2000 ± 1000) kg entry mass was destroyed already at 0.02 MPa, another 25–40%, according to different models, survived without fragmentation up to the relatively large dynamic pressures of 3–5 MPa. These pressures are only slightly lower than the measured tensile strengths of hydrated carbonaceous chondrite (CC) meteorites and are comparable with usual atmospheric fragmentation pressures of ordinary chondritic (OC) meteoroids. While internal cracks weaken OC meteoroids in comparison with meteorites, this effect seems to be absent in CC, enabling meteorite delivery even at high speeds, though in the form of only small fragments.

^1,2Emilie T. Dunham,^3,4,5J. Brian Balta,^1,2 Meenakshi Wadhwa,² Thomas G. Sharp,⁵ Harry Y. McSween Jr.
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13262]
¹Center for Meteorite Studies, Arizona State University, Tempe, Arizona, 8528 USA
²School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, 8528 USA
³Department of Geology & Geophysics, Texas A&M University, College Station, Texas, 77843 USA
⁴ Department of Earth and Environmental Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania, 15260 USA
⁵Department of Earth and Planetary Sciences, Planetary Geosciences Institute, University of Tennessee Knoxville, Knoxville, Tennessee, 37996 USA
Published by arrangement with John Wiley & Sons

Larkman Nunatak (LAR) 12095 and LAR 12240 are recent olivine‐phyric shergottite finds. We report the results of petrographic and chemical analyses of these two samples to understand their petrogenesis on Mars. Based on our analyses, we suggest that these samples are likely paired and are most similar to other depleted olivine‐phyric shergottites, particularly Dar al Gani (DaG) 476 and Sayh al Uhaymir (SaU) 005 (and samples paired with those). The olivine megacryst cores in LAR 12095 and LAR 12240 are not in equilibrium with the groundmass olivines. We infer that these megacrysts are phenocrysts and their major element compositions have been homogenized by diffusion (the cores of the olivine megacrysts have Mg# ~70, whereas megacryst rims and groundmass olivines typically have Mg# ~58–60). The rare earth element (REE) microdistributions in the various phases (olivine, low‐ and high‐Ca pyroxene, maskelynite, and merrillite) in both samples are similar and support the likelihood that these two shergottites are indeed paired. The calculated parent melt (i.e., in equilibrium with the low‐Ca pyroxene, which is one of the earliest formed REE‐bearing minerals) has an REE pattern parallel to that of melt in equilibrium with merrillite (i.e., one of the last‐formed minerals). This suggests that the LAR 12095/12240 paired shergottites represent the product of closed‐system fractional crystallization following magma emplacement and crystal accumulation. Utilizing the europium oxybarometer, we estimate that the magmatic oxygen fugacity early in the crystallization sequence was ~IW. Finally, petrographic evidence indicates that LAR 12095/12240 experienced extensive shock prior to being ejected from Mars.