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¹⁸O/¹⁶O, D/H, and ¹³C/¹²C studies of the Tertiary igneous complex of Skye, Scotland

Forester, Richard W. and Taylor, Hugh P., Jr. (1977) ¹⁸O/¹⁶O, D/H, and ¹³C/¹²C studies of the Tertiary igneous complex of Skye, Scotland. American Journal of Science, 277 (2). pp. 136-177. ISSN 0002-9599. doi:10.2475/ajs.277.2.136. https://resolver.caltech.edu/CaltechAUTHORS:20221021-458531000.6

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Abstract

In southern Skye, δ18O analyses of 350 samples show that almost all the rocks within 4 km of the central intrusive complexes are depicted in 18O due to extensive interaction with heated meteoric ground waters. Because of higher permeabilities, the plateau lavas are most widely affected (outward to 8 km). Whole-rock δ18O values (SMOW) of basalts (-7.1 to +8.4), Mesozoic shales (-0.6 to +12.4), and Precambrian sandstones (-6.2 to + 10.8) systematically decrease inward toward the center of the plutonic complex. In the Cuillin gabbro, δ18O of plagioclase (-7.1 to +2.5) and pyroxene (-0.5 to +3.2) decrease outward toward the margins of the pluton. Most of these 18O depletions were produced by subsolidus exchange, but it also appears that the original Cuillin gabbro magma was depleted in 18O, with a δ18O ≅ +3.5. The Red Hills epigranite plutons have δ18O quartz (-2.7 to +7.6) and feldspar (-6.7 to +6.0) that suggest about 80 percent of the exchange took place at subsolidns temperatures; profound disequilibrium quartz-feldspar fractionations (up to 12.0) are characteristic. The early epigranites were, however, apparently intruded as low-18O magmas (18O depletions of up to 3 per mil), with δ18O of quartz decreasing progressively with time. Successively later intrusions exhibit increasing δ18O quartz, but a major δ18O discontinuity is observed at the geologic break between intrusion of the granites of the Western and Eastern Red Hills. The Southern Porphyritic epigranite was intruded as a low-18O magma with an extremely non-homogeneous δ18O of -3.0 to +3.0, suggesting that it was formed by partial melting of hydrothermally altered country rocks at depth; a similar origin may apply to the Coire Uaigneich granophyre. A good correlation exists between grain size and δ18O for the unique, relatively norma1-18O Beinn an Dubhaich granite, which intrudes limestone having a δ18O range of +0.5 to +20.8 and δ18O (PDB) of -4.9 to -1.0. The δD values (SMOW) of sericites (-104 to -107), and amphiboles, chlorites, and biotites (-105 to -128) from the igneous rocks are all low compared to "normal" igneous rocks, indicating that Eocene surface waters at Skye had δD ≅ -85 and δ18O ≅ -12. The average integrated water/rock ratio for the Skye hydrothermal system is approximately unity; at least 2000 km3 of heated meteoric waters were cycled through these rocks.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.2475/ajs.277.2.136DOIArticle
Alternate Title:18O/16O, D/H, and 13C/12C studies of the Tertiary igneous complex of Skye, Scotland
Additional Information:We thank S. Epstein, L. T. Silver, and S. M. F. Sheppard for valuable discussions of this work. S. M. F. Sheppard kindly helped us collect some of the samples, and John Esson, Ros Todhunter, Anne Bateman, and Steven Moorbath generously contributed others. Sam Epstein made available certain laboratory facilities, and Paul Yanagisawa did some of the laboratory work. This research was principally supported by National Science Foundation Grants GA-12945 and GA-30997X; additional support came from the Phelps Dodge Fund and the National Research Council of Canada Grant A8698.
Funders:
Funding AgencyGrant Number
NSFGA-12945
NSFGA-30997X
Phelps Dodge FundUNSPECIFIED
National Research Council of CanadaA8698
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Caltech Division of Geological and Planetary Sciences2704
Issue or Number:2
DOI:10.2475/ajs.277.2.136
Record Number:CaltechAUTHORS:20221021-458531000.6
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20221021-458531000.6
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:117517
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:25 Oct 2022 20:04
Last Modified:25 Oct 2022 20:04

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