¹Alvaro Penteado Crósta,²Neivaldo Araújo de Castro,³Marcos Alberto Rodrigues Vasconcelos,⁴Airton Natanael Coelho Dias,^5,6Ludovic Ferrière,⁷Wolf Uwe Reimold,⁸Ana Maria Góes,⁹Jackson Alves Martins,²Liliana Sayuri Osako,⁹Raimundo Mariano Gomes Castelo Branco
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70144]
¹Institute of Geosciences, Universidade Estadual de Campinas, Campinas, Brazil
²Geology Department, Federal University of Santa Catarina-UFSC, Florianopolis, Brazil
³Institute of Geosciences, Federal University of Bahia, Salvador, Brazil
⁴Physics, Chemistry and Mathematics Department, Federal University of Sao Carlos, Sorocaba, Brazil
⁵Natural History Museum Abu Dhabi, Abu Dhabi, United Arab Emirates
⁶Department of Lithospheric Research, University of Vienna, Vienna, Austria
⁷Instituto de Geociencias, Universidade de Brasilia, Brasilia, Brazil
⁸Institute of Geosciences, University of Sao Paulo, Sao Paulo, Brazil
⁹Geophysics Laboratory, Centro de Ciencias, Federal University of Ceara, Fortaleza, Brazil
Published by arrangement with John Wiley & Sons

The São Miguel do Tapuio structure (SMT) is a remarkable, nearcircular feature of about 21 km diameter, centered at 5°37.6′ S, 41°23.3′ W in Piauí state, northeastern Brazil. The structure is located within the sedimentary strata of the Paleozoic–Mesozoic Parnaíba Basin and predominantly comprises sandstones of the Devonian Pimenteiras and Cabeças formations. SMT exhibits a rugged morphology, in contrast to the smoother surrounding terrain. An impact origin has been suggested for SMT since the 1980s based on indirect aspects, such as the structure’s morphology with an annular outer rim, inner rings, and an elevated central area. Some of the sandstones found in the inner region were structurally deformed and recrystallized, in contrast to the undeformed equivalent strata outside the structure. A field survey conducted in 2017 yielded a few samples of sandstone and monomict sandstone breccia from near the center of the structure. Here we report the discovery of multiple shocked quartz grains with planar fractures (PFs), feather features (FFs), and planar deformation features (PDFs) in four thin sections of two samples from this central area. Universal stage measurements on all shocked quartz grains (25 grains in total; 16 with PFs, five with PDFs, and four with both PFs and PDFs) confirm that these planar microstructures occur in distinct crystallographic orientations that are indicative of shock pressures up to 20 GPa. Our investigations, thus, have confirmed the impact origin of the SMT. It represents the ninth confirmed impact structure in Brazil and, at 21 km diameter, is the second largest of its kind in South America.

¹S.P. Goudy, ¹M. Telus, ²K. Nagashima, ²G.R. Huss
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2026.04.014]
¹Earth and Planetary Sciences, University of California at Santa Cruz, 1156 High Street, Room A232, Santa Cruz, CA 95064, United States
²Hawaii Institute of Geophysics and Planetology, University of Hawai’i at Mānoa, 1680 East-West Road, POST Building, Office 602, Honolulu, HI 96822, United States
Copyright Elsevier

Here we present petrographic, O isotope, and C isotope data on calcite and petrographic and O isotope data on magnetite in Aguas Zarcas (CM2) and Miller Range (MIL) 13005 (CM1/2) in an effort to test CM aqueous alteration models. Our O and C isotope data for Aguas Zarcas and MIL 13005 are within the ranges reported in previous work for calcite in CMs. Using O isotope data from Δ17O-matched calcite and magnetite grains in our samples, we calculated equilibrium-model formation temperatures for the analyzed calcites in each meteorite. Combining our isotopic and temperature data with literature data, we sort the data into less altered (CM2) and more altered (CM1/2 and CM1) categories, and examine that data for differences between the categories by analyzing the δ18O-δ17O, δ18O-δ13C, and model formation temperature data of the categories. We find potential differing cluster patterns between δ18O and δ13C in calcites between our two CM alteration categories, and find that sparse extant temperature data imply that more altered CMs (types 1 and 1/2) may have undergone alteration at a lower average temperature than CM2s. We also find that the O isotopic compositions between CM1s, CM2s, and CM1/2s do not differ significantly. Through use of a novel mass-balance model, we infer a pre-alteration ice Δ17O within the range of 5.3–13.3‰. We find two generations of calcite formation in MIL 13005 with different proportions of their O being sourced from ices and anhydrous silicates, and one generation of calcites within Aguas Zarcas. We created concentration mixing models through an original machine-learning-based analytical approach, which show that the calcite data can be readily explained using three optimally determined C- and O-bearing sources.