Noble gas elemental abundances in three solar wind regimes as recorded by the Genesis mission

1Nadia Vogel,1,2Veronika S.Heber,3Peter Bochsler,4Donald S.Burnett,1Colin Maden,1Rainer Wieler
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.08.007]
1ETH Zürich, Institute for Geochemistry and Petrology, Department of Earth Sciences, Clausiusstrasse 25, CH-8092 Zürich, Switzerland
2Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095-1567, USA
3Physikalisches Institut, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland
4California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA 91125, USA
Copyright Elsevier

We discuss elemental abundances of noble gases in targets exposed to the solar wind (SW) onboard the “Genesis” mission during the three different SW “regimes”: “Slow” (interstream, IS) wind, “Fast” (coronal hole, CH) wind and solar wind related to coronal mass ejections (CME). To this end we first present new Ar, Kr, and Xe elemental abundance data in Si targets sampling the different regimes. We also discuss He, Ne, and Ar elemental and isotopic abundances obtained on Genesis regime targets partly published previously. Average Kr/Ar ratios for all three regimes are identical to each other within their uncertainties of about 1% with one exception: the Fast SW has a 12% lower Xe/Ar ratio than do the other two regimes. In contrast, the He/Ar and Ne/Ar ratios in the CME targets are higher by more than 20% and 10%, respectively, than the corresponding Fast and Slow SW values, which among themselves vary by no more than 2-4%.
Earlier observations on lunar samples and Genesis targets sampling bulk SW wind had shown that Xe, with a first ionisation potential (FIP) of ∼12 eV, is enriched by about a factor of two in the bulk solar wind over Ar and Kr compared to photospheric abundances, similar to many “low FIP” elements with a FIP less than ∼10 eV. This behaviour of the “high FIP” element Xe was not easily explained, also because it has a Coulomb drag factor suggesting a relatively inefficient feeding into the SW acceleration region and hence a depletion relative to other high FIP elements such as Kr and Ar. The about 12% lower enrichment of Xe in Genesis’ Fast SW regime observed here is, however, in line with the hypothesis that the depletion of Xe in the SW due to the Coulomb drag effect is overcompensated as a result of the relatively short ionisation time of Xe in the ion-neutral separation region in the solar chromosphere. We will also discuss the rather surprising fact that He and Ne in CME targets are quite substantially enriched (by 20% and 10%, respectively) relative to the other solar wind regimes, but that this enrichment is not accompanied by an isotopic fractionation. The Ne isotopic data in CMEs are consistent with a previous hypothesis that isotopic fractionation in the solar wind is mass-dependent.

A nanoscale study of the formation of Fe-(hydr)oxides in a volcanic regolith: Implications for the understanding of soil forming processes on Earth and Mars

1Michael Schindler,1Sophie Michel,2Daniel Batcheldor,3,4Michael F.Hochella
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.08.008]
1Department of Earth Sciences, 935 Ramsey Lake Road, Laurentian University, Sudbury, ON, Canada, P3E2C6
2Physics and Space Sciences, Florida Institute of Technology, Melbourne, FL, 32901, USA
3Department of Geosciences, Virginia Tech, Blacksburg, VA, 24061, USA
4Subsurface Science and Technology Group, Pacific Northwest National Laboratory, Richland, WA, 99352, USA
Copyright Elsevier

Iron(hydr)oxides are one of the most important constituents of regoliths and soils derived from volcanic rocks both on Earth and Mars, often giving them their characteristic red color. This study deciphers for the first time an underlying mechanism for the formation of Fe-(hydr)oxides in a regolith which can occur during the weathering of basaltic glass; Fe-(hydr)oxides are prominent alteration products of regoliths under low water/rock ratios. An excellent example of these conditions is the early stage of basaltic glass weathering in the Martian regolith simulant JSC MARS-1A. In this study, a combination of focused ion beam technology and analytic transmission electron microscopy is employed in order to characterize basaltic glass weathering down to the nanometer level. Our results show that the formation of Fe-(hydr)oxide phases such as ferrihydrite, magnetite/maghemite and hematite during alteration of basaltic glass is based on complex and formerly unknown sequences of dissolution-precipitation reactions and pressure induced coalescence, segregation, aggregation, densification and growth processes. The weathering of the glass starts with its dissolution and subsequent precipitation of hydrous amorphous silica-bearing pockets rimmed by nano-size domains of ferrihydrite. An increase in molar volume during this process leads to an overall volume expansion, which promotes (a) growth of the hydrous silica through coalescence of individual pockets, (b) agglomeration of ferrihydrite domains to larger and denser aggregates in between layers or along the surfaces of plagioclase, hydrous amorphous silica and amorphous Al-(hydr)oxides, (c) formation of hematite within dense aggregates of ferrihydrite or as larger nanoparticles within an hydrous amorphous Si-Al-rich phase and (d) the break-up of plagioclase crystals and the replacement of these fragments by an hydrous amorphous Fe-Al-Si-bearing phase. At a later weathering stage, ferrihydrite nano-domains can also transform into magnetite/maghemite nanoparticles, which occur as layers around and on the surface of larger plagioclase crystals. This study also indicates the presence of past nano-environments in close proximity to each other, as for example layers of imogolite and ferrihydrite/hydrous amorphous silica occur only nanometers apart from each other on the opposite sides of unaltered glass. In accord with previous mineralogical studies of JSC MARS-1, the observed bulk and nano-mineralogical composition indicate that early alteration processes of basaltic glass under dry and cold conditions are mainly controlled by the formation of Fe-(hydr)oxides and minor imogolite and kaolinite. Recent mineralogical studies indicate that alteration processes at these conditions may have been the dominant weathering processes over long time periods on the Martian surface.

The condensation temperatures of the elements: A reappraisal

1Wood, B.J.,1Smythe, D.J.,1Harrison, T.
American Mineralogist 104, 844-856 Link to Article [DOI: 10.2138/am-2019-6852CCBY]
1Department of Earth Sciences, University of Oxford, South Parks Road, Oxford, OX1 3AN, United Kingdom
Copyright: The Mineralogical Society of America

As part of a project to investigate the volatilities of so-called “moderately volatile elements” such as Zn, In, Tl, Ga, Ag, Sb, Pb, and Cl during planetary formation, we began by re-calculating the condensation temperatures of these elements from a solar gas at 10-4 bar. Our calculations highlighted three areas where currently available estimates of condensation temperature could be improved. One of these is the nature of mixing behavior of many important trace elements when dissolved in major condensates such as silicates, Fe-rich metals, and sulfides. Nonideal solution of the trace elements can alter (generally lower) condensation temperatures by up to 500 K. Second, recent measurements of the halogen contents of CI chondrites (Clay et al. 2017) indicate that the solar system abundance of chlorine is significantly overestimated, and this affects the stabilities of gaseous complexes of many elements of interest. Finally, we have attempted to improve on previous estimates of the free energies of chlorine-bearing solids since the temperature of chlorine condensation has an important control on the condensation temperatures of many trace elements. Our result for the 50% condensation temperature of chlorine, 472 K is nearly 500 K lower than the result of Lodders (2003), and this means that the HCl content of the solar gas at temperatures <900 K is higher than previously estimated. We based our calculations on the program PHEQ (Wood and Hashimoto 1993), which we modified to perform condensation calculations for the elements H, O, C, S, Na, Ca, Mg, Al, Si, Fe, F, Cl, P, N, Ni, and K by free energy minimization. Condensation calculations for minor elements were then performed using the output from PHEQ in conjunction with relevant thermodynamic data. We made explicit provision for nonidealities using information from phase diagrams, heat of solution measurements, partitioning data and by using the lattice strain model for FeS and ionic solids and the Miedema model for solutions in solid Fe. We computed the relative stabilities of gaseous chloride, sulfide, oxide, and hydroxide species of the trace elements of interest and used these, as appropriate in our condensation calculations. In general, our new 50% condensation temperatures are similar to or, because of the modifications noted above, lower than those of Lodders (2003).

Selenium isotopes as tracers of a late volatile contribution to Earth from the outer Solar System

1María Isabel Varas-Reus,1Stephan König,1Aierken Yierpan,2Jean-Pierre Lorand,1Ronny Schoenberg
Nature Geoscience (in Press) Link to Article [DOI
https://doi.org/10.1038/s41561-019-0414-7]
1Isotope Geochemistry, Department of Geosciences, University of Tuebingen, Tuebingen, Germany
2Laboratoire de Planétologie et Géodynamique à Nantes, CNRS UMR 6112, Université de Nantes, Nantes, France

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The Hamburg meteorite fall: Fireball trajectory, orbit, and dynamics

1P.G.Brown et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13368]
1Department of Physics and Astronomy, University of Western Ontario, London, Ontario, N6A 3K7 Canada
2Centre for Planetary Science and Exploration, University of Western Ontario, London, Ontario, N6A 5B7 Canada
Published by arrangement with John Wiley & Sons

The Hamburg (H4) meteorite fell on 17 January 2018 at 01:08 UT approximately 10 km north of Ann Arbor, Michigan. More than two dozen fragments totaling under 1 kg were recovered, primarily from frozen lake surfaces. The fireball initial velocity was 15.83 ± 0.05 km s−1, based on four independent records showing the fireball above 50 km altitude. The radiant had a zenith angle of 66.14 ± 0.29° and an azimuth of 121.56 ± 1.2°. The resulting low inclination (<1°) Apollo‐type orbit has a large aphelion distance and Tisserand value relative to Jupiter (Tj) of ~3. Two major flares dominate the energy deposition profile, centered at 24.1 and 21.7 km altitude, respectively, under dynamic pressures of 5–7 MPa. The Geostationary Lightning Mapper on the Geostationary Operational Environmental Satellite‐16 also detected the two main flares and their relative timing and peak flux agree with the video‐derived brightness profile. Our preferred total energy for the Hamburg fireball is 2–7 T TNT (8.4–28 × 109 J), which corresponds to a likely initial mass in the range of 60–225 kg or diameter between 0.3 and 0.5 m. Based on the model of Granvik et al. (2018), the meteorite originated in an escape route from the mid to outer asteroid belt. Hamburg is the 14th known H chondrite with an instrumentally derived preatmospheric orbit, half of which have small (<5°) inclinations making connection with (6) Hebe problematic. A definitive parent body consistent with all 14 known H chondrite orbits remains elusive.

 

Documentation of shock features in impactites from the Dhala impact structure, India

1,2Jayanta Kumar Pati,3Michael H. Poelchau,4,5Wolf Uwe Reimold,6,7Norihiro Nakamura,6Yutaro Kuriyama,8Anuj Kumar Singh
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13369]
1Department of Earth and Planetary Sciences, Nehru Science Centre, University of Allahabad, Allahabad, 211 002 India
2National Center of Experimental Mineralogy and Petrology, University of Allahabad, 14 Chatham Lines, Allahabad, 211 002 India
3Institute of Earth and Environmental Science‐Geology, Albert‐Ludwigs‐Universität Freiburg, Albertstraße 23‐B, D‐79104 Freiburg, Germany
4Museum für Naturkunde ‐ Leibniz Institute for Evolution and Biodiversity Science, Invalidenstraße 43, 10115 Berlin, Germany
5Laboratory of Geochronology, Instituto de Geociências, Universidade de Brasília, CEP 70910 900 Brasília, DF, Brazil
6Department of Earth Science, Tohoku University, 6‐3 Aoba, Aramaki, Sendai, 980‐8578 Japan
7Institute for Excellence in Higher Education, Tohoku University, 42 Kawauchi, Sendai, 980‐8576 Japan
8Department of Earth and Planetary Sciences, Nehru Science Centre, University of Allahabad, Allahabad, 211 002 India
Published by arrangement with John Wiley & Sons

The fundamental approach for the confirmation of any terrestrial meteorite impact structure is the identification of diagnostic shock metamorphic features, together with the physical and chemical characterization of impactites and target lithologies. However, for many of the approximately 200 confirmed impact structures known on Earth to date, multiple scale‐independent tell‐tale impact signatures have not been recorded. Especially some of the pre‐Paleozoic impact structures reported so far have yielded limited shock diagnostic evidence. The rocks of the Dhala structure in India, a deeply eroded Paleoproterozoic impact structure, exhibit a range of diagnostic shock features, and there is even evidence for traces of the impactor. This study provides a detailed look at shocked samples from the Dhala structure, and the shock metamorphic evidence recorded within them. It also includes a first report of shatter cones that form in the shock pressure range from ~2 to 30 GPa, data on feather features (FFs), crystallographic indexing of planar deformation features, first‐ever electron backscatter diffraction data for ballen quartz, and further analysis of shocked zircon. The discovery of FFs in quartz from a sample of the MCB‐10 drill core (497.50 m depth) provides a comparatively lower estimate of shock pressure (~7–10 GPa), whereas melting of a basement granitoid infers at least 50–60 GPa shock pressure. Thus, the Dhala impactites register a strongly heterogeneous shock pressure distribution between <2 and >60 GPa. The present comprehensive review of impact effects should lay to rest the nonimpact genesis of the Dhala structure proposed by some earlier workers from India.

The composition and mineralogy of rocky exoplanets: A survey of >4000 stars from the Hypatia Catalog

1Putirka, K.D.,1Rarick, J.C.
American Mineralogist 104, 817-829 Link to Article [DOI: 10.2138/am-2019-6787]
1Department of Earth and Environmental Sciences, Fresno State, 2345 E. San Ramon Avenue, MS/MH24, Fresno, CA 93720, United States
Copyright: The Mineralogical Society of America

Combining occurrence rates of rocky exoplanets about sun-like stars, with the number of such stars that occupy possibly hospitable regions of the Milky Way, we estimate that at least 1.4 × 108 near-Earth-sized planets occupy habitable orbits about habitable stars. This number is highly imprecise to be sure, and it is likely much higher, but it illustrates that such planets are common, not rare. To test whether such rocky exoplanets might be geologically similar to Earth, we survey >4000 star compositions from the Hypatia Catalog – the most compositionally broad of such collections. We find that rocky exoplanets will have silicate mantles dominated by olivine and/or orthopyroxene, depending upon Fe partitioning during core formation. Some exoplanets may be magnesiowüstite- or quartz-saturated, and we present a new classification scheme based on the weight percent ratio (FeO+MgO)/SiO2, to differentiate rock types. But wholly exotic mantle mineralogies should be rare to absent; many exo-planets will have a peridotite mantle like Earth, but pyroxenite planets should also be quite common. In addition, we find that half or more of the range of exoplanet mantle mineralogy is possibly controlled by core formation, which we model using αFe = FeBSP/FeBP, where FeBSP is Fe in a Bulk Silicate Planet (bulk planet, minus core), on a cation weight percent basis (elemental weight proportions, absent anions) and FeBP is the cation weight percent of Fe for a Bulk Planet. This ratio expresses, in this case for Fe, the fraction of an element that is partitioned into the silicate mantle relative to the total amount available upon accretion. In our solar system, αFe varies from close to 0 (Mercury) to about 0.54 (Mars). Remaining variations in theoretical exoplanet mantle mineralogy result from non-trivial variations in star compositions. But we also find that Earth is decidedly non-solar (non-chondritic); this is not a new result, but appears worth re-emphasizing, given that current discussions often still use carbonaceous or enstatite chondrites as models of Bulk Earth. While some studies emphasize the close overlap of some isotope ratios between certain meteoritic and terrestrial (Earth-derived) samples, we find that major oxides of chondritic meteorites do not precisely explain bulk Earth. To allow Earth to be chondritic (or solar), there is the possibility that Earth contains a hidden component that, added to known reservoirs, would yield a solar/chondritic bulk Earth. We test that idea using a mass balance of major oxides using known reservoirs, so that the sum of upper mantle, metallic core, and crust, plus a hidden component, yields a solar bulk composition. In this approach, the fractions of crust and core are fixed and the hidden mantle component, F h, is some unknown fraction of the entire mantle (so if FDM is the fraction of depleted mantle, then F h + F DM = 1). Such mass balance shows that if a hidden mantle component were to exist, it must comprise >28% of Earth’s mantle, otherwise it would have negative abundances of TiO2 and Al2O3. There is no clear upper limit for such a component, so it could comprise the entire mantle. But all estimates from Fh = 0.28 to Fh = 1.0 yield a hidden fraction that does not match the inferred sources of ocean island or mid-ocean ridge basalts, and would be geologically unusual, having higher Na2O, Cr2O3, and FeO and lower CaO, MgO, and Al2O3 compared to familiar mantle components. We conclude that such a hidden component does not exist. © 2019 Walter de Gruyter GmbH, Berlin/Boston 2019.