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      Single chondrule K-Ar and Pb-Pb ages of Mexican ordinary chondrites as tracers of extended impact events Translated title: Edades K-Ar y Pb-Pb de condros individuales de condritas ordinarias mexicanas como trazadores de eventos de impacto prolongadas

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          Abstract

          There is a good consensus about the age of the most primitive materials in the Solar System represented by carbonaceous chondrites. From these, a canonical age of the Solar System of 4567 Ma has been defined. Nevertheless, the age of the subsequent events that modified the accreted material that formed meteorites has been only partially constrained by geochronological methods. In this work, we report K-Ar and Pb-Pb ages for individual chondrules from eight Mexican chondrites: three H5 (Cosina, Nuevo Mercurio and Aldama), one LL5 (Tuxtuac), one L4 (Zapotitlán Salinas), one L5 (El Pozo), one L6 (Pácula), and one carbonaceous CV3 (Allende). Analysis were performed by using combined isotope dilution thermal ionization mass spectrometry and noble gas mass spectrometry. The purpose was to obtain some understanding of the thermal history of these meteorites and their parent bodies. The analyzed chondrules and matrix show a comprehensive K-Ar age range from the Solar System formation to 442 Ma, demonstrating that ordinary chondrites preserve an extensive record of disruption and accretion of their parent bodies. The large time span recorded in the analyzed meteorites suggests that some chondrules were able to maintain a closed isotopic system, whereas the whole rock and other chondrules did not. To explain these ages, we suggest that partial heating from impact events, presumably from protracted bombardment of the parent body surface that also affected the surfaces of the Moon and Mars, is responsible for resetting the K-Ar chronometer. It is remarkable that some of the studied chondrules show pre-solar ages similar to those reported for the Allende meteorite by the Ar-Ar method, implying that primordial bodies incorporated some kind of pre-solar materials, as suggested previously.

          Translated abstract

          La edad del Sistema Solar se ha establecido en 4567 Ma, tomando en cuenta la edad obtenida en condritas carbonáceas consideradas como el material más primitivo del Sistema Solar. Sin embargo, la edad de los eventos posteriores que modificaron los materiales primigenios ha sido definida sólo parcialmente a través del uso de diferentes métodos geocronológicos. Para establecer la historia térmica de ocho condritas mexicanas y sus cuerpos parentales, en este trabajo se ha utilizado una combinación de análisis isotópicos por espectrometría de masas con ionización térmica, dilución isotópica y espectrometría de masas de gases nobles aplicada a condros individuales a fin de obtener edades K-Ar y Pb-Pb. Las condritas analizadas son tres del tipo H5 (Cosina, Nuevo Marcurio y Aldama), una del tipo LL5 (Tuxtuac), una L4 (Zapotitlán Salinas), una L5 (El Pozo), una L6 (Pácula), todas ellas del tipo ordinario, además de una carbonácea CV3 (Allende). Los condros analizados y la matriz de algunos de ellos muestran un rango amplio de edades K-Ar que van desde 4581 hasta 442 Ma, señalando que las condritas preservan un registro extenso de perturbaciones en el proceso de acreción de sus cuerpos parentales. Se sugiere que el calentamiento parcial de los cuerpos parentales es el responsable de la gran diversidad de edades registradas en los condros, roca total y matrices de los meteoritos analizados. Los eventos prolongados e intensos de impacto bien documentados en las superficies de Marte y la Luna pudieron generar calentamiento y reiniciar parcialmente el cronómetro K-Ar de los cuerpos parentales de estos meteoritos. Algunos condros de este estudio muestran edades presolares, similares a aquéllas de Ar-Ar reportadas previamente en el meteorito de Allende, sugiriendo que los cuerpos primordiales incorporaron algún tipo de material presolar, como ya se ha propuesto anteriormente.

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                Author and article information

                Journal
                rmcg
                Revista mexicana de ciencias geológicas
                Rev. mex. cienc. geol
                Instituto de Geología, UNAM (México, DF, Mexico )
                1026-8774
                2007-2902
                April 2010
                : 27
                : 1
                : 123-133
                Affiliations
                México D.F. orgnameUniversidad Nacional Autónoma de México orgdiv1Instituto de Geología Mexico msol@ 123456geologia.unam.mx
                Article
                S1026-87742010000100011 S1026-8774(10)02700100011

                This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

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                Figures: 0, Tables: 0, Equations: 0, References: 33, Pages: 11
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