ABSTRACT

The partitioning of Y and Ho between CaCO₃ (calcite and aragonite respectively) and seawater was experimentally investigated at 25°C and 1 atm. Both Y and Ho were observed to be strongly partitioned into the overgrowths of calcite or aragonite. Their partition coefficients, D_Y and D_Ho, were determined to be ∼520-1400 and ∼700-1900 in calcite, ∼1200-2400 and ∼2400-4300 in aragonite, respectively. Y fractionates from Ho during the coprecipitation with either calcite or aragonite. Within our experimental conditions, the fractionation factor, k = D_Y/D_Ho, was determined to be ∼0.62-0.77 in calcite and ∼0.50-0.57 in aragonite, respectively. The aqueous complexation of Y and Ho, which is a function of solution chemistry, probably plays an important role in both the partitioning and the fractionation. Further analyses suggest that the difference in covalency between Y and Ho associated with changes in their coordination environments is the determinant factor to the Y-Ho fractionation in the H₂CO₃-CaCO₃ system.