The effects of terrestrial weathering on samarium‑neodymium isotopic composition of ordinary chondrites

Hamed Pourkhori, Vinciane Debaille, Rosalind M.G. Armytage, Matthias van Ginneken, Pierre Rochette, Jérôme Gattacceca

Chemical Geology
Available online 4 January 2021

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“Following their fall to Earth, meteorites experience weathering. In this systematic study, we evaluate the trace element composition of ordinary chondrites from the Antarctic cold desert, and Atacama (Chile) and Lut (Iran) hot deserts, with an emphasis on rare earth elements (REE). Our data confirms that terrestrial weathering of meteorites in hot deserts changes their trace element (Sr, Ba, REE, Hf, Th, and U) concentrations. However, weathering effects in majority of Antarctic samples are limited to slight Ba, REE, Hf, and Th depletions and in some case to U enrichment. In comparison to the Antarctic meteorites, hot desert samples show greater disturbances and REE fractionation relative to the average fall values. We measured the Sm-Nd isotopic composition of the hot desert meteorites that have heavily affected REE compositions. Our Sm-Nd isotopic data show a significant effect of terrestrial weathering evidenced by non-CHUR 147Sm/144Nd and 143Nd/144Nd ratios. Measurements show a higher variation and lower values of 147Sm/144Nd for the Atacama samples than those from the Lut Desert. Deviations from CHUR 147Sm/144Nd value are in positive accordance with the degree of La/Lu fractionation caused by weathering. The ɛNd values of Atacama and Lut deserts meteorites range from −2.20 to +1.61, which is wider than the −1.07 to +0.64 range for falls. We suggest that disturbance of primary Sm/Nd ratios resulting from mixing with terrestrial components originating from soil during weathering is responsible for lower 147Sm/144Nd ratio in these meteorites. The majority of the Atacama meteorites regardless of their weathering degrees have their REE compositions and ɛNd affected by terrestrial contamination. Both 147Sm/144Nd ratio and ɛNd values show no straightforward relationship with weathering degree. However, in both cases the samples with the highest negative isotopic disturbances are H chondrites from the Atacama and Lut deserts. In addition, Ba concentration shows a negative correlation with 147Sm/144Nd ratio. Care must be taken into account while dealing with samples collected from hot deserts, even fresh-looking ones.”