Mineralogy and Spectroscopy (Visible Near Infrared and Fourier Transform Infrared) of Mukundpura CM2: Implications for asteroidal aqueous alteration

Chemie der Erde (Geochemistry)(in Press) Link to Article [https://doi.org/10.1016/j.chemer.2020.125729]
1Physical Research Laboratory, Ahmedabad, 380009, India
2Indian Institute of Technology, Kharagpur, 721302, India
3Space Application Centre, Ahmedabad, 380015, India
Copyright Elsevier

We report the textures, mineralogy and mineral chemistry of the Mukundpura matrix component, a clast-bearing, brecciated, new CM2 carbonaceous chondrite. Like other CMs, Mukundpura is matrix-enriched and has experienced different degrees of aqueous alteration with evidences of fracturing and compaction of clasts due to the impact. A few relict chondrule clasts and CAIs (diopside and spinel) survived despite of the alteration amidst accessory phases of olivine, magnetite, sulphides and calcite. X-Ray Diffraction (XRD), Visible Near Infrared (VNIR) and Fourier Transform Infrared (FTIR) spectroscopic studies reveal higher phyllosilicate content (∼90%) comprising of both Mg and Fe-serpentine and abundant serpentine-sulphide intergrowths. Even then, the presence of accessory olivine as relict clasts can be interpreted from the presence of certain typical olivine absorptions in the FTIR spectra. The non-stoichiometric, Tochilinite-Cronstedtite occurrences probably relate to broadening of XRD and FTIR spectra and can be explained by coupled Al–Si and Mg–Al substitutions in talc and serpentine. The FTIR spectra suggest widespread transformation of olivine to serpentine, unlike the largely unaltered chondrules. The correlations of mineralogical alteration index with FeO/SiO2 and S/SiO2 in different domains of matrix suggest different extent of alterations. Thus, the aqueous alteration is extensive but not pervasive. The majority of alteration seems to have occurred within the asteroidal parent body. The Mukundpura CM2 thus preserves a unique combination of relict chondrules and highly aqueous altered variegated matrix clasts, although the surface mineralogy resembles the C-type asteroids recently probed by OSIRIS-REx and Hayabusa-2 missions.

CM carbonaceous chondrite falls and their terrestrial alteration

1Martin R. Lee,1,2,3Luke Daly,1Cameron Floyd,1Pierre‐Etienne Martin
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13607]
1School of Geographical & Earth Sciences, University of Glasgow, Glasgow, G12 8QQ UK
2Space Science and Technology Centre, School of Earth and Planetary Sciences, Curtin University, GPO BOC U1987, Perth, Western Australia, 6845 Australia
3Australian Centre for Microscopy and Microanalysis, The University of Sydney, Camperdown, New South Wales, 2006 Australia
Published by arrangement with John Wiley & Sons

The CM carbonaceous chondrites provide unique insights into the composition of the protoplanetary disk, and the accretion and geological history of their parent C‐complex asteroid(s). Of the hundreds of CMs that are available for study, the majority are finds and so may have been compromised by terrestrial weathering. Nineteen falls have been recovered between 1838 and 2020, and there is a hint of two temporal clusters: 1930–1942 and 2009–2020. Falls are considered preferable to finds to study because they should be near pristine, and here this assumption is tested by investigating their susceptibility to alteration before recovery and during curation. CMs falling on the land surface are prone to contamination by organic compounds from soil and vegetation. Where exposed to liquid water prior to collection, minerals including oldhamite can be dissolved and most fluid mobile elements leached. Within days of recovery, CMs adsorb water from the atmosphere and are commonly contaminated by airborne hydrocarbons. Interaction with atmospheric water and oxygen during curation over year to decadal timescales can produce Fe‐oxyhydroxides from Fe,Ni metal and gypsum from indigenous gypsum and oldhamite. Relationships between the petrologic (sub)types of pre‐1970 falls and their terrestrial age could be due to extensive but cryptic alteration during curation, but are more likely a sampling bias. The terrestrial history of a CM fall, including circumstances of its collection and conditions of its curation, must be taken into account before it is used to infer processes on C‐complex parent bodies such as Ryugu and Bennu.

Fe‐redox changes in Itokawa space‐weathered rims

1L. J. Hicks,1J. C. Bridges,2T. Noguchi,3A. Miyake,1J. D. Piercy,1S. H. Baker
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13611]
1Space Research Centre, School of Physics & Astronomy, University of Leicester, Leicester, LE1 7RH UK
2Faculty of Arts and Science, Kyushu University, 744 Motooka, Nishi‐ku, Fukuoka, 819‐0395 Japan
3Division of Earth and Planetary Sciences, Kyoto University, Kitashirakawaoiwake‐cho, Kyoto, 606‐8502 Japan
Published by arrangement with John Wiley & Sons

Synchrotron Fe‐K X‐ray absorption spectroscopy and transmission electron microscopy have been used to investigate the mineralogy and Fe‐redox variations in the space‐weathered (SW) rims of asteroidal samples. This study focuses on the FIB lift‐out sections from five Itokawa grains, returned by the Hayabusa spacecraft, including samples RB‐QD04‐0063, RB‐QD04‐0080, RB‐CV‐0011, RB‐CV‐0089, and RB‐CV‐0148. Each of the samples featured partially amorphized SW rims, caused by irradiation damage from implanted low mass solar wind ions, and the impacting of micrometeorites. Using bright‐field and HAADF‐STEM imaging, vesicular blistering and nanophase Fe metal (npFe0) particles were observed within grain rims, and solar flare tracks were observed in the substrate host grain, confirming the presence of SW zones. We use Fe‐K XANES mapping to investigate Fe‐redox changes between the host mineral and the SW zones. All SW zones measured show some increases in the ferric‐ferrous ratio (Fe3+/ΣFe) relative to their respective host grains, likely the result of the implanted solar wind H+ ions reacting with the segregated ferrous Fe in the surface material.

Presolar stardust in highly pristine CM chondrites Asuka 12169 and Asuka 12236

1Larry R. Nittler,1Conel M. O’D. Alexander,1,2Andrea Patzer,1,3Maximilien J. Verdier‐Paoletti
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13618]
1Earth and Planets Laboratory, Carnegie Institution of Washington, 5241 Broad Branch Rd NW, Washington, District of Columbia, 20015 USA
2Geosciences Center Göttingen, University of Göttingen, Goldschmidtstr. 1, 37077 Göttingen, Germany
3Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie (IMPMC), Sorbonne Université, Muséum national d’Histoire naturelle, UPMC Université Paris 06, UMR CNRS 7590, IRD, UMR 206, 75005 Paris, France
Published by arrangement with John Wiley & Sons

We report a NanoSIMS search for presolar grains in the CM chondrites Asuka (A) 12169 and A12236. We found 90 presolar O‐rich grains and 25 SiC grains in A12169, giving matrix‐normalized abundances of 275 (+55/−50, 1σ) ppm or, excluding an unusually large grain, 236 (+37/−34) ppm for O‐rich grains and 62 (+15/−12) ppm for SiC grains. For A12236, 18 presolar silicates and 6 SiCs indicate abundances of 58 (+18/−12) and 20 (+12/−8) ppm, respectively. The SiC abundances are in the typical range of primitive chondrites. The abundance of presolar O‐rich grains in A12169 is essentially identical to that in CO3.0 Dominion Range 08006, higher than in any other chondrites, while in A12236, it is higher than found in other CMs. These abundances provide further strong support that A12169 and A12236 are the least‐altered CMs as indicated by petrographic investigations. The similar abundances, isotopic distributions, silicate/oxide ratios, and grain sizes of the presolar O‐rich grains found here to those of presolar grains in highly primitive CO, CR, and ungrouped carbonaceous chondrites (CCs) indicate that the CM parent body(ies) accreted a similar population of presolar oxides and silicates in their matrices to those accreted by the parent bodies of the other CC groups. The lower abundances and larger grain sizes seen in some other CMs are thus most likely a result of parent‐body alteration and not heterogeneity in nebular precursors. Presolar silicates are unlikely to be present in high abundances in returned samples from asteroids Ryugu and Bennu since remote‐sensing data indicate that they have experienced substantial aqueous alteration.

Experimental investigation of structural OH/H2O in different lunar minerals and glass via solar-wind proton implantation

1,2,3Hong Tang,1,2,3XiongyaoLi,1Xiaojia Zeng,1,2,3Yang Li,1,2,3Wen Yu,1,2,3Bing Mo,1,2,3Jianzhong Liu,4Shijie Wang,5Yongliao Zou
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114322]
1Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
2CAS Center for Excellence in Comparative Planetology, China
3Key Laboratory of Space Manufacturing Technology, Chinese Academy of Sciences, Beijing 100094, China
4State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
5National Space Science Center, Chinese Academy of Sciences, Beijing 100190, China
Copyright Elsevier

The possibility of OH/H2O formation on the lunar surface has been proposed because of the interaction between protons from the solar wind and oxygen in the regolith. In this study, we examined olivine, pyroxene, plagioclase, and volcanic glass samples together irradiated with 7 keV H+ at a dose of 1017 ions/cm2 under the same experimental conditions to simulate the solar-wind proton implantation process on the moon. By comparing the infrared spectral characteristics of these samples before and after H+ implantation through an infrared spectrometer, we confirm that OH forms in all minerals and glass after H+ implantation, with a remarkable amount of OH/H2O found in plagioclase. This indicates that plagioclase can capture more H+ than other silicate phases to form the OH/H2O. The absorption characteristics of OH/H2O formed by H+ implantation are distinct and associated with the mineral structure. The efficiency of OH/H2O formation by H+ implantation is affected by crystal structure. We conclude that OH/H2O formed by solar-wind implantation in the lunar soil is likely to be mainly preserved in plagioclase, and the estimated OH/H2O absorption strength from 0.7 to 3.6% at 3356 cm−1 and from 0.9 to 4.8% at 3622 cm−1 of plagioclase is consistent with those found by recent lunar spacecraft missions

Geochemical data indicate highly similar sediment compositions for the Grasberg and Burns formations on Meridiani Planum, Mars

1Thomas M.McCollom,1,2Brian Hynek
Earth and Planetary Science Letters 557, 116729 Link to Article [https://doi.org/10.1016/j.epsl.2020.116729]
1Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80309, United States of America
2Department of Geological Sciences, University of Colorado, Boulder, CO 80309, United States of America
Copyright Elsevier

The hematite-bearing, sulfate-rich sandstones of the Burns formation at Meridiani Planum are underlain by a thin stratigraphic unit referred to as the Grasberg formation. The sulfate-bearing Grasberg rocks are fine-grained and lack bedding structures, and were previously interpreted to be a distinct lithologic unit based on morphological and chemical differences from the overlying Burns formation. However, reanalysis of the data indicates that, except for variable amounts of Mg, Ni, SO3 and Mn, the chemical compositions of the Grasberg and Burns rocks are very similar. The relatively low levels of Mg, Ni, and SO3 in the Grasberg rocks indicates that they have experienced diagenetic loss of Mg sulfates similar to that observed in a subset of eleven Burns formation rocks depleted in the same elements, including two Burns rocks immediately above the Grasberg contact. The Grasberg formation and Burns rocks near the contact have also evidently lost Mn during diagenesis. When compensated for diagenetic losses, rocks from the Grasberg and Burns formations are found to have nearly identical chemical compositions, albeit Grasberg rocks contained a few wt.% less SO3. These observations suggest that the sediment sources for the Grasberg and Burns formations are genetically related, and that both formations experienced some of the same diagenetic processes after deposition. Furthermore, the apparent loss of Mg, Ni, SO3, and Mn from the Grasberg formation and immediately overlying Burns rocks is mirrored by enrichments of these same elements in fractures within the underlying Shoemaker formation, suggesting downward movement of fluids during some diagenetic events.

Stirred not shaken; critical evaluation of a proposed Archean meteorite impact in West Greenland

1Chris Yakymchuk,2Christopher L.Kirkland,3Aaron J.Cavosie,4Kristoffer Szilas,5Julie Hollis,6Nicholas J.Gardiner,4Pedro Waterton,7Agnete Steenfelt,8LaureMartin
Earth and Planetary Science Letters 557, 116730 Link to Article [https://doi.org/10.1016/j.epsl.2020.116730]
1Department of Earth and Environmental Sciences, University of Waterloo, Canada
2Timescales of Mineral Systems Group, Centre for Exploration Targeting – Curtin Node, School of Earth and Planetary Sciences, Curtin University, Perth, Australia
3Space Science and Technology Centre, School of Earth and Planetary Sciences, Curtin University, Perth, Australia
4Department of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen K, Denmark
5Department of Geology, Ministry of Mineral Resources, Government of Greenland, P.O. Box 930, 3900 Nuuk, Greenland
6School of Earth and Environmental Sciences, University of St Andrews, St Andrews, KY16 9AL, United Kingdom
7The Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen K, Denmark
8Centre for Microscopy, Characterisation & Analysis, The University of Western Australia, Perth, Western Australia 6009, Australia
Copyright Elsevier

Large meteorite impacts have a profound effect on the Earth’s geosphere, atmosphere, hydrosphere and biosphere. It is widely accepted that the early Earth was subject to intense bombardment from 4.5 to 3.8 Ga, yet evidence for subsequent bolide impacts during the Archean Eon (4.0 to 2.5 Ga) is sparse. However, understanding the timing and magnitude of these early events is important, as they may have triggered significant change points to global geochemical cycles. The Maniitsoq region of southern West Greenland has been proposed to record a ∼3.0 Ga meteorite impact, which, if confirmed, would be the oldest and only known impact structure to have survived from the Archean. Such an ancient structure would provide the first insight into the style, setting, and possible environmental effects of impact bombardment continuing into the late Archean. Here, using field mapping, geochronology, isotope geochemistry, and electron backscatter diffraction mapping of 5,587 zircon grains from the Maniitsoq region (rock and fluvial sediment samples), we test the hypothesis that the Maniitsoq structure represents Earth’s earliest known impact structure. Our comprehensive survey shows that previously proposed impact-related geological features, ranging from microscopic structures at the mineral scale to macroscopic structures at the terrane scale, as well as the age and geochemistry of the rocks in the Maniitsoq region, can be explained through endogenic (non-impact) processes. Despite the higher impact flux, intact craters from the Archean Eon remain elusive on Earth.

Iron‐rich olivine in the unequilibrated ordinary chondrite, MET 00526: Earliest stages of formation

1,2Elena Dobrică,2Adrian J. Brearley
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13610]
1Hawai’i Institute of Geophysics and Planetology, School of Ocean, Earth Science, and Technology, University of Hawai’i at Mānoa, Honolulu, Hawaii, 96822 USA
2Department of Earth and Planetary Sciences, MSC03‐2040, University of New Mexico, Albuquerque, New Mexico, 87131‐0001 USA
Published by arrangement with John Wiley & Sons

In order to understand the effects of the earliest stages of hydrothermal alteration and fluid‐assisted metamorphism on the matrices of unequilibrated ordinary chondrites (UOCs), we have investigated the fine‐grained matrix of MET 00526 (L3.05) using multiple electron microscope techniques. Iron‐rich olivines (Fa50‐100) are present in all four representative fine‐grained matrix regions analyzed in this study. This study shows for the first time the occurrence of FeO‐rich olivines in distinct submicron veins that crosscut regions of matrix consisting of amorphous silicates and phyllosilicates, providing evidence for elemental mass transport in a hydrothermal fluid. Our detailed transmission electron microscopy study reinforces the idea that FeO‐rich olivines are formed on asteroidal parent bodies by the interaction between a hydrothermal fluid and the pristine solar nebular materials that may be the product of condensation processes in the protoplanetary disk, that is, amorphous silicates. We propose that the FeO‐rich olivines currently observed in MET 00526 matrix are the products of three possible reaction mechanisms, (1) replacement of amorphous silicates, (2) precipitation from an SiO‐rich fluid, and (3) replacement of phyllosilicates; all these mechanisms take place in the presence of an iron‐rich fluid. The chemical evolution of the hydrothermal fluid could trigger the formation of either fayalite or phyllosilicates depending on the Si/Fe ratios. A low Si/Fe ratio is required to form FeO‐rich olivines, rather than phyllosilicates, which form at high Si/Fe ratio. Although MET 00526 records the effects of secondary alteration processes, its fine‐grained matrix still preserves some evidence of its pristine, solar nebular characteristics.

Pluto’s Sputnik Planitia: Composition of geological units from infrared spectroscopy

1F.Scipioni et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114303]
1SETI Institute, Mountain View, CA 94040, USA
Copyright Elsevier

We have compared spectroscopic data of Sputnik Planitia on Pluto, as acquired by New Horizons’ Linear Etalon Imaging Spectral Array (LEISA) instrument, to the geomorphology as mapped by White et al. (2017) using visible and panchromatic imaging acquired by the LOng-Range Reconnaissance Imager (LORRI) and the Multi-spectral Visible Imaging Camera (MVIC). We have focused on 13 of the geologic units identified by White et al. (2017), which include the plains and mountain units contained within the Sputnik basin. We divided the map of Sputnik Planitia into 15 provinces, each containing one or more geologic units, and we use LEISA to calculate the average spectra of the units inside the 15 provinces. Hapke-based modeling was then applied to the average spectra of the units to infer their surface composition, and to determine if the composition resulting from the modeling of LEISA spectra reflects the geomorphologic analyses of LORRI data, and if areas classified as being the same geologically, but which are geographically separated, share a similar composition. We investigated the spatial distribution of the most abundant ices on Pluto’s surface – CH4, N2, CO, H2O, and a non-ice component presumed to be a macromolecular carbon-rich material, termed a tholin, that imparts a positive spectral slope in the visible spectral region and a negative spectral slope longward of ~1.1 μm. Because the exact nature of the non-ice component is still debated and because the negative spectral slope of the available tholins in the near infrared does not perfectly match the Pluto data, for spectral modeling purposes we reference it generically as the negative spectral slope endmember (NSS endmember). We created maps of variations in the integrated band depth (from LEISA data) and areal mass fraction (from the modeling) of the components. The analysis of correlations between the occurrences of the endmembers in the geologic units led to the observation of an anomalous suppression of the strong CH4 absorption bands in units with compositions that are dominated by H2O ice and the NSS endmember. Exploring the mutual variation of the CH4 and N2 integrated band depths with the abundance of crystalline H2O and NSS endmember revealed that the NSS endmember is primarily responsible for the suppression of CH4 absorptions in mountainous units located along the western edge of Sputnik Planitia. Our spectroscopic analyses have provided additional insight into the geological processes that have shaped Sputnik Planitia. A general increase in volatile abundance from the north to the south of Sputnik Planitia is observed. Such an increase first observed and interpreted by Protopapa et al., 2017 and later confirmed by climate modeling (Bertrand et al., 2018) is expressed geomorphologically in the form of preferential deposition of N2 ice in the upland and mountainous regions bordering the plains of southern Sputnik Planitia. Relatively high amounts of pure CH4 are seen at the southern Tenzing Montes, which are a natural site for CH4 deposition owing to their great elevation and the lower insolation they are presently receiving. The NSS endmember correlates the existence of tholins within certain units, mostly those coating the low-latitude mountain ranges that are co-latitudinal with the tholin-covered Cthulhu Macula. The spectral analysis has also revealed compositional differences between the handful of occurrences of northern non-cellular plains and the surrounding cellular plains, all of which are located within the portion of Sputnik Planitia that is presently experiencing net sublimation of volatiles, and which do not therefore exhibit a surface layer of bright, freshly-deposited N2 ice. The compositional differences between the cellular and non-cellular plains here hint at the effectiveness of convection in entraining and trapping tholins within the body of the cellular plains, while preventing the spread of such tholins to abutting non-cellular plains.

The lifecycle of hollows on mercury: An evaluation of candidate volatile phases and a novel model of formation

Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114306]
1Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, United States of America
2Planetary Exploration Group, Johns Hopkins University Applied Physics Laboratory, United States of America
3Department of Astronomy and Planetary Sciences, Northern Arizona University, United States of America
Copyright Elsevier

On Mercury, high-reflectance, flat-floored depressions called hollows are observed nearly globally within low-reflectance material, one of Mercury’s major color units. Hollows are thought to be young, or even currently active, features that form via sublimation, or a “sublimation-like” process. The apparent abundance of sulfides within LRM combined with spectral detections of sulfides associated with hollows suggests that sulfides may be the phase responsible for hollow formation. Despite the association of sulfides with hollows, it is still not clear whether sulfides are the hollow-forming phase. To better understand which phase(s) might be responsible for hollow formation, we calculated sublimation rates for 57 candidate hollow-forming volatile phases from the surface of Mercury and as a function of depth beneath regolith lag deposits of various thicknesses. We found that stearic acid (C18H36O2), fullerenes (C60, C70), and elemental sulfur (S) have the appropriate thermophysical properties to explain hollow formation. Stearic acid and fullerenes are implausible hollow-forming phases because they are unlikely to have been delivered to or generated on Mercury in high enough volume to account for hollows. We suggest that S is most likely the phase responsible for hollow formation based on its abundance on Mercury and its thermophysical properties. We discuss the possibility that S is the phase responsible for hollow formation within the hollow-formation model framework proposed by Blewett et al. (2013). However, several potential limitations with that model lead us to suggest an alternative hollow-formation model: a subsurface heat source (most often impact-induced) generates thermal systems that drive sulfur-rich fumaroles in which S and other phases accumulate on and within the surface at night and sublimate during the day to create hollows. We call this hollow-formation model “Sublimation Cycling Around Fumarole Systems” (SCArFS). We suggest that thermal decomposition of sulfides within LRM is a main contributor to S and S-bearing gases within the proposed fumarole systems and that (re-)precipitation of sulfides may occur at the surface along hollow floors and rims.