¹Xeynab Mouti Al-Hashimi,¹Jemma Davidson,¹Devin L. Schrader,²Emma S. Bullock
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14076]
¹Buseck Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA
²Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC, USA
Published by arrangement with John Wiley & Sons

The Mighei-like carbonaceous (CM) chondrites, the most abundant carbonaceous chondrite group by number, further our understanding of processes that occurred in their formation region in the protoplanetary disk and in their parent body/bodies and provide analogs for understanding samples returned from carbonaceous asteroids. Chondrules in the CMs are commonly encircled by fine-grained rims (FGRs) whose origins are debated. We present the abundances, sizes, and petrographic observations of FGRs in six CMs that experienced varying intensities of parent body processing, including aqueous and thermal alteration. The samples studied here, in approximate order of increasing thermal alteration experienced, are Allan Hills 83100, Murchison, Meteorite Hills 01072, Elephant Moraine 96029, Yamato-793321, and Pecora Escarpment 91008. Based on observations of these CM chondrites, we recommend a new average apparent (2-D) chondrule diameter of 170 μm, which is smaller than previous estimates and overlaps with that of the Ornans-like carbonaceous (CO) chondrites. Thus, we suggest that chondrule diameters are not diagnostic for distinguishing between CM and CO chondrites. We also argue that chondrule foliation noted in ALH 83100, MET 01072, and Murchison resulted from multiple low-intensity impacts; that FGRs in CMs formed in the protoplanetary disk and were subsequently altered by both aqueous and thermal secondary alteration processes in their parent asteroid; and that the heat experienced by some CM chondrites may have originated from solar radiation of their source body/bodies during close solar passage as evidenced by the presence of evolved desiccation cracks in FGRs that formed by recurrent wetting and desiccation cycles.

¹M. V., ²G. Varga, Z. Dankházi,¹A. V. Chukin,³I. Felner,²E. Kuzmann,¹V. I. Grokhovsky,²Z. Homonnay,¹M. I. Oshtrakh
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14070]
¹Institute of Physics and Technology, Ural Federal University, Ekaterinburg, Russian Federation
²Department of Materials Physics, Eötvös Loránd University, Budapest, Hungary
³Racah Institute of Physics, The Hebrew University, Jerusalem, Israel
⁴Laboratory of Nuclear Chemistry, Institute of Chemistry, Eötvös Loránd University, Budapest, Hungary
Published by arrangement with John Wiley & Sons

A fragment of Mundrabilla IAB-ung iron meteorite was analyzed using optical microscopy, scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), magnetization measurements, and Mössbauer spectroscopy. The polished section of meteorite fragment characterization by optical microscopy and SEM shows the presence of the γ-Fe(Ni, Co) phase lamellae, plessite structures and schreibersite inclusions in the α-Fe(Ni, Co) phase. EDS indicates variations in the Ni concentrations in the following ranges: (i) ∼6.3–6.5 atom% in the α-Fe(Ni, Co) phase and (ii) ∼22 to ∼45 atom% in the γ-Fe(Ni, Co) phase lamellae including the range of ∼29–33 atom% of Ni leading to the paramagnetic state of the γ-Fe(Ni, Co) phase. Schreibersite inclusions contain ∼23 atom% of P, ∼33 atom% of Fe, ∼43 atom% of Ni, and ∼0.7 atom% of Co. Plessite structure contains the average Ni concentration of ∼17 atom% while detailed EDS analysis shows: (i) the lowest Ni concentrations of ∼5 to ∼8 atom%, (ii) the intermediate Ni concentrations of ∼9 to ∼19 atom%, and (iii) the highest Ni concentration up ∼38 atom% (some individual micro-grains demonstrate up to ∼47 and ∼59 atom% of Ni) that may indicate the presence of the (i) α-Fe(Ni, Co), (ii) α₂-Fe(Ni, Co), and (iii) γ-Fe(Ni, Co) phases. These may be a result of the γ-phase decomposition with mechanism γ → α + α₂ + γ that indicates a slow cooling rate for Mundrabilla IAB-ung iron meteorite. The presence of ∼98.6 wt% of the α-Fe(Ni, Co) phase and ∼1.4 wt% of the γ-Fe(Ni, Co) phase is found by XRD while schreibersite is not detected. Magnetization measurements show the saturation magnetization moment of Mundrabilla IAB-ung of 188(2) emu g⁻¹ indicating a low average Ni concentration in Fe-Ni-Co alloy. Mössbauer spectrum of the bulk Mundrabilla powder demonstrates five magnetic sextets related to the ferromagnetic α₂-Fe(Ni, Co), α-Fe(Ni, Co), and γ-Fe(Ni, Co) phases and one singlet associated with the paramagnetic γ-Fe(Ni, Co) phase, however, there are no spectral components corresponding to schreibersite. Basing on relatively larger and smaller values of the magnetic hyperfine field, two magnetic sextets associated with γ-Fe(Ni, Co) phase can be related to the disordered and more ordered γ-phases. The iron fractions in the detected phases can be roughly estimated as follows: (i) ∼17.6% in the α₂-Fe(Ni, Co) phase, (ii) ∼68.5% in the α-Fe(Ni, Co) phase, (iii) ∼11.5% in the disordered γ-Fe(Ni, Co) phase, (iv) ∼2.0% in the more ordered γ-Fe(Ni, Co) phase, and (v) ∼0.4% in the paramagnetic γ-Fe(Ni, Co) phase.