Exploring the efficiency of stepwise dissolution in removal of stubborn non-radiogenic Pb in chondrule U-Pb dating

1Renaud Merle,1Yuri Amelin,2Qing-Zhu Yin,2Magdalena H.Huyskens,2Matthew E.Sanborn,3Kazuhide Nagashima,4Katsuyuki Yamashita,1Trevor R.Ireland,3Alexander N.Krot,1,5Melanie J.Siebera
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.03.010]
1Research School of Earth Sciences, The Australian National University, Canberra, 2601 Australia
2Department of Earth and Planetary Sciences, University of California-Davis, One Shields Avenue, Davis, CA, USA
3Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
4Graduate School of Natural Science and Technology, Okayama University, Japan
5GFZ, German Research Centre for Geoscience, Telegrafenberg, D.-14473 Potsdam,Germany
Copyright Elsevier

Chondrules in chondritic meteorites are unique witnesses of nebular and asteroidal processes that preceded large-scale planetary accretion. Together with refractory calcium-aluminium-rich inclusions (CAIs), they are the sources of our knowledge of the initial evolution of the early Solar System. We have investigated a single very large (>10 mm in longer dimension) chondrule, hereafter, the mega-chondrule A25-2, extracted from the Allende CV3 chondrite. We characterised texture, mineralogy and mineral chemistry of this chondrule, and studied its Al-Mg, U-Pb and U-isotope systematics. We also studied the distribution of U, Th and Pb, and measured Pb isotopic composition in individual minerals of A25-2 by secondary ion mass-spectrometry (SIMS). The main difficulty in absolute age determination was the presence of pervasive and resilient non-radiogenic Pb. In the search for the best way to separate radiogenic Pb from non-radiogenic Pb components of terrestrial and asteroidal origins, we used various protocols of multi-step leaching and assessed their efficiency in generating data suitable for the construction of an isochron. Testing the data filtering procedure led us to explore the behaviour of the stepwise leaching method in the presence of pervasive and resilient non-radiogenic Pb. The model age patterns observed in the final HF partial dissolution steps were probably induced by isotopic fractionation. Although step leaching did not yield fractions with highly radiogenic Pb, a Pb-Pb isochron age corrected for measured 238U/235U was obtained by: (1) data filtering process based on strict analytical and geochemical criteria to include in the Pb-Pb isochron only leaching steps free from terrestrial contamination and (2) arithmetically recombined analyses to cancel the effects of leaching-induced isotopic fractionation.

This extensive data processing yielded the age of 4568.5±3.0 Ma, which we consider reliable within its uncertainty limits, although it is not as precise as, and more model dependent than, the age that could have been obtained if Pb isotopic compositions were more radiogenic. The 238U/235U ratio of the mega-chondrule is 137.764±0.016, which is similar to the ratios obtained from single chondrules yet slightly different from small pooled Allende chondrules. The initial 27Al/26Al ratio inferred from internal isochron obtained from SIMS Al-Mg isotope measurements is (5.4±6.5)×10–6, which corresponds to 4565.0 +0.8/-∞ Ma, assuming homogeneous distribution of 26Al throughout the protoplanetary disk at the canonical level (∼5.2×10−5). This age is 3.5±3.1 Ma younger than the Pb-isotopic age. Calculation of 26Al-26Mg age assuming initial (27Al/26Al)0 of (1.36±0.72)×10–5 in the chondrule-forming region yields the age of 4566.4+0.8/-∞, which is consistent with the Pb-isotopic age.

Composition, Stratigraphy, and Geological History of the Noachian Basement Surrounding the Isidis Impact Basin

1Eva L. Scheller,1,2Bethany L. Ehlmann
Journal of Geophysical Research (Planets) (In Press) Link to Article [https://doi.org/10.1029/2019JE006190]
1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
2Jet Propulsion Laboratory, Pasadena, California Institute of Technology, Pasadena, California, USA
Published by arrangement with John Wiley & Sons

The western part of the Isidis basin structure hosts a well‐characterized Early Noachian to Amazonian stratigraphy. The Noachian Basement comprises its oldest exposed rocks (Early to Mid‐Noachian), and was previously considered a single LCP‐ and Fe/Mg‐smectite‐bearing unit. Here, we divide the Noachian Basement Group into 5 distinct geological units (Stratified Basement Unit, Blue Fractured Unit, Mixed Lithology Plains Unit, LCP‐bearing Plateaus Unit, Fe/Mg‐smectite‐bearing Mounds Unit), 2 geomorphological features (megabreccia and ridges), and a mineral deposit (kaolinite‐bearing bright materials), based on geomorphology, spectral characteristics, and stratigraphic relationships. Megabreccia contain four different pre‐Isidis lithologies, possibly including deeper crust or mantle materials, formed through mass‐wasting associated with transient crater collapse during Isidis basin formation. The Fe/Mg‐smectite‐bearing Stratified Basement Unit and LCP‐bearing Blue Fractured Unit likewise represent pre‐Isidis units within the Noachian Basement Group. Multiple Fe/Mg‐smectite‐bearing geological units with different stratigraphic positions and younger kaolinite‐bearing bright materials indicate several aqueous alteration episodes of different ages and styles. Units with slight changes in pyroxene spectral properties suggest a transition from low‐Ca pyroxene‐containing materials to those with higher proportions of pyroxenes higher in Ca and/or glass that could be related to different impact‐ and/or igneous processes, or provenance. This long history of Noachian and potentially Pre‐Noachian geological processes, including impact basin formation, aqueous alteration, and multiple igneous and sedimentary petrogeneses, records changing ancient Mars environmental conditions. All units defined by this study are available 20 km outside of Jezero crater for in‐situ analysis and sampling during a potential extended mission scenario for the Mars 2020 rover.

Observations, meteorites, and models: A pre‐flight assessment of the composition and formation of (16) Psyche

1L.T.Elkins-Tanton et al. (>10)
Journal of Geophysical Research (Planets) (In Press) Link to Article [https://doi.org/10.1029/2019JE006296]
1Arizona State University
Published by arrangement with John Wiley & Sons

Some years ago the consensus was that asteroid (16) Psyche was almost entirely metal. New data on density, radar properties, and spectral signatures indicate that the asteroid is something perhaps even more enigmatic: a mixed metal and silicate world. Here we combine observations of Psyche with data from meteorites and models for planetesimal formation to produce the best current hypotheses for Psyche’s properties and provenance. Psyche’s bulk density appears to be between 3,400 and 4,100 kg m‐3. Psyche is thus predicted to have between ~30 vol% and ~60 vol% metal, with the remainder likely low‐iron silicate rock and not more than ~20% porosity. Though their density is similar, mesosiderites are an unlikely analog to bulk Psyche because mesosiderites have far more iron‐rich silicates than Psyche appears to have. CB chondrites match both Psyche’s density and spectral properties, as can some pallasites, although typical pallasitic olivine contains too much iron to be consistent with the reflectance spectra. Final answers, as well as resolution of contradictions in the dataset of Psyche physical properties, for example, the thermal inertia measurements, may not be resolved until the NASA Psyche mission arrives in orbit at the asteroid. Despite the range of compositions and formation processes for Psyche allowed by the current data, the science payload of the Psyche mission (magnetometers, multi‐spectral imagers, neutron spectrometer, and a gamma‐ray spectrometer) will produce datasets that distinguish among the models.

Volatile element chemistry during accretion of the earth

1BruceFegleyJr,1Katharina Lodders,2Nathan S.Jacobson
Geochemistry [Chemie der Erde] (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2019.125594]
1Planetary Chemistry Laboratory, Dept. of Earth & Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, St. Louis, MO 63130, USA
2Materials Division, NASA Glenn Research Center, MS106-1, 21000 Brookpark Road, Cleveland, OH 44135, USA
Copyright Elsevier

We review some issues relevant to volatile element chemistry during accretion of the Earth with an emphasis on historical development of ideas during the past century and on issues we think are important. These ideas and issues include the following: (1) whether or not the Earth accreted hot and the geochemical evidence for high temperatures during its formation, (2) some chemical consequences of the Earth’s formation before dissipation of solar nebular gas, (3) the building blocks of the Earth, (4) the composition of the Earth and its lithophile volatility trend, (5) chemistry of silicate vapor and steam atmospheres during Earth’s formation, (6) vapor – melt partitioning and possible loss of volatile elements, (7) insights from hot rocky extrasolar planets. We include tabulated chemical kinetic data for high-temperature elementary reactions in silicate vapor and steam atmospheres. We finish with a summary of the known and unknown issues along with suggestions for future work.

Mineralogical survey of the anorthositic Feldspathic Highlands Terrane crust using Moon Mineralogy Mapper data

1,2M.Martinot,3J.Flahaut,4S.Besse,2C.Quantin-Nataf,3W.van Westrenen
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113747]
1Faculty of Science, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands
2Université Lyon 1, ENS-Lyon, CNRS, UMR 5276 LGL-TPE, F-69622, Villeurbanne, France
3Centre de Recherches Pétrographiques et Géochimiques, CNRS/Université de Lorraine F-54500, Vandoeuvre-lès-Nancy, France
4European Space Astronomy Centre, P.O. Box 78, 28691 Villanueva de la Canada, Madrid, Spain
Copyright Elsevier

Spectroscopic data from the Moon Mineralogy Mapper (M3) instrument are used to study the mineralogy of the central peak or peak ring of 75 craters located in the lunar anorthositic Feldspathic Highlands Terrane (FHT-a), as defined by Jolliff et al. (2000). The thickness of South-Pole Aitken (SPA) ejecta at the location of the selected craters is estimated. Crustal thickness models are used with empirical cratering equations to estimate the depth of origin of the material excavated in the studied central peaks, and its distance to the crust-mantle interface. The goal of this survey is to study the composition of the FHT-a crust, and the extent of its potential lateral and vertical heterogeneities. High-Calcium Pyroxene (HCP) and featureless spectra are mostly detected throughout the entire FHT-a, whereas the number of pure plagioclase detections is small. No relationship between the central peak composition and the distance to SPA or the depth within the SPA ejecta is observed. The SPA ejecta material cannot be spectrally distinguished from crustal material. We interpret the paucity of plagioclase spectra in the FHT-a, which contrasts with more frequent plagioclase detections in the central peaks of craters sampling the crust in younger lunar terranes using identical spectroscopic techniques Martinot et al. (2018b), as a possible effect of terrane maturation, or of mixing with mafic components that mask their signature in the visible near-infrared. Overall, the FHT-a appears homogeneous laterally. However, data hint at a pyroxene compositional change with increasing depth, from high-calcium content in the upper crust towards less calcic compositions with increasing depth, which is consistent with prior studies of the architecture of the lunar crust.

Micrometeorites: Insights into the flux, sources and atmospheric entry of extraterrestrial dust at Earth

1,3Matthew J.Genge,2Matthias Van Ginneken,3Martin D.Suttle
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2020.104900]
1Department of Earth Science and Engineering, Imperial College London, Exhibition Road, London, SW7 2AZ, UK
2Department of Physics, University of Kent, Canterbury, Kent, UK
3Department of Earth Sciences, The Natural History Museum, Cromwell Road, London, SW7 2BD, UK

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Linking studies of tiny meteoroids, zodiacal dust, cometary dust and circumstellar disks

Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2020.104896]
1LATMOS, Sorbonne Université, CNRS, CNES, Paris, France
2IRAP, Université de Toulouse, CNES, CNRS, UPS, Toulouse, France
3European Southern Observatory, Alonso de Córdova 3107, Casilla, 19001, Santiago, Chile
4Univ. Grenoble Alpes, IPAG, Grenoble, France
5LPC2E, Université d’Orléans, CNRS, Orléans, France

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