Patterns and processes of oxygen isotope exchange in a fossil meteoric hydrothermal system, Cuillins Gabbro Complex, Isle of Skye, Scotland

Abstract

Oxygen isotope compositions of whole rock specimens and mineral separates from the Cuillins Gabbro Complex, Isle of Skye, Scotland, are employed to determine the patterns and processes of¹⁸O depletion in the Outer Unlayered Gabbro (OUG) and associated dikes. Whole rockδ ¹⁸O values range from +4.8 to −1.1‰ (SMOW) and dikeδ ¹⁸O values range from +4.7 to −2.8‰ Mineral separates from three OUG samples yieldδ ¹⁸O values from +5.3 to +4.8‰ for augite and +4.1 to +0.8‰ for plagioclase. An early, small-scale hydrothermal circulation system was initiated by the OUG prior to the large-scale hydrothermal convection established by the later Layered Cuillins Complex (LCC). Dikes were emplaced in the OUG after intrusion of the LCC and had only a minor effect on hydrothermal circulation in the OUG. There is evidence of enhanced fluid flow along dike/gabbro contacts. Isotopic compositions of augite separates demonstrate a normalδ ¹⁸O value for the OUG magma with all¹⁸O depletion in the OUG due to subsolidus exchange processes including diffusion and surface reaction. The mineral separates yield a pattern of¹⁸O depletion consistent with a diffusion mechanism, the bulk of the exchange having occurred in the plagioclase. Secondary mineral formation played a subordinate role in the¹⁸O depletion of the OUG. The calculated water to rock mass ratio necessary to effect the observed¹⁸O depletion in the OUG is on the order of 0.2, although a much greater amount of water circulation probably occurred. The cooling duration required to explain the measured¹⁸O depletion in the OUG by diffusion is very short (140 years at 750° C, 2400 years at 550° C) compared to the duration necessary for pure conductive cooling (10⁵ to 10⁶ years). Rapid local cooling rates in the OUG due to meteoric water convection are consistent with the observed¹⁸O depletion in OUG samples.