The role of ignimbrites and fine sediments in the lithium distribution and isotopic fractionation in hyperarid environments: Insights from Li-isotopes in the Atacama Desert

The Atacama region in Northern Chile hosts the driest desert on Earth and provides the second highest global Li production, containing the largest reported lithium reserves in the world. The occurrence of extremely elevated Li concentration is restricted to brine deposits contained in the halite nucleus of the Salar de Atacama (SDA). The origin of Li and other solutes in the Andean salt flats has been attributed to different processes, where weathering of Neogene and Quaternary volcanic rocks has been considered to be the most important. Despite recent advances, there is still limited geochemical and isotopic Li data in ignimbrites, particularly surrounding the SDA, hence the potential of these units as a source has remained elusive. In this study, we provide new Li concentration and isotopic data in ignimbrites from the Western Cordillera, fine sediments from the marginal zone of the SDA, and suspended load in shallow groundwater. A moderate enrichment (between 20 and 50 ppm) is recorded in volcanic rocks, averaging 33 ppm, while fine sediments present Li contents one order of magnitude higher, with a mean concentration of similar to 290 ppm. Due to the Li contents recorded in ignimbrites, which is slightly higher than the upper crust average (24-30 ppm; Rudnick and Gao, 2004), we propose that the potential of these units as the main source of Li resides in their large areal extent, volume, and hydraulic gradient with respect to the SDA. The delta Li-7 values in ignimbrites vary from -1.5 parts per thousand to +12.8 parts per thousand Too. In contrast, marginal zone sediments show negative values of delta Li-7 (from -1.9 parts per thousand to -5.3 parts per thousand), except for fine sediments from the Peine Block where a delta(7) Li is +4.4 parts per thousand. Our results reveal that the measured delta Li-7 in ignimbrites would indicate volatile exsolution as the main syn-eruptive factor controlling the isotope Li distribution. Regarding post-eruptive processes, water-rock interactions trigger partition of Li-7 from ignimbrite to water. Both syn-eruptive and post-eruptive proposed were tested by numerical modeling, supporting our interpretation. We propose that the predominant constituent in ignimbrites, crystals or glass, impacts the subsequent distribution and fractionation of Li in inflowing waters and fine sediments covering the marginal zone. The analysis presented here allows us to define three subzones where different processes are controlling Li fractionation. These factors together with the geological, hydrologic, and climatic history of the region have led to the world-renowned accumulation of Li in the SDA.

Automated summary

The Atacama region in Northern Chile is known for its extremely dry desert and significant lithium reserves. This study suggests that ignimbrites, a type of volcanic rock, may be the main source of lithium in the region due to their high lithium concentrations. The study also highlights the importance of volcanic eruptions and subsequent water-rock interactions in determining the distribution and isotopic composition of lithium.