THE REE AS INDICATORS OF HYDROTHERMAL PROCESSES: A STUDY OF SKARN GARNETS FROM THE BEINN AN DUBHAICH GRANITE AUREOLE, SKYE

LA-ICPMS analysis of zoned grossular-andradite garnets from skarns in the Beinn an Dubhaich aureole, Skye, indicate dramatic changes in the REE patterns across growth discontinuities, which correlate broadly with the U content. Small-scale oscillatory zonation of major and trace elements is the result of self-organisation during growth from a supersaturated fluid. Mesoscale changes in pattern at growth discontinuities can be interpreted as the result of the contrast between equilibrium fractionation control on the REE content of garnet during slight supersaturation, and rapid growth kinetics during periods of high supersaturation resulting in a surface speciation control on REE adsorption into garnet. Equilibrium patterns are typically LREE enriched, with a normalised abundance peak at Pr, a negative or positive Eu anomaly, and a slight positive Y anomaly. The HREE from Ho-Lu show a hump, with Er, Tm and Yb enriched relative to Ho and Lu. The sign and magnitude of the Eu anomaly is related to the redox conditions, and to fractional crystallisation processes. Y contents are anomalously high as the increase in the stability of Y-F complexes relative to Ho and Dy, increase its relative concentration in the fluid. In patterns controlled by sorption processes La and Ce are depleted relative to the normalised abundance of most other REE. Pr to Sm and Gd to Dy show very enriched abundances with relatively flat patterns, and Ho to Lu show a curved pattern with a gradual decrease in normalised abundance to Lu. Both Eu and Y show marked negative anomalies. In this instance the Y anomaly forms due to the preferential retention of Y in the fluid, again due to the high relative stability of Y-F complexes. The growth discontinuities may be related to periods of boiling, or to influxes of fresh fluid due to rapid permeability changes on hydrofracturing. The correlation of U with the total REE content and with features within the REE patterns indicates that sorption processes are controlling U uptake into garnet.