Y-Ho fractionation during basalt alteration in hydrothermal system: An implication for superchondritic Y/Ho signature recorded in Precambrian banded iron formations

The concentration of rare earth elements (REE) in Precambrian sedimentary rocks is widely used to estimate their depositional environment. Specifically, superchondritic Y/Ho value is a powerful indicator for determining whether a sedimentary rock was deposited in a marine environment. Currently, superchondritic Y/Ho value in seawater is believed to result from the preferential adsorption of Y onto Fe-Mn oxides. In modern oceans, Fe-Mn oxides are considered the sink for subchondritic Y/Ho value. On the contrary, Precambrian banded iron formations (BIFs) display superchondritic Y/Ho value, suggesting that Precambrian seawater also possessed a superchondritic Y/Ho value. However, Precambrian BIFs cannot account for the sink of subchondritic Y/Ho value. Thus, other processes must have been responsible for maintaining the superchondritic Y/Ho value of Precambrian seawater. A hydrothermal experiment involving mid-ocean ridge basalt (MORB) and a Cl-rich fluid was conducted at 300 °C and 500 bars to assess the role of basalt-hosted hydrothermal systems on the Y/Ho value of seawater. Over time, the fluid's Ca concentration increased while Na decreased, suggesting plagioclase albitization occurred. The REE pattern of the reacted fluids exhibited superchondritic Y/Ho values up to 63.2. Based on the presence of positive Eu anomaly and the degree of light REE depletion, the REE in the hydrothermal fluids was mainly sourced from plagioclase during albitization. The superchondritic Y/Ho values in the fluids can be explained by the preferential leaching of these elements due to differences in chloride complexing strength between REE. Subchondritic Y/Ho values observed in Precambrian hydrothermally altered basalts may be interpreted as remnants generating superchondritic Y/Ho values in coexisting Precambrian hydrothermal fluids. Therefore, we believe that the superchondritic Y/Ho value of Precambrian seawater was maintained by basalt-hosted hydrothermal system. • Fe-Mn oxide wasn't a major cause of superchondritic Y/Ho in the Precambrian seawater. • We reacted Precambrian seawater-like fluid with MORB under high P-T conditions. • This experiment shows REE leaching from plagioclase yields superchondritic Y/Ho. • Precambrian seawater had superchondritic Y/Ho due to MOR hydrothermal systems. • Archean seafloor hydrothermal metabasalt with subchondritic Y/Ho supports our idea.