^1,2Ritesh Kumar Mishra
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.70050]
¹Independent Researcher, Bhagalpur, Bihar, 813211 India
²Veer Kunwar Singh University, Arrah, Bihar, 802301 India
Published by arrangement with John Wiley & Sons

Ca-Al-rich inclusions (CAIs), amoeboid-olivine aggregates (AOAs), and chondrules from the lowest petrographic type unequilibrated chondrites hold the potential to provide the best-preserved records of the origin and cosmochemical evolution of the solar system. Six CAIs, and three chondrules from Yamato (Y) 81020 (CO3.05), and one AOA and one spinel-bearing chondrule from Allan Hills (ALHA)77307 (CO3.03) were analyzed for 26Al-26Mg (t1/2 = 0.72 Ma) short-lived now-extinct radioisotope decay systematics. Five CAIs from Y-81020 and an AOA from ALHA77307 show a small range of abundance of 26Al/27Al from ~4.5 × 10−5 to 3.2 × 10−5. The inferred abundances in these relatively small-sized CAIs and AOA suggest their formation and/or resetting during distinct episodes spanning a few million years. The inferred time of formation of these small CAIs and AOA from the lowest petrographic type in Y-81020 and ALHA77307 is consistent with the previous results of high-precision analyses of three CAIs from Y-81020. The obtained results in CO chondrites are also in agreement with CR chondrites and with an order of magnitude larger-sized CAIs in CV (Vigarano) chondrites. 26Al/27Al abundances in the three analyzed chondrules imply their formation within the typical range of ~1 to 2 million years after the formation of CAIs. The observed 26Al/27Al abundances and initial magnesium isotopic compositions of these small CAIs and AOA in the weakly metamorphosed CO chondrites are in consonance with the previous studies of CAIs and AOAs in CV chondrites that inferred the formation and evolution of these objects from a homogeneous reservoir that existed at the birth of the solar system.

¹Nicolas P. Walte,²Max Collinet,³Cyrena A. Goodrich
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70063]
¹Heinz Meier-Leibnitz Centre for Neutron Science (MLZ), Technical University Munich, Garching, Germany
²Institute of Life, Earth and Environment (ILEE), University of Namur, Namur, Belgium
³Lunar and Planetary Institute, USRA, Houston, Texas, USA
Published by arrangement with John Wiley & Sons

Ureilites are carbon-rich ultramafic achondrites that display unique textures, including strips of metal and carbon phases situated along grain boundaries and in fractures. Shock metamorphism observed in ureilites suggests an episode of brittle deformation caused by impact disruption of their parent body. The origin of carbon and metal has long been debated; in particular, whether either is endogenous or at least partly exogenous. We conducted experiments to simulate the metal-carbon textures and constrain their origin. Two model systems were investigated: (A) intrusion of FeS melt (analog for metal) into an olivine matrix containing dispersed graphite and (B) intrusion of graphite into a matrix containing dispersed FeS. After static annealing at 0.5–2 GPa and 1300°C, the samples were deformed at high strain rates to simulate an impact event. The microstructures of system A most closely resembled the textures observed in medium to low-shock main group ureilites, supporting an endogenous origin of carbon and a largely exogenous origin of metal. The grain boundary linings of ureilites were formed by impactor metal that intruded along grain boundaries and mixed with locally mobilized carbon. Hence, we establish a direct connection between the metal-carbon textures in ureilites and the collision history of their parent body.