¹Jian Chen, ²Tiekuang Dong, ^1,3Zhongzhou Ren
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12817]
¹Department of Physics and Key Laboratory of Modern Acoustics, Institute of Acoustics, Nanjing University, Nanjing, China
²Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, CAS, Nanjing, China
³Center of Theoretical Nuclear Physics, National Laboratory of Heavy-Ion Accelerator, Lanzhou, China
Published by arrangement with John Wiley & Sons

A physical model based on the open-source toolkit Geant4 for production rates of cosmogenic nuclei on the lunar surface is proposed and calibrated. The fluxes of proton and neutron beneath the lunar surface are obtained by simulating the physical processes between the cosmic-ray particles and the lunar surface material. By combining the experimental proton cross sections and the a posteriori neutron cross sections, we calculate the production rate depth profiles of long-lived nuclei (10Be, 14C, 26Al, 36Cl, and 53Mn). Through comparing experimental and theoretical data for these nuclei, we find that for all the selected nuclei, experimental and theoretical production rate depth profiles agree well with each other by introducing a single normalization factor. It means that the physical model based on Geant4 can also reproduce the depth profiles of cosmogenic nuclei, and that this model can be used by everyone worldwide. In addition, we predict the production rates of three stable nuclei (21Ne, 22Ne, and 38Ar).

¹I. S. Sanders, ²E. R. D. Scott, ³J. S. Delaney
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12820]
¹Department of Geology, Trinity College, Dublin 2, Ireland
²Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Mānoa, Honolulu, Hawaii, USA
³Department of Geological Sciences, Rutgers University, Piscataway, New Jersey, USA
Published by arrangement with John Wiley & Sons

Ureilite meteorites are abundant, carbon-rich, primitive achondrites made of coarse-grained, equilibrated olivine and pyroxene (usually pigeonite). They probably sample the baked, heterogeneous, melt-depleted mantle of a large, once-chondritic parent body that was broken up catastrophically while still young and hot. Heterogeneity in the parent body is inferred from a considerable “slope-1” variation from one meteorite to another in oxygen isotopes (−2.5‰ < Δ17O < −0.2‰), which correlates with both molar FeO/MgO (range 0.03–0.35) and molar FeO/MnO (range 3–57), i.e., Δ17O correlates with the redox state. No consensus has yet emerged on the cause of these correlated trends. One view favors their inheritance via silicates from hot nebular (preaccretion) processes. Another invokes smelting (reduction of FeO by C in the hot parent body). Here, guided mainly by similar trends among equilibrated ordinary and R chondrites, studies of their unequilibrated counterparts, and work on other primitive achondrites, we propose a new model for ureilites in which the parent body accreted nebular ice with high-∆17O, Mg-rich silicates with low ∆17O, and varying amounts of metallic iron. Water from the thawing ice then oxidized the metal yielding secondary FeO-bearing minerals with high ∆17O that, with metamorphism, became incorporated into the ureilite silicates. FeO/MgO, FeO/MnO, and ∆17O correlate because they rose in unison by amounts that varied spatially, depending on the local amount of metal that was oxidized. We suggest that the parent body was so large (radius ≫ 100 km) that smelting was inhibited and that carbon played a passive role in ureilite evolution. Although ureilites are regarded as complicated meteorites, we believe our analysis explains their mass-independent oxygen isotope trend and related FeO variation through well-understood processes and enlightens our understanding of the evolution of early planetesimals from cold, wet bodies to hot, dry ones.

¹Krzysztof Szopa, ¹Tomasz Brachaniec, ¹Łukasz Karwowski, ¹Tomasz Krzykawski
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12815]
¹Department of Geochemistry, Mineralogy and Petrography, Faculty of Earth Science, University of Silesia, Sosnowiec, Poland
Published by arrangement with John Wiley & Sons

Fossil iron meteorites are extremely rare in the geological sedimentary record. The paleometeorite described here is the first such finding at the Cretaceous-Paleogene (K-Pg) boundary. In the boundary clay from the outcrop at the Lechówka quarry (Poland), fragments of the paleometeorite were found in the bottom part of the host layer. The fragments of meteorite (2–6 mm in size) and meteoritic dust are metallic-gray in color and have a total weight of 1.8181 g. Geochemical and petrographic analyses of the meteorite from Lechówka reveal the presence of Ni-rich minerals with a total Ni amount of 2–3 wt%. The identified minerals are taenite, kamacite, schreibersite, Ni-rich magnetite, and Ni-rich goethite. No relicts of silicates or chromites were found. The investigated paleometeorite apparently represents an independent fall and does not seem to be derived from the K-Pg impactor. The high degree of weathering did not permit the chemical classification of the meteorite fragments. However, the recognized mineral inventory, lack of silicates, and their pseudomorphs and texture may indicate that the meteorite remains were an iron meteorite.