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.

Microstructures of enstatite in fine-grained CAIs from CV3 chondrites: Implications for mechanisms and conditions of formation

1Shaofan Che,1Adrian J.Brearley
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.12.027]
1Department of Earth and Planetary Sciences, MSC03-2040, 1 University of New Mexico, Albuquerque, NM 87131-0001, USA
Copyright Elsevier

Enstatite is a ubiquitous phase in chondritic meteorites, interplanetary dust particles, and cometary samples. In equilibrium condensation models, enstatite is predicted to condense via a reaction between pre-condensed forsterite and gaseous SiO. However, previous studies have shown that some enstatites in chondrite matrices and AOAs do not have a genetic relationship with forsterite, arguing against formation by the predicted forsterite-gas reaction. Here we report the occurrence of enstatite in a unique, fine-grained, spinel-rich inclusion (FGI) Ef1014-01 in the Efremovka CV3 chondrite. Enstatite in this FGI is present as an outer layer on spinel-anorthite-diopside nodules and separates the FGI from an amoeboid olivine aggregate (AOA) -like material. Enstatite shows elevated CaO and Al2O3 contents (up to a few weight percent). Four FIB sections were extracted from this FGI to investigate the microstructures of enstatite and its relationship with other phases using TEM techniques. The TEM observations show that the enstatite is dominantly low-temperature clinoenstatite (LCLEN), which displays abundant twinning, and is sometimes associated with thin orthoenstatite (OREN) lamellae. Clinoenstatite grains commonly have a crystallographic orientation relationship with adjacent diopside, but do not exhibit any replacement relationship with forsterite in the AOA-like material surrounding the FGI. Investigations of several other fine-grained CAIs from the Efremovka and Leoville CV3 chondrites show that enstatite is more common in these inclusions than previously thought and typically forms discontinuous layers or islands on the diopside layers.

Based on SEM and TEM observations, we suggest that the LCLEN-OREN intergrowths in Ef1014-01 formed by transformation from a protoenstatite (PEN) precursor, which may be a product of direct condensation or reheating in the solar nebula. The crystallographic orientation relationship between enstatite and diopside suggests that epitaxial growth of enstatite occurred, lowering the activation energy for nucleation and facilitating direction condensation of enstatite from the gas phase, rather than by reaction of the gas with forsteritic olivine. The microstructures of enstatite are indicative of an extremely rapid cooling rate (∼104 K/h) that is within the range of chondrule cooling rates. Such a rapid cooling rate may imply that the cooling rates of FGIs are indeed much higher than other types of refractory inclusions. Alternatively, the rapid cooling rate may not reflect the primary cooling of the FGIs, but is the result of rapid cooling after a short-lived secondary reheating event in the solar nebula.

A fractionated gas with a lower Mg/Si ratio than the solar value is required to condense enstatite. Such a gas could be produced by isolation of pre-condensed forsterite or repeated evaporation-recondensation processes. The presence of both enstatite-bearing and enstatite-free CAIs in CV3 chondrites suggests that at least two gaseous reservoirs with different Mg/Si ratios were present in the CAI-forming regions.

A deuterium-poor water reservoir in the asteroid 4 Vesta and the inner Solar System

Geochimica et cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.01.004]
1School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA
2School of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK
3Istituto di Astrofisica e Planetologia Spaziali – INAF, 00111 Roma, Italy
Copyright Elsevier

Recent investigations of meteorites thought to originate from the asteroid 4 Vesta have suggested an early accretion of water on rocky bodies in the inner Solar System from a carbonaceous chondrite-like source. However, these studies have been based on the hydrogen isotope compositions (δD) of late-crystallizing apatite grains in eucrites that likely do not record the primary magmatic composition. We have determined the δD and H2O concentrations in some of the earliest-formed silicates (clinopyroxenes) from several eucrites with the goal of constraining the hydrogen isotope composition of their source reservoir on their parent body. The H2O concentrations in clinopyroxenes from eucrites Juvinas, Stannern and Tirhert range from 5 to 18 μg/g, with a weighted average δD of –263 ± 70 ‰. Their apatites and whitlockites exhibit a higher weighted average δD of –165 ± 73 ‰, possibly as a result of H2 degassing during or after phosphate crystallization. Thermal metamorphism of these eucrites has most probably resulted in the loss of H, and an increase in their original δD values. While the weighted average δD value for the eucrite clinopyroxenes reported here is inferred to reflect an upper limit for the isotopic composition of the silicate mantle reservoir on their parent asteroid 4 Vesta, the average δD value of Stannern clinopyroxenes is considered to be closest to the initial δD of the source mantle (i.e., –373 ±127 ‰), which is lighter than that of Earth’s depleted upper mantle and most carbonaceous chondrites. We suggest that at least some of the water in 4 Vesta (and possibly other rocky bodies in the inner Solar System) was derived from a relatively deuterium-poor reservoir in the protosolar nebula, which was incorporated into planetesimals formed early in Solar System history.

Collisions and compositional variability in chondrule-forming events

1Emmanuel Jacquet
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.12.025]
1Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), Muséum national d’Histoire naturelle, Sorbonne Université, CNRS; CP52, 57 rue Cuvier, 75005 Paris, France
Copyright Elsevier

Compound chondrules, i.e. chondrules fused together, make a powerful probe of the density and compositional diversity in chondrule-forming environments, but their abundance among the dominating porphyritic textures may have been drastically underestimated. I report herein microscopic observations and LA-ICP-MS analyses of lobate chondrules in the CO3 chondrites Miller Range 07193 and 07342. Lobes in a given chondrule show correlated volatile and moderately volatile element abundances but refractory element concentrations are essentially independent. This indicates that they formed by the collision of preexisting droplets whose refractory elements behaved in closed system, while their more volatile elements were buffered by the same gaseous medium. The presence of lobes would otherwise be difficult to explain, as surface tension should have rapidly imposed a spherical shape at the temperature peak. In fact, since most chondrules across chondrite groups are nonspherical, a majority are probably compounds variously relaxed toward sphericity. The lack of correlation of refractory elements between conjoined compound chondrule components is inconsistent with derivation of chondrules from the disruption of homogenized melt bodies as in impact scenarios and evokes rather the melting of independent mm-size nebular aggregates. Yet a “nebular” setting for chondrule formation would need to involve not only increased solid concentration, e.g. by settling to the midplane, but also a boost in relative velocities between droplets during chondrule-forming events to account for observed compound chondrule frequencies .

Lithium pollution of a white dwarf records the accretion of an extrasolar planetesimal

1B. C. Kaiser,1J. C. Clemens,2S. Blouin,3,4P. Dufour,1R. J. Hegedus,1J. S. Reding,3A. Bédard
Science 371, 168-172 Link to Article [DOI: 10.1126/science.abd1714]
1Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC, USA.
2Los Alamos National Laboratory, Los Alamos, NM, USA.
3Département de Physique, Université de Montréal, Montreal, QC, Canada.
4Institut de Recherche sur les Exoplanètes, Université de Montréal, Montreal, QC, Canada.
Reprinted with Permission from AAAS

Tidal disruption and subsequent accretion of planetesimals by white dwarfs can reveal the elemental abundances of rocky bodies in exoplanetary systems. Those abundances provide information on the composition of the nebula from which the systems formed, which is analogous to how meteorite abundances inform our understanding of the early Solar System. We report the detection of lithium, sodium, potassium, and calcium in the atmosphere of the white dwarf Gaia DR2 4353607450860305024, which we ascribe to the accretion of a planetesimal. Using model atmospheres, we determine abundance ratios of these elements, and, with the exception of lithium, they are consistent with meteoritic values in the Solar System. We compare the measured lithium abundance with measurements in old stars and with expectations from Big Bang nucleosynthesis.