Experimental Evaluation Of Yttrium And Rare Earth Element (YREE) Coprecipitation With Calcium Carbonates In Seawater At 5℃, 15℃, 25℃ And 1 Atm., And Low Dissolved Concentrations

Using the Constant Addition System, the coprecipitation of YREEs with calcite or aragonite in seawater was simulated, with concerning on the migration, transformation and reallocation of these elements in such water-solid interactions. Based on quantitative descriptions of their partitioning between CaCO3 and seawater, we studied their responses to experimental conditions, with attempts to study the key factors that controlling the partitioning and extract the mechanism.Basic parameters of experiments, namely [H⁺], [Ca²⁺] and total alkalinity were first measured by pH measurements and high precision titrations. Subsequently key factors of the carbonic system were calculated and the knetic expressions of calcite or aragonite precipitation in seawater at 5℃, 15℃, 25℃and pCO₂=0.003atm were built. Our results presented that:1) Under any given condition, the precipitation rates of calcite or aragonite have excellent correlativity with CaCO3 saturation states of seawater, which can be expressed as LogRate = k * Log (Ω- 1)+ b2) High concentration of YREEs will inhibit the precipitation of aragonite or calcite, therefore effect the partitioning and fractionation of YREEs during the coprecipitation. The precipitation of aragonite, as compared with that of calcite, appears to be more endurable to YREEs.3) The kinetic expressions of calcite or aragonite at different temperature are quite different, proving that relative processes are thermodynamic.Different from former experiments, the concentrations of individual YREEs was initialized to a considerable low level that no significant disturbance to the kinetic of carbonate by YREEs was observed. And the final condition of the steady solution was much closed to the nature situation. Some conclusion can be drive from this study as flows:1) All YREEs are strongly partitioned into calcite or aragonite. The partitioning coefficient of YREEs were about 3000 for calcite, while 3000～10000 for aragonite.2) The precipitation rates of calcite or aragonite have a kinetic control over the partitioning of YREEs. In calcite, partition coefficients of all YREEs decrease with the increase of calcite precipitation rates. In aragonite, the precipitation rates are have observed to have distinct impacts on YREE partitioning, which is negative for elements from La to Gd and positive for Y, Ho, Tm, Yb and Lu.3) In calcite, partition coefficients of YREEs have excellent correlation with each other. The YREE spectrum is slightly fractionated in calcite with a convex kink at the position from Ce to Eu. In aragonite, YREEs are severely but differentially fractionated due to their different response to aragonite precipitation rates. Over investigated temperature scales, well restricted ratios are only found among Y, Ho, Tm, Yb and Lu.4) The carbonate complexes of YREEs in solutions and on calcite surfaces are vital to YREE partitioning between calcite and seawater solutions. The adsorption of YREEs on calcite surface dominates the fractionation of YREE spectrum. The bong length between YREE ions and O, instead of ionic radii, is the regulator of YREE coordination in calcite.5) M₂(CO₃)₃-CaCO₃ and MNa(CO₃)₂-CaCO₃ are two possible models of YREE-calcite solid solutions in this experiments.It is more inspiring to observe that our DYREE patterns display striking consistency with those inferred from natural calcites, including limestones, microbialites and foraminifers. Therefore, these natural calcites formed in seawater are promising proxy recorders for the reconstruction of ancient seawater YREE patterns.