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In-situ CO₂ mineralisation has been demonstrated as a rapid and secure mechanism of geological CO₂ storage through a series of field demonstrations in Icelandic basalts by Carbfix. A key indicator of success in the first of these tests was the discovery of newly precipitated calcite on the monitoring well downhole pump that contained the ¹⁴C tracer added to the injected CO₂. In this study we use clumped, carbon and oxygen isotope measurements to determine the temperature of CO₂ mineralisation and source of parental fluids and CO₂ for the calcite found on the pump. Clumped isotope values (Δ₄₇) range from 0.526‰ to 0.540‰, which correspond to mineralisation temperatures of 45–51 °C. These are 10–16 °C warmer than pre-injection temperatures measured at the pump depth. We attribute the warmer temperatures to continuous water pumping from the monitoring well, drawing up waters from deeper in the …