ARGON AND HALOGEN GEOCHEMISTRY OF HYDROTHERMAL FLUIDS IN THE LOCH-AINORT GRANITE, ISLE-OF-SKYE, SCOTLAND

Highly turbid alkali feldspars from the Loch Ainort granite (59 Ma), I sle of Skye, have been analysed using the Ar-40-Ar-39 method to obtain chronological and chemical (K, Cl, Br, I) information concerning thei r origin and hydrothermal alteration. Three methods of gas extraction have been applied to neutron-irradiated samples: laser probe spot fusi on of feldspars, in vacuo crushing of a feldspar/quartz separate, and laser stepped heating of the crushed residue. Apparent ages obtained b y laser probe spot fusion are mostly similar to the 59 Ma intrusion ag e. Analyses of relatively pristine regions give some high apparent age s (> 59 Ma) indicating the presence of small amounts of Ar-40(E) (exce ss Ar-40). Crushing releases significant amounts of Ar-40(E), but is d ominated by an Ar-40(A) (atmospheric Ar-40) component. Kr-84/Ar-36 val ues obtained by crushing are higher than air and are consistent with a ir equilibration with fresh water at low temperature (approximately 20 -degrees-C). Therefore, Ar-40(A) was most probably introduced as palae oatmospheric argon dissolved in the circulating hydrothermal fluids th at interacted with the granite, thus supporting a meteoric origin for the fluids. Stepped heating gives a flat age spectrum and an age of 56 +/- 4 Ma. Crushing and stepped heating both released significant amou nts of halogens with high Br/Cl and I/Cl ratios; excess Xe is also pre sent as indicated by the high Xe-132/Ar-36 values. It seems likely tha t the halogen (and possibly Xe) enrichments resulted from interaction of the meteoric fluids with Jurassic sedimentary country rocks.