Oxygen isotope compositions of calcite foraminifera tests are considered to be a robust palaeotemperature proxy at earth-surface conditions, where diffusion is slow. Recrystallization of tests during diagenesis may be optically identified and is often the basis for excluding such samples from paleotemperature calculations e.g. “glassy” vs. “frosty” foraminifera. However, biogenic and abiogenic calcite can isotopically reequilibrate and recrystallize in calcite-saturated fluids, even at low temperatures (<100 °C) with no visible indicators of changes in the texture of the tests (Bernard et al. 2017). Such a process challenges the premise that textural discrimination is sufficient to determine the suitability of calcite for paleothermometry. To determine the rates and extent of this process, abiogenic and biogenic (foraminiferal) calcites were analyzed for their stable isotope compositions before and after low temperature (<100 °C) exposure to calcite-saturated solutions (Ωcalcite = 1) of known isotopic composition (−12 to 1000‰VSMOW) in autoclaves. Experiments took place over different periods, from days to months. Two stages of exchange are identified: (1) rapid exchange during the first days followed by (2) a gradually slowing exchange reaction as the experiment progresses. If the exchange rate during the slowing phase is extrapolated, after ~40 Myr >10 mol.% of a calcite’s oxygen could exchange with oxygen from the surrounding pore fluids. The calculated temperature from the resulting reequilibrated calcite could change by as much as 4 °C depending on the pore-fluid isotopic composition, compared to the original precipitation temperature. These results compel a reconsideration of the reliability of many calcitic paleoclimate proxies even in the absence of any evidence for recrystallization. Calcite precipitated at low temperatures, e.g. benthic foraminifera, are particularly vulnerable to this process, which may, in part, explain their anomalously-high apparent equilibration temperatures during the Paleocene–Eocene.