Martian hydrothermal fluids recorded in the Sm-Nd isotopic systematics of apatite in regolith breccia meteorites

1Sheng Shang,1,2Hejiu Hui,2Yueheng Yang,1Tianyu Chen
Earth and Planetary Science Letters 581, 117413 Link to Article [https://doi.org/10.1016/j.epsl.2022.117413]
1State Key Laboratory for Mineral Deposits Research & Lunar and Planetary Science Institute, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China
2CAS Center for Excellence in Comparative Planetology, Hefei 230026, China
3State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
Copyright Elsevier

The observations of Martian orbiters and rovers have suggested that there were fluids on the surface of early Mars. However, the geochemical properties of those fluids are unclear. The Martian regolith breccia meteorites (MRB), the source materials of which are thought to have formed at 4.4 Ga, may have recorded interactions with fluids on Mars. Here, we have analyzed the in situ Sm-Nd isotopic compositions and trace element contents of apatite in MRB and have obtained a Sm-Nd isochron age of 1490±480 Ma. This young age indicates that the MRB apatites were altered by fluids and have exchanged trace elements with fluids. The very negative initial Nd, combined with the previously reported positive δD and Cl in the MRB apatites, indicate that Martian fluids originated from a geochemically enriched reservoir in the crust. The large ranges of rare earth element abundance (ΣREE) and of the chondrite normalized ratio of La and Yb [(La/Yb)N] indicate the chemical complexity of the fluids that interacted with the apatites in the MRB. The apatite REE compositions were used to further determine the pH values of Martian fluids equilibrated with the MRB apatites, which varied from ∼3 to ∼8. The apatite X-site Cl contents indicate that the Cl contents in Martian fluids in equilibrium with the MRB apatites at 400 °C and 1 bar could be up to 1857 ppm, within the range of terrestrial hydrothermal fluids. Combined with previously reported geochemical data from Martian meteorites, our study suggests that fluids may have been present throughout the early geological history of Mars.

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