The Effects Of Terrestrial Weathering On Samarium-Neodymium Isotopic Composition Of Ordinary Chondrites

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 Sm-147/Nd-144 and Nd-143/Nd-144 ratios. Measurements show a higher variation and lower values of Sm-147/Nd-144 for the Atacama samples than those from the Lut Desert. Deviations from CHUR Sm-147/Nd-144 value are in positive accordance with the degree of La/Lu fractionation caused by weathering. The epsilon Nd 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 Sm-147/Nd-144 ratio in these meteorites. The majority of the Atacama meteorites regardless of their weathering degrees have their REE compositions and epsilon Nd affected by terrestrial contamination. Both Sm-147/Nd-144 ratio and epsilon Nd 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 Sm-147/Nd-144 ratio. Care must be taken into account while dealing with samples collected from hot deserts, even fresh-looking ones.