Assessing the Heterogeneity of the Tissint Shergottite Strewn Field Using Rb-Sr, Sm-Nd and Lu-Hf Isotope Systematics



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Tissint, the 5th witnessed Martian meteorite fall, occurred on July 18, 2011 east of Tata, Morocco. While most studies of Tissint classify it as a shergottite relatively depleted in lithophile incompatible trace elements (e.g., light REE), crystallization age determinations have been variable. Currently, there is a discrepancy in crystallization age determination amongst three separate labs (UH: University of Houston; LLNL: Lawrence Livermore National Labs; and NASA-JSC: NASA Johnson Space Center) using combinations of Lu-Hf, Sm-Nd, and Rb-Sr analyses. Each lab tested one single fragment from the entire Tissint meteorite strewn field, which included samples (UWB1), (ASU#1744), (UNM#645). Sm-Nd analyses were performed at all three labs and produced two different dates. Analyses from two of the three labs, UH and LLNL, are in agreement and give a combined Sm-Nd age of 593±25 Ma for samples UWB1 and ASU#1744. Analysis at NASA-JSC provided a date of 472±36 Ma (sample UNM#645). This is approximately a 120 Ma difference in crystallization ages for separate samples analyzed from the strewn field. To confirm whether materials of different isotopic compositions and ages were co mingled resulting in a heterogeneous strewn field, eight separately-collected fragments from the Tissint strewn field, including the sample analyzed at JSC (UNM#645) were examined using Rb-Sr, Sm-Nd and Lu-Hf isotopic systems. Mineral and leached bulk rocks analyzed at UH and LLNL define isochrons of 571±84 Ma, 559±39 Ma and 590±49 Ma, respectively. In all three isotopic systems, the analyses performed at UH and LLNL plot along the same isochron with slopes consistent with a ~590 Ma igneous crystallization age, including UNM#645 dated at 495±35 Ma by NASA-JSC. While the Lu-Hf isotopic system indicates no evidence of contamination or element mobility, the Rb-Sr and Sm-Nd isotopic systems show evidence for element mobility and potential mixing with an isotopic component not in equilibrium with the igneous phases. Despite these issues, there is no evidence for multiple igneous sources. Fragments identified so far are cogenetic and the Tissint strewn field appears to be homogeneous.



Meteorites, Martian, Shergottites, Tissint