¹M. Patzek, ¹A. Bischoff, ²R. Visser, ²T. John
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13175]
¹Institut für Planetologie, Westfälische Wilhelms‐Universität Münster, Münster, Germany
²Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstr. 74‐100, Berlin, Germany
Published by arrangement with John Wiley & Sons

Meteoritic breccias are valuable samples as they can contain rare materials from the early solar system as clasts. Volatile‐rich, CI‐ and CM‐like clasts may represent parent body lithologies, which cannot be found as individual meteorites in today’s meteorite collections. In order to reveal a better knowledge about the presence and chemical characteristics and variability of volatile (water‐bearing) materials in the early solar system these clasts play an important role. Such materials may have been available as the volatile component during the accretion of terrestrial planets. To understand the distribution of volatile‐rich materials in the solar system, we studied CI‐ and CM‐like clasts in brecciated meteorites including polymict ureilites, HEDs, CR, CB, CH, and ordinary chondrites. CI‐like clasts occur throughout all of the mentioned meteorite groups, whereas the CM‐like clasts have only been identified in HEDs and ordinary chondrites. The abundance of volatile‐rich clasts in general decreases in the order CH > CR > ureilites > HEDs > CB > OC > R. The mineralogy of CI‐like clasts is similar to CI chondrites, but their compositions of phyllosilicates differ. The mineralogy of CM‐like clasts clearly links them to CM chondrites. They must have been delivered to the HED parent body by low‐velocity impacts after differentiation and volcanism, as there is no evidence for high shock and heating processes. Additionally, we propose that CI‐like clasts in the CR, CB, and CH chondrites are a primary component of the appropriate parent bodies (accretionary breccias). Conversely, the CI‐like clasts in polymict ureilites and HEDs represent an infall as (micro)meteorites or as low‐velocity impactors, which happened after the accretion and differentiation of the appropriate parent bodies.

^1,2Ninja Braukmüller, ^1,2Frank Wombacher, ^1,3Dominik C.Hezel, ^1,2Raphaelle Escoube, ^1,2Carsten Münker
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.07.023]
¹Institut für Geologie und Mineralogie, Universität zu Köln, Zülpicher Str. 49b, 50674 Köln, Germany
²Steinmann Institut für Geologie, Mineralogie und Paläontologie, Universität Bonn, Poppelsdorfer Schloss, 53115 Bonn, Germany
³Natural History Museum, Department of Mineralogy, Cromwell Road, SW7 5BD, London, UK
Copyright Elsevier

In Earth and planetary sciences, the chemical composition of chondritic meteorites provides an essential reference to constrain the composition and differentiation history of planetary reservoirs. Yet, for many trace elements, and in particular for volatile trace elements the composition of chondrites is not well constrained. Here we present new compositional data for carbonaceous chondrites with an emphasis on the origin of the volatile element depletion pattern. Our database includes 25 carbonaceous chondrites from 6 different groups (CI, CM, CR, CV, CO, CK), two ungrouped carbonaceous chondrites and Murchison powder samples heated up to 1000°C in O₂ or Ar gas streams, respectively. A total of 51 major and trace elements were analyzed by sector field inductively coupled plasma mass spectrometry (SF-ICP-MS), using chondrite-matched calibration solutions. Our results confirm that parent body alteration and terrestrial weathering only have minor effects on the bulk chondrite compositions. Thermal metamorphism can lead to the loss of some volatile elements, as best observed in the heating experiments and two thermally overprinted chondrites Y-980115 (CI) and EET 96026 (CV4/5 or CK4/5). The effects of aqueous alteration and terrestrial weathering on the Antarctic samples are difficult to discriminate. Both processes may redistribute fluid mobile elements such as K, Na, Rb, U and LREE within the meteorite. In hot desert finds, the typical weathering effects are enrichments of Sr, Ba and U and a depletion of S.

In general, moderately volatile elements with 50% condensation temperatures (T_C) ranging from 1250 K to 800 K show an increasing depletion, whereas 11 moderately volatile elements with 50% T_C between 800 K and 500 K are unfractionated from each other in most samples. Their extent of depletion is characteristic for the different chondrite groups. Because of this well-defined “hockey stick” pattern, we propose to divide the moderately volatile elements into two subgroups, the ‘slope volatile elements’ and the unfractionated ‘plateau volatile elements’ with lower T_C. Notably, the abundances of plateau volatile elements exhibit a co-variation with the matrix abundances of the respective host meteorites. Carbonaceous chondrite matrices are likely mixes of: (i) CI-like material and (ii) chondrule-related matrix. Chondrule-related matrix is expected to be depleted in volatile elements relative to CI and likely formed contemporaneously with chondrules, leading to chondrule-matrix complementarity. The addition of CI-like material only changed the absolute elemental concentrations of bulk matrix and bulk chondrite, while refractory and main component element ratios such as Mg/Si remain unaffected. Such a model can also account for the co-existence of low temperature CI-like material and high temperature chondrule and chondrule-related matrix. However, elevated volatile element abundances observed in chondrules still provide a challenge for the model as proposed here.