Experimental insights into Stannern‐trend eucrite petrogenesis

1,2S. D. Crossley, 3N. G. Lunning, 4R. G. Mayne, 3T. J. McCoy, 4S. Yang, 4M. Humayun, 5R. D. Ash, 6J. M. Sunshine, 7R. C. Greenwood, 7I. A. Franchi
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13114]
1Monnig Meteorite Collection, Texas Christian University, , Fort Worth, Texas, USA
2Department of Geology, University of Maryland, , Maryland, USA
3Department of Mineral Sciences, Smithsonian Institution, National Museum of Natural History, Washington, DC, USA
4National High Magnetic Field Laboratory and Department of Earth, Ocean & Atmospheric Science, Florida State University, Tallahassee, Florida, USA
5Department of Geology, University of Maryland, , Maryland, USA
6Department of Astronomy, University of Maryland, , Maryland, USA
7Planetary and Space Sciences, School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, UK
Published by arrangement with John Wiley & Sons

The incompatible trace element‐enriched Stannern‐trend eucrites have long been recognized as requiring a distinct petrogenesis from the Main Group‐Nuevo Laredo (MGNL) eucrites. Barrat et al. (2007) proposed that Stannern‐trend eucrites formed via assimilation of crustal partial melts by a MGNL‐trend magma. Previous experimental studies of low‐degree partial melting of eucrites did not produce sufficiently large melt pools for both major and trace element analyses. Low‐degree partial melts produced near the solidus are potentially the best analog to the assimilated crustal melts. We partially melted the unbrecciated, unequilibrated MGNL‐trend eucrite NWA 8562 in a 1 atm gas‐mixing furnace, at IW‐0.5, and at temperatures between 1050 and 1200 °C. We found that low‐degree partial melts formed at 1050 °C are incompatible trace element enriched, although the experimental melts did not reach equilibrium at all temperatures. Using our experimental melt compositions and binary mixing modeling, the FeO/MgO trend of the resultant magmas coincides with the range of known Stannern‐trend eucrites when a primary magma is contaminated by crustal partial melts. When experimental major element compositions for eucritic crustal partial melts are combined with trace element concentrations determined by previous modeling (Barrat et al. 2007), the Stannern‐trend can be replicated with respect to both major, minor, and trace element concentrations.

Accumulation mechanisms of micrometeorites in an ancient supraglacial moraine at Larkman Nunatak, Antarctica

1,2Matthew J. Genge, 1,2Matthias van Ginneken, 1,2Martin D. Suttle, 3Ralph P. Harvey
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13107]
1Department of Earth Science and Engineering, Imperial College London, , London, UK
2Department of Earth Science, The Natural History Museum, London, UK
3Department of Geological Sciences, Case Western Reserve University, Cleveland, Ohio, USA
Published by arrangement wit John Wiley & Sons

We report the discovery of a large accumulation of micrometeorites (MMs) in a supraglacial moraine at Larkman Nunatak in the Grosvenor Mountains of the Transantarctic Range in Antarctica. The MMs are present in abundances of ~600 particles kg−1 of moraine sediment and include a near‐complete collection of MM types similar to those observed in Antarctic blue ice and within bare‐rock traps in the Antarctic. The size distribution of the observed particles is consistent with those collected from snow collections suggesting the moraine has captured a representative collection of cosmic spherules with significant loss of only the smallest particles (<100 μm) by wind. The presence of microtektites with compositions similar to those of the Australasian strewn field suggests the moraine has been accumulating for 780 ka with dust‐sized debris. On the basis of this age estimate, it is suggested that accumulation occurs principally through ice sublimation. Direct infall of fines is suggested to be limited by snow layers that act as barriers to accumulation and can be removed by wind erosion. MM accumulation in many areas in Antarctica, therefore, may not be continuous over long periods and can be subject to climatic controls. On the basis of the interpretation of microtektites as Australasian, Larkman Nunatak deposit is the oldest known supraglacial moraine and its survival through several glacial maxima and interglacial periods is surprising. We suggest that stationary ice produced by the specific ice flow conditions at Larkman Nunatak explains its longevity and provides a new type of record of the East Antarctic ice sheet.

D/H fractionation during sublimation of water ice at low temperatures into a vacuum

1James Mortimer, 2Christophe Lécuyer, 2François Fourel, 3James Carpenter
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2018.05.010]
1Planetary and Space Sciences, School of Physical Sciences, The Open University, Walton Hall, MK7 6AA, Milton Keynes, Buckinghamshire, United Kingdom
2Laboratoire de Géologie de Lyon, CNRS UMR 5276, Université Claude Bernard Lyon 1, 69622, Villeurbanne, France
3ESA ESTEC, Keplerlaan 1, 2401, AZ, Noordwijk, the Netherlands

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

What is controlling the reflectance spectra (0.35- 150 µm) of hydrated (and dehydrated) carbonaceous chondrites?

1,2Pierre Beck,3A.Maturilli,1A.Garenne,4P.Vernazza,3J.Helbert,1E.Quirico,1B.Schmitt
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.05.010]
1Institut de Planétologie et d’Astrophysique de Grenoble, France.
2Institut Universitaire de France.
3DLR, Berlin, Germany.
4Aix Marseille Univ, CNRS, LAM, Laboratoire d’Astrophysique de Marseille, Marseille, France
Copyright Elsevier

In order to determine the controls on the reflectance spectra of hydrated carbonaceous chondrites, reflectance spectra were measured for a series of samples with well-determined mineralogy, water-content, and thermal history. This includes 5 CR chondrites, 11 CM chondrites, and 7 thermally metamorphosed CM chondrites. These samples were characterized over the 0.35 to 150 µm range by reflectance spectroscopy in order to cover the full spectral range accessible from ground based observation, and that will be determined in the near-future by the Hayabusa-2 and Osiris-REx missions. While spectra show absorption features shortward of 35 µm, no strong absorption bands were identified in this suite of samples longward of 35 µm. This work shows that the 0.7-µm band observed in hydrated carbonaceous chondrites is correlated with the total water content as well as with the band depth at 2.7 µm, confirming the suggestion that they are related to Mg-rich, Fe-bearing phyllosilicates. A feature at 2.3 µm, diagnostic of such phyllosilicates was found for all samples with a detectable 0.7-µm band, also indicative of Mg-rich phyllosilicates.
A strong variability is found in the shape of the 3-µm band among CM chondrites, and between CM, CR and thermally metamorphosed CM chondrites. Heavily altered CM chondrites show a single strong band around 2.72 µm while more thermally metamorphosed CM samples show an absorption band at higher wavelength. The CR chondrite GRO 95577 has a 3-µm feature very similar to those of extensively altered CM chondrites while other CR chondrite rather shows goethite-like signatures (possibly due to terrestrial weathering of metals). Thermally metamorphosed CM chondrites all have 3-µm features, which are not purely due to terrestrial adsorbed water. The band shape ranges from heavily altered CM-like to goethite-like.
The overall reflectance was found to be significantly higher for CR chondrites than for CM chondrites. This is also true for the hydrated CR chondrite GRO 95577 whose reflectance spectrum is almost identical to spectra obtained for CM chondrites except that it is brighter by about 40 % in the visible. Another possibility to distinguish hydrated CM from hydrated CR chondrites is to use the combination of band depths at 0.7 and 2.3 µm.
When comparing the spectra obtained with Cg and Cgh spectral end member, it is found that the band depth determined for hydrated chondrites (0.7 and 2.3 µm) are always higher than calculated for these spectral endmembers. If one considers only asteroids with unambiguous hydration detection, band depth at 0.7 µm are of similar values to those measured for hydrated carbonaceous chondrites.

Er, Yb, and Hf isotopic compositions of refractory inclusions: An integrated isotopic fingerprint of the Solar System’s earliest reservoir

1Quinn R.Shollenberger,1Jan Render, 1Gregory A.Brennecka
Earth and Planetary Science Letters 495, 12-23 Link to Article [https://doi.org/10.1016/j.epsl.2018.05.007]
1Institut für Planetologie, University of Münster, Wilhelm-Klemm-Straße 10, Münster, 48149 Germany
Copyright Elsevier

The oldest dated solids in our Solar System, calcium–aluminum-rich inclusions (CAIs), contain isotopic anomalies in a whole suite of elements relative to later formed Solar System materials. Previous work has reported differences in the proportions of nucleosynthetic components between CAIs and terrestrial rocks as a function of mass. However, the nucleosynthetic fingerprint of the CAI-forming region is still lacking significant data in the heavier mass range (A > 154). Therefore, we present the first erbium (Er) and ytterbium (Yb) isotopic data along with hafnium (Hf) isotopic compositions in a wide variety of CAIs derived from a variety of CV and CK chondrites. This work presents new methods for Er and Yb isotopic investigation that were explored using both thermal ionization mass spectrometry (TIMS) and multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). Relative to terrestrial rock standards, CAIs—regardless of host rock, petrologic or chemical classification—have uniform and resolvable Er, Yb, and Hf isotopic compositions. The CAI isotopic patterns correspond to r-process deficits (or s-process excesses) relative to terrestrial values of 9 ppm for Er, 18 ppm for Yb, and 17 ppm for Hf. This new Er, Yb, and Hf data help complete the nucleosynthetic fingerprint of the CAI-forming region, further highlighting the systematic difference between the CAIs and later formed bulk planetary bodies. Such a systematic difference between CAIs and terrestrial rocks cannot be caused by different amounts of any known single presolar phase but is likely the result of a well-mixed reservoir made of diverse stellar sources.