¹John P.Bradley,¹Hope A.Ishii,²Karen Bustillo,²James Ciston,³Ryan Ogliore,^4,5Thomas Stephan,⁶Donald E.Brownlee,⁶David J.Joswiak
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.06.036]
¹Hawai‘i Institute of Geophysics and Planetology, University of Hawai ‘i at Mānoa, 1680 East-West Road, Honolulu, HI 96822, USA
²National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
³Laboratory for Space Sciences, Washington University, St. Louis, MO 63130, USA
⁴Department of Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USA
⁵Chicago Center for Cosmochemistry, Chicago, IL, USA
⁶Department of Astronomy, University of Washington, Seattle WA 98195, USA
Copyright Elsevier

The provenance of GEMS (glass with embedded metal and sulfides) in cometary type interplanetary dust particles is investigated using analytical scanning transmission electron microscopy and secondary ion mass spectrometry. We review the current state of knowledge and closely examine the densities, elemental compositions and distributions, iron oxidation states, and isotopic compositions of a subset of GEMS in chondritic porous interplanetary dust. We find that GEMS are underdense with estimated densities that are 35–65% of compositionally equivalent crystalline aggregates. GEMS low densities result in a lower contribution to the bulk compositions of IDPs than has been assumed based on their volume fraction. We also find that element/Si ratios, assumed to be primary (indigenous), are instead perturbed by contamination and secondary alteration, including pulse heating during atmospheric entry. Fe in pyrrhotite inclusions was oxidized and Mg, S, Ca, and Fe were depleted relative to lithophile Al and Si, resulting in reduction in element/Si ratios. Because they trap outgassing elements, Fe-rich oxide rims that formed on the surface of GEMS are serendipitous “witness plates” to the changes in composition that accompany atmospheric entry. As a result of alteration, GEMS elemental compositions cannot reliably inform about their provenance. Except for highly anomalous oxygen isotope ratios measured in some large GEMS grains that indicate a contribution from circumstellar dust, oxygen isotope compositions are generally poor indicators of provenance. Prior work indicates that most GEMS fall close to the terrestrial oxygen isotope composition, which, however, does not exclude a presolar interstellar origin. Nitrogen isotopic compositions are more diagnostic. Elevated 15N/14N ratios indicate that GEMS accreted in conjunction with formation of organic matter by ion–molecule reactions in a cold (<50 K) presolar environment like the extreme outer nebula or interstellar medium. Considering all observations, we conclude that GEMS are most likely processed interstellar silicates.

^1,2L.Folco,³L.Carone,^1,2M.D’Orazio,⁴C.Cordier,^5,1M.D.Suttle,⁶M.van Ginneken,^1,2M.Masotta
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.06.035]
¹Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, Pisa, Italy
²CISUP, Centro per la Integrazione della Strumentazione dell’Università di Pisa, Lungarno Pacinotti, Pisa, Italy
³Università di Camerino, Scuola di Scienze e Tecnologie, Sezione di Geologia, Via Gentile III da Varano 7, 62032 Camerino, Italy
⁴Université Grenoble Alpes, Université Savoie Mont Blanc, CNRS, IRD, IFSTTAR, ISTerre, 38000 Grenoble, France
⁵School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
⁶Centre for Astrophysics and Planetary Science, School of Physical Sciences, Ingram Building, University of Kent, Canterbury CT2 7NH, UK
Copyright Elsevier

Kamil crater (Egypt) is a natural laboratory for the study of processes and products associated with the impacts of small iron projectiles on the Earth’s crust. In particular, because of the distinctive composition of the impactor (an ungrouped Ni-rich ataxite) and the target (Cretaceous sandstones and minor wackes) it offers a unique opportunity to study impactor–target physical–chemical interactions. Continuing the study of impact melt ejecta, we investigated the mineralogy and geochemistry of 25 Fe-Ni spherules – representative of a suite of 135 – recovered from the soil around the crater. Samples were collected during our 2010 geophysical expedition and investigated by combining scanning electron microscope imaging, electron probe microanalyzer and Raman spectroscopy analyses. Spherules range in size from 100 to 500 µm and show a variety of dendritic textures and mineral compositions dominated by Fe-Ni oxides of the wüstite – bunsenite and magnetite – trevorite series or Fe-Ni metal. All these features indicate quenching of high temperature (1600–1500 °C) oxide or metal liquid droplets under varying oxidizing conditions. A geochemical affinity with the iron impactor recorded by the Fe, Co, Ni ratios in the constituent phases (average Ni/Co element ratio of 25.1 ± 7.6; average Ni/(Ni + Fe) molar ratio of 0.21 ± 0.13), combined with target contamination (i.e., the ubiquitous occurrence of Si and Al from trace to minor amounts), document their origin as impact melt spherules formed through the physical and chemical interaction between metal projectile and silicate target melts and air. We propose a petrogenetic model that envisions formation as liquid droplet residues of immiscible projectile in a mixed silicate melt and their subsequent separation as individual spherules by stripping during hypervelocity ejection. We also argue that this model applies to all impact events produced by small iron projectiles and that such individual Fe-Ni oxide and metal spherules should be common impact products, despite little documentation in the literature. Our detailed mineralogical and geochemical characterization will facilitate their distinction from other, similar spherules of different origin (cosmic spherules, ablation spherules) often encountered in the geologic record.

¹Andrew K. Matzen,²Alan Woodland,³John R. Beckett,¹Bernard J. Wood
American Mineralogist 107, 1442-1452 Link to Article [http://www.minsocam.org/MSA/AmMin/TOC/2022/Abstracts/AM107P1442.pdf]
¹Department of Earth Sciences, University of Oxford, Oxford, OX1 3AN, U.K.
²Goethe-Universität Institut für Geowissenschaften, Frankfurt, D-60438, Germany
³California Institute of Technology, MC 170-25, Pasadena, California 91125, U.S.A.
Copyright: The Mineralogical Society of America

We performed a series of experiments at 1 atm pressure and temperatures of 1300–1500 °C to
determine the effect of oxygen fugacity on the oxidation state of Fe in a synthetic martian basalt. Ferricferrous ratios were determined on the quenched glasses using Mössbauer spectroscopy. Following the conventional doublet assignments in the spectrum, we obtain a Fe3+/ΣFe value of 0.19 at 1450 °C and
an oxygen fugacity corresponding to the QFM buffer. If we apply the Berry et al. (2018) assignments
the calculated Fe3+/ΣFe drops to 0.09, and the slope of log(XFe melt O1.5/XFe melt O) vs. log( fO2) changes from 0.18 to 0.26.
Combining oxidation state data together with results of one additional olivine-bearing experiment
to determine the appropriate value(s) for the olivine (Ol)-liquid (liq) exchange coefficient, KD,Fe2+-Mg
= (FeO/MgO)Ol/(FeO/MgO)liq (by weight), suggests a KD,Fe2+-Mg of 0.388 ± 0.006 (uncertainty is one
median absolute deviation) using the traditional interpretation of Mössbauer spectroscopy and a value
of 0.345 ± 0.005 following the Mössbauer spectra approach of Berry et al. (2018).
We used our value of KD,Fe2+-Mg to test whether any of the olivine-bearing shergottites represent liquids.
For each meteorite, we assumed a liquid composition equal to that of the bulk and then compared that
liquid to the most Mg-rich olivine reported. Applying a KD,Fe2+-Mg of ~0.36 leads to the possibility that
bulk Yamato 980459, NWA 5789, NWA 2990, Tissint, and EETA 79001 (lithology A) represent liquids.