Lu-Hf and Sm-Nd Ages and Source Compositions for Depleted Shergottite Tissint
Grosshans, Therica Esther 1983-
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Tissint is the fifth witnessed Martian meteorite fall and was collected in Morocco in 2011. The quick recovery of this meteorite limits the possibility of terrestrial weathering or contamination by terrestrial components. Tissint is classified as a depleted olivine-phyric shergottite similar in bulk composition to Dar al Gani 476 and EETA79001A. It has a porphyritic texture with primary mineral phases of olivine, plagioclase (maskelynite), and pyroxene (augite and pigeonite). Other phases include oxides (chromite, ilmenite, magnetite) and phosphate (Irving et al., 2012). In situ trace element analyses of plagioclase and pyroxene show a depletion of light REE relative to heavy REE, similar to bulk Tissint and other depleted shergottites. Microprobe analyses indicate that the core compositions of macrocrystic olivine were likely in equilibrium with the melt from which they crystallized. Oxybarometry calculations utilizing equilibrium olivine-pyroxene-spinel and late-stage oxides reveal that the Tissint melt became oxidized during crystallization. The plagioclase, pyroxene, and whole rock fractions yielded Lu-Hf and Sm-Nd ages of 570 ± 84 Ma and 690 ± 140 Ma, respectively. The initial 176Hf/177Hf isotope ratio is 0.284060 ± 0.000046 and the initial 143Nd/144Nd isotope ratio is 0.51376 ± 0.00034. Using the weighted average of all internal isochron ages (597 ± 27 Ma) and whole rock isotope compositions, initial ε176Hf and ε143Nd are calculated to be +57.8 ± 0.9 and +41.6 ± 1.5, respectively. Initial ε176Hf and ε143Nd values and calculated 176Lu-177Hf and 147Sm-144Nd source ratios of 0.0548 and 0.279, respectively, indicate Tissint is the most depleted in incompatible trace elements of known shergottites and was derived from one of the most depleted sources of shergottites that has been recognized. The Lu-Hf and Sm-Nd internal isochron ages of this study are consistent with previously published Sm-Nd and Rb-Sr (Brennecka et al., 2012; 2013) and Ar-Ar (Park et al., 2013) ages. These ages indicate Tissint is one of the oldest known shergottites, except possibly for Dhofar 019 which is 586 ± 9 Ma (Borg et al., 2001).