A complex history of silicate differentiation of Mars from Nd and Hf isotopes in crustal breccia NWA 7034

1Rosalind M.G.Armytage, 1Vinciane Debaille, 2Alan D.Brandon, 3Carl B.Agee
Earth and Planetary Science Letters 502, 274-283 Link to Article [https://doi.org/10.1016/j.epsl.2018.08.013]
1Laboratoire G-Time, CP 160/02, Université Libre de Bruxelles, Av. F. Roosevelt 50, 1050 Bruxelles, Belgium
2Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX, 77204, USA
3Institute of Meteoritics, University of New Mexico, Albuquerque, NM, 87131, USA
Copyright Elsevier

Resolving the possible mantle and crustal sources for shergottite meteorites is crucial for understanding the formation and early differentiation of Mars. Orbiter and rover characterization of the martian surface reveal that the major element composition of most of its surface does not match the shergottites (McSween et al., 2009) leaving the relationship between them poorly understood. The identification of the meteorite NWA 7034 and its pairs as a Mars surface rock (Cartwright et al., 2014) provides access to a representative sample of Mars’ crust (Agee et al., 2013, Humayun et al., 2013). Utilizing the short-lived 146Sm–142Nd, and long-lived 147Sm–143Nd and 176Lu–176Hf chronometers, which are sensitive to silicate differentiation, we analyzed three fragments of NWA 7034. The very negative mean isotopic compositions for this breccia,μ142NdJNdi-1=−45±5
(2SD), ε143NdCHUR=−16.7±0.4(2SD) and ε176HfCHUR=−61±9(2SD) point to an ancient origin for this martian crust. However, modeling of the data shows that the crust sampled by NWA 7034 possesses a Hf/Nd ratio and coupled ε143Nd–μ142Nd
model age that are incompatible with this crustal reservoir being an end-member that generated the shergottite source mixing array. In addition, this crust is not juvenile, despite its rare earth element profile, but has had a multistage formation history. Therefore, early crustal extraction alone was not responsible for the creation of the reservoirs that produced the shergottites. Instead mantle reservoirs formed via other early differentiation processes such as in a Mars magma ocean must be responsible for the trace element and isotopic signatures present in shergottites.

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