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Hydrothermal Sr contamination of the Dippin sill, Isle of Arran, Western Scotland

Published online by Cambridge University Press:  05 July 2018

A. P. Dickin ,
C. M. B. Henderson  and
F. G. F. Gibb
A. P. Dickin
Affiliation:
Department of Geology and Mineralogy, University of Oxford, Oxford OX1 3PR
C. M. B. Henderson
Affiliation:
Department of Geology, The University, Manchester M13 9PL
F. G. F. Gibb
Affiliation:
Department of Geology, University of Sheffield, Sheffield S1 3JD
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Abstract

The Dippin sill, which is emplaced into the Triassic sediments of SE Arran, is an alkaline basic sheet which displays pronounced hydrothermal alteration. The 40-m-thick sill has suffered pervasive contamination with radiogenic Sr, introduced from the Triassic sediments by hydrothermal fluids. Stable isotope measurements suggest that fluids were of meteoric origin, but were restricted to a small closed-system circulation. Initial 87Sr/86Sr ratios in the sill were raised from an original value of 0.7032 to a maximum of 0.7091, contamination being especially pronounced near the contacts at Dippin Head itself (localities 12 and 14) and in a drill core section through the sill above Dippin. Hydrothermal Sr was incorporated into an early-formed high-CaO, high-Sr analcime, which replaced unstable high-silica nepheline in interstitial patches. However, this high-CaO analcime, along with plagioclase, was later replaced by a low-CaO, low-Sr analcime, allowing Sr leaching from the margins of the sill. Hydrothermal fluids are thought to have migrated up to 1 km laterally, up the dip of the sill, mainly via tension joints forming in the cooling intrusion. Pooling of hot fluids at the upper end of the sill probably raised water/rock ratios in this region and allowed greater Sr contamination during mineralogical alteration. The undersaturated mineralogy of the sill accounts for its pervasive hydrothermal Sr contamination, which contrasts markedly with the relatively undisturbed Sr isotope compositions of Hebridean granites involved in hydrothermal systems.

Type
Research Article
Information
Mineralogical Magazine , Volume 48 , Issue 348 , September 1984 , pp. 311 - 322
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1984

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Footnotes

*

Present address: Isotope Geology Unit, Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 0QU.

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