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^(18)O/^(16)O and D/H studies on the interactions between heated meteoric ground waters and igneous intrusions: Western San Juan Mountains, Colorado and the Isle of Skye, Scotland

Citation

Forester, Richard W. (1975) ^(18)O/^(16)O and D/H studies on the interactions between heated meteoric ground waters and igneous intrusions: Western San Juan Mountains, Colorado and the Isle of Skye, Scotland. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/KMXR-6155. https://resolver.caltech.edu/CaltechTHESIS:02202014-160719532

Abstract

Detailed oxygen, hydrogen and carbon isotope studies have been carried out on igneous and metamorphic rocks of the Stony Mountain complex, Colorado, and the Isle of Skye, Scotland, in order to better understand the problems of hydrothermal meteoric water-rock interaction.

The Tertiary Stony Mountain stock (~1.3 km in diameter), is composed of an outer diorite, a main mass of biotite gabbro, and an inner diorite. The entire complex and most of the surrounding country rocks have experienced various degrees of 18O depletion (up to 10 per mil) due to interaction with heated meteoric waters. The inner diorite apparently formed from a low-18O magma with δ18O ≃ +2.5, but most of the isotopic effects are a result of exchange between H2O and solidified igneous rocks. The low-18O inner diorite magma was probably produced by massive assimilation and/or melting of hydrothermally altered country rocks. The δ18O values of the rocks generally increase with increasing grain size, except that quartz typically has δ18O = +6 to +8, and is more resistant to hydrothermal exchange than any other mineral studied. Based on atom % oxygen, the outer diorites, gabbros, and volcanic rocks exhibit integrated water/rock ratios of 0.3 ± 0.2, 0.15 ± 0.1, and 0.2 ± 0.1, respectively. Locally, water/rock ratios attain values greater than 1.0. Hydrogen isotopic analyses of sericites, chlorites, biotites, and amphiboles range from -117 to -150. δD in biotites varies inversely with Fe/Fe+Mg, as predicted by Suzuoki and Epstein (1974), and positively with elevation, over a range of 600 m. The calculated δD of the mid-to-late-Tertiary meteoric waters is about -100. Carbonate δ13C values average -5.5 (PDB), within the generally accepted range for deep-seated carbon.

Almost all the rocks within 4 km of the central Tertiary intrusive complex of Skye are depleted in 18O. Whole-rock δ18O values 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 towards the center of the complex. The Cuillin gabbro may have formed from a 18O-depleted magma (depleted by about 2 per mil); δ18O of plagioclase (-7.1 to + 2.5) and pyroxene (-0.5 to + 3.2) decrease outward toward the margins of the pluton. The Red Hills epigranite plutons have δ18O quartz (-2.7 to + 7.6) and feldspar (-6.7 to + 6.0) that suggest about 3/4 of the exchange took place at subsolidus temperatures; profound disequilibrium quartz-feldspar fractionations (up to 12) are characteristic. The early epigranites were intruded as low-18O melts (depletions of up to 3 per mil) with δ18O of the primary, igneous quartz decreasing progressively with time. The Southern Porphyritic Epigranite was apparently intruded as a low-18O magma with δ18O ≃ -2.6. A good correlation exists between grain size and δ18O for the unique, high-18O Beinn an Dubhaich granite which intrudes limestone having a δ18O range of +0.5 to +20.8, and δ13C of -4.9 to -1.0. The δD values of sericites (-104 to -107), and amphiboles, chlorites, and biotites (-105 to -128) from the igneous rocks , indicate that Eocene surface waters at Skye had δD ≃ -90. The average water/rock ratio for the Skye hydrothermal system is approximately one; at least 2000 km3 of heated meteoric waters were cycled through these rocks.

Thus these detailed isotopic studies of two widely separated areas indicate that (1) 18O-depleted magmas are commonly produced in volcanic terranes invaded by epizonal intrusions; (2) most of the 18O-depletion in such areas are a result of subsolidus exchange (particularly of feldspars); however correlation of δ18O with grain size is generally preserved only for systems that have undergone relatively minor meteoric hydrothermal exchange; (3) feldspar and calcite are the minerals mos t susceptible to oxygen isotopic exchange, whereas quartz is very resistant to oxygen isotope exchange; biotite, magnetite, and pyroxene have intermediate susceptibilities; and (4) basaltic country rocks are much more permeable to the hydrothermal convective system than shale, sandstone, or the crystalline basement complex.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Geology
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Geochemistry
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Taylor, Hugh P.
Thesis Committee:
  • Unknown, Unknown
Defense Date:21 January 1975
Non-Caltech Author Email:richwf (AT) gmail.com
Additional Information:Supplemental Files Information: δ18O analysis of mineral and rocks of Skye: Supplement 1 from "^(18)O/^(16)O and D/H studies on the interactions between heated meteoric ground waters and igneous intrusions: Western San Juan Mountains, Colorado and the Isle of Skye, Scotland" (Thesis). δ18O analysis of mineral and rocks of Skye: Supplement 2 from "^(18)O/^(16)O and D/H studies on the interactions between heated meteoric ground waters and igneous intrusions: Western San Juan Mountains, Colorado and the Isle of Skye, Scotland" (Thesis)
Funders:
Funding AgencyGrant Number
Phelps Dodge FundUNSPECIFIED
ARCS ScholarshipUNSPECIFIED
NSFGA-12945
NSFGA 30997X
Record Number:CaltechTHESIS:02202014-160719532
Persistent URL:https://resolver.caltech.edu/CaltechTHESIS:02202014-160719532
DOI:10.7907/KMXR-6155
Related URLs:
URLURL TypeDescription
https://doi.org/10.22002/D1.892DOISupplement 1 in CaltechDATA: δ18O analysis of mineral and rocks of Skye
https://doi.org/10.22002/D1.893DOISupplement 2 in CaltechDATA: δ18O analysis of mineral and rocks of Skye
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:8093
Collection:CaltechTHESIS
Deposited By: Benjamin Perez
Deposited On:24 Feb 2014 15:52
Last Modified:21 Dec 2019 03:51

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