Thermal stability and compressibility of bastnaesite

An Unusually Warm Upper‐Crust in the Late Paleozoic North China Continental Arc: Implications for the Thermal Modification of the Giant Bayan Obo REE Deposit

“…As shown by the late Paleozoic hornblende and biotite 40 Ar/ 39 Ar ages from the upper‐crustal basement complexes and Permian plutons around Bayan Obo, the upper‐crust was heated above the closure temperatures of hornblende (530 ± 50°C) and biotite (300 ± 50°C) and was characterized by a high paleogeothermal gradient of >50.0 ± 8.3°C/km with a maximal geothermal gradient of 88.3 ± 8.3°C/km around Bayan Obo during this period. These upper‐crust temperatures from 530 ± 50°C to 300 ± 50°C are higher than the decomposition temperature of bastnäsites from 400 to 310°C (Janka & Schleid, 2009; X. Li et al., 2020), and resulted in strong thermal perturbations and recrystallization of the thermal sensitive REE‐bearing minerals such as monazite and bastnäsite. This warm upper‐crust and high paleogeothermal gradients may provide a steady force to drive the hydrothermal fluids in the surrounding sedimentary rocks of the Bayan Obo Group and resulted in complex textures of ore bodies and enrichment of REE‐bearing minerals in the veined, banded or massive ores.…”

“…Li et al., 2021, 2022; Song et al., 2018; K. F. Yang et al., 2019; Y. H. Yang et al., 2019; S. H. Zhang, Zhao & Liu, 2017). Although a few studies suggest that the closure temperatures of the Th‐(U‐)Pb isotopic system of monazite and bastnäsite could be higher than 700°C (Cherniak et al., 2004; Copeland et al., 1988; Parrish, 1990), other studies show that these minerals are sensitive to hydrothermal alteration at temperatures well below diffusional closure temperatures, and disturbed and meaningless Th‐(U‐)Pb ages may be obtained even under temperatures lower than 400°C (Gysi & Williams‐Jones, 2015; X. Li et al., 2020; Seydoux‐Guillaume et al., 2012; Song et al., 2018; Teufel & Heinrich, 1997; Townsend et al., 2000; Williams et al., 2011; W. Zhang et al., 2023). For example, monazite dating results on the ca.…”

“…Thermogravimetric analysis and high‐temperature Raman spectroscopy demonstrate that the decomposition temperature of bastnäsit‐(Ce) is 400°C (673 K) and bastnäsite undergoes a decarbonization transition at above 400°C (X. Li et al., 2020). The decomposition temperature of bastnäsite‐(La) is ca.…”

“…583 K, Janka & Schleid, 2009). The presence of F − can largely enhance the thermal stability and incompressibility of bastnäsite (X. Li et al., 2020).…”

“…S. Zhang & Tao, 1986) provides solid evidence for presence of F − that may largely enhanced the thermal stability and incompressibility of bastnäsite (X. Li et al, 2020).…”