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Hydrothermal alteration of plagioclase and growth of secondary feldspar in the Hengill Volcanic Centre, SW Iceland

Larsson, Daniel LU ; Gronvold, K; Oskarsson, N and Gunnlaugsson, E (2002) In Journal of Volcanology and Geothermal Research 114(3-4). p.275-290
Abstract
Dissolution of igneous feldspar and the formation and occurrence of secondary feldspar in tholeiitic basalts from the Hengill volcanic centre, in SW Iceland was studied by microprobe analysis of cuttings from two ca. 2000 m deep geothermal wells. Well NG-7 in Nesjavellir represents a geothermal system in a rift zone where the intensity of young, insignificantly altered intrusions increases with depth. Well KhG-1 in Kolvioarholl represents the margin of a rift zone where the intensity of intrusives is lower and the intensity of alteration higher. This marginal well represents altered basaltic crust in an early retrograde state. The secondary plagioclase in both wells is mainly oligoclase, occurring in association with K-feldspar and... (More)
Dissolution of igneous feldspar and the formation and occurrence of secondary feldspar in tholeiitic basalts from the Hengill volcanic centre, in SW Iceland was studied by microprobe analysis of cuttings from two ca. 2000 m deep geothermal wells. Well NG-7 in Nesjavellir represents a geothermal system in a rift zone where the intensity of young, insignificantly altered intrusions increases with depth. Well KhG-1 in Kolvioarholl represents the margin of a rift zone where the intensity of intrusives is lower and the intensity of alteration higher. This marginal well represents altered basaltic crust in an early retrograde state. The secondary plagioclase in both wells is mainly oligoclase, occurring in association with K-feldspar and chlorite actinolite. The texture of this assemblage depends on the lithology and intensity of alteration. In Nesjavellir (NG-7) the composition of secondary albite-oligoclase is correlated with the host-rock composition. This connection is not apparent in more intensely altered samples from Kolvioarholl (KhG-1). The influence of temperature on composition of secondary Na-feldspar is unclear in both wells although Ca is expected to increase with temperature. Any temperature dependence may be suppressed by the influence of rock composition in Nesjavellir and by retrograde conditions at Kolvioarhall. The absence of clear compositional gradients between igneous plagioclase and secondary feldspar and between Na-feldspar and K-feldspar suggests that secondary feldspars formed by dissolution precipitation reactions. (C) 2002 Elsevier Science B.V. All rights reserved. (Less)
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author
publishing date
type
Contribution to journal
publication status
published
subject
keywords
hydrothermal alteration, feldspar, Hengill, plagioclase
in
Journal of Volcanology and Geothermal Research
volume
114
issue
3-4
pages
275 - 290
publisher
Elsevier
external identifiers
  • wos:000176528000002
  • scopus:0037198728
ISSN
0377-0273
DOI
10.1016/S0377-0273(01)00267-0
language
English
LU publication?
no
id
00f99492-7d1f-4581-bc1d-c60dc0f5eeb2 (old id 334460)
date added to LUP
2007-10-31 11:08:58
date last changed
2017-01-01 06:54:50
@article{00f99492-7d1f-4581-bc1d-c60dc0f5eeb2,
  abstract     = {Dissolution of igneous feldspar and the formation and occurrence of secondary feldspar in tholeiitic basalts from the Hengill volcanic centre, in SW Iceland was studied by microprobe analysis of cuttings from two ca. 2000 m deep geothermal wells. Well NG-7 in Nesjavellir represents a geothermal system in a rift zone where the intensity of young, insignificantly altered intrusions increases with depth. Well KhG-1 in Kolvioarholl represents the margin of a rift zone where the intensity of intrusives is lower and the intensity of alteration higher. This marginal well represents altered basaltic crust in an early retrograde state. The secondary plagioclase in both wells is mainly oligoclase, occurring in association with K-feldspar and chlorite actinolite. The texture of this assemblage depends on the lithology and intensity of alteration. In Nesjavellir (NG-7) the composition of secondary albite-oligoclase is correlated with the host-rock composition. This connection is not apparent in more intensely altered samples from Kolvioarholl (KhG-1). The influence of temperature on composition of secondary Na-feldspar is unclear in both wells although Ca is expected to increase with temperature. Any temperature dependence may be suppressed by the influence of rock composition in Nesjavellir and by retrograde conditions at Kolvioarhall. The absence of clear compositional gradients between igneous plagioclase and secondary feldspar and between Na-feldspar and K-feldspar suggests that secondary feldspars formed by dissolution precipitation reactions. (C) 2002 Elsevier Science B.V. All rights reserved.},
  author       = {Larsson, Daniel and Gronvold, K and Oskarsson, N and Gunnlaugsson, E},
  issn         = {0377-0273},
  keyword      = {hydrothermal alteration,feldspar,Hengill,plagioclase},
  language     = {eng},
  number       = {3-4},
  pages        = {275--290},
  publisher    = {Elsevier},
  series       = {Journal of Volcanology and Geothermal Research},
  title        = {Hydrothermal alteration of plagioclase and growth of secondary feldspar in the Hengill Volcanic Centre, SW Iceland},
  url          = {http://dx.doi.org/10.1016/S0377-0273(01)00267-0},
  volume       = {114},
  year         = {2002},
}