Controls on cold-climate critical minerals: Regolith-hosted REE at three polar desert salt ponds (McMurdo Dry Valleys, Antarctica)

Rare earth elements (REE) are a group of metals considered critical minerals for the energy transition and for numerous advanced technologies essential to modern society. However, despite continually increasing resource demand, REE accumulation processes (especially low-temperature processes) remain understudied and relatively poorly understood. Here, we investigate low-temperature REE behavior in the shallow, supra-permafrost regolith of three salt ponds (Don Juan Pond, VXE-6 Pond, and Brine Pond #1) in Antarctica's McMurdo Dry Valleys (MDV). We characterize REE distributions at five soil pit sites across the three separate salt pond basins. At the scale of an entire basin (i.e., at 100-m-scale) across three Don Juan Pond (DJP) soil pits, we document decreasing REE abundances toward the pond center (with the center of DJP being the most REE-depleted site in this study). This is compatible with an overall cross-basin trend of increased chemical leaching with increasing proximity to the center of DJP. At the scale of individual sediment profiles (i.e., at cm-scale) among all five soil pits, we document two soil pit locations where relative REE enrichment occurs at a clay-rich sediment interval: at the soil pit 300 m W of Don Juan Pond, the clay-enriched 10–12 cm depth interval has ΣREE ~17 % higher than average for the soil pit, and at the VXE-6 Pond soil pit, the clay-enriched 4–7 cm interval has ΣREE ~46 % higher than average. The clay-enriched, REE-enriched intervals are compatible with zones of active chemical alteration and concentration due to intermittent aqueous activity including supra-permafrost freeze-thaw cycling, pond fluctuations, and possible surface meltwaters. Overall, this work demonstrates the potential for limited water availability (and attendant leaching, aqueous alteration, and clay formation) in cold desert regions to nonetheless influence the mobility and concentration of regolith-hosted REE in the shallow subsurface. While MDV regolith-hosted REE are far below economic concentrations, Antarctic permafrost- and salt pond-associated geochemical processes may be relevant to understanding REE behavior (and REE accumulation) in desert and cold regions elsewhere on Earth, as well as to guiding future in-situ resource utilization in analogous planetary environments such as on Mars.