Hydrothermal Sr contamination of the Dippin sill, Isle of Arran, western Scotland.

This 40 m-thick alkaline basic sheet (M.A. 78-5005) has suffered pervasive contamination with radiogenic Sr introduced from the Triassic sediments by hydrothermal fluids. Stable-isotope measurements for 44 specimens suggest that these fluids were of meteoric origin; initial ⁸⁷Sr/⁸⁶Sr ratios were raised from 0.7032 to 0.7091. The hydrothermal Sr was incorporated in an early high-Ca, high-Sr analcite which replaced unstable high- silica nepheline in interstitial patches. This high-Ca analcite, along with plagioclase, was later replaced by a low-Ca, low-Sr analcite. Hydrothermal fluids are thought to have migrated laterally up to 1 km, up the dip of the sill, mainly via tension joints in the cooling intrusion.-R.A.H.