Berggrundvattnets evolution återspeglad i kalcitens spårämnessammansättning i frakturer, Olkiluoto, Euraåminne

Over the last 30 years, the geology and paleohydrology has been examined at Olkiluoto as to predict how they may change in the future and affect the final placement of nuclear waste. Some knowledge about the hydrology can be seen directly from present groundwater and the fracture calcite reflect older environments from where they were precipitated. Calcite is the most used fracture mineral within paleohydrogeochemical studies since it easily precipitates during different conditions, including colder climate. The calcite at Olkiluoto has also earlier been examined and dating and grouping of the calcite has been carried out. The aim of the study was to further analyze the trace element composition in fracture calcite samples from earlier studies, in order to understand the influences of groundwater in calcite. The REE concentrations and their anomalies were of special interest. 31 calcite samples from a depth of about 12–660 m under the surface, from 20 different drill cores were analyzed. The calcite had been precipitated at pegmatitc granite, mica gneiss, migmatitic mica gneiss, quartz gneiss and veined gneiss. The calcite was analyzed with ICP-AES (Agilent MP4100), ICP-MS (Agilent 7500ce/cx) and LA-ICP-MS (Coherent GeoLasPro MV and Agilent 7900s). The ICP-AES method was found unsuited for trace element analyses of calcite. The trace element concentrations showed large variations. The later calcite had larger concentrations and more variation towards the surface than deeper in the ground. REE concentrations decreased over depth which can reflect the Ca in the groundwater that increases with depth. LREE showed higher concentrations than HREE since soluble HREE tend to stay in the groundwater during calcite precipitation as LREE migrates to the calcite. Larger LREE concentrations may have occurred from hydrothermal conditions. Usually the calcite had negative Eu anomaly but also positive occurred. Reducing conditions create negative Eu anomalies when Eu3+ takes divalent form. The positive Eu anomalies may also have occurred during hydrothermal conditions or may reflect plagioclase in the bedrock. No direct influence between bedrock and calcite trace element composition could however be determined. The calcite usually lacked Ce anomaly except for a couple near the surface which may be caused by oxidizing conditions where Ce3+ changes to soluble Ce4+. The negative Ce anomalies in calcite from meteoric water, may have inherited the anomaly from earlier seawater. A few La anomalies were observed. Negative La anomaly may occur if La has been taken by other minerals. Usually the calcite lacked Y anomaly except for the oldest calcite that had positive Y anomaly. Acidic conditions tend to cause Fe oxyhydroxation and reduce REE in comparison to Y which may have produced positive Y anomalies. Reducing conditions may also have caused the higher Mn and Fe concentrations in calcite while oxidizing conditions may have created the higher U concentrations. Bacterial activity and clay accumulations during calcite precipitation may have affected Mn concentrations and sulfide precipitation may have affected Fe. The U concentrations in groundwater tend to decrease with depth and at high salinity which may have affected the U concentration in calcite. Variations in Mg concentrations may have been caused by Mg ion exchange. High Sr concentrations may reflect hydrothermal conditions while low Sr concentrations may reflect low temperatures or precipitation of other minerals.