Early Impact Events on Chondritic Parent Bodies: Insights From NWA 11004, Reclassified as an LL7 Breccia

1,2,3Y. Li,3,4A. E. Rubin,1W. Hsu,4K. Ziegler
Journal of Geophysical Research (Planets) Link to Article [https://doi.org/10.1029/2019JE006360]
1Center for Excellence in Comparative Planetology, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, China
2Macau University of Science and Technology, Macau, China
3Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA, USA
4Maine Mineral and Gem Museum, Bethel, ME, USA
5Institute of Meteoritics, University of New Mexico, Albuquerque, NM, USA
Published by arrangement with John Wiley & Sons

The NWA 11004 ordinary chondrite (OC) can provide insights into the complex petrogenetic processes of the early solar system. Although originally classified as an L7 chondrite, it is reclassified as LL based on kamacite Ni (4.9 ± 0.3 wt.%) and Co (3.6 ± 0.5 wt.%) and bulk O‐isotopic composition (δ17O = 3.76‰; δ18O = 5.39‰). NWA 11004 is characterized by (1) the occurrence of 3‐ to 5‐mm‐sized poikilitic pyroxene, (2) scattered low‐Ca pyroxene data in a TiO2 versus Al2O3 diagram, (3) relatively magnesian olivine and low‐Ca pyroxene (Fa25.4, Fs21.3), (4) low abundances of high‐Ca pyroxene, plagioclase, troilite and Ca‐phosphate, and (5) low rare earth element contents in low‐Ca pyroxene. The geochemical features of olivine and low‐Ca pyroxene in NWA 11004 differ from literature data for grains that crystallized from a melt in an OC impact melt breccia. We suggest that in NWA 11004, a plagioclase‐phosphate high‐Ca pyroxene‐troilite melt migrated away during partial melting. Some high‐Ca pyroxene grains crystallized from the residual melt, as indicated by a positive linear trend in a TiO2 versus Al2O3 diagram. Whereas poikilitic low‐Ca pyroxene in NWA 11004 exhibits undulose‐to‐weak mosaic extinction, the olivine chadacrysts exhibit sharp optical extinction; this implies that NWA 11004 experienced a late‐stage shock event (S4) followed by annealing. The Ca‐phosphate 207Pb/206Pb age of 4546 ± 34 Ma most likely dates this late‐stage shock event. We suggest that the presence of type 7 OC in the early solar system may be attributable to impacts on warm chondritic asteroids that were initially heated by the decay of 26Al.

Evidence for Adsorption of Chlorine Species on Iron (III) (Hydr)oxides in the Sheepbed Mudstone, Gale Crater, Mars

1T. S. Peretyazhko,1S. J. Ralston,1B. Sutter,2D. W. Ming
Journal of Geophysical Research (Planets) Link to Article [https://doi.org/10.1029/2019JE006220]
1Jacobs, NASA Johnson Space Center, Houston, TX, USA
2NASA Johnson Space Center, Houston, TX, USA
Published by arrangement with John Wiley & Sons

Ancient aquatic environments in Yellowknife Bay, Gale crater, Mars, could create favorable conditions for adsorption of chlorine compounds (perchlorate and chloride) on Fe (III) (hydr)oxides present in the Sheepbed mudstone, such as akaganeite and ferrihydrite. In this work, 5.2 mM ClO4− and 1.7 to 12 mM Cl− were adsorbed onto ferrihydrite and 5.2 mM ClO4− was adsorbed onto akaganeite at ultraacidic (pH 2–2.5), acidic (pH 3.8–4.5), and near‐neutral (pH 6.2–7.7) pH. Samples were characterized by evolved gas analysis and compared to the data collected for the Cumberland sample from the Sheepbed mudstone. Evolved gas analysis showed that ferrihydrite with 0.5–1 wt.% ClO4− adsorbed under ultraacidic and acidic conditions had a well‐resolved O2 peak at 306 °C due to the thermal decomposition of adsorbed ClO4−. All akaganeite samples with 0.5 wt.% adsorbed ClO4− had a weak peak at 312 °C tentatively assigned to decomposing perchlorate. Evolved gas analysis demonstrated that 0.5–2 wt.% Cl− adsorbed on ferrihydrite at ultraacidic and acidic pH was the main contributor to HCl evolved at >470 °C. Comparison with martian observations indicated that the temperature of the O2 peak originating from ClO4− adsorbed on ferrihydrite matched well with the thermal evolution of O2 from the Cumberland. Evolved HCl originating from Cl− adsorbed on ferrihydrite was within the temperature range of the high‐temperature HCl release from Cumberland (~770 °C). These observations suggest that ferrihydrite containing adsorbed ClO4− and Cl− could exist in the mudstone. Experimental results are consistent with adsorption at acidic pH < 4 environments through postdepositional water‐rock interactions of ferrihydrite with acid‐sulfate groundwater containing dissolved chloride and perchlorate.

Hydrogen Variability in the Murray Formation, Gale Crater, Mars

1N.H. Thomas,1B.L. Ehlmann,1W. Rapin,2F. Rivera‐Hernández,1N.T. Stein,3J. Frydenvang,4T. Gabriel,5P.‐Y. Meslin,5S. Maurice,6R.C. Wiens
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2019JE006289]
1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
2Dartmouth College, Hanover, NH, USA
3Natural History Museum, University of Copenhagen, Denmark
4Arizona State University, Tempe, AZ, USA
5Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, UPS, CNES, Toulouse, France
6Los Alamos National Laboratory, Los Alamos, NM, USA
Published by arrangement with John Wiley & Sons

The Mars Science Laboratory (MSL) Curiosity rover is exploring the Murray formation, a sequence of heterolithic mudstones and sandstones recording fluvial deltaic and lake deposits that comprise over 350 meters of sedimentary strata within Gale crater. We examine >4500 Murray formation bedrock points, employing recent laboratory calibrations for ChemCam laser‐induced breakdown spectroscopy H measurements at millimeter scale. Bedrock in the Murray formation has an interquartile range of 2.3‐3.1 wt. % H2O, similar to measurements using the DAN and SAM instruments. However, specific stratigraphic intervals include high H targets (6‐18 wt. % H2O) correlated with Si, Mg, Ca, Mn, or Fe, indicating units with opal, hydrated Mg‐sulfates, hydrated Ca‐sulfates, Mn‐enriched units, and akageneite or other iron oxyhydroxides, respectively. One stratigraphic interval with higher hydrogen is the Sutton Island unit and Blunts Point unit contact, where higher hydrogen is associated with Fe‐rich, Ca‐rich, and Mg‐rich points. A second interval with higher hydrogen occurs in the Vera Rubin ridge portion of the Murray formation, where higher hydrogen is associated with Fe‐rich, Ca‐rich, and Si‐rich points. We also observe trends in the H signal with grain size, separate from chemical variation, whereby coarser‐grained rocks have higher hydrogen. Variability in the hydrogen content of rocks points to a history of water‐rock interaction at Gale crater that included changes in lake water chemistry during Murray formation deposition and multiple subsequent groundwater episodes.

Sampling interplanetary dust from Antarctic air

1S.Taylor et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13483]
1CRREL, 72 Lyme Road, Hanover, New Hampshire, 03755 USA
Published by arrangement with John Wiley & Sons

We built a collector to filter interplanetary dust particles (IDPs) larger than 5 μm from the clean air at the Amundsen Scott South Pole station. Our sampling strategy used long duration, continuous dry filtering of near‐surface air in place of short duration, high‐speed impact collection on flags flown in the stratosphere. We filtered ~107 m3 of clean Antarctic air through 20 cm diameter, 3 µm filters coupled to a suction blower of modest power consumption (5–6 kW). Our collector ran continuously for 2 years and yielded 41 filters for analyses. Based on stratospheric concentrations, we predicted that each month’s collection would provide 300–900 IDPs for analysis. We identified 19 extraterrestrial (ET) particles on the 66 cm2 of filter examined, which represented ~0.5% of the exposed filter surfaces. The 11 ET particles larger than 5 µm yield about a fifth of the expected flux based on >5 µm stratospheric ET particle flux. Of the 19 ET particles identified, four were chondritic porous IDPs, seven were FeNiS beads, two were FeNi grains, and six were chondritic material with FeNiS components. Most were <10 µm in diameter and none were cluster particles. Additionally, a carbon‐rich candidate particle was found to have a small 15N isotopic enrichment, supporting an ET origin. Many other candidate grains, including chondritic glasses and C‐rich particles with Mg and Si and FeS grains, require further analysis to determine if they are ET. The vast majority of exposed filter surfaces remain to be examined.

Kinetic condensation of metals in the early solar system: Unveiling the cooling history of solar nebula by refractory metal nuggets

1,2MingenPan(潘明恩)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113851]
1Department of Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA
2Department of Computer Science, Columbia University, 500 West 120th Street, New York, NY 10027, USA
Copyright Elsevier

Refractory Metal Nuggets (RMNs; submicrometer highly siderophile element rich metal alloys) are observed in Ca, Al-rich inclusions (CAIs) and other components of primitive meteorites, and some RMNs could have condensed from the Solar Nebula. In order to study the condensation of RMNs in the Solar Nebula, NUCON – a kinetic condensation model – has been developed to simulate the nucleation and condensation of refractory siderophile metal phases. NUCON treats RMNs as solid solutions where multiple elements can accrete onto one RMN. To achieve this goal, the homogeneous nucleation theory is modified to compute the nucleation of solid solutions. Also, a numerical method is developed to compute the integration of condensation and evaporation rates of an RMN. Equilibrium among gaseous phases is also considered, including monatomic gases and oxides. The oxygen fugacity of the simulated Solar Nebula can also be modified by adjusting carbon abundance. NUCON shows that the nucleation of RMNs was inhibited even when the cooling rate of the Solar Nebula was below 0.1 K/year, and RMNs experienced kinetic condensation largely deviated from the equilibrium condensation.

This study modeled the condensation of RMNs in the RMN-forming regions with different cooling rates, total pressures, and oxygen fugacities, and explored how these parameters affect the radii and Ni/Fe ratios of RMNs. To reproduce the RMNs reported in literature, most of which have radii from 100 to 1000 nm, the cooling rate during the accretion of refractory siderophile metals in RMN-forming regions should be in the order of 1 K/year. The timescale of refractory-metal condensation is in the order of 102 years. In addition, RMNs have been measured to have Ni/Fe ratios from almost zero to over unity, and NUCON shows that the cooling rate during FeNi accretion in RMN-forming regions should be in the order of 10 K/h so that the observed Ni/Fe ratios of RMNs can be reproduced. The timescale of FeNi condensation is in the order of 10 h. Thus, NUCON predicts a transition from slow cooling to rapid cooling that is likely to have occurred during RMN condensation.

Classification of CM chondrite breccias—Implications for the evaluation of samples from the OSIRIS‐REx and Hayabusa 2 missions

1Sarah Lentfort,1Addi Bischoff,1,2Samuel Ebert,1Markus Patzek
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13486]
1Institut für Planetologie, Westfälische Wilhelms‐Universität Münster, Wilhelm‐Klemm Str. 10, D‐48149 Münster, Germany
2SOEST/HIGP, University of Hawaii at Manoa, University of Hawai’i, 1680 East‐West Road, POST 516B, Honolulu, HI, 96822 USA
Published by arrangement with John Wiley & Sons

CM chondrites are complex impact (mostly regolith) breccias, in which lithic clasts show various degrees of aqueous alteration. Here, we investigated the degree of alteration of individual clasts within 19 different CM chondrites and CM‐like clasts in three achondrites by chemical analysis of the tochilinite‐cronstedtite‐intergrowths (TCIs; formerly named “poorly characterized phases”). To identify TCIs in various chondritic lithologies, we used backscattered electron (BSE) overview images of polished thin sections, after which appropriate samples underwent electron microprobe measurements. Thus, 75 lithic clasts were classified. In general, the excellent work and specific criteria of Rubin et al. (2007) were used and considered to classify CM breccias in a similar way as ordinary chondrite breccias (e.g., CM2.2‐2.7). In BSE images, TCIs in strongly altered fragments in CM chondrites (CM2.0‐CM2.2) appear dark grayish and show a low contrast to the surrounding material (typically clastic matrix), and can be distinguished from TCIs in moderately (CM2.4‐CM2.6) or less altered fragments (CM2.7‐CM2.9); the latter are bright and have high contrast to the surroundings. We found that an accurate subclassification can be obtained by considering only the “FeO”/SiO2 ratio of the TCI chemistry. One could also consider the TCIs’ S/SiO2 ratio and the metal abundance, but these were not used for classification due to several disadvantages. Most of the CM chondrites are finds that have suffered terrestrial weathering in hot and cold deserts. Thus, the observed abundance of metal is susceptible to weathering and may not be a reliable indicator of subtype classification. This study proposes an extended classification scheme based on Rubin’s scale from subtypes CM2.0‐CM2.9 that takes the brecciation into account and includes the minimum to maximum degree of alteration of individual clasts. The range of aqueous alteration in CM chondrites and small spatial scale of mixing of clasts with different alteration histories will be important for interpreting returned samples from the OSIRIS‐REx and Hayabusa 2 missions in the future.

Outward migration of chondrule fragments in the Early Solar System: O-isotopic evidence for rocky material crossing the Jupiter Gap?

1Devin L.Schrader,2Kazuhide Nagashima,1Jemma Davidson,3Timothy J.McCoy,4Ryan C.Ogliore,5Roger R.Fu
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.05.014]
1Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, 781 East Terrace Road, Tempe, AZ 85287, USA
2HIGP/SOEST, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
3Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, 10th & Constitution Avenue NW, Washington, DC 20560-0119, USA
4Department of Physics, Washington University in St. Louis, St. Louis, MO 63130, USA
5Department of Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, MA 02138, USA
Copyright Elsevier

Determining the origins of chondrule precursors is key to constraining how material migrated in the early Solar System. Chondrules that were only partially melted during their formation retain portions of their solid precursors, termed relict grains. By measuring the chemical and O-isotopic compositions of relict grains in chondrules from an unequilibrated ordinary chondrite (UOC), and Renazzo-like carbonaceous (CR) and Mighei-like carbonaceous (CM) chondrites we constrain their origins and discuss implications for disk transport within the first 4 million years of the Solar System. For all three chondrite groups, the chemical and O-isotopic compositions of dusty olivine grains are sometimes consistent with the reduction of type I (FeO-poor) and/or type II (FeO-rich) chondrules from the same meteorite group. However, other dusty olivine grains from the CM chondrites and the UOC are found to be xenocrysts that require an origin from a source distinct from the host meteorite. This material plausibly originated as fragments of earlier-formed chondrules from another chondrite group or of partially or fully differentiated planetesimals that migrated into an active chondrule-forming region. Multiple CM chondrite dusty olivine chondrules have O-isotope compositions that match those of UOC chondrule olivine (Δ17O ∼ 0‰), suggesting an origin from an UOC source. This implies that UOC chondrules and/or chondrule fragments migrated from the inner Solar System outwards to CM chondrite chondrule-forming region, likely beyond the orbit of Jupiter. These UOC chondrules or chondrule fragments could have migrated outwards in the protoplanetary disk before the formation of the Jupiter Gap, or <300 μm diameter fragments could have migrated outwards after Gap formation as CM chondrite chondrule dusty olivine grains with Δ17O ∼ 0‰ were small enough to pass through Jupiter Gap. The identification of xenocrysts in each meteorite group studied here argues for widespread migration of material in the early Solar System, potentially crossing the Jupiter Gap.