Boron isotopic investigation of the Bayan Obo carbonatite complex: Insights into the source of mantle carbon and hydrothermal alteration

Abstract

Earth's largest rare earth element (REE) ore deposit at Bayan Obo, China, is hosted in a dolomitic marble of Mesoproterozoic age, the genesis of which remains controversial, whether it is of sedimentary or igneous origin. Evidence of Caledonian-age recrystallization further complicates the interpretation of geochemical data from this complex. This study provides new insights into the petrogenetic history of the Bayan Obo host dolomite by documenting major and trace element abundances, and the first-reported δ11B values combined with radiogenic (Sr, Nd, Pb) and stable (C, O) isotope results. Boron abundances of the carbonates investigated are typical for carbonatites worldwide (0.4–1.1 ppm) and overlap with those for fresh (unaltered) mid-ocean ridge basalts (MORBs). Boron isotope ratios do not exhibit any systematic correlations with their corresponding B contents, light rare earth element (LREE) concentrations, δ13CPDB or δ18OSMOW compositions. For seven dolomite samples characterized by pristine 87Sr/86Sr(i) values (<0.70340), the δ11B compositions range from −3.6 to −9.0‰, which vary between those for typical asthenospheric (MORB) mantle (−7.1 ± 0.9‰) and an enriched mantle source. The δ11B values for the remaining samples span a wide range (−14.5 to −1.0‰) and given their corresponding more radiogenic 87Sr/86Sr(i) compositions (>0.70340) suggest hydrothermal alteration. Additionally, εNdCHUR values (1.3 Ga) define two groups corresponding to pristine samples (−1.5 to 1.2) and those containing recrystallized REE-bearing phases (−3.0 to −12.6). Together, the major and trace element abundances, δ11B values, and radiogenic isotope data for the host dolomitic marble at Bayan Obo are consistent with an upper mantle origin, and are best explained through the involvement and mixing of at least three distinct mantle reservoirs. When combined with radiogenic and B isotope results from other carbonatite complexes within China (Maoniuping, HYC- Huayangchuan, QMC- Qiganbulake mafic–ultramafic‑carbonatite complex, Miaoya, TNCC- Trans-North China Carbonatites) of varying emplacement ages (between ~26 and ~2000 Ma), these all suggest derivation from enriched upper mantle source regions during the past ~2 billion years. Comparison of the initial Pb isotope ratios reported here for Bayan Obo carbonates to those from the 440 Ma Miaoya carbonatite complex may be attributed to either a significant increase in the upper mantle's 238U/204Pb ratio under present-day China over a ~900-million-year interval, or simply reflects derivation from distinct upper mantle sources.