Partition coefficients of Mg and Sr (KMg and KSr) between calcite and fluid were determined in crystals grown under different rates (V), where V is defined as the crystal extension per unit of time (nm/s). Addition of rare earth elements (REE) into the fluid, leads to REE preservation in the calcite. This preservation allows for a direct determination of the values of V by measuring the widths of the REE spiked zones. The edge to edge spot analyses of Mg/Ca, Sr/Ca, and REE/Ca in the individual crystals were conducted using Secondary Ion Mass Spectrometry (SIMS). The Mg/Ca and Sr/Ca ratios in the fluids, from which calcites precipitated, were also measured with Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-OES). This allowed for the first time, to evaluate KMg in situ in the individual crystals without averaging the data from the multiple calcite crystals. Results show that KSr increases by a factor of six with increasing calcite growth rate from 0.001 to 4nm/s. KMg decreases by a factor of three with the same increase of V values. These KSr and KMg behaviors were described quantitatively using a growth entrapment model (GEM). According to the GEM simulations, Sr is enriched and Mg is depleted in the near-surface layer of calcite relative to the bulk lattice. This distinct outermost region of the crystal may be fully or partially captured by its lattice during rapid growth. This process causes disequilibrium partitioning of Sr and Mg through growth entrapment.