Oxygen isotope fractionation between calcite and fluid as a function of growth rate and temperature: An in situ study

The 18O content of carbonates (expressed as δ18O) is widely used in paleothermometry, yet it is clear from comparison of experimental results and natural carbonate samples that chemical (isotopic) equilibrium is not always achieved in nature. This observation underscores the importance of exploring possible effects of growth rate on isotopic fractionation, which is the focus of this study. In situ Secondary Ion Mass Spectrometry (SIMS) analyses of δ18O were performed on single crystals of experimentally grown calcite. The change in growth rate (V) over time within each crystal was monitored by addition of multiple rare earth element (REE) spikes into the fluid from which the calcite grew. The values of δ18O in bulk calcites and experimental fluids were measured by stable isotope ratio mass spectrometers. The average SIMS δ18O overlaps with those determined by conventional mass spectrometry within analytical uncertainty. The fractionation factor expressed as Δ18O approaches its equilibrium value in slowly grown calcites (V < 0.02 nm/s) and decreases by 1.5‰ with increasing growth rate from 10− 3 to 0.34 nm/s. Here Δ18O = 103⋅ ln(α18O), and α18O = (18O/16O)calcite/(18O/16O)fluid. Our results provide the first in situ evidence that under equilibrium conditions 18O may be depleted in the near-surface region of calcite relative to the bulk crystal lattice, and that this 18O-depleted zone can be “captured” during rapid crystal growth. Crystal growth rate is therefore a potentially important consideration when using δ18O in natural carbonates as a proxy for ocean and terrestrial temperature.