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Geological Society, London, Special Publications; 1992; v. 62; p. 93-96;
DOI: 10.1144/GSL.SP.1992.062.01.09
© 1992 Geological Society of London

The Hebridean Basins and Adjacent Areas

Authigenic K-Feldspar in the Permo-Triassic of northwest Britain: a pilot oxygen isotope study

P. J. McKeever1, P. Carey2 & J. Quinn2

1 Department of Geology, The University, Manchester MB 9PL, UK
2 Department of Geology, Queen’s University, Belfast BT7 1NN, UK

One of the most important authigenic precipitates found in the Permo-Triassic sediments of northwest Britain is K-feldspar. The feldspar is an extremely pure orthoclase end-member of composition Or99 to Or100 occurring as whole overgrowths on mostly fresh detrital grains or as pore-filling adularia. This phase is common in many basins in western Scotland and Northern Ireland. Petrographic studies show that the K-feldspar is an early diagenetic phase, whose precipitation commenced before compaction was completed (McKeever 1990; Parnell 1992). The K-feldspar occupies up to 4% of rock volume, and is important in inhibiting porosity and permeability in potential gas reservoirs. Pores are infilled and pore-throats are blocked within large volumes of sandstone (Fig. Ala-f). Given the abundance of the authigenic K-feldspar, it was considered plausible that it could be separated out and subjected to 18O/16O and K/Ar isotope analysis. To date, only oxygen isotope analyses have been achieved on a sample from the Permian Thornhill Sandstone Formation in Dumfries & Galloway, in which overgrowths occur on nearly all detrital feldspar grains. The Thornhill Sandstone Formation includes tabular and trough-shaped, cross-stratified sets representing aeolian dune deposits in which authigenic K-feldspar appears to be particularly common. By analysing the oxygen isotope composition of authigenic mineral phases it is possible to determine the oxygen isotope composition of the pore water from which that phase precipitated. The procedure used for the separation and isolation of the K-feldspar overgrowths from their detrital cores is described in detail in McKeever (1990). Oxygen isotope analyses were performed on

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C. Spotl, M. J. Kunk, K. Ramseyer, and F. J. Longstaffe
Authigenic potassium feldspar: a tracer for the timing of palaeofluid flow in carbonate rocks, Northern Calcareous Alps, Austria
Geological Society, London, Special Publications, 1998; 144: 107 - 128.
[Abstract] [PDF]