¹L. M. Barge,¹E. Flores,²D. VanderVelde,¹J. M. Weber,³M. M. Baum,³A. Castonguay
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2020JE006423]
¹NASA Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, 91109 USA
²Department of Chemistry, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA, 91125 USA
³Department of Chemistry, Oak Crest Institute of Science, 128‐132 W. Chestnut Ave., Monrovia, CA, 91016 USA
Published by arrangement with John Wiley & Sons

Geological conditions play a significant role in prebiotic / abiotic organic chemistry, especially when reactive minerals are present. Previous studies of the prebiotic synthesis of amino acids and other products in mineral‐containing systems have shown that a diverse array of compounds can be produced, depending on the experimental conditions. However, these previous experiments have not simulated the effects of varying geochemical conditions, in which factors such as pH, iron redox state, or chemical concentrations may vary over time and space in a natural environment. In geochemical systems that contain overlapping gradients, many permutations of individual conditions could exist and affect the outcome of an organic reaction network. We investigated reactions of pyruvate and glyoxylate, two compounds that are central to the emergence of metabolism, in simulated geological gradients of redox, pH, and ammonia concentration. Our results show that the positioning of pyruvate/glyoxylate reactions in this environmental parameter space determines the organic product distribution that results. Therefore, the distribution pattern of amino acids and alpha‐hydroxy acids produced prebiotically in a system reflects the specific reaction conditions, and would be distinct at various locations in an environment depending on local geochemistry. This is significant for origin of life chemistry in which the composition and function of oligomers could be affected by the environmentally‐driven distribution of monomers available. Also, for astrobiology and planetary science where organic distribution patterns are sometimes considered as a possible biosignature, it is important to consider environmentally‐driven abiotic organic reactions that might produce similar effects.

¹Shaofan Che,¹Adrian J.Brearley
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.10.031]
¹Department of Earth and Planetary Sciences, MSC03-2040, 1 University of New Mexico, Albuquerque, NM 87131-0001, USA
Copyright Elsevier

Type C CAIs are a rare group of refractory inclusions in carbonaceous chondrites that are compositionally and isotopically distinct from the more commonly observed igneous CAIs (i.e., Type As and Type Bs). We have investigated a forsterite-bearing Type C CAI ALNH-04 from the Allende CV3 chondrite. This CAI has both textural and compositional similarities to some of the Type C CAIs previously reported; however, there are notable differences that imply that ALNH-04 may have formed from a different precursor from other Type C inclusions. Based on the bulk composition of ALNH-04 and the minor element contents of forsterite, we suggest that the forsterite grains were inherited from a Forsterite-bearing Type B CAI (FoB) precursor. The presence of augite on the periphery of ALNH-04 implies a re-melting event that probably occurred in a chondrule-forming region.

Another interesting feature of ALNH-04 is the secondary iron-alkali-halogen zoning sequence as manifested by varying proportions of nepheline, sodalite, fayalitic olivine, and sulfides in different regions of the CAI. Nepheline ± sodalite have replaced anorthite in the outer part of the inclusion, giving way to the presence of ubiquitous sodalite with minor nepheline, partially replacing anorthite at grain boundaries and fractures in the interior of the inclusion. Sulfides and Fe-bearing olivine form an iron-rich alteration zone. The textural relationships between nepheline and sodalite show no evidence of a direct replacement relationship between the two phases. Combined with the SEM observations, the microstructures are most consistent with a two-stage fluid alteration process: (1) nepheline replaced anorthite in the outer part of the CAI via a fluid with within the stability range of nepheline; (2) a later-stage fluid, with elevated that could preferentially stabilize sodalite, penetrated further into the CAI interior, replacing anorthite with sodalite. The lack of a nepheline-sodalite replacement relationship indicates that the conditions and fluid chemistry were suitable for nepheline and/or sodalite to be stable. Together with other Fe-rich secondary phases, fayalitic olivine may have recorded an increase in pH and of the fluid. These changes were probably induced by the extensive alteration of the outer part of the CAI by feldspathoids. The observed alteration microstructures are consistent with a coupled dissolution-precipitation alteration mechanism. The fluid alteration was also responsible for the formation of Na- and Ca-rich halos in the matrix surrounding the CAI.

We compared ALNH-04 with other CAIs and chondrules showing alkali-halogen-(iron) zoning sequences in Allende, and found that the observed zoning structures are consistent with the two-stage fluid-assisted metasomatic process mentioned above. The different distribution patterns of nepheline and sodalite in plagioclase-rich CAIs, chondrules, and melilite-rich CAIs may be explained by different chemical potential gradients in SiO2 in the fluid. Precipitation of nepheline and sodalite may require a higher SiO2 activity compared to grossular and dmisteinbergite (±secondary anorthite), which controlled the formation location of sodalite during the second fluid alteration event. Fluids with different compositions may be produced by fluid percolation along different directions and pathways, changing convection patterns, or release of water from a differentiated asteroidal interior.

^1,2Jörg Fritz,³Ansgar Greshake,⁴Mariana Klementova,⁵Richard Wirth,⁴Lukas Palatinus,⁶Reidar G. Trønnes,^3,7,8Vera Assis Fernandes,⁹Ute Böttger,¹⁰Ludovic Ferrière
American Mineralogist 105, 1704–1711 Link to Article [http://www.minsocam.org/msa/ammin/toc/2020/Abstracts/AM105P1704.pdf]
¹Zentrum für Rieskrater und Impaktforschung, Nördlingen, Vordere Gerbergasse 3, D-86720 Nördlingen,
Germany. ORCID 0000-0002-6333-4775
²Saalbau Weltraum Projekt, Liebigstraße 6, D-64646 Heppenheim, Germany
³Museum für Naturkunde Berlin, Invalidenstrasse 43, D-10115 Berlin, Germany. ORCID 0000-0001-6475-9751
⁴Institute of Physics of the Czech Academy of Science, v.v.i., Na Slovance 2, 182 21 Prague, Czech Republic. † ORCID 0000-0002-8987-8164
⁵Helmholtz-Zentrum Potsdam–Deutsches GeoForschungsZentrum, Sektion 3.5 Grenzflächen-Geochemie, Telegrafenberg, D-14473 Potsdam, Germany
⁶Natural History Museum and Centre for Earth Evolution and Dynamics (CEED), University of Oslo, N-0315 Oslo,
Norway. ORCID 0000-0002-4458-5624
⁷Department of Earth and Environmental Sciences, University of Manchester, Williamson Building, Oxford Road, M13 9PL Manchester, U.K.
ORCID 0000-0003-0848-9229
⁸Instituto Dom Luiz (IDL), Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016, Lisboa, Portugal
⁹Institut für Optische Sensorsysteme, Deutsches Zentrum für Luft und Raumfahrt Berlin, Rutherfordstrasse 2, D-12489 Berlin, Germany
¹⁰Natural History Museum, Burgring 7, A-1010 Vienna, Austria. ORCID 0000-0002-9082-6230
Copyright: The Mineralogical Society of America

We report on the occurrence of a new high-pressure Ca-Al-silicate in localized shock melt pockets found in the feldspatic lunar meteorite Oued Awlitis 001 and discuss the implications of our discovery.
The new mineral crystallized as tiny, micrometer-sized, acicular grains in shock melt pockets of roughly anorthitic bulk composition. Transmission electron microscopy based three-dimensional electron diffraction (3D ED) reveals that the CaAl4Si2O11 crystals are identical to the calcium aluminum silicate (CAS) phase first reported from static pressure experiments. The new mineral has a hexagonal structure, with a space group of P63/mmc and lattice parameters of a = 5.42(1) Å; c = 12.70(3) Å; V = 323(4) Å3;Z = 2. This is the first time 3D ED was applied to structure determination of an extraterrestrial mineral.
The International Mineralogical Association (IMA) has approved this naturally formed CAS phase as the new mineral “donwilhelmsite” [CaAl4Si2O11], honoring the U.S. lunar geologist Don E. Wilhelms.
On the Moon, donwilhelmsite can form from the primordial feldspathic crust during impact cratering events. In the feldspatic lunar meteorite Oued Awlitis 001, needles of donwilhelmsite crystallized in ~200 mm sized shock melt pockets of anorthositic-like chemical composition. These melt pockets quenched within milliseconds during declining shock pressures. Shock melt pockets in meteorites serve as natural crucibles mimicking the conditions expected in the Earth’s mantle. Donwilhelmsite forms in the Earth’s mantle during deep recycling of aluminous crustal materials, and is a key host for Al and Ca of subducted sediments in most of the transition zone and the uppermost lower mantle (460–700 km). Donwilhelmsite bridges the gap between kyanite and the Ca-component of clinopyroxene at low pressures and the Al-rich Ca-ferrite phase and Ca-perovskite at high-pressures. In ascending buoyant mantle plumes, at about 460 km depth, donwilhelmsite is expected to break down into minerals such as garnet, kyanite, and clinopyroxene. This process may trigger minor partial melting, releasing
a range of incompatible minor and trace elements and contributing to the enriched mantle (EM1 and EM2) components associated with subducted sedimentary lithologies.

^1,2Ryoichi Nakada,²Tomohiro Usui,³Masashi Ushioda,⁴Yoshio Takahashi
American Mineralogist 105, 1695–1703 Link to Article [http://www.minsocam.org/msa/ammin/toc/2020/Abstracts/AM105P1695.pdf]
¹Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Monobe 200, Nankoku, Kochi 783-8502, Japan
²Earth-Life Science Institute, Tokyo Institute of Technology, Meguro, Tokyo 152-8550, Japan
³Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan
⁴Department of Earth and Planetary Science, The University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-0033, Japan
Copyright: The Mineralogical Society of America

^1,2A.P.Jordan
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.114199]
¹Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH, USA
²Solar System Exploration Research Virtual Institute, NASA Ames Research Center, Moffett Field, CA, USA
Copyright Elsevier

Soil on the Moon is darkened by space weathering, a process generally assumed to be dominated by the solar wind and/or micrometeoroid impacts. Recent work, however, predicts that another process darkens the soil: large solar energetic particle events may cause dielectric breakdown (or “sparking”), melting, and vaporizing soil at a rate comparable to that of micrometeoroids. Unlike the solar wind and/or micrometeoroids, a combination of dielectric breakdown and micrometeoroid weathering can explain how the reflectance of the lunar maria varies with latitude at 750 and 1064 nm, and this combination provides a reasonable mechanism to explain how magnetic anomalies form prominent swirls in the maria. Consequently, space weathering in the lunar maria seems to be dominated by micrometeoroid impacts and dielectric breakdown.

¹Victoria E. Hamilton,²Christopher W. Haberle,^3,4Thomas G. Mayerhöfer
American Mineralogist 105, 1756–1760 Link to Article [http://www.minsocam.org/msa/ammin/toc/2020/Abstracts/AM105P1756.pdf]
¹Department of Space Studies, Southwest Research Institute, 1050 Walnut Street, no. 300, Boulder, Colorado 80302, U.S.A ²School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, U.S.A.
³Leibniz Institute of Photonic Technology (IPHT), Albert-Einstein-Str. 9, D-07745 Jena, Germany 4
⁴Institute of Physical Chemistry and Abbe Center of Photonics, Friedrich Schiller University, Helmholtzweg 4, D-07743 Jena, Germany
Copyright: The Mineralogical Society of America

The thermal infrared (TIR, or vibrational) emission spectra of a suite of synthetic Mg-Fe olivines exhibit notable differences from their natural igneous counterparts in terms of their band shapes, relative depths, and reduced shifts in some band positions with Mg-Fe solid solution. Comparable reflectance spectra acquired from olivine-dominated matrices and fusion crusts of some carbonaceous chondrite meteorites exhibit similar deviations. Here we show that these unusual spectral characteristics are consistent with crystallite sizes much smaller than the resolution limit of infrared light. We hypothesize that these small crystallites denote abbreviated crystal growth and also may be linked to the size of nucleation sites. Other silicates and non-silicates, such as carbonates, exhibit similar spectral behaviors. Because the spectra of mineral separates are commonly used in the modeling and analysis of comparable bulk rock, meteorite, and remote sensing data, understanding these spectral variations is important to correctly identifying the
minerals and interpreting the origin and/or secondary processing histories of natural materials.

¹Guillaume Siron,¹Kohei Fukuda,²Makoto Kimura,¹Noriko T. Kita
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.10.025]
¹WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, USA
²National Institute of Polar Research, Meteorite Research Center, Midoricho 10-3, Tachikawa, Tokyo 190-8518, Japan
Copyright Elsevier

26Al-26Mg ages were determined for 14 anorthite-bearing chondrules from five different unequilibrated ordinary chondrites (UOCs) with low petrologic subtypes (3.00-3.05). In addition, oxygen three isotopes of these chondrules were also measured. The selected chondrules are highly depleted in alkali elements, and anorthite is the only mesostasis phase, though they show a range of mafic mineral compositions (Mg# 76-97 mole%) that are representative of chondrules in UOCs. The mean Δ17O values in these chondrules range from –0.44 ± 0.23‰ to 0.49 ± 0.15‰, in good agreement with previous studies of plagioclase-bearing chondrules from UOCs. Anorthite in all chondrules exhibit resolvable excess 26Mg (> 1.0 ± 0.4‰). Their inferred (27Al/26Al)0 range from (6.3 ± 0.7)×10–6 to (8.9 ± 0.3)×10–6 corresponding to a timescale for chondrule formation of 1.8 ± 0.04 Ma to 2.16 ± 0.12/0.11 Ma after CAIs using a canonical (27Al/26Al)0 value of 5.25×10–5. The ages from six chondrules in LL chondrites are restricted to between 1.8 Ma and 1.9 Ma, whereas eight chondrules in L chondrites show ages from 1.8 Ma to 2.2 Ma, including three chondrules at ∼2.0 Ma and two chondrules at ∼2.15 Ma.

The inferred chondrule formation ages from this study are at the peak of those previously determined for UOC chondrules, though with much shorter durations. This is potentially due to the time difference between formation of anorthite-bearing chondrules and typical UOC chondrules with alkali-rich compositions. Alternatively, younger chondrules ages in previous studies could have been the result of disturbance to the Al-Mg system in glassy mesostasis even at the low degree of thermal metamorphism in the parent bodies. Nevertheless, the high precision ages from this study (with uncertainties from 0.04 Ma to 0.15 Ma) indicate that there was potentially more than one chondrule forming event represented in the studied population. Considering data from LL chondrites only, the restricted duration (≤0.1 Ma) of chondrule formation ages suggests an origin in high density environments that subsequently lead to parent body formation. However, the unusually low alkali contents of the studied chondrules compared to common alkali-rich chondrules could also represent earlier chondrule formation events under relatively lower dust densities in the disk. Major chondrule forming events for UOCs might have postdated or concurrent with the younger anorthite-bearing chondrule formation at 2.15 Ma after CAIs, which are very close to the timing of accretion of ordinary chondrite parent bodies that are expected from thermal evolution of ordinary chondrite parent bodies.

¹Anna Milillo et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.114179]
¹INAF/IAPS
Copyright Elsevier

The Na exosphere of Mercury is characterized by the variability of the emission lines intensity and of its distribution in time scales from less than one hour to seasonal variations. While the faster variations, accounting for about 10–20% of fluctuations are probably linked to the planetary response to solar wind and Interplanetary Magnetic Field variability, the seasonal variations (up to about 80%) should be explained by complex mechanisms involving different surface release processes, loss, source and migrations of the exospheric Na atoms. Eventually, a Na annual cycle can be identified. In the past, ground-based observations and equatorial density from MESSENGER data have been analyzed. In this study, for a more extensive investigation of the exospheric Na features, we have studied the local time and latitudinal distributions of the exospheric Na column density as a function of the True Anomaly Angle (TAA) of Mercury by means of the extended dataset of images, collected from 2009 to 2013, by the THEMIS solar telescope. Our results show that the THEMIS images, in agreement with previous results, registered a strong general increase in sodium abundance at aphelion and a dawn ward emission predominance with respect to dusk ward and subsolar region between 90° and 150° TAA. This behavior can be explained by desorption of a sodium surface reservoir consisting of sodium that is pushed anti-sunward and condenses preferentially in the coldest regions. Our analyses shows a predominance of subsolar line-of-sight column density along the rest of Mercury’s orbit. An unexpected relationship between Northward or Southward peak emission and both TAA and local time is also shown by our analysis. This result seems to contradict previous results obtained from different data sets and it is not easily explained, thus it requires further investigations.

¹Yohei Igami,^2,3Akira Tsuchiyama,⁴Tomoya Yamazaki,⁵Megumi Matsumoto,⁴Yuki Kimura
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.10.023]
¹Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
²Research Organization of Science and Technology, Ritsumeikan University, Kusatsu 525-8577, Japan
³Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China
⁴Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan
⁵Department of Earth and Planetary Materials Science, Tohoku University, Sendai 980-8578, Japan
Copyright Elsevier

Amorphous silicates, abundant in primitive carbonaceous chondrites, are among the most primitive materials from the early Solar System. They show evidence of some aqueous alteration in the meteorite parent bodies, but it is not clear how this highly reactive material changed at an early stage after contact with water. Herein, we report in-situ experiments on the aqueous alteration of amorphous silicate nanoparticles (typically 70 nm in diameter); we used two different compositions that are similar to forsterite (MgO/SiO2 = 2.02) and enstatite (MgO/SiO2 = 1.15) in the simple MgO–SiO2 system to understand basic reaction principles at the onset of the aqueous alteration. The experiments were performed in pure water at room temperature using X-ray diffraction (XRD), transmission electron microscopy (TEM), and pH measurements. The in-situ TEM images of the nanoparticles—in particular those with the forsterite composition—gradually became difficult to recognize in water. The pH value of the water also increased with time, suggesting that preferential Mg2+ dissolution occurred from the amorphous silicates right after mixing with water. The in-situ XRD patterns showed that magnesium silicate hydrate (M-S-H), which is a poorly crystalline phase like a phyllosilicate, newly appeared. The M-S-H seems to have been formed via a dissolution–precipitation process. Its formation rate from amorphous silicates was considerably higher than from crystalline silicates, because amorphous silicates are highly metastable and have high solubility in water. M-S-H formation from the forsterite composition, which has a highly unstable amorphous structure, is ten times faster than from the enstatite composition. The M-S-Hs show string-like or tiny fragmental textures in the final dried products that are very similar to those observed in the matrices of some primitive carbonaceous chondrites. M-S-H would have been the initial product formed in the aqueous alteration of amorphous silicates in the meteorites; thus, it is an important tracer of early aqueous activity at low temperatures in the early Solar System. By comparing the in-situ observations with those obtained after drying the experimental samples, we found two types of M-S-Hs: epigenetic M-S-Hs—which have a slightly Si-rich composition—formed during drying, and syngenetic M-S-Hs formed by in-situ alteration. Carbonaceous chondrites may also contain these two types of hydrous silicates, and this should be investigated to understand the conditions for aqueous alteration in the early Solar System in more detail. The present study clearly showed the importance of Mg/Si ratio in the precursor materials, although the actual chondrites are in more complicated multi-component system. Future experiments based on the present results can extend the investigation to the system containing Fe, S, and other components as in carbonaceous chondrites.

¹Marine Paquet,¹James M.D.Day,²Arya Udry,¹Ruan Hattingh,¹Ben Kumler,²Rachel R.Rahib,³Kimberly T.Tait,⁴Clive R.Neal
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.10.024]
¹Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0244, USA
²Department of Geoscience, University of Nevada, Las Vegas, 4505 S. Maryland Parkway, Las Vegas, NV 89154, USA
³Department of Natural History, Royal Ontario Museum, 100 Queens Park, Toronto, ON M5S 2C6, Canada
⁴Department of Civil Engineering and Geological Sciences, University of Notre Dame, Notre Dame, IN 46556, USA
Copyright Elsevier

Shergottite meteorites are ultramafic to mafic igneous rocks derived from partial melting of distinct regions of the martian mantle. As such, they trace magmatic processes, including fractional crystallization and mixing processes in Mars. New chalcophile (Cu, Se, Zn, Pb), siderophile (Ni, Co, W), and highly siderophile element (HSE: Au, Re, Pd, Rh, Pt, Ru, Ir, Os) abundance data are reported for sulfide assemblages in a suite of thirteen incompatible trace element depleted, intermediate and enriched shergottites, along with new whole-rock HSE abundance and 187Os/188Os data for seven shergottites. Sulfide grains in depleted and intermediate shergottites typically have the highest absolute abundances of the HSE, with broadly flat CI-chondrite normalized patterns. Enriched shergottite sulfide grains typically have highly variable Au, elevated Pd and Rh and are relatively depleted in Zn, Ir and Os. The new HSE whole-rock data for enriched (Northwest Africa [NWA] 7397, NWA 7755, NWA 11043), and intermediate shergottites (NWA 10961, NWA 11065, NWA 12241, and NWA 12536) are generally consistent with existing 187Os/188Os and HSE abundance data for these geochemical groupings. Enriched shergottites with > 1 ppb Os have measured 187Os/188Os ranging between 0.1296 and 0.1471, with variable Pd and Pt contents. Intermediate shergottites with > 1 ppb Os have chondrite-relative proportions of the HSE at ∼ 0.01 to 0.001 × CI chondrites and 187Os/188Os from 0.1284 and 0.1295. Sulfides are the major host of the HSE, and they control the behavior of the HSE during petrogenetic processes in shergottite magmas, enabling the determination of HSE compatibility for martian magmatism in the order: Os > Ir ≥ Ru ≥≥ Rh ≥ Pd ≥ Re ≥ Pt ≥ Au. Fractionation models of removal of an olivine-dominated cumulate recreate HSE patterns for the whole-rock shergottites. Enriched shergottites are best reproduced by 25 to 30% of fractionation from a degassed parent melt (250 ± 50 ppm of S), whereas depleted and intermediate shergottites can be explained by slightly lower fractionation (10 to 15%) from higher S content parent melts (350 ± 100 ppm of S). Sulfur contents in the melt ∼ 50% higher than these estimates yield earlier S-saturation during fractional crystallization, leading to an abrupt decrease of the more compatible HSE (Ru, Ir, Os), which is not observed. These results indicate that the martian mantle and partial melts from it, are probably not anomalously enriched in S, and instead are similar to slightly higher than those of the terrestrial mantle and its partial melts.