Magnesium and zinc isotopic evidence for the involvement of recycled carbonates in the petrogenesis of Gaussberg lamproites, Antarctica

• Published in: Chemical Geology
• Main Authors: Liu, Jian Qiang, Chen, Li Hui, Wang, Xiao Jun, Zhang, Xiao-Yu, Zeng, Gang, Erdmann, Saskia, Murphy, David T., Collerson, Kenneth D., Komiya, Tsuyoshi, Krmíček, Lukáš
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Elsevier 2022
• Subjects: Lamproites; Magnesium isotopes; Mantle transition zone; Recycled carbonates; Zinc isotopes; Antarc*; Antarctica
• Online Access: https://eprints.qut.edu.au/235324/

Lamproites are rare mantle-derived peralkaline ultrapotassic rocks, and they are commonly geographically associated with the ultramafic lamprophyres and kimberlites. Their unique geochemistry and mineralogy make determining their mantle source and origin important because of the significance for inferring specific geodynamic processes. In this study, we further examine lamproite petrogenesis using new Mg and Zn isotopic data for the typical Gaussberg lamproites, Antarctica, the source of which were thought to be contributed by recycled crustal materials. Results show that these lamproites have lower δ 26 Mg (−0.44‰ to −0.39‰) and higher δ 66 Zn (0.36‰ to 0.39‰) values than terrestrial mantle (δ 26 Mg = −0.25 ± 0.04‰, δ 66 Zn = 0.18 ± 0.05‰). The post-magmatic alteration and crustal contamination as well as fractional crystallization and partial melting cannot account for these anomalous Mg and Zn isotopic values. By contrast, the involvement of sedimentary carbonates which are characterized by light δ 26 Mg (average approximately −2.0‰) and heavy δ 66 Zn (average ~ +0.91‰) values in their mantle source can explain these Mg and Zn isotopic anomalies. Quantitative modelling suggests that addition of 10–15% subducted dolomite into the source of Gaussberg lamproites can well reproduce their Mg and Zn isotopic values. The source component with light Mg and heavy Zn isotopic compositions can either be sub-continental lithospheric mantle metasomatized by carbonate melts or residue of subducted carbonate-bearing sediments after deep melting in the mantle transition zone. A lithospheric mantle contribution is indeed required to explain their strongly enriched radiogenic isotopic compositions. However, in terms of carbonate component, their positive Zr–Hf anomalies (Hf/Hf* = 1.28–2.19), and extremely high K/U (~40, 000) and Ba/Th (~400) ratios lead us to favor the latter deep recycling model in which the recycled carbonate-bearing sediments subducted as K-hollandite and majorite underwent partial melting within the mantle ...