Plumes originating in high-temperature hydrothermal discharges entrain background water and often entrain diffuse-source fluids during convective ascent. The mixing of fluid from those three different sources determines the chemical composition of the ensuing neutrally buoyant plumes. Here convection models and aluminum data from the Trans-Atlantic Geotraverse (TAG) site are used to examine relative contributions of focused, diffuse, and background sources to anomalies in the plume. Our analysis shows that anomalously high Al values observed just above the seafloor at TAG, which we assume stem from diffuse flows, can contribute as much as 50% to the Al anomaly in the neutrally buoyant plume found >350 m above bottom. Background water contributes nearly an equal fraction of Al to the anomaly. More generally, entrained contributions depend on gradients in profiles external to the rising plume. Entrainment also depends on effective source area; distributed high-temperature vents entrain more fluid than a single, composite high-temperature vent. Larger effective source area is particularly significant for entrainment of diffuse-source or other tracers trapped within a few tens of meters of the seafloor. Downstream, cross-stream, and temporal variabilities exhibited in the three-dimensional model plume also underscore problems of sampling and interpreting actual plume data. ¿ 1999 American Geophysical Union