Isotopic measurement of Sr and O exchange between meteoric-hydrothermal fluid and the Coire Uaigneich Granophyre, Isle of Skye, N.W. Scotland

To determine the scale and magnitude of hydrothermal Sr contamination in an 18O-depleted granite from the Isle of Skye, Sr and O isotopic and electron microprobe mineral analyses were performed on samples of Coire Uaigneich Granophyre (CUG), including two samples cut by mineralised fractures. Unfractured whole-rock samples have model initial 87Sr/86Sr ratios between 0.7298 and 0.7320 and δ18O values within 2% of SMOW. Depletions in18O are most severe near intrusive contacts, indicating structural control on fluid flux and attesting to vigorous subsolidus O exchange with circulating fluids. Initial 87Sr/86Sr ratios do not correlate with δ18O values, suggesting that 18O depletion was not generally accompanied by significant Sr isotopic contamination, a probable consequence of low fluid Sr concentrations. Fractures in two CUG samples are fringed by bleached zones up to about 1 cm wide which contain epidote, chlorite and albitised plagioclase. Sr isotopic gradients exist within these zones, where initial87Sr/86Sr ratios are depressed by as much as 0.0036. The coincidence of marked Sr isotopic contamination and plagioclase albitisation suggests enhanced, but localised, Sr exchange during mineral reactions at temperatures near 400°C. O isotopic profiles are essentially flat within 10 cm of the fractures. The final fluids to circulate along the fractures precipitated epidote, effectively plugging the plumbing system within the CUG. Mn and Sr zoning in these epidotes reflects changes in the Sr isotopic and trace element chemistries of these late fluids, and suggests increased buffering of the fluid composition by the CUG as fluid flux diminished. The high initial 87Sr/86Sr ratios exhibited by the CUG cannot have resulted from subsolidus fluid/rock interactions. Provided that sampling is conducted with care, Sr isotopic analyses of 18O-depleted granites can have petrogenetic significance.