Nano-scale investigation of granular neoblastic zircon, Vredefort impact structure, South Africa: Evidence for complete shock melting

1Elizaveta Kovaleva,2Monika A.Kusiak,3Gavin G.Kenny,3Martin J.Whitehouse,4Gerlinde Habler,5Anja Schreiber,2Richard Wirth
Earth and Planetary Science Letters 565, 116948 Link to Article [https://doi.org/10.1016/j.epsl.2021.116948]
1Department of Earth Sciences, University of the Western Cape, Robert Sobukwe Road, Bellville 7535, South Africa
2Institute of Geophysics, Polish Academy of Sciences, Księcia Janusza 64, PL-01452 Warsaw, Poland
3Department of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
4Department of Lithospheric Research, University of Vienna, 1090 Vienna, Austria
5Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Telegrafenberg, 3.5 Surface Geochemistry, D-14473 Potsdam, Germany
Copyright Elsevier

Granular neoblastic zircon (ZrSiO4) with systematically oriented granules has been proposed as evidence for extreme shock pressures (>30 GPa) and subsequent high temperatures (>1200 °C). It is widely agreed to reflect the solid-state phase transition from zircon to its high-pressure polymorph reidite and subsequent reversion to zircon. This model is based on crystallographic relationships between granules of a single type of granular zircon and does not explain the formation of other types of granular zircon textures, for example, grains with randomly oriented granules or with large, often euhedral granules. Here we report the first nano-scale observations of granular neoblastic zircon and the surrounding environment. We conducted combined microstructural analyses of zircon in the lithic clast from an impact melt dike of the Vredefort impact structure. Zircon granules have either random or systematic orientation with three mutually orthogonal directions of their c-axes coincident with [110] axes. Each 1-2 μm zircon granule is a mosaic crystal composed of nanocrystalline subunits. Granules contain round inclusions of baddeleyite (monoclinic ZrO2) and amorphous silica melt. Tetragonal and cubic ZrO2 also occur as sub-μm-sized inclusions (<50 nm). Filament-like aggregates of nanocrystalline zircon are present as “floating” in the surrounding silicate matrix. They are aligned with each other, apparently serving as the building blocks for the mosaic zircon crystals (granules). Our results indicate shock-related complete melting of zircon with the formation of immiscible silicate and oxide melts. The melts reacted and crystallized rapidly as zircon granules, some of which experienced growth alignment/twinning and parallel growth, causing the characteristic systematic orientation of the granules observed for some of the aggregates. In contrast to the existing model, in which this type of granular zircon is considered to be a product of reversion from the high-pressure polymorph reidite, our nano-scale observations suggest a formation mechanism that does not require phase transition via reidite but is indicative of instant incongruent decomposition, melting and rapid crystallization from the melt.

Insoluble organic matter in chondrites: Archetypal melanin-like PAH-based multifunctionality at the origin of life?

1d’Ischia, M.,1Manini, P.,2Martins, Z.,3Remusat, L.,4O’D. Alexander, C.M.,5Puzzarini, C.,6Barone, V.,7Saladino, R.
Physics of Life Reviews 37, 65-93 Link to Article [DOI: 10.1016/j.plrev.2021.03.002]
1Department of Chemical Sciences, University of Naples Federico II, Via Cintia 4, Naples, 80126, Italy
2Centro de Química Estrutural, Departamento de Engenharia Química, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, Lisboa, 1049-001, Portugal
3Institut de minéralogie, de physique des matériaux et de cosmochimie, UMR CNRS 7590, Sorbonne Université, Muséum National d’Histoire Naturelle, 61 rue Buffon, Paris, 75005, France
4Earth and Planets Laboratory, Carnegie Institution for Science, 5241 Broad Branch Road, NW Washington, DC 20015-1305, United States
5Department of Chemistry “Giacomo Ciamician”, University of Bologna, Via F. Selmi 2, Bologna, I-40126, Italy
6Scuola Normale Superiore, Piazza dei Cavalieri 7, Pisa, I-56126, Italy
7Biological and Ecological Sciences Department (DEB), University of Tuscia, Via S. Camillo de Lellis, Viterbo, 01100, Italy

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Manganese oxides in Martian meteorites Northwest Africa (NWA) 7034 and 7533

1Yang Xiu et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114471]
1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
Copyright Elsevier

We report the discovery of indigenous Mn-oxides in Martian regolith breccias Northwest Africa (NWA) 7034 and 7533. These Mn-oxides occur in Mn-rich regions as nanocrystals mixed with silicates, FeOOH, and possible phosphates. The Mn-rich regions contain up to 34 wt% Mn and typically display large chemical gradients on the scale of 10–20 μm. The Martian origin of Mn-oxides was established by the presence of Mn-rich glass (4.8–5.6 wt% Mn) in the fusion crust that crosscuts a Mn-oxides-bearing monzonite clast and by the absence of Mn-oxides on the environmentally exposed surfaces (exterior and fractures) of the meteorites. Manganese K-edge X-ray absorption spectrum (XAS) of the Mn-rich glass in the fusion crust indicated that this glass included high-valent Mn species. Synchrotron micro-X-ray diffraction of a Mn-rich region in a basalt clast and XAS of Mn-rich regions in three monzonite clasts indicate Mn-oxides in these regions are dominantly hollandite-structured with 67–85 mol% of the total Mn being Mn4+. The fact that Mn-rich regions are present in diverse petrological associations but are absent in the matrix of the breccias indicates that the Mn-oxides formed through surface alteration prior to the final brecciation event that assembled NWA 7034 and 7533. Thus, the age of the Mn-oxides is older than the lithification age (arguably 1.35 Ga) of NWA 7034 and 7533. Together with findings of Mn-rich phases within Noachian and Hesperian sedimentary strata in Endeavor and Gale craters, our results suggest that Mn-oxides are a common weathering product on Mars surface, suggesting aqueous environment on the Martian surface with high redox potential.

Albite dissolution rates in brines: Implications for late-stage weathering on Mars

1,2Charity M.Phillips-Lander,1Jamie L.Miller,1Megan Elwood Madden
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114478]
1School of Geology and Geophysics, University of Oklahoma, Norman, OK, United States of America
2Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238, United States of America
Copyright Elsevier

Plagioclase minerals have been detected over large portions of the Mars surface. Average plagioclase mineral compositions are approximately An60 for dust free areas on Mars; however, these compositions likely reflect a mixture of more and less sodic plagioclase minerals at the surface. Plagioclase minerals have also been observed in association with chloride and sulfate salts on the Mars surface and in meteorites. Understanding plagioclase dissolution in brines provides insight into post-Noachian weathering on Mars. Batch reactor dissolution experiments were conducted at 298 K to compare albite dissolution rates in water (18 MΩ cm−1ultrapure water (UPW) adjusted to pH 2 with H2SO4; activity of water (ɑH2O) = 1.0), 2.7 mol kg−1 MgSO4 (aH2O = 0.92), 1.24 mol kg−1 MgCl2 (aH2O = 0.92), 2.9 mol kg−1 MgCl2 (aH2O = 0.75), and 5.8 mol kg−1 MgCl2 (aH2O = 0.33) brines at pH 2 to determine how changing solution chemistry and activity of water influence albite dissolution. Aqueous Si-based dissolution rates indicate albite dissolution rates decreased from −8.80 ± 0.02 to −9.49 ± 0.40 log mol m−2 s−1 as the activity of water decreased from 1 to 0.33. Na-based dissolution rates followed the same trend, decreasing from −8.58 ± 0.04 in UPW to −9.45 ± 0.15 log mol m−2 s−1 in 2.9 mol kg−1 MgCl2. Anion chemistry did not appear to effect albite dissolution in high salinity brines at acid pH. Estimated 1 mm albite grain lifetimes increase from ~60–100 to ~587 of years as activity of water decreases, suggesting that post-Noachian weathering on Mars was limited in duration and/or extent.

Characterizing the spectral, microstructural, and chemical effects of solar wind irradiation on the Murchison carbonaceous chondrite through coordinated analyses

1D.L.Laczniak,1M.S.Thompson,2R.Christoffersen,3C.A.Dukes,2S.J.Clemett,4R.V.Morris,4L.P.Keller
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114479]
1Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907, United States of America
2Jacobs, NASA Johnson Space Center, Mail Code X13, Houston, TX 77058, United States of America
3Materials Science and Engineering, University of Virginia, 395 McCormick Road, Charlottesville, VA 22904, United States of America
4ARES, Mail Code X13, NASA Johnson Space Center, Houston, TX 77058, United States of America
Copyright Elsevier

We performed H+ and He+ ion irradiation experiments on slabs of the Murchison CM2 meteorite to simulate solar wind irradiation of carbonaceous asteroids. Two separate 6 mm × 6 mm regions were irradiated with 1 keV H+ and 4 keV He+, respectively, to fluences of 8.1 × 1017 ions/cm2 for H+ and 1 × 1018 ions/cm2 for He+. Unirradiated and irradiated surfaces were analyzed using X-ray photoelectron spectroscopy (XPS), visible to near infrared spectroscopy (VNIR; 0.35–2.5 μm), and microprobe two-step laser-desorption mass spectrometry (μL2MS). We also performed analytical field-emission scanning transmission electron microscopy (FE-STEM) of focused ion beam (FIB) cross-sections extracted from olivine grains and matrix material within the H+- and He+-irradiated regions. In situ XPS analyses suggest that ion irradiation results in the removal of most surface carbon and the partial reduction of surface iron to lower oxidation states. In response to He+-irradiation, we observed reddening and brightening of reflectance spectra, which is a departure from typical lunar-style space weathering. Additionally, H+- and He+-irradiation have opposing effects on organic carbon content: H+-irradiation increases the abundance of some free organic species by breaking down macromolecular material while He+-irradiation causes a decrease in overall organic content by cleaving bonds and sputtering constituent atoms. This suggests that solar wind H+-irradiation and solar wind He+-irradiation change the organic functional group chemistry of asteroidal regolith in different ways. In contrast to some previous experimental space weathering studies, we observe an increase in H2O and OH− abundances in our sample in response to both types of ion irradiation. FE-STEM and energy dispersive X-ray spectroscopy (EDX) analyses show complete amorphization of matrix phyllosilicates in ion-affected rims, partial amorphization of olivine, and changes in Si and Mg concentrations at and/or near the surface. We discuss the implications of these results for understanding the complex nature of space weathering of primitive, carbon-rich asteroids and for analyzing future returned samples from carbonaceous asteroids Bennu and Ryugu.

The Loongana (CL) group of carbonaceous chondrites

1Knut Metzler et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.04.007]
1Institut für Planetologie, University of Münster, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany
Copyright Elsevier

A coordinated study of the petrology, mineral chemistry, and bulk chemical and isotopic composition of the five ungrouped carbonaceous chondrites Coolidge, Loongana 001, Los Vientos (LoV) 051, Northwest Africa (NWA) 033, and NWA 13400 reveals that these meteorites have a similar set of properties that distinguishes them from the other carbonaceous chondrite groups and allows definition of the new Loongana (CL) group of carbonaceous chondrites. The basic characteristics of the investigated samples are: (1) Lithophile element ratios (e.g., Al/Mg, Si/Mg) are within the typical range of other carbonaceous chondrite groups. (2) Fe-Ni metal abundances are considerably higher than for CV, but similar to CR chondrites. (3) Chondrule size-frequency distributions are similar to CV, but dissimilar to CR chondrites. (4) The mean CAI abundance is ∼1.4 vol%, i.e., lower than in CV but much higher than in CR chondrites. (5) Very low amounts of matrix (17-21 vol%), the lowest among the main carbonaceous chondrite groups (CI, CM, CO, CV, CR, CK). (6) Olivine is nearly equilibrated, with mean fayalite (Fa) values between 12.5 mol% (Loongana 001) and 14.7 mol% (NWA 13400) as a metamorphic effect. (7) Lower Al2O3 and higher MgO and Cr2O3 concentrations in matrix, compared to matrix in CV, CK, and CR chondrites. (8) Volatile elements (Mn, Na, K, Rb, Cs, Zn, Se, Te, Pb, Tl) are considerably depleted compared to all other main carbonaceous chondrite groups, reflecting the low matrix abundance. (9) Bulk O isotope compositions plot along the CCAM line (Δ17O -3.96 to -5.47‰), partly overlapping with the CV and CK chondrite field but including samples that are more 16O-rich. (10) Unique positions of CL values in the є54Cr-є50Ti isotope plot, with є54Cr values similar to CV, CK, and CO, but є50Ti values similar to CR chondrites. All CL chondrites studied here are of petrologic type 3.9 to 4, indicating that they have been thermally metamorphosed on the parent body. The diagnostic features of CL chondrites detailed here provide a basis for identifying CL members of lower petrologic types. Such samples will be important for determining the pristine state of these meteorites and their components.

Testing models for the composition of chondrites and their components: I. CO chondrites

1Andrea Patzer,1Emma S.Bullock,1Conel M. O’D.Alexander
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.04.004]
1Earth and Planets Laboratory, Carnegie Institution for Science, 5241 Broad Branch Rd. NW, Washington D.C. 20015, USA
Copyright Elsevier

We present the first results of a comprehensive investigation aimed at testing the hypothesis of chondrule-matrix complementarity and the four-component model for the compositions of the carbonaceous chondrites and their components. Combining point-counting with electron microprobe analyses, we have determined the bulk compositions of thin sections, as well as the average abundances and compositions of the major chondritic components (chondrules, matrix, refractory inclusions, isolated silicate grains and isolated opaque grains). To minimize the potential for element exchange between components during parent body processing, the two most primitive COs, DOM 08006 and ALH 77307, and the primitive ungrouped CO/CM-like Acfer 094 were selected for this study. To verify our method, we also examined one section of the well-studied CO3.2 Kainsaz, a fall that is free of weathering. We were able to reproduce all major and many minor elemental concentrations reported in the literature for average bulk COs and Kainsaz to better than 10 %. The elements most commonly cited as displaying evidence for complementarity are Mg, Si, Al, Ca, Fe and Ti. Iron, however, can be easily affected by chondrule metal-silicate fractionation, redistribution in the parent body and weathering, and our Ti data for matrix are likely compromised by an analytical artifact. Hence, we focused on Mg, Al, Si and Ca – four elements that we can determine very accurately – and show that their relative abundances in chondrules are on average CI-like within the uncertainties of the method. The matrix is not CI-like, but its composition can be explained by the loss of 10-15 wt.% of forsterite from an initially CI-like material prior to or during parent body accretion. These results are inconsistent with chondrule-matrix complementarity. Our average CO chondrule compositions, as well as chondrule and matrix abundances, are in line with the predictions of the four-component model. However, the four-component model assumes a CI-like composition for matrix, and also predicts refractory inclusion abundances that are higher and compositions that are less refractory than we observe. While similar studies of the other carbonaceous chondrite groups are needed, these differences may indicate the limitations of the simplifying assumptions made in the four-component model.

Resolving the age of the Haughton impact structure using coupled 40Ar/39Ar and U-Pb geochronology

1,2Timmons M.Erickson,2Christopher L.Kirkland,2,3Fred Jourdan,4Martin Schmieder,2MichaelI. H. Hartnady,2Morgan A.Cox,2Nicholas E.Timms
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.04.008]
1Jacobs – JETS Contract, Astromaterials Research and Exploration Science Division, NASA Johnson Space Center, Houston, TX, 77058, USA
2The Institute for Geoscience Research (TIGeR), (Timescales of Mineral Systems Group), School of Earth and Planetary Sciences, Curtin University, GPO Box 1984, Perth, WA, 6845, Australia
3Western Australian Argon Isotope Facility, John de Laeter Centre and Space Science and Technology Centre, Curtin University, GPO Box 1984, Perth, WA, 6845, Australia
4HNU Neu-Ulm University of Applied Sciences, Wileystraße 1, D-89231 Neu-Ulm, Germany
Copyright Elsevier

The Haughton Dome located on Devon Island, in the Canadian Archipelago represents a well-preserved, moderate-sized, complex impact crater. Previous age constraints for the 24 km-diameter impact structure have ranged from ca. 21 Ma to ca. 39 Ma. Herein, we present a coordinated microstructural and in situ U-Pb study of zircon and monazite coupled with 40Ar/39Ar laser step heating of shock-melted K-feldspar clasts from shock metamorphosed gneissic fragments collected from the allochthonous impact breccia at Haughton. Moderately shocked zircon and monazite grains yield an age distribution consistent with an Archean protolith metamorphosed at ca. 1.9 Ga, whereas shock recrystallized zircon and monazite yield a lower intercept age of 31.8 ± 1.7 Ma (n=48, MSWD = 0.58, P = 0.99). Four inverse isochron 40Ar/39Ar ages of shocked feldspar clasts yield a weighted mean age of 31.04 ± 0.37 Ma (MSWD = 0.98, P = 0.40), within uncertainty of the U-Pb lower concordia intercept. Ar diffusion modelling supports the interpretation of the impact age and helps resolve impact-driven age resetting. These results highlight the power of coupling multiple geochronologic techniques for determining impact ages, especially from targets with complex geologic histories. Furthermore, they resolve previous discrepancies in the age of the Haughton Dome and the interpretation of the post impact stratigraphy of the crater fill.

Excellent mechanical properties of taenite in meteoric iron

1,2Ueki, S.,1Mine, Y.,1Takashima, K.
Scienctific Reports 11, 4750 Link to Article [DOI: 10.1038/s41598-021-83792-y]
1Department of Materials Science and Engineering, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto, 860-8555, Japan
2Institute of Science and Engineering, Shimane University, 1060 Nishikawatsu, Matsue, Shimane 690-8504, Japan

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Jarosite formation in deep Antarctic ice provides a window into acidic, water-limited weathering on Mars

1,2Baccolo, G. et al. (>10)
Nature Communications 12, 436 Link to Article [DOI: 10.1038/s41467-020-20705-z]
1Department of Environmental and Earth Sciences, University of Milano-Bicocca, Milan, 20126, Italy
2INFN, section of Milano-Bicocca, Milan, 20126, Italy

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