Oscillatory zoning and trace element incorporation in hydrothermal minerals: insights from calcite growth experiments

Oscillatory zoning and fine‐scale variations in trace element chemistry are commonly observed in hydrothermal minerals. It has been suggested that fine‐scale chemical variations are caused by extrinsic changes in the parent hydrothermal system, such as varying fluid composition, pressure or temperature, as well as changes in mineral growth rate. In this study, LA–ICP–MS (laser ablation, inductively coupled plasma mass spectrometer) analyses were carried out on calcite crystals grown in Ca–NH₃–Cl solutions doped with rare earth elements (REE). The variety of crystal morphologies observed (euhedral to acicular), likely relate to variations in trace element abundance and calcite supersaturation state. Crystals display oscillatory and sector zoning, with significant variations in REE concentrations among zones. Cyclic variations in REE concentrations (exceeding 10‐fold) occur over distances of <1 mm along the growth direction of acicular calcite crystals. In general, trace element concentrations decrease during progressive crystal growth, implying that the concentration of trace and REEs within crystals reflects the overall composition of the growth solution. However, bulk changes in crystal composition are modulated by fine‐scale (<1 mm) variations, which are inferred to be caused by growth‐rate‐controlled incorporation of trace elements. These results have important implications for using hydrothermal minerals to infer fluctuations in fluid compositions in ancient, exhumed hydrothermal systems.