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Chlorine-rich amphibole in deep layered gabbros as evidence for brine/rock interaction in the lower oceanic crust: a case study from the Wadi Wariyah, Samail Ophiolite, Sultanate of Oman

Chlorine-rich amphibole in deep layered gabbros as evidence for brine/rock interaction in the lower oceanic crust: a case study from the Wadi Wariyah, Samail Ophiolite, Sultanate of Oman
Chlorine-rich amphibole in deep layered gabbros as evidence for brine/rock interaction in the lower oceanic crust: a case study from the Wadi Wariyah, Samail Ophiolite, Sultanate of Oman

Hydrothermal veins and dykelets that cross-cut layered olivine gabbros deep in the plutonic section of the Samail Ophiolite, Sultanate of Oman, point towards the occurrence of hydrothermal circulation in the deep oceanic crust, and these features record interactions between rock and high temperature seawater-derived fluids or brines. Deep penetration of seawater-derived fluids down to 100 m above the Moho transition zone and the consequent interactions with the host rock lead to hydrothermal alteration from granulite facies grading down to greenschist facies conditions. Here we present a study of veins and dykelets formed by hydrothermal interaction cutting layered gabbro in the Wadi Wariyah, using petrographic, microanalytical, isotopic, and structural methods. We focus on amphiboles, which show a conspicuous compositional variation from high-Ti magnesiohastingsite and pargasite via magnesiohornblende and edenite, to Cl-rich ferropargasite and hastingsite (up to 1.5 a.p.f.u. Cl) and actinolite. These minerals record a wide range of formation conditions from magmatic to hydrothermal, and reveal a complex history of interactions between rock and hydrothermal fluid or brine in a lower oceanic crustal setting. Large variations in Cl content and cation configurations in amphibole suggest formation in equilibrium with fluids of different salinities at variable fluid/rock ratios. The presence of subsolidus amphibole extremely enriched in chlorine implies phase separation and brine/rock interactions. 87Sr/86Sr values of 0.7031 to 0.7039 and stable δ18O isotopic compositions of 4.1 to 5.6‰ of the different amphibole types suggest a rock-dominated environment, i.e. with low fluid/rock ratios. However, the slight departure from mean Oman isotope values may indicate there was some influence of seawater in the aforementioned fluid-rock interactions. Our study provides new petrological data for the subsolidus evolution of gabbro-hosted amphibole-rich veins in the presence of a seawater-derived fluid.

Brine-rock interactions, Cl-bearing amphibole, High-temperature hydrothermal circulation, Lower oceanic crust, Phase separation, Samail Ophiolite
0024-4937
125-136
Currin, Adriana
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Wolff, Paul Eric
32af3a2a-74f1-4a3c-af6d-29c0a0a9784b
Koepke, Juergen
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Almeev, Renat R.
ff4398ac-658c-46bd-899b-aed9bd0fb082
Zhang, Chao
275a6acd-9b7c-4c10-b894-2085af9ecd7c
Zihlmann, Barbara
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Ildefonse, Benoit
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Teagle, Damon A.H.
396539c5-acbe-4dfa-bb9b-94af878fe286
Currin, Adriana
719ec8a4-2826-47ae-8547-67a08c999b94
Wolff, Paul Eric
32af3a2a-74f1-4a3c-af6d-29c0a0a9784b
Koepke, Juergen
0588b06e-3cd2-409d-ab47-850450bab8dd
Almeev, Renat R.
ff4398ac-658c-46bd-899b-aed9bd0fb082
Zhang, Chao
275a6acd-9b7c-4c10-b894-2085af9ecd7c
Zihlmann, Barbara
4a1d8360-0f4d-4f0d-ae4e-bd1ffd426e61
Ildefonse, Benoit
fbff4be0-d1fb-47d1-9174-33989a8da5d8
Teagle, Damon A.H.
396539c5-acbe-4dfa-bb9b-94af878fe286

Currin, Adriana, Wolff, Paul Eric, Koepke, Juergen, Almeev, Renat R., Zhang, Chao, Zihlmann, Barbara, Ildefonse, Benoit and Teagle, Damon A.H. (2018) Chlorine-rich amphibole in deep layered gabbros as evidence for brine/rock interaction in the lower oceanic crust: a case study from the Wadi Wariyah, Samail Ophiolite, Sultanate of Oman. Lithos, 323, 125-136. (doi:10.1016/j.lithos.2018.09.015).

Record type: Article

Abstract

Hydrothermal veins and dykelets that cross-cut layered olivine gabbros deep in the plutonic section of the Samail Ophiolite, Sultanate of Oman, point towards the occurrence of hydrothermal circulation in the deep oceanic crust, and these features record interactions between rock and high temperature seawater-derived fluids or brines. Deep penetration of seawater-derived fluids down to 100 m above the Moho transition zone and the consequent interactions with the host rock lead to hydrothermal alteration from granulite facies grading down to greenschist facies conditions. Here we present a study of veins and dykelets formed by hydrothermal interaction cutting layered gabbro in the Wadi Wariyah, using petrographic, microanalytical, isotopic, and structural methods. We focus on amphiboles, which show a conspicuous compositional variation from high-Ti magnesiohastingsite and pargasite via magnesiohornblende and edenite, to Cl-rich ferropargasite and hastingsite (up to 1.5 a.p.f.u. Cl) and actinolite. These minerals record a wide range of formation conditions from magmatic to hydrothermal, and reveal a complex history of interactions between rock and hydrothermal fluid or brine in a lower oceanic crustal setting. Large variations in Cl content and cation configurations in amphibole suggest formation in equilibrium with fluids of different salinities at variable fluid/rock ratios. The presence of subsolidus amphibole extremely enriched in chlorine implies phase separation and brine/rock interactions. 87Sr/86Sr values of 0.7031 to 0.7039 and stable δ18O isotopic compositions of 4.1 to 5.6‰ of the different amphibole types suggest a rock-dominated environment, i.e. with low fluid/rock ratios. However, the slight departure from mean Oman isotope values may indicate there was some influence of seawater in the aforementioned fluid-rock interactions. Our study provides new petrological data for the subsolidus evolution of gabbro-hosted amphibole-rich veins in the presence of a seawater-derived fluid.

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Currin_et_al_2018_Lithos_Wariyah - Accepted manuscript
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More information

Accepted/In Press date: 12 September 2018
e-pub ahead of print date: 25 September 2018
Published date: 15 December 2018
Keywords: Brine-rock interactions, Cl-bearing amphibole, High-temperature hydrothermal circulation, Lower oceanic crust, Phase separation, Samail Ophiolite

Identifiers

Local EPrints ID: 427199
URI: http://eprints.soton.ac.uk/id/eprint/427199
ISSN: 0024-4937
PURE UUID: 332ade05-31c7-49c1-93ee-a41993c5679d
ORCID for Damon A.H. Teagle: ORCID iD orcid.org/0000-0002-4416-8409

Catalogue record

Date deposited: 07 Jan 2019 17:30
Last modified: 26 Nov 2021 02:43

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Contributors

Author: Adriana Currin
Author: Paul Eric Wolff
Author: Juergen Koepke
Author: Renat R. Almeev
Author: Chao Zhang
Author: Barbara Zihlmann
Author: Benoit Ildefonse

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