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

• Published in: Earth and planetary science letters 2018-11, Vol.502, p.274-283
• Main Authors: Armytage, Rosalind M.G., Debaille, Vinciane, Brandon, Alan D., Agee, Carl B.
• Format: Article
• Language: English
• Subjects: Hf isotopes; Lunar And Planetary Science And Exploration; Mars; Nd isotopes; shergottites; silicate differentiation
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2018.08.013

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. •ε176Hf, ε143Nd, and μ142Nd in NWA 7034 meteorite consistent with ancient crust.•Sm–Nd and Lu–Hf rule out bulk NWA 7034 as partial melt of primitive mantle.•NWA 7034 crustal reservoir is not the enriched end member for shergottites.•Martian magma ocean necessary to generate enriched end-member for shergottites.