REE mobility and fractionation during shale weathering along a climate gradient

We systematically investigated six soil profiles developed on a climosequence of gray shale to constrain the mobility and fractionation of rare earth elements (REE) during chemical weathering processes. In addition, one site developed on black shale (Marcellus Formation) was included to document REE behaviors in organic-rich versus organic-poor shale end members under the same environmental conditions. Our study shows that REE are mobilized intensively during shale weathering and the extent of depletion is larger under warm/humid climates. However, the integrated release rates calculated from six soil profiles are not directly correlated to mean annual precipitation or temperature. Instead, the primary control might be the REE concentrations in the most reactive minerals. REE-bearing phases in shale (sulfides, phosphates and organic matter) probably react quickly at first, mobilizing REE. Following that, REE are then released more slowly during dissolution reactions of clay minerals. Consistent with this interpretation, black shale weathers much faster and releases more REE than gray shale under the same climate conditions, due to the higher organic matter and sulfide contents and lower soil pH. REE are not 100% depleted in any of the investigated soil sites; in northern sites, depletion is minimal whereas in the southern (warm and humid) sites, surface depletion is higher and re-deposition is observed at depth. Retention of REE is likely caused by adsorption to mineral surfaces as pH increases and dissolved organic matter content decreases with depth. This case study quantified loss, redistribution and fractionation of REE during shale weathering, improved our understanding of REE mobility in surficial environments, and contributed to the exploration of REE as strategic mineral resources.