^1,2 Courtney J. Sprain,^1,3Paul R. Renne⁴Loÿc Vanderkluysen,⁵Kanchan Pande,¹Stephen Self,¹Tushar Mittal
Science 363, 866-870 Link to Article [DOI: 10.1126/science.aav1446]
¹Department of Earth and Planetary Science, University of California, Berkeley, 307 McCone Hall, Berkeley, CA 94720-4767, USA.
²Geomagnetism Laboratory, Department of Earth, Ocean and Ecological Sciences, University of Liverpool, Liverpool L69 7ZE, UK.
³Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA.
⁴Department of Biodiversity, Earth and Environmental Science, Drexel University, 3245 Chestnut Street, PISB 123, Philadelphia, PA 19104, USA.
⁵Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India.
Reprinted with permission of AAAS

Late Cretaceous records of environmental change suggest that Deccan Traps (DT) volcanism contributed to the Cretaceous-Paleogene boundary (KPB) ecosystem crisis. However, testing this hypothesis requires identification of the KPB in the DT. We constrain the location of the KPB with high-precision argon-40/argon-39 data to be coincident with changes in the magmatic plumbing system. We also found that the DT did not erupt in three discrete large pulses and that >90% of DT volume erupted in <1 million years, with ~75% emplaced post-KPB. Late Cretaceous records of climate change coincide temporally with the eruption of the smallest DT phases, suggesting that either the release of climate-modifying gases is not directly related to eruptive volume or DT volcanism was not the source of Late Cretaceous climate change.

¹Blair Schoene, ¹Michael P. Eddy, ²Kyle M. Samperton, ³C. Brenhin Keller, ¹Gerta Keller, ⁴Thierry Adatte, ⁵Syed F. R. Khadri
Science 363, 862-866 Link to Article [DOI: 10.1126/science.aau2422]
¹Department of Geosciences, Princeton University, Princeton, NJ, USA.
²Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA, USA.
³Berkeley Geochronology Center, Berkeley, CA, USA.
⁴ISTE, Institut des Sciences de la Terre, Université de Lausanne, GEOPOLIS, Lausanne, Switzerland.
⁵Department of Geology, Amravati University, Amravati, India.
Reprinted with permission from AAAS

Temporal correlation between some continental flood basalt eruptions and mass extinctions has been proposed to indicate causality, with eruptive volatile release driving environmental degradation and extinction. We tested this model for the Deccan Traps flood basalt province, which, along with the Chicxulub bolide impact, is implicated in the Cretaceous-Paleogene (K-Pg) extinction approximately 66 million years ago. We estimated Deccan eruption rates with uranium-lead (U-Pb) zircon geochronology and resolved four high-volume eruptive periods. According to this model, maximum eruption rates occurred before and after the K-Pg extinction, with one such pulse initiating tens of thousands of years prior to both the bolide impact and extinction. These findings support extinction models that incorporate both catastrophic events as drivers of environmental deterioration associated with the K-Pg extinction and its aftermath.

Udry¹ et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13252]
¹Department of Geoscience, University of Nevada Las Vegas, Las Vegas, Nevada, 89154 USA
Published by arrangement with John Wiley & Sons

We present petrologic and isotopic data on Northwest Africa (NWA) 4799, NWA 7809, NWA 7214, and NWA 11071 meteorites, which were previously classified as aubrites. These four meteorites contain between 31 and 56 vol% of equigranular, nearly endmember enstatite, Fe,Ni metal, plagioclase, terrestrial alteration products, and sulfides, such as troilite, niningerite, daubréelite, oldhamite, and caswellsilverite. The equigranular texture of the enstatite and the presence of the metal surrounding enstatite indicate that these rocks were not formed through igneous processes like the aubrites, but rather by impact processes. In addition, the presence of pre‐terrestrially weathered metal (7.1–14 vol%), undifferentiated modal abundances compared to enstatite chondrites, presence of graphite, absence of diopside and forsterite, low Ti in troilite, and high Si in Fe,Ni metals suggest that these rocks formed through impact melting on chondritic and not aubritic parent bodies. Formation of these meteorites on a parent body with similar properties to the EHa enstatite chondrite parent body is suggested by their mineralogy. These parent bodies have undergone impact events from at least 4.5 Ga (NWA 11071) until at least 4.2 Ga (NWA 4799) according to ³⁹Ar‐⁴⁰Ar ages, indicating that this region of the solar system was heavily bombarded early in its history. By comparing NWA enstatite chondrite impact melts to Mercury, we infer that they represent imperfect petrological analogs to this planet given their high metal abundances, but they could represent important geochemical analogs for the behavior and geochemical affinities of elements on Mercury. Furthermore, the enstatite chondrite impact melts represent an important petrological analog for understanding high‐temperature processes and impact processes on Mercury, due to their similar mineralogies, Fe‐metal‐rich and FeO‐poor silicate abundances, and low oxygen fugacity.

Karel ZAK¹, Roman SKALA¹, Andreas PACK², Lukas ACKERMAN¹, and Sarka KRIZOVA^1,3
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13251]
¹Institute of Geology of the Czech Academy of Sciences, Rozvojova 269, CZ-165 00 Praha 6, Czech Republic
²Geowissenschaftliches Zentrum, Abteilung Isotopengeologie, Universität Göttingen,Goldschmidtstraße 1, D-37077 Göttingen, Germany
³Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science,Charles University, Albertov 6, CZ-128 43 Praha 2, Czech Republic
Published by arrangement with John Wiley & Sons

Major and trace element analyses and triple oxygen isotope measurements were performed on 11 individual specimens of Australasian tektites (AAT) with exactly known field positions from Laos. The sample set was dominated by Muong Nong‐type tektites (MNAAT), including separated layers of glass of different appearance and chemistry from four samples. This first larger set of oxygen isotope data of MNAAT revealed the δ¹⁸O range 8.7 ≤ δ¹⁸O ≤ 11.6‰ on VSMOW2 scale (12 analyses), only slightly wider than the previously reported range for splash‐form AAT. The Δ’¹⁷O values of MNAAT (−0.098 ≤ Δ’¹⁷O ≤ −0.069‰; 12 analyses) and splash‐form AAT (−0.080 ≤ Δ’¹⁷O ≤ −0.068‰; three analyses) are all in the range of data typical for terrestrial crustal rocks, with no mass‐independent oxygen isotope fractionation (from impactor or from exchange with atmospheric O₂) being observed.

M. R. Mumpower¹, T. Kawano¹, T. M. Sprouse², N. Vassh², E. M. Holmbeck², R. Surman², and P. Möller¹
Astrophysical Journal 869, 14 Link to Article [DOI: 10.3847/1538-4357/aaeaca]
¹Theoretical Division, Los Alamos National Laboratory Los Alamos, NM 87545, USA
²Department of Physics, University of Notre Dame Notre Dame, IN 46556, USA

We present β-delayed neutron emission and β-delayed fission (βdf) calculations for heavy, neutron-rich nuclei using the coupled Quasi-Particle Random Phase Approximation plus Hauser-Feshbach (QRPA+HF) approach. From the initial population of a compound nucleus after β-decay, we follow the statistical decay, taking into account competition between neutrons, γ-rays, and fission. We find a region of the chart of nuclides where the probability of βdf is ~100%, which likely prevents the production of superheavy elements in nature. For a subset of nuclei near the neutron dripline, neutron multiplicity and the probability of fission are both large, leading to the intriguing possibility of multi-chance βdf, a decay mode for extremely neutron-rich heavy nuclei. In this decay mode, β-decay can be followed by multiple neutron emission, leading to subsequent daughter generations that each have a probability to fission. We explore the impact of βdf in rapid neutron-capture process (r-process) nucleosynthesis in the tidal ejecta of a neutron star–neutron star merger and show that it is a key fission channel that shapes the final abundances near the second r-process peak.

Sakari¹ et al. (>10)
Astrophysical Journal 868, 110 Link to Article [DOI: 10.3847/1538-4357/aae9df]
Space Science Division, U.S. Naval Research Laboratory, Washington, DC 20375, USA
¹ Department of Astronomy, University of Washington, Seattle, WA 98195-1580, USA

This paper presents the detailed abundances and r-process classifications of 126 newly identified metal-poor stars as part of an ongoing collaboration, the R-Process Alliance. The stars were identified as metal-poor candidates from the RAdial Velocity Experiment (RAVE) and were followed up at high spectral resolution (R ~ 31,500) with the 3.5 m telescope at Apache Point Observatory. The atmospheric parameters were determined spectroscopically from Fe i lines, taking into account non-LTE corrections and using differential abundances with respect to a set of standards. Of the 126 new stars, 124 have [Fe/H] < −1.5, 105 have [Fe/H] < −2.0, and 4 have [Fe/H] < −3.0. Nine new carbon-enhanced metal-poor stars have been discovered, three of which are enhanced in r-process elements. Abundances of neutron-capture elements reveal 60 new r-I stars (with +0.3 ≤ [Eu/Fe] ≤ +1.0 and [Ba/Eu] < 0) and 4 new r-II stars (with [Eu/Fe] > +1.0). Nineteen stars are found to exhibit a “limited-r” signature ([Sr/Ba] > +0.5, [Ba/Eu] < 0). For the r-II stars, the second- and third-peak main r-process patterns are consistent with the r-process signature in other metal-poor stars and the Sun. The abundances of the light, α, and Fe-peak elements match those of typical Milky Way (MW) halo stars, except for one r-I star that has high Na and low Mg, characteristic of globular cluster stars. Parallaxes and proper motions from the second Gaia data release yield UVW space velocities for these stars that are consistent with membership in the MW halo. Intriguingly, all r-II and the majority of r-I stars have retrograde orbits, which may indicate an accretion origin.

János Kodolányi¹, Christian Vollmer², Peter Hoppe¹, and Maren Müller³
Astrophysical Journal 868, 34 Link to Article [DOI: 10.3847/1538-4357/aae482]
¹Max Planck Institute for Chemistry, Hahn-Meitner-Weg 1, D-55128 Mainz, Germany
²University of Münster, Institute for Mineralogy, Corrensstrasse 24, D-48149 Münster, Germany
³Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany

We analyzed seven presolar SiC grains of supernova origin (average diameter: 1–2 μm) with transmission electron microscopy. Five grains are polycrystalline, whereas two grains are single crystals. Individual crystal domains of polycrystalline grains are in epitaxial relationship, with two grains consisting almost entirely of twinned crystal domains. Most grains are free of inclusions (only one TiC inclusion and one iron- and nickel-rich inclusion were found in two separate grains). Almost all crystals have cubic symmetry (3C polytype), but we found hexagonal SiC (6H polytype) in two grains. The large range of crystal domain sizes (average diameter: 50–970 nm), as well as the larger fraction of noncubic SiC polytypes in supernova grains relative to SiC grains that crystallized in the winds of asymptotic giant branch (AGB) stars, suggest that SiC condensation in supernova ejecta occurs at a larger range of chemical and physical conditions, including supersaturation, than in the winds of AGB stars. Modeling condensation of SiC struggles to produce SiC grains as large as, or bigger than, observed here, if condensation of large (i.e., several μm in diameter) graphite grains is to precede that of SiC, which is suggested by the presolar grain record and published equilibrium condensation models. We propose that future models of graphite and SiC condensation in SN ejecta explore higher ejecta densities than before, as well as gas compositions that are more silicon- and carbon-rich. Furthermore, we infer that some supernova SiC grains may have formed without prior condensation of graphite from their parent gas.

Shu-Zhou WANG¹, Ai-Cheng ZHANG^1,2, Run-Lian PANG¹, Yang LI³, and Jia-Ni CHEN¹
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13254]
¹State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University,Nanjing 210046, China
²Lunar and Planetary Science Institute, Nanjing University, Nanjing 210046, China
³Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
Published by arrangement with John Wiley & Sons

Records of space weathering are important for understanding the formation and evolution of surface regolith on airless celestial bodies. Current understanding of space weathering processes on asteroids including asteroid‐4 Vesta, the source of the howardite–eucrite–diogenite (HED) meteorites, lags behind what is known for the Moon. In this study, we studied agglutinates, a vesicular glass‐coating lithic clast, and a fine‐grained sulfide replacement texture in the polymict breccia Northwest Africa (NWA) 1109 with electron microscopy. In agglutinates, nanophase grains of FeNi and FeS were observed, whereas npFe⁰ was absent. We suggested that the agglutinates in NWA 1109 formed from fine‐grained surface materials of Vesta during meteorite/micrometeorite bombardment. The fine‐grained sulfide replacement texture (troilite + hedenbergite + silica) should be a result of reaction between S‐rich vapors and pyroxferroite. The unique Fe/Mn values of relict pyroxferroite indicate a different source from normal HED pyroxenes, arguing that the reaction took place on or near the surface of Vesta. The fine‐grained sulfide replacement texture could be a product of nontypical space weathering on airless celestial bodies. We should pay attention to this texture in future returned samples by asteroid exploration missions.

Shinya Wanajo^1,2,3
Astrophysical Journal 868, 65 Link to Article [DOI: 10.3847/1538-4357/aae0f2]
¹Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Am Mühlenberg 1, Potsdam-Golm, D-14476, Germany
²Department of Engineering and Applied Sciences, Sophia University, Chiyoda-ku, Tokyo 102-8554, Japan
³iTHEMS Research Group, RIKEN, Wako, Saitama 351-0198, Japan

Radioactive energies from unstable nuclei made in the ejecta of neutron star mergers play principal roles in powering kilonovae. In previous studies, power-law-type heating rates (e.g., ) have frequently been used, which may be inadequate if the ejecta are dominated by nuclei other than the A ~ 130 region. We consider, therefore, two reference abundance distributions that match the r-process residuals to the solar abundances for A ≥ 69 (light trans-iron plus r-process elements) and A ≥ 90 (r-process elements). Nucleosynthetic abundances are obtained by using free-expansion models with three parameters: expansion velocity, entropy, and electron fraction. Radioactive energies are calculated as an ensemble of weighted free-expansion models that reproduce the reference abundance patterns. The results are compared with the bolometric luminosity (> a few days since merger) of the kilonova associated with GW170817. We find that the former case (fitted for A ≥ 69) with an ejecta mass 0.06 M _⊙ reproduces the light curve remarkably well, including its steepening at 7 days, in which the mass of r-process elements is ≈0.01 M _⊙. Two β-decay chains are identified: ⁶⁶Ni ⁶⁶Cu ⁶⁶Zn and ⁷²Zn ⁷²Ga ⁷²Ge with similar halflives of parent isotopes (≈2 days), which leads to an exponential-like evolution of heating rates during 1–15 days. The light curve at late times (>40 days) is consistent with additional contributions from the spontaneous fission of ²⁵⁴Cf and a few Fm isotopes. If this is the case, the GW170817 event is best explained by the production of both light trans-iron and r-process elements that originate from dynamical ejecta and subsequent disk outflows from the neutron star merger.

Guillermo Stenborg, Johnathan R. Stauffer¹, and Russell A. Howard
Astrophysical Journal 868, 34 Link to Article [DOI: 10.3847/1538-4357/aae6cb]
Space Science Division, U.S. Naval Research Laboratory, Washington, DC 20375, USA
¹Current address: Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309, USA.

To test a technique to be used on the white-light imager onboard the recently launched Parker Solar Probemission, we performed a numerical differentiation of the brightness profiles along the photometric axis of the F-corona models that are derived from STEREO Ahead Sun Earth Connection Heliospheric Investigation observations recorded with the HI-1 instrument between 2007 December and 2014 March. We found a consistent pattern in the derivatives that can be observed from any S/C longitude between about 18° and 23° elongation with a maximum at about 21°. These findings indicate the presence of a circumsolar dust density enhancement that peaks at about 23° elongation. A straightforward integration of the excess signal in the derivative space indicates that the brightness increase over the background F-corona is on the order of 1.5%–2.5%, which implies an excess dust density of about 3%–5% at the center of the ring. This study has also revealed (1) a large-scale azimuthal modulation of the inner boundary of the pattern, which is in clear association with Mercury’s orbit; and (2) a localized modulation of the inner boundary that is attributable to the dust trail of Comet 2P/Encke, which occurs near ecliptic longitudes corresponding to the crossing of Encke’s and Mercury’s orbital paths. Moreover, evidence of dust near the S/C in two restricted ranges of ecliptic longitudes has also been revealed by this technique, which is attributable to the dust trails of (1) comet 73P/Schwassmann–Wachmann 3, and (2) 169P/NEAT.