Trace and rare earth element geochemistry of Holocene hydromagnesite from Dujiali Lake, central Qinghai-Tibetan Plateau, China

The genesis of hydromagnesite [Mg-5(CO3)(4)(OH)(2).4H(2)O] has attracted great interest as a pathway for sequestering anthropogenic CO2 and because of its importance to Mg carbonate depositional environments; however, there remain uncertainties regarding the chemical environment for hydromagnesite precipitation in modern and ancient geologic systems. Trace and rare earth element (REE) concentrations in hydromagnesite from Dujiali Lake, central Qinghai-Tibetan Plateau, China identified the formation conditions in the context of the depositional environment. The analyzed hydromagnesite samples had low total REE concentrations, varying from 0.62 to 3.11 ppm, with an average Sigma REE value of 1.75 ppm. Comparisons of Ce/Ce* with La-N/Sm-N, Dy-N/Sm-N, and Sigma REE showed no correlation indicating preservation of the original redox conditions during hydromagnesite precipitation. Redox-sensitive trace element ratios (U/Th, Ni/Co, V/Cr and V/V + Ni), negative Mn* values, and low authigenic uranium (U-a) values all indicate oxic conditions at the time of hydromagnesite formation. Furthermore, the Post-Archean Australian Shale-normalized REE patterns of the hydromagnesite display slight heavy REE enrichment, a slightly negative Ce anomaly, and a consistently positive Eu anomaly, which are consistent with precipitation in a predominantly oxidizing environment. Data indicate that hydromagnesite precipitated from waters influenced by both Mg-rich hydrothermal fluids and meteoric water with a similar composition to the lake water. This study provides new insights into the conditions of hydromagnesite formation at Dujiali Lake with implications for the understanding of the genesis of modern and ancient Mg carbonate deposits.