^1,2Elishevah M.M.E. van Kooten,¹Edith Kubik,¹Julien Siebert,³Benjamin D.Heredia,³Tonny B.Thomsen,¹Frédéric Moynier
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.01.008]
¹Université de Paris, Institut de Physique du Globe de Paris, CNRS UMR 7154, 1 rue Jussieu, 75238 Paris, France
²Center for Star and Planet Formation, Globe Institute, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen, Denmark
³Geological Survey of Denmark, GEUS, Øster Voldgade 10, 1350 Copenhagen, Denmark
Copyright Elsevier

FeNi metals represent an important fraction of chondritic components that remains relatively unexplored within most carbonaceous chondrite groups. The compositions of these metals can place constraints on the nature of their precursor materials as well as the physicochemical conditions of chondrule formation. In this study, we have analyzed the major, minor and trace element compositions of metal grains from relatively unaltered carbonaceous chondrites NWA 801 (CR), Leoville (CV3.1), Paris (CM2.9), Maribo (CM2.8) and Bells (CM-an). We observe a predominant and constant sub-solar Co/Ni ratio of CR, CM and CM-an metal grains. In Ni versus Co space, the metal grains fall below modelled curves for equilibrium condensation of metals from a solar gas. From Ni versus Cr plots, we infer that Paris (and possibly Leoville) metal grains could have maintained a primary condensation signature, although for most grains, condensation must have occurred under disequilibrium conditions. CR and isolated CM-an metals mostly fall outside of the predicted condensation fields. Based on metal-silicate partition coefficients of Ni and Co that vary with pressure, we interpret their Co/Ni signatures as having a planetary origin, with presumable extraction by impact jetting. Considering that almost all CM and CR metal grains have the same Co/Ni ratio, we cannot rule out a planetary origin for CM metal grains. We relate the highly siderophile element (HSE) patterns of carbonaceous chondrite metal to mixing and subsequent equilibration of refractory metal nuggets (RMN), FeNi alloys and silicate chondrule precursors. As with the Co/Ni ratios, the HSE patterns of CM, CM-an, CR and CV metal grains are nearly identical, suggesting that the abundance and nature of the metal precursor materials were similar for carbonaceous chondrites. The overall volatility patterns of CV, CM and CR chondrites, suggest that the latter form under more oxidizing conditions than CV chondrites. The volatility patterns of Paris metal grains overlap with CV and CR chondrule metals, implying variable P-T-fO2 conditions during CM chondrule formation. Finally, we comment on the origin of metal grains in various petrological settings. Chondrule rim and isolated metal grains are likely derived and expelled from the equilibrated core metal and were subsequently altered to include and re-equilibrate with materials from the disk. Trace element analyses of the anomalous CM chondrite Bells metal grains show potential relationships with CM chondrite and CH chondrite metal for the chondrule cores and isolated grains, respectively. Small metal grains from CM chondrite Maribo, which are located in the chondrite matrix, potentially have distinct volatility patterns from CR and Paris isolated grains, hinting at a distinct origin for small metal grains and large chondrule-derived metal. Future work on carbonaceous chondrite metal should include an extensive dataset of metal-silicate equilibration calculations on individual chondrules and an investigation of small (micron scale) versus large isolated metal grains.

¹F. Zambon,¹C. Carli,²J. Wright,²D. A. Rothery,¹F. Altieri,^3,4M. Massironi, F. Capaccioni,⁴G. Cremonese
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2021JE006918]
¹INAF-Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere, 100 I-00 133 Rome
²The Open University, Walton Hall, Milton Keynes, MK7 6AA UK
³University of Padua, Department of Geoscience, Via Gradenigo 6, I-35 131 Padua Italy
⁴INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio, 5, 35 122 Padua Italy
Published by arrangement with John Wiley & Sons

MESSENGER mission data allowed the entire surface of Mercury to be mapped at various spatial scales, from both geological and compositional stand points. Here, we present a spectral analysis of the H05-Hokusai quadrangle, using data acquired by the Mercury Dual Imaging System-Wide-Angle Camera. We defined a suitable set of parameters, such as reflectance and spectral slopes, to study the spectral variation though the definition of spectral units. The determination of spectral units permits to infer the physical and compositional properties of a surface by processing several parameters simultaneously, instead of the more traditional approach of interpreting each single parameter separately. We identified 11 spectral units within H05, 6 large scale and 5 localized units. The large scale units include the northern smooth plains of Borealis Planitia. South-western H05 is characterized by two widespread spectral units, partially overlapping intercrater plains and intermediate plains. Furthermore, we found very localized spectral units corresponding to the low-reflectance blue material of Rachmaninoff basin and the high-reflectance red material of Nathair Facula. We investigated the link between spectral units and compositional maps obtained by GRS and XRS, to associate compositional information to the spectral units. We found some spectral units are correlated with Mg and Al variations displayed in the elemental maps. This implies that spectral variations associated to these units are mainly linked with composition rather than terrain maturity and/or grain size effects.

¹John O.Edgar,²Katie Gilmour,²Maggie L.White,¹Geoffrey D.Abbott,¹Jon Telling
Earth and Planetary Science Letters 579, 117361 Link to Article [https://doi.org/10.1016/j.epsl.2021.117361]
¹School of Natural and Environmental Sciences, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK
²School of Engineering, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK
Copyright Elsevier

The surface of Mars is a dynamic, cold environment where aeolian abrasion leads to the fracturing of silicate minerals which can produce oxidants upon exposure to water. Here we report results of a series of laboratory experiments where the abrasion of sand sized (125 – 300 μm) quartz, labradorite, forsterite and opal were conducted under a simulated Martian atmosphere at a range of temperatures common to Mars’ surface (193 to 273 K). Our results suggest that abrasion rates are controlled by temperature; an observation that may have potential for providing insight into Martian paleo-temperatures. On the addition of water, detectable H2O2 was generated in all abraded experiments with crystalline quartz, labradorite and forsterite, but not amorphous opal – supporting previous inferences that mineral crystal structure plays a role in oxidant production. Dissolved Fe concentrations also indicated a strong additional control on net H2O2 production by Fenton reactions. Detectable H2 was similarly measured in abraded experiments with crystalline minerals and not for amorphous opal. Labradorite and forsterite generated minimal H2 and only in more abraded samples, likely due to the reaction of Si• with water. In quartz experiments H2 was only present in samples where a black magnetic trace mineral was also present, and where H2O2 concentrations had been reduced to close to detection. In the quartz samples we infer a mechanism of H2 generation via the previously proposed model of spinel-surface-promoted-electron transfer to water. The presence of H2O2 may exert an additional control on net H2 production rates either directly (via reaction of H2 with OH• and H2O2) or indirectly (by the oxidation of H2 generating sites on mineral surfaces). Overall, our data supports previous inferences that aeolian abrasion can produce additional oxidants within the Martian regolith that can increase the degradation of organic molecules. We further suggest that the apparent control of H2O2 concentrations on net H2 generation in our experiments may help explain some previous apparently contradictory evidence for mineral-water H2 generation at low temperatures.

^1,2M.J.Loeffler,³B.S.Prince
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.114881]
¹Department of Astronomy and Planetary Science, Northern Arizona University, Flagstaff, AZ 86011, United States of America
²Center for Materials Interfaces in Research and Applications, Northern Arizona University, Flagstaff, AZ 86011, United States of America
³Department of Applied Physics and Materials Science, Northern Arizona University, Flagstaff, AZ 86011, United States of America
Copyright Elsevier

In an effort to better understand the role dark material plays in the reflectance spectrum of carbonaceous asteroids, we performed laboratory studies focusing on quantifying how the addition of relevant dark material (graphite, magnetite and troilite) can alter the ultraviolet-visible and near-infrared spectrum of a neutral silicate mineral. We find that addition of graphite, magnetite and troilite all darken the reflectance spectrum of our forsterite samples and cause the spectral slope to decrease (become blue). These spectral changes can be caused by both nm- and μm-sized grains. In the ultraviolet-visible region, we find that graphite is most efficient at altering the spectral slope, while in the near-infrared, magnetite is the most efficient. At all wavelengths studied, graphite is the most efficient at darkening our sample spectrum. However, the observation that troilite also alters the slope and albedo of our samples suggests that the spectral changes caused by magnetite and graphite may not be unique. In addition, we find that the spectral slopes in our mixtures compare generally well to what has been observed on Bennu suggesting that a significant portion of fine-grained dark material, including sulfides, present in the regolith can cause the observed negative (blue) slope found on B-type asteroids.

¹Addi Bischoff,¹Jakob Storz,^2,3Jean-Alix Barrat,⁴Dieter Heinlein,^5,6A. J. Timothy Jull,^7,8Silke Merchel,⁹Andreas Pack,⁷Georg Rugel
Meteorotics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13779]
¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm Str. 10, Münster, D-48149 Germany
²CNRS, IRD, Ifremer, LEMAR, University of Brest, Plouzané, F-29280 France
³Institut Universitaire de France, Paris, 75005 France
⁴German Fireball Network, Lilienstraße 3, Augsburg, D-86156 Germany
⁵University of Arizona AMS Laboratory, 1118 East Fourth St, Tucson, Arizona, 85721 USA
⁶Isotope Climatology and Environmental Research Centre (ICER), Institute for Nuclear Research, Hungarian Academy of Sciences, Bem ter 18/c, Debrecen, 4026 Hungary
⁷Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstr. 400, Dresden, D-01328 Germany
⁸Faculty of Physics, Isotope Physics, VERA Laboratory, University of Vienna, Währinger Str. 17, Vienna, 1090 Austria
⁹Geowissenschaftliches Zentrum, Universität Göttingen, Goldschmidtstr. 1, Göttingen, D-37077 Germany
Published by Arrangement with John Wiley & Sons

In the last 7 years, three meteorites (Blaubeuren, Cloppenburg, and Machtenstein) found in Germany were identified as chondrites. Two of these rocks had been recovered from the impact sites decades ago but not considered to be meteorites. The aim of this study is to fully characterize these three meteorites. Based on the compositional data on the silicates, namely olivine and low-Ca pyroxene, these meteorites fit nicely within the H-group ordinary chondrites. The brecciated texture of Blaubeuren and Cloppenburg (both H4-5) is perfectly visible, whereas that of Machtenstein, officially classified as an H5 chondrite, is less obvious but was detected and described in this study. Considering chondrites in general, brecciated rocks are very common rather than an exception. The bulk rock degree of shock is S2 for Blaubeuren and Machtenstein and S3 for Cloppenburg. All samples show significant features of weathering. They have lost their original fusion crust and more than half (W3) or about half (W2-3) of their original metal abundances. The oxygen isotope compositions of the three chondrites are consistent with those of other H chondrites; however, the Cloppenburg values are heavily disturbed and influenced by terrestrial weathering. This is supported by the occurrence of the very rare hydrated iron phosphate mineral vivianite (Fe2+Fe2+2[PO4]2·8H2O), which indicates that the chondrite was weathered in a very wet environment. The terrestrial ages of Blaubeuren (~9.2 ka), Cloppenburg (~5.4 ka), and Machtenstein (~1.8 ka) show that these chondrites are very similar in their degree of alteration and terrestrial age compared to meteorite finds from relatively wet terrestrial environments. They still contain abundant metal, although, as noted, the oxygen isotope data indicate substantial weathering of Cloppenburg. The bulk compositions of the three meteorites are typical for H chondrites, although terrestrial alteration has slightly modified the concentrations, leading in general to a loss of Fe, Co, and Ni due to preferential alteration of metals and sulfides. As exceptions, Co and Ni concentrations in Machtenstein, which has the shortest terrestrial age, are typical for H chondrites. The chemical data show no enrichments in Ba and Sr, as is often observed in different meteorite groups of desert finds.

¹Anthony Lagain,²Mikhail Kreslavsky,^3,4David Baratoux,⁵Yebo Liu,¹Hadrien Devillepoix,¹Philip Bland,^6,7Gretchen K.Benedix,⁵Luc S.Doucet,⁸Konstantinos Servish
Earth and Planetary Science Letters 579, 117362 Link to Article [https://doi.org/10.1016/j.epsl.2021.117362]
¹Space Science and Technology Centre, School of Earth and Planetary Sciences, Curtin University, Kent St, Bentley, 6102, WA, Australia
²Earth and Planetary Sciences, University of California – Santa Cruz, CA, USA
³Géosciences Environnement Toulouse, University of Toulouse, 14, Avenue Edouard Belin, Toulouse, 31400, France
⁴University Félix Houphouët-Boigny, UFR des Scinces de la Terre et des Ressoures Minières, Cocody, Abidjan, Côte d’Ivoire
⁵Earth Dynamics Research Group, The Institute for Geoscience Research (TIGeR), Department of Earth and Planetary Sciences, Curtin University, Kent St, Bentley, 6102, WA, Australia
⁶Planetary Sciences Institute, Tucson, AZ, USA
⁷Department of Earth and Planetary Sciences, Western Australian Museum, WA, Australia
⁸CSIRO – Pawsey Supercomputing Centre, WA, Australia
Copyright Elsevier

The impact flux over the last 3 Ga in the inner Solar System is commonly assumed to be constant through time due to insufficient data to warrant a different choice for this range of time. However, asteroid break-up events in the main belt may have been responsible for cratering spikes over the last ∼2 Ga on the Earth-Moon system. Due to its proximity with the main asteroid belt, i.e., the main impactors reservoir, Mars is at the outpost of these events with respect to the other inner planets. We investigate here, from automatic crater identification, the possible variations of the size frequency distributions of impactors from the record of small craters of 521 impact craters larger than 20 km in diameter. We show that 49 craters (out of the 521) correspond to the complete crater population of this size formed over the last 600 Ma. Our results on Mars show that the flux of both small (> 5 m) and large asteroids (> 1 km) are coupled, does not vary between each other over the last 600 Ma. Existing data sets for large craters on the Earth and the Moon are analyzed and compared to our results on Mars. On Earth, we infer the formation location of a set of impact craters thanks to plate tectonic reconstruction and show that a cluster of craters formed during the Ordovician period, about 470 Ma ago, appears to be a preservation bias. On the Moon, the late increase seen in the crater age signal can be due to the uncertain calibration method used to date those impacts (i.e. rock abundance in lunar impact ejecta), and other calibrations are consistent with a constant crater production rate. We conclude to a coupling of the crater production rate between kilometer-size craters (∼100 m asteroids) and down to ∼100 m in diameter (∼5 m asteroids) in the inner Solar System. This is consistent with the traditional model for delivering asteroids to planet-crossing obits: the Yarkovsky effect slowly pushes the large debris from asteroid break-ups towards orbital resonances while smaller debris are grinded through collisional cascades. This suggests that the long-term impact flux of asteroids > 5 m is most likely constant over the last 600 Ma, and that the influence of past asteroid break-ups in the cratering rate for D > 100 m is limited or inexistent.

^1,2Bastian Baecker,^1,2,3Ulrich Ott,²Mario Trieloff,⁴Cécile Engrand,⁵Jean Duprat
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.114884]
¹Max-Planck Institut für Chemie, Hahn-Meitner-Weg 1, 55128 Mainz, Germany
²Klaus-Tschira-Labor für Kosmochemie, Institut für Geowissenschaften, Universität Heidelberg, Im Neuenheimer Feld 234-236, 69120 Heidelberg, Germany
³MTA Atomki, Bem tér 18/C, 4026 Debrecen, Hungary
⁴IJCLab, CNRS-Paris-Saclay, Orsay, France
⁵IMPMC, CNRS-MNHN, Paris, France
Copyright Elsevier

We have performed a comprehensive noble gas study, including the isotopes of krypton and xenon, on a set of micrometeorites (MMs) collected from surface snow at Dome C (DC) on the Antarctic plateau. He and Ne are generally dominated by a solar component, with lower 4He concentrations and 4He/20Ne ratios in crystalline (Xtal) compared to fine-grained carbonaceous (FgC) MMs. Concentrations of (surface-correlated) solar wind (SW) He and Ne in FgC MMs are at the high end of what has been seen in earlier work, whereas the abundances of (volume-correlated) Kr and Xe are similar to what has been found in previous studies of MMs. In most samples, isotopic ratios for Kr and Xe are in the usual range of Q-Kr and single bondXe (the Q component is the dominating component in primitive macroscopic meteorites) and air. When quantifiable, cosmic ray exposure (CRE) ages based on cosmogenic 21Ne and 3He, in combination with the Poynting-Robertson effect, are broadly consistent with an origin of the MMs from the asteroid belt. An exception is an Xtal MM, which exhibits a cosmogenic 21Ne concentration in agreement with an origin from beyond Saturn, consistent with a possible cometary origin. In addition, data for trapped noble gases in three (out of ten analyzed) DC MMs provide hints that these may be related to a cometary source. One sample, a fragment of a FgC MM, is of particular interest. This fragment exhibits a Xe composition, although with large analytical uncertainties, deficient in the heavy isotopes 134Xe and 136Xe. This is similar to the Xe isotopic pattern, probably related to cometary ice, measured by Rosetta in the coma of comet 67P/Churyumov-Gerasimenko. The same MM also has an unusually high 36Ar/38Ar ratio, consistent with Rosetta’s Ar measurement (in this case the latter having a large uncertainty). The other hints are for two MMs, of crystalline (Xtal) type, that show Ne similar to that found in laboratory analysis of refractory grains captured from comet 81P/Wild 2 by the Stardust mission. Additionally, a FgC/Xtal MM may contain excess 3He, similar to what has been seen in some cluster interplanetary dust particles (cluster IDPs).

¹Rhianna D.Moore,¹Anna Szynkiewicz
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.114880]
¹Dept. of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN, United States of America
Copyright Elsevier

Acidic hydrothermal and fumarolic surface deposits within the Columbia Hills in Gusev crater on Mars were found to have elevated concentrations of Fe-Mg-Ca-sulfate minerals. However, this is inconsistent with analogous terrestrial hydrothermal settings that are usually enriched in elemental sulfur (S). Consequently, this raises questions about the origin and hydrothermal history of the Gusev sediments. To address this discrepancy, we analyzed quantities and S isotope compositions of S-bearing minerals in hydrothermal sediment samples from acidic hot springs, mud pots, and fumaroles with elevated H2S emissions in Iceland and the United States (e.g., Valles Caldera, Lassen, and Yellowstone). Our results indicate that the typical concentrations (e.g., inter-quartile range) of elemental S and sulfide minerals (0.3 to 10.5 wt% S, but as high as ~75 wt% S; and 0.1 to 1.7 wt% S, but as high as ~10 wt% S) are significantly higher compared to sulfate (0.1 to 1.1 wt% S, but as high as ~4.5 wt% S) in the surface hydrothermal deposits. In most cases, the concentrations of elemental S, sulfides, and sulfates in the sediments decreased with increasing hydrological connectivity and in wetter climates. Similar δ34S values between sulfate (−0.1 to +1.4‰) and elemental S (−0.4 to +1.6‰) compared to lower δ34S of sulfide (−2.4 to +0.4‰) suggest that more sulfate is likely derived from the subsequent oxidation of elemental S than sulfide. Conversely, minor amounts of sulfate are formed via direct oxidation of H2S which had higher δ34S values (+1.1 to +5.9‰). Our laboratory experiments carried over a wide range of temperatures (25, 65, and 85 °C) and low pH (~2) indicate that elemental S and pyrite undergo subsequent oxidation to sulfate via both ferric iron (Fe3+) and O2. While the amount of sulfate increased with increasing temperature in the presence of both Fe3+ and O2, Fe3+ appears to be a more efficient oxidizer than O2. For example, pyrite oxidation by only Fe3+ resulted in ~1.5× more sulfate (~80 to 180 mg/L SO42−) than by only O2 (~40 to 140 mg/L SO42−). In contrast, considerably less sulfate was formed during the oxidation of elemental S, although in the presence of O2 ~ 10× more sulfate (~0.1 to 45 mg/L SO42−) was formed than when Fe3+ was present (~0.3 to 7.5 mg/L SO42−).

Despite the prevalence of sulfate minerals rather than elemental S and sulfides in the hydrothermal Gusev deposits on Mars, the total S concentrations measured by the Spirit rover (2.9 to 9.3 wt% S) are highly comparable to the total S in hydrothermal sediments formed in colder and moderately wet climates such as coastal Iceland (1.8 to 10.7 wt% S). This contrasts with sediments formed in the high-altitude and drier climate of Valles Caldera (9.9 to 37.6 wt% S), or the wetter climates of Yellowstone (4.1 to 17.3 wt% S) and Lassen (0.5 to 3.5 wt% S). Because water is needed to further oxidize the hydrothermal elemental S and sulfide to sulfate, we infer that the aqueous conditions must have persisted in Gusev crater for a period of time after the main hydrothermal activity ceased. Later, under low water-to-rock conditions with little (or no) H2S emission, complete oxidation of the Gusev hydrothermal deposits likely took place and led to the formation of the sulfate minerals that were identified by the Spirit rover.

¹E. Dobrică,²J. A. Nuth,³A. J. Brearley
Meteoritics&Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13765]
¹Hawai’i Institute of Geophysics and Planetology, School of Ocean, Earth Science, and Technology, University of Hawai’i at Mānoa, Honolulu, Hawaii, 96822 USA
²Solar System Exploration Division, Code 690, NASA Goddard Space Flight Center, Greenbelt, Maryland, 20771 USA
³Department of Earth and Planetary Sciences, MSC03-2040, 1 University of New Mexico, Albuquerque, New Mexico, 87131–0001 USA
Published by arrangemengt with Jophn Wiley & Sons

In order to understand the effects of the earliest fluid-assisted hydration processes on asteroids, we performed one hydrothermal experiment using three different reactants (FeO-rich amorphous silicates, iron metal powder, and water) at conditions informed by our current state of knowledge of asteroidal alteration. This experiment provides, for the first time, clear evidence that the growth of fayalite can occur during hydrothermal alteration, as described previously in meteorites. These newly formed fayalite crystals are elongated and porous, similar to the ones described in CV3, CK, and ordinary chondrites. The results show that (1) fayalite could form even if chemical equilibrium was not reached in the experiment, at a water to rock mass ratio (0.4 W/R at the beginning of the experiment) higher than the values calculated to be thermodynamically viable at equilibrium (W/R > 0.2); (2) the composition and the texture of the reactants changed during the hydrothermal alteration process, suggesting that the reactants, especially the amorphous silicates, underwent dissolution and reprecipitation; (3) fayalite can form at low temperature (220 °C), which is at the transition between hydrothermal alteration and fluid-assisted metamorphism in chondrites. The results are consistent with previous mineralogical observations and thermodynamic models, which suggest that fayalite crystals are formed on asteroidal parent bodies by the interaction between a hydrothermal fluid and disequilibrium assemblages that compose the pristine materials that condensed in the early solar nebula. This experiment suggests that two variables play a very important role in the formation of fayalite during the hydrothermal growth (W/R mass ratio and the fluid composition). These results are similar to the recent observations of the fine-grained matrix of ordinary chondrites.

¹C. A. Lorenz,¹E. V. Korochantseva,¹M. A. Ivanova,²J. Hopp,³I. A. Franchi,⁴M. Humayun,¹M. O. Anosova,¹S. N. Teplyakova,²M. Trieloff
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13774]
¹Vernadsky Institute RAS, Kosygin St. 19, Moscow, 119991 Russia
²Institut für Geowissenschaften, Klaus-Tschira-Labor für Kosmochemie, Universität Heidelberg, Im Neuenheimer Feld 234-236, Heidelberg, 69120 Germany
³Planetary & Space Sciences, School of Physical Sciences, Open University, Milton Keynes, MK7 6AA UK
⁴National High Magnetic Field Laboratory and Department of Earth, Ocean & Atmospheric Science, Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, Florida, 32310 USA
Published by arrangement with John Wiley & Sons

We report the results of petrological, geochemical, and geochronological investigations of the unusual K-rich L chondrite melt rock Northwest Africa 6486 (NWA 6486). The rock has slightly fractionated siderophile elements and a mostly unfractionated L chondrite pattern of lithophile elements with the exceptions of enrichments in K and Rb and chondritic Sr abundance similar to the K-rich inclusions found in the ordinary chondrites and indicating a fractionation of alkaline elements through the vapor. We suggest that NWA 6486 and related K-rich chondritic inclusions were formed in situ on the OC parent bodies and that K and Rb enrichment of these rock most probably is a result of the selective impact evaporation of volatile alkali elements followed by the reaction of a vapor with shock melt. NWA 6486 recorded a breakup event of the L chondrite parent asteroid at 470 Ma during which it was formed. Unusual veins, depleted in K, Na, Ca, and Al relative to the host rock were found in NWA 6486. We suggest that NWA 6486 was affected by aqueous fluids that produced alteration zones depleted in a feldspar component on the walls of opened fractures. The melt veins could be formed during a subsequent impact event by in situ melting of the fracture walls or due to decomposition of an injected supercritical aqueous silicate fluid. The aqueous alteration and the second impact event had no detectable effect on Ar and oxygen isotopic systems. Cosmic ray exposure ages indicate that NWA 6486 was ejected from its parent asteroid ~3–4 Ma ago.