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

Boron isotope compositions establish the origin of marble from metamorphic complexes: Québec, New York, and Sri Lanka

“…). These concentrations are also higher than the ≤1 ppm B reported for the vast majority of carbonatites worldwide (Hulett et al 2016;Çimen et al 2018, 2019Kuebler et al 2020). The total rare earth element (REE) contents for the samples vary from 0.5 to 1068 ppm (Table 1).…”

“…It is reasonable to expect that if the investigated samples are offshoots of primary carbonatites, then these would have mantle-like geochemical signatures, since it is commonly accepted that carbonatites represent low-degree partial-melts of metasomatized upper mantle (Bell and Simonetti 2010). Some of these characteristics include REE enrichment (i.e., 100 -1000 ppm; Le Bas et al 2002) Hulett et al 2016;Cimen et al 2018Cimen et al , 2019Kuebler et al 2020). Based on the geochemical data reported here for the Grenville Province samples, these clearly have a metasedimentary rather than a mantle-derived origin.…”

“…A comparison of the δ 11 B values vs. the corresponding strontium isotopic ratios for the Grenville Province samples is shown in Figure 8b. The boron isotopic values for all samples of marble examined here are plotted against their corresponding δ 13 C V-PDB and Sr isotopic ratios in Figure 9b and 9c, respectively, along with those for carbonatites worldwide (Chen and Simonetti 2015;Hulett et al 2016;Çimen et al 2018, 2019Kuebler et al 2020 Marschall et al 2017). Figure 9a compares the δ 13 C V-PDB and δ 18 O V-SMOW compositions for the samples studied to those for worldwide carbonatites; all of the data plot above and to the right relative to those for the carbonatite complexes.…”

“…Boron-related investigations of geological samples are advantageous for many reasons, including its use as a paleo-pH proxy in marine carbonates (e.g., Deegan et al 2016;Rasbury and Hemming 2017;Rae 2018), isotopic sensitivity, especially in fluid-mediated processes (e.g., Spivack and Edmond 1987;Lemarchand et al 2000Lemarchand et al , 2002Gaillardet and Lemarchand 2018;De Hoog and Savov 2018), and limited isotopic fractionation during high-temperature metamorphism and/or hydrothermal activity associated with mantle-derived carbonates (e.g., Çimen et al 2018, 2019Kuebler et al 2020). Boron is a widespread trace element in natural carbonates (~1 to 100 ppm; Kowalski and Wunder 2018 and references therein); its incorporation depends on the conditions of the precipitating fluid (e.g., pH, temperature), the presence of other elements (e.g., Mg, Sr), and the type of carbonate present (biogenic or inorganic; Hemming and Hanson 1992;Hemming et al 1995;Sanyal et al 2000;Penman et al 2013;Rasbury and Hemming 2017;Sutton et al 2018).…”

“…In recent studies, boron has proven to be a powerful diagnostic tool even in complicated geologic settings. For instance, it has been possible to elucidate the mantle source region(s) of the Miaoya and Bayan Obo carbonatite complexes in China despite extensive hydrothermal activity or high-grade metamorphism or both (Çimen et al 2018, 2019Kuebler et al 2020). Furthermore, combining boron isotope values with C, O and Sr isotope compositions has established an effective means for identifying pristine (unaltered) mantle-derived carbonate samples that can then be used to decipher the chemical nature of their upper mantle source regions (Hulett et al 2016;Çimen et al 2018, 2019Kuebler et al 2020).…”