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Seafloor hydrothermal alteration at an Archaean mid-ocean ridge.

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Title: Seafloor hydrothermal alteration at an Archaean mid-ocean ridge.
Authors: Kitajima, K.1, Maruyama, S.1, Utsunomiya, S.2, Liou, J. G.3
Source: Journal of Metamorphic Geology. Sep2001, Vol. 19 Issue 5, p583-599. 17p.
Subjects: Complexes (Stratigraphy), Chert, Facies, Hydrogeology
Geographic Terms: Western Australia, Pilbara (W.A.)
Abstract: Abstract A hydrothermally metamorphosed/altered greenstone complex capped by bedded cherts exposed in the North Pole, Pilbara Carton, Western Australia, is interpreted as an accretionary complex. It is distinctive in being characterised by both duplex structure and an oceanic crust stratigraphy. This complex is shown to represent an Archaean upper oceanic crust with a mid-ocean ridge hydrothermal metamorphism that increases in grade stratigraphically downward. Three mineral zones have been defined; Zone A of the zeolite facies, the prehnite-pumpellyite facies or the lower-greenschist facies at high-XCO2 condition, Zone B of the greenschist facies, and Zone C of the greenschist/amphibolite transition facies. In Zone A metabasites, Ca-Al silicates including Ca-zeolites, prehnite and pumpellyite are absent and epidote/clinozoisite is extremely rare. Instead, abundant carbonates are present with chlorite suggesting high-XCO2 composition in the fluid. On the other hand, in Zones B and C metabasites, where Ca-amphibole + epidote/clinozoisite + chlorite + Ca-Na plagioclase are the dominant assemblages, carbonate is not identified. The metamorphic conditions boundary of Zones B/C were estimated to be about 350 °C at a pressure of <0.5 kbar. Fluid compositions coexisting with Archaean greenstones at the transition between Zones B and C were estimated by thermodynamic calculation in the CaFMASCH system (T = 350–370 °C, P = 150–1000 bar) at XCO2 of 0.012–0.140, such values are higher than present-day vent fluids collected near mid-ocean ridges with low-XCO2 values, up to 0.005. The Archaean seawater depth at the mid-ocean ridge was estimated to be 1600 m at XCO2 = 0.06 using a depth-to-boiling point curve for a fluid. The carbonation due to high-XCO2 hydrothermal fluids occurred near the ridge-axis before or was... [ABSTRACT FROM AUTHOR]
Copyright of Journal of Metamorphic Geology is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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DbLabel: Engineering Source
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  Data: Abstract A hydrothermally metamorphosed/altered greenstone complex capped by bedded cherts exposed in the North Pole, Pilbara Carton, Western Australia, is interpreted as an accretionary complex. It is distinctive in being characterised by both duplex structure and an oceanic crust stratigraphy. This complex is shown to represent an Archaean upper oceanic crust with a mid-ocean ridge hydrothermal metamorphism that increases in grade stratigraphically downward. Three mineral zones have been defined; Zone A of the zeolite facies, the prehnite-pumpellyite facies or the lower-greenschist facies at high-XCO2 condition, Zone B of the greenschist facies, and Zone C of the greenschist/amphibolite transition facies. In Zone A metabasites, Ca-Al silicates including Ca-zeolites, prehnite and pumpellyite are absent and epidote/clinozoisite is extremely rare. Instead, abundant carbonates are present with chlorite suggesting high-XCO2 composition in the fluid. On the other hand, in Zones B and C metabasites, where Ca-amphibole + epidote/clinozoisite + chlorite + Ca-Na plagioclase are the dominant assemblages, carbonate is not identified. The metamorphic conditions boundary of Zones B/C were estimated to be about 350 &#176;C at a pressure of &lt;0.5 kbar. Fluid compositions coexisting with Archaean greenstones at the transition between Zones B and C were estimated by thermodynamic calculation in the CaFMASCH system (T = 350–370 &#176;C, P = 150–1000 bar) at XCO2 of 0.012–0.140, such values are higher than present-day vent fluids collected near mid-ocean ridges with low-XCO2 values, up to 0.005. The Archaean seawater depth at the mid-ocean ridge was estimated to be 1600 m at XCO2 = 0.06 using a depth-to-boiling point curve for a fluid. The carbonation due to high-XCO2 hydrothermal fluids occurred near the ridge-axis before or was... [ABSTRACT FROM AUTHOR]
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  Data: &lt;i&gt;Copyright of Journal of Metamorphic Geology is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites without the copyright holder&#39;s express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.&lt;/i&gt; (Copyright applies to all Abstracts.)
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RecordInfo BibRecord:
  BibEntity:
    Identifiers:
      – Type: doi
        Value: 10.1046/j.0263-4929.2001.00330.x
    Languages:
      – Code: eng
        Text: English
    PhysicalDescription:
      Pagination:
        PageCount: 17
        StartPage: 583
    Subjects:
      – SubjectFull: Complexes (Stratigraphy)
        Type: general
      – SubjectFull: Chert
        Type: general
      – SubjectFull: Facies
        Type: general
      – SubjectFull: Hydrogeology
        Type: general
      – SubjectFull: Western Australia
        Type: general
      – SubjectFull: Pilbara (W.A.)
        Type: general
    Titles:
      – TitleFull: Seafloor hydrothermal alteration at an Archaean mid-ocean ridge.
        Type: main
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            NameFull: Kitajima, K.
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            NameFull: Maruyama, S.
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            NameFull: Utsunomiya, S.
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            NameFull: Liou, J. G.
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          Dates:
            – D: 01
              M: 09
              Text: Sep2001
              Type: published
              Y: 2001
          Identifiers:
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              Value: 02634929
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              Value: 19
            – Type: issue
              Value: 5
          Titles:
            – TitleFull: Journal of Metamorphic Geology
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