Iron content determines how space weathering flux variations affect lunar soils

1James McFadden,1,2IanGarrick-Bethell,2Chae K.Sim,2Sungsoo S.Kim,3DougHemingway
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.05.033]
1Earth and Planetary Sciences, University of California, Santa Cruz, USA
2School of Space Research, Kyung Hee University, Republic of Korea
3Carnegie Institution for Science, Washington, DC, USA
Copyright Elsevier

Previous work has established that the solar wind and micrometeoroids produce spectral changes on airless silicate bodies. However, the relative importance of these two weathering agents, the timescales over which they operate, and how their effects depend on composition have not yet been well determined. To help address these questions we make use of the fact that solar wind and micrometeoroid fluxes vary with latitude on the Moon. Previous work has shown that this latitudinally varying flux leads to systematic latitudinal variations in the spectral properties of lunar soils. Here we find that the way in which a lunar soil’s spectral properties vary with latitude is a function of its iron content, when we consider soils with 14–22 wt% FeO. In particular, a 50% reduction in flux corresponds to a significant increase in reflectance for 14 wt% FeO soils, while the same flux reduction on 21 wt% FeO soils is smaller by a factor of ~5, suggesting that this brightening effect saturates for high FeO soils. We propose that lower iron soils may not approach saturation because grains are destroyed or refreshed before sufficient nano- and micro-phase iron can accumulate on their rims. We compare our results to the spectral variations observed across the Reiner Gamma swirl, which lies on a high‑iron surface, and find it has anomalous brightness compared to our predictions. Swirls in Mare Marginis, which lie on a low iron surface, exhibit brightness differences that suggest reductions in solar wind flux between 20 and 40%. Our inferences suffer from the limited latitudinal extent of the maria and the convolution of micrometeoroid flux and solar wind flux changes with latitude. Superior constraints on how space weathering operates throughout the inner solar system would come from in situ measurements of the solar wind flux at lunar swirls.

Calcite and dolomite formation in the CM parent body: Insight from in situ C and O isotope analyses

1M.Telus,1C.M.O’D.Alexander,1E.H.Hauri,1J.Wang
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.06.012]
1Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, Washington, DC 20015, USA
Copyright Elsevier

To constrain the conditions of aqueous alteration in early planetesimals, we carried out in situ C and O isotope analyses of calcite and dolomite and O isotope analyses of magnetite from the highly altered CM chondrites ALH 83100, ALH 84034, and MET 01070. Petrographic and isotopic analyses of these samples support previous findings of multiple generations of carbonate growth. We observe wide ranges in the C and O isotope compositions of carbonates of up to 80‰ and 30‰, respectively, that span the full range of previously reported bulk carbonate values for CM chondrites. Variations in the Δ17O values indicate that fluid evolution varied for each chondrite. ALH 83100 dolomite-magnetite δ18O fractionation of 23‰ ± 7‰ (2SD) corresponds to dolomite formation temperature of 125°C ± 60°C. δ13C vs δ18O values fall into two groups, one consisting of primary calcite and the other consisting of dolomite and secondary calcite. The positive correlation between δ13C and δ18O for primary calcite is consistent with the precipitation of calcite in equilibrium with a gas mixture of CO (or CH4) and CO2. The isotopic composition of calcite in CM1s and CM2s overlap significantly; however, many CM1 calcite grains are more depleted in δ18O compared to CM2s. Altogether, the data indicate that the fluid composition during calcite formation was initially the same for both CM1s and CM2s. CM1s experienced more episodes of carbonate dissolution and reprecipitation where some fraction of the carbonate grains survive each episode resulting in a highly disequilibrium assemblage of carbonates on the thin-section scale.

Identification of a meteoritic component using chromium isotopic composition of impact rocks from the Lonar impact structure, India

1,2Berengere Mougel,1,3Frederic Moynier,4,5Christian Koeberl,6Daniel Wielandt,6Martin Bizzarro
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13312]
1Institut de Physique du Globe de Paris, Université Sorbonne Paris Cité, CNRS UMR7154, 1 rue Jussieu, 75238 Paris Cedex 05, France
2Centro de Geociencias, Universidad Nacional Autónoma de México, Blvd. Juriquilla No 3001, Querétaro, 76230 Mexico
3Institut Universitaire de France, 1 rue Descartes, 75231 Paris Cedex 05, France
4Department of Lithospheric Research, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria
5Natural History Museum, Burgring 7, 1010 Vienna, Austria
6Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5‐7 DK‐1350, Copenhagen, Denmark
Published by arrangement with John Wiley & Sons

The existence of mass‐independent chromium isotope variability of nucleosynthetic origin in meteorites and their components provides a means to investigate potential genetic relationship between meteorites and planetary bodies. Moreover, chromium abundances are depleted in most surficial terrestrial rocks relative to chondrites such that Cr isotopes are a powerful tool to detect the contribution of various types of extra‐terrestrial material in terrestrial impactites. This approach can thus be used to constrain the nature of the bolide resulting in breccia and melt rocks in terrestrial impact structures. Here, we report the Cr isotope composition of impact rocks from the ~0.57 Ma Lonar crater (India), which is the best‐preserved impact structure excavated in basaltic target rocks. Results confirm the presence of a chondritic component in several bulk rock samples of up to 3%. The impactor that created the Lonar crater had a composition that was most likely similar to that of carbonaceous chondrites, possibly a CM‐type chondrite.

Vapor‐deposited minerals contributed to the martian surface during magmatic degassing

1H. Nekvasil,2N.J. DiFrancesco,1A.D. Rogers,3A.E. Coraor,4P.L. King
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005911]
1Stony Brook University, Department of Geosciences, Stony Brook, NY, USA
2SUNY Oswego, Department of Atmospheric and Geological Sciences, Oswego, NY, USA
3Institute for Molecular Engineering, The University of Chicago, Chicago, IL, USA
4Research School of Earth Sciences, The Australian National University, Canberra, ACT, Australia
Published by arrangement with John Wiley & Sons

Martian magmas were likely enriched in S and Cl with respect to H2O. Exsolution of a vapor phase from these magmas and ascent of the gas bubbles through the magma plumbing system would have given rise to shallow magmas that were gas‐charged. Release and cooling of this gas from lava flows during eruption may have resulted in the addition of a significant amount of vapor‐deposited phases to the fines of the surface. Experiments were conducted to simulate degassing of gas‐charged lava flows and shallow intrusions in order to determine the nature of vapor‐deposited phases that may form through this process. The results indicate that magmatic gas may have contributed a large amount of Fe, S, and Cl to the martian surface through the deposition of iron oxides (magnetite, maghemite, hematite), chlorides (molysite, halite, sylvite), sulfur and sulfides (pyrrhotite, pyrite). Primary magmatic vapor‐deposited minerals may react during cooling to form a variety of secondary products, including iron oxychloride (FeOCl), akaganéite (Fe3+O (OH,Cl)), and jarosite (KFe3+3(OH)6(SO4)2). Vapor‐deposition does not transport significant amounts of Ca, Al, or Mg from the magma and hence, this process does not directly deposit Ca‐ or Mg‐sulfates.

The deposition and alteration history of the northeast Syrtis Major layered sulfates

1D.P. Quinn,2B.L. Ehlmann
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005706]
1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
2Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
Published by arrangement with John Wiley & Sons

Ancient stratigraphy on Isidis Basin’s western margin records the history of water on early Mars. Noachian units are overlain by layered, basaltic‐composition sedimentary rocks that are enriched in polyhydrated sulfates and capped by more resistant units. The layered sulfates –uniquely exposed at northeast Syrtis Major – comprise a sedimentary sequence up to 600‐m thick that has undergone a multi‐stage history of deposition, alteration, and erosion. Siliciclastic sediments enriched in polyhydrated sulfates are bedded at m‐scale and were deposited on slopes up to 10°, embaying and thinning against pre‐existing Noachian highlands around the Isidis basin rim. The layered sulfates were modified by volume‐loss fracturing during diagenesis. Resultant fractures hosted channelized flow and jarosite mineral precipitation to form resistant ridges upon erosion. The structural form of the layered sulfates is consistent with packages of sediment fallen from either atmospheric or aqueous suspension; coupling with substantial diagenetic volume‐loss may favor deepwater basin sedimentation. After formation, the layered sulfates were capped by a “smooth capping unit” and then eroded to form paleovalleys. Hesperian Syrtis Major lavas were channelized by this paleotopography, capping it in some places and filling it in others. Later fluvial features and phyllosilicate‐bearing lacustrine deposits, sharing a regional base level of ~‐2300m, were superimposed on the sulfate‐lava stratigraphy. The layered sulfates suggest surface bodies of water and active groundwater upwelling during the Noachian–Hesperian transition. The northeast Syrtis Major stratigraphy records at least four distinct phases of surface and subsurface aqueous activity spanning from late Noachian to early Amazonian time.