LA-ICP-MS imaging on secondary mineral phases as tool to derive palaeohydrological fluctuations and estimate radionuclide retention in future scenarios: A case study from Granitoid Rocks (Sweden)

Coprecipitation of trace elements (TE) and rare-earth elements (REE) in secondary mineral phases is sensitive to changes of the fluid chemistry and system parameters at the times of their formation.

Fracture filling carbonates are therefore a rewarding study base, as elemental compositions of the water column are well preserved in form of coprecipitates. Based on the spatial resolved high resolution analysis of laser ablation – inductive coupled plasma – mass spectrometry (LA-ICP-MS), trace element fluctuations of even a few ppm and within µm distance are detectable. The multielement analysis of cations elements especially sensitive to redox potential, temperature, and salinity in combination with enrichments in elements, that reflect the fingerprint of certain (paleo-) groundwater types, provide information about the palaeohydrological evolution of the site.

In the example presented here, LA-ICP-MS analysis was applied to map concentration of the trace elements Na, Mn, Fe, Sr, Y, La, Ce, Yb, Pb, Th and U coprecipitated in low-temperature calcite fracture fillings originating from exploration drill cores from the sites Simpevarp, Laxemar and Forsmark (Sweden) [1]. The samples were taken by the Swedish Nuclear Fuel and Waste Management Co (SKB) in the context of extensive studies on potential radionuclide transport and retention of radionuclide deriving from deep underground storage of high-level waste (HLW) in crystalline base rock. Based on these results, the partition coefficient D_TE  calculated from the trace and rare earth element assemblage of the most recent growth zone and associated formation waters were calculated. Furthermore, the correlation of the element partitioning with the precipitation rate of the calcites, which is in reversed for Mn and Sr [2] was applied to evaluate the previously assumed precipitation from solutions comparable to recent waters.

Moreover, the findings were used to draw conclusion about the radionuclide retention potential of crystalline host rock fractures. The REE enrichments were used as analogues for trivalent actinides, as they share similar chemical properties.