¹T.J.McCoy, ¹C.M.Corrigan,^1,2K.Nagashim, ^1,3V.S.Reynolds, ⁴R.D.Ash, ⁴W.F.McDonough,^5,6,7J.Yang, ⁵J.I.Goldstein, ⁴C.D.Hilton
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.06.005]
¹Department of Mineral Sciences, Smithsonian Institution, Washington, D.C. 20560-0119, USA
²Hawai’i Institute of Geophysics and Planetology, Univ. of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
³Dept. of Geography and Earth Sciences, UNC-Charlotte, Charlotte, NC 28223 USA
⁴Department of Geology, University of Maryland, College Park, Maryland 20742 USA
⁵Dept. of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, Massachusetts 01003 USA
⁶Key Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China 100029
⁷Institutions of Earth Science, University of Chinese Academy of Sciences, Beijing, China 100029
Copyright Elsevier

The link between the Milton pallasite and the South Byron Trio irons is examined through metallography and metallogaphic cooling rates; major, minor, and trace element compositions of metal; inclusion mineralogy and mineral compositions; and oxygen isotopic compositions. The metallic hosts of these Ni-rich meteorites (18.2-20.3 wt.% Ni) are dominated by plessite with spindles of kamacite and schreibersite. The presence of ∼50 nm wide tetrataenite and absence of high-Ni particles in the cloudy zone in Milton suggest cooling of ∼2,000 K/Myr or >10,000 K/Myr. Compositionally, the metallic host in all four meteorites exhibits modest (1-2 orders of magnitude compared to CI chondrites) depletions of volatile elements relative to refractory elements, and marked depletions in the redox sensitive elements W, Mo, Fe, and P. Oxygen isotopic compositions (Δ17O) are, within uncertainty, the same for the Milton and the South Byron Trio and for IVB irons. Similarities in metallography, metal composition, inclusion mineralogy, and oxygen (Δ17O), molybdenum and ruthenium isotopic composition suggest that the Milton pallasite and South Byron Trio irons could have originated on a common parent body as chemically distinct melt, or on separate parent bodies that experience similar cosmochemical and geochemical processes. The Milton pallasite and South Byron Trio irons share a number of properties with IVB irons, including metallography, enrichment in highly siderophile elements and nickel, inclusion mineralogy and oxygen isotopic composition, suggesting they formed in a similar nebular region through common processes, although Milton and the South Byron Trio did not experience the dramatic volatile loss of the IVB irons. Depletions in W, Mo, Fe, and P relative to elements of similar volatility likely result from oxidation, either in the nebula prior to accretion or on the parent body during melting. Oxidation ∼73 wt.% of Fe is indicated, with a correspondingly FeO-rich mantle and smaller core. If Milton and the South Byron Trio sample a common core, Milton formed near the surface of the core after stripping of the silicate shell and may have experienced rapid solidification and contamination by an impactor. The molten core, from which the South Byron Trio irons crystallized, solidified from the outside in.

¹James F.J.Bryson,^2,3,4Jerome A.Neufeld,⁵Francis Nimmo
Earth and Planetary Science Letters 521, 68-78 Link to Article [https://doi.org/10.1016/j.epsl.2019.05.046]
¹Department of Earth Sciences, University of Cambridge, Cambridge, UK
²BP Institute, University of Cambridge, Cambridge, UK
³Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Cambridge, UK
⁴Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge, UK
⁵Department of Earth and Planetary Sciences, University of California, Santa Cruz, CA, USA
Copyright Elsevier

Paleomagnetic measurements of ancient terrestrial and extraterrestrial samples indicate that numerous planetary bodies generated magnetic fields through core dynamo activity during the early solar system. The existence, timing, intensity and stability of these fields are governed by the internal transfer of heat throughout their parent bodies. Thus, paleomagnetic records preserved in natural samples can contain key information regarding the accretion and thermochemical history of the rocky bodies in our solar system. However, models capable of predicting these field properties across the entire active lifetime of a planetary core that could relate the processes occurring within these bodies to features in these records and provide such information are limited. Here, we perform asteroid thermal evolution models across suites of radii, accretion times and thermal diffusivities with the aim of predicting when fully and partially differentiated asteroids generated magnetic fields. We find that dynamo activity in both types of asteroid is delayed until ∼4.5-5.5 Myr after calcium-aluminium-rich inclusion formation due to the partitioning of ²⁶Al into the silicate portion of the body during differentiation and large early surface heat fluxes, followed by a brief period (<12.5 Myr for bodies with radii <500 km) of thermally-driven dynamo activity as heat is convected from the core across a partially-molten magma ocean. We also expect that gradual core solidification produced compositionally-driven dynamo activity in these bodies, the timing of which could vary by tens to hundreds of millions of years depending on the S concentration of the core and the radius of the body. There was likely a pause in core cooling and dynamo activity following the cessation of convection in the magma ocean. Our predicted periods of magnetic field generation and quiescence match eras of high and low paleointensities in the asteroid magnetic field record compiled from paleomagnetic measurements of multiple meteorites, providing the possible origins of the remanent magnetisations carried by these samples. We also compare our predictions to paleomagnetic results from different meteorite groups to constrain the radii of the angrite, CV chondrite, H chondrite, IIE iron meteorite and Bjürbole (L/LL chondrite) parent bodies and identify a likely nebula origin for the remanent magnetisation carried by the CM chondrites.

¹Chavrit, D.,¹Moreira, M.A.,²Fike, D.A.,^1,3Moynier, F.
Chemical Geology 520, 52-59 Link to Article [DOI: 10.1016/j.chemgeo.2019.04.025]
¹Université de Paris, Institut de physique du globe de Paris, CNRS, Paris, F-75005, France
²Department of Earth and Planetary Sciences, Washington University, St Louis, MO 63130, United States
³Institut Universitaire de France, Paris, France

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

¹Xunyu Zhang,²Meng‐Hua Zhu,¹Roberto Bugiolacchi
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005870]
¹Space Science Institute, Macau University of Science and Technology, Macau, China
²Chinese Academy of Sciences Center for Excellence in Comparative Planetology, Hefei, China
Published by arrangement with John Wiley & Sons

The formation of the South Pole‐Aitken (SPA) basin is thought to excavate the deep crust or mantle because of its large size. The pervasive orthopyroxene‐dominated materials found across the basin suggest that they either represent the SPA impact melt or the excavated materials from the lower crust and/or upper mantle. This study analyzes the relative content and distribution of mafic minerals in the SPA area based on the spectra from small fresh craters. The orthopyroxene‐dominated materials in the non‐mare regions are classified into two types based on their distribution and different composition. One is distributed from the center to the edge across the SPA basin and interpreted as the SPA impact melt. The other is Mg‐richer and generally located in some plagioclase‐rich regions (e.g., some large impact craters/basins and the SPA edge), thought to represent materials from the lower crust and/or upper mantle. For the maria in the SPA area, the basaltic materials in the northwest are found to be richer in olivine and/or clinopyroxene than the southern ones.

¹C.T. Unterborn,²W.R. Panero
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005844]
¹School of Earth and Space Exploration, Arizona State University
²School of Earth Sciences, The Ohio State University
Published by arrangement with John Wiley & Sons

The interior composition of exoplanets is not observable, limiting our direct knowledge of their structure, composition, and dynamics. Recently described observational trends suggest that rocky exoplanets, that is, planets without significant volatile envelopes, are likely limited to <1.5 Earth radii. We show that given this likely upper limit in the radii of purely‐rocky super‐Earth exoplanets, the maximum expected core‐mantle boundary pressure and adiabatic temperature is relatively moderate, 630 GPa and 5000 K, while the maximum central core pressure varies between 1.5 and 2.5 TPa. We further find that for planets with radii less than 1.5 Earth radii, core‐mantle boundary pressure and adiabatic temperature are mostly a function of planet radius and insensitive to planet structure. The pressures and temperatures of rocky exoplanet interiors, then, are less than those explored in recent shock‐compression experiments, ab‐initio calculations, and planetary dynamical studies. We further show that the extrapolation of relevant equations of state does not introduce significant uncertainties in the structural models of these planets. Mass‐radius models are more sensitive to bulk composition than any uncertainty in the equation of state, even when extrapolated to TPa pressures.

¹Ustinova, G.K.,¹Alexeev, V.A.
Doklady Physics 64, 139-143 Link to Article [DOI: 10.1134/S1028335819030029]
¹Institute of Geochemistry and Analytical Chemistry (GEOKHY), Russian Academy of Science, Moscow, 119991, Russian Federation

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

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.

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.

^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.

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.