FLUID-FLOW AND FRACTURE-ZONE EVOLUTION THROUGH COMBINED GEOCHRONOLOGY, ISOTOPIC AND ELEMENTAL COMPOSITION OF CALCITE

Calcite is an abundant fracture-filling mineral in many geological settings. Traditional methods employed to investigate calcite composition include stable isotopes (carbon and oxygen), radiogenic isotopes (Rb-Sr), trace element contents and fluid inclusion studies. These are used collectively to inform about fluid composition and the temperature of formation. Recent advances include measurement of these same isotopic and elemental signatures, but using higher spatial resolution techniques such as SIMS (Secondary Ionisation Mass Spectrometry) and LA-ICP-MS (Laser Ablation Inductively Coupled Mass Spectrometry), enabling the determination of detailed intra-crystal variation. Additionally, we have recently developed the application of in-situ U-Pb geochronology to calcite using LA-ICP-MS. The combination of these in-situ high spatial resolution techniques, whereby chronological information can be tied to specific isotopic and elemental compositions of specific calcite generations, is hoped to improve our understanding of fluid-flow and fracture fill evolution, including: 1) tying the source of fluids to particular fracture events; 2) elucidating changes in fluid source through time; and 3) investigation of elemental uptake, exchange and mobility in calcite. Here we present results from three studies: 1) the Laxemar and Forsmark nuclear repository investigation sites in Sweden; 2) the Sellafield nuclear decommissioning site in England; and 3) the Faroe Islands rifted continental margin in the NE Atlantic. From these sites we present a range of observations on fracture and fluid-flow evolution using chronological information tied to in-situ trace element and stable isotope compositions.