The rare earth elements and uranium in garnets from the Beinn an Dubhaich Aureole, Skye, Scotland, UK: constraints on processes in a dynamic hydrothermal system

Garnets from skarns in the Beinn an Dubhaich granite aureole, Isle of Skye, Scotland, have a large range of concentrations of uranium (0·2–358 ppm) and the rare earth elements (REE) (23–4724 ppm). Variations in these concentrations correlate with major element zonation within the garnets, and with changes in the shape of REE patterns. Typical patterns in most garnets display light REE (LREE) enrichment, flat heavy REE (HREE) distribution and a negative Eu anomaly. These patterns are interpreted to represent equilibrium trace element exchange between pre-existing pyroxene, hydrothermal fluid and calcic garnets. Iron-rich zones are characterized by positive Eu anomalies and an increase in the abundance of the LREE relative to the HREE. These patterns are interpreted as resulting from changes in REE speciation related to the introduction of externally buffered fluid to the skarn system. Relatively Fe-poor zones show strongly HREE-enriched patterns with negative Eu anomalies and in some instances depletions in Y relative to Ho and Dy, which are interpreted as resulting from surface sorption of the REE during rapid, disequilibrium garnet growth. Strong correlations between U abundance and the REE patterns indicate that the same processes have affected U distribution. Both types of pattern can be modified by the effects of closed-system crystallization on REE abundance in the fluid, and changes in fluid major element chemistry.