¹Tasha L. Dunn,¹Oriana K. Battifarano,^2,3Juliane Gross,¹Emma J. O’Hara
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13118]
¹Department of Geology, Colby College Waterville, Maine, USA
²Department of Earth and Planetary Sciences, Rutgers University Piscataway, New Jersey, USA
³Department of Earth and Planetary Sciences, American Museum of Natural History New York, New York, USA
Published by arrangement with John Wiley & Sons

Based on the chemical heterogeneity of chondrule and matrix olivine, Northwest Africa (NWA) 5343 is the least metamorphosed CK chondrite reported so far. To better constrain the lower limit of metamorphism in the CK chondrites, we performed a detailed analysis of matrix material in NWA 5343, including characterization of the texture and bulk composition and analyses of individual silicate minerals. Results suggest that NWA 5343 is petrologic type 3.6 or 3.7. Although silicate minerals in the matrix seem to be equilibrated to roughly the same extent throughout the sample, there are recognizable differences in grain size and shape. These textural differences may be the result of transient heating events during impacts, which would be likely on the CK chondrite parent body. The difference between the extent of chemical equilibration and texture may also suggest that grain size and shape are still sensitive to metamorphism at petrologic subtypes where silicate mineral equilibration is nearly complete (e.g., >3.7). Carbonate material present in NWA 5343 is a product of terrestrial weathering; however, infiltration of a Ca‐bearing fluid did not influence the composition of silicate minerals in the matrix. To evaluate the possibility of a continuous metamorphic sequence between the CV and CK chondrites, the bulk matrix composition of NWA 5343 is compared to the CVred chondrite, Vigarano. Although the matrix composition of NWA 5343 could be derived by secondary processing of a Vigarano‐like precursor, porosity and texture of matrix olivine in NWA 5343 are hard to reconcile with a continuous metamorphic sequence.

¹J. Leitner, ²C. Vollmer, ³T. Henkel, ^1,4U. Ott, ¹P. Hoppe
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.05.025]
¹Max Planck Institute for Chemistry, Particle Chemistry Department, Hahn-Meitner-Weg 1, 55128 Mainz, Germany
²Institute for Mineralogy, Westfälische Wilhelms-Universität, Correnstrasse 24, 48149 Münster, Germany
³The University of Manchester, School of Earth and Environmental Sciences, Williamson Building, Oxford Road, Manchester, M13 9PL, UK
⁴MTA Atomki, Bem tér 18/c, 4826 Debrecen, Hungary
Copyright Elsevier

We report an in-situ investigation of silicon nitride (Si3N4) grains from several enstatite chondrites of low petrologic types (EL3, EH3, and EH4). The grains occurred in various host phases, including Fe,Ni metal, schreibersite, sulfides, and also in the silicate fraction, and are of Solar System origin. Energy-dispersive X-ray spectroscopy (EDS) showed that carbon and oxygen are present in all investigated grains. Carbon- and N-isotopic compositions of 288 grains were measured by NanoSIMS. Nitrogen is isotopically light compared to terrestrial air, with an average δ15N = –60±1 ‰. The average carbon isotopic composition does not deviate significantly from the terrestrial PDB standard. TOF-SIMS investigation of one particularly large Si3N4 grain (10 µm ×2 µm) showed that the O is located within the grain and not in adjacent particles, and also revealed the presence of chromium. Transmission electron microscopy (TEM) analysis showed that the Cr is present as carlsbergite (CrN) inclusions. TEM investigation of three Si3N4 grains showed them to be polycrystalline, with no consistent crystallographic relationship with the host material. Estimated Si3N4 abundances for four metal-sulfide assemblages demonstrate that the amount of nitrogen bound in the nitrides exceeds the maximum concentration of N that can be stored in Fe,Ni metal in solid solution. Thus, even if all of the N in the metal would have been exsolved, it would not have been enough to form the observed amounts of Si3N4 grains, giving further evidence against formation by exsolution. This clearly shows that they did not form by exsolution from the host materials, as has been suggested in earlier studies. Instead, the Si3N4 must have formed prior to incorporation into the enstatite chondrite parent bodies, either by shock wave-induced condensation processes, or by precipitation from the host phases in the presence of NH3.