The role of carbonate complexes and crystal habit on rare earth element uptake in low-temperature calcite in fractured crystalline rock

• Low-temperature calcite is enriched in light REE.
• The REE concentrations in calcite vary considerably and decrease weekly with depth.
• The REE distribution patterns in calcite depend on its crystal habit.
• Existing water–mineral partitioning coefficients correctly describe REE uptake in calcite.
• Carbonate complexes play an important role in REE fractionation in calcite.

This study focuses on rare earth element (REE) geochemistry of low-temperature calcite coatings occurring on the walls of fractures throughout the upper kilometer of crystalline rocks of the Baltic Shield. Fifty one calcite coatings were sampled from cores drilled with the triple-tube technique which successfully preserved the fragile calcite coatings on the fracture walls. The calcites, which based on geological and isotopic evidence were precipitated over the last 10 million years, had highly variable ΣREE concentrations (0.61–2276 ppm) that decreased weakly with the depth the calcite was sampled from. When normalized to shale (and host rock), the REE concentrations of habits with c-axis ≈ a-axes and the closely associated c-axis > a-axes, the most abundant crystal morphologies in the system, decreased strongly and smoothly across the series. In contrast, the REEs of habits with c-axis ≫ a-axes, identified only in fractures in the uppermost 260 m of the bedrock, were flatter and occasionally expressed a weak middle REE enrichment. By using calcite–water partition coefficients derived for REEs in previous laboratory experiments, the La/Yb of the paleogroundwater from which the calcites precipitated was back-calculated and found to be overall similar (range 0.15–452) overlap to the corresponding ratio of the present groundwater (range: 2.1–36.4). In terms of REE/Ca, the values for the back-calculated paleogroundwater (La / Ca 9.9 ∗ 10− 11–3.9 ∗ 10− 7; Yb / Ca 1.5 ∗ 10− 10–2.2 ∗ 10− 7) were similar to those of LaCO3+ / Ca (4.5 ∗ 10− 10–8.5 ∗ 10− 7) and (YbCO3+ + Yb(CO3)2−) / Ca (5.4 ∗ 10− 11–1.8 ∗ 10− 8), respectively, in the present groundwater. These patterns indicate that the LREE to HREE and REE to Ca ratios in the groundwater at the site are broadly similar to those existing when the calcites precipitated, and that carbonate complexes present in the paleogroundwater played a crucial role in sequestration and fractionation of REEs in calcite. The findings have implications for bedrock storage of high-level radioactive waste, which contains actinides for which the REEs can be used as natural analogues.