Seafloor hydrothermal alteration at an Archaean mid-ocean ridge

K. Kitajima, S. Maruyama, S. Utsunomiya, J. G. Liou

    Research output: Contribution to journalArticle

    79 Citations (Scopus)

    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 coincident with ridge metamorphism.

    Original languageEnglish
    Pages (from-to)583-599
    Number of pages17
    JournalJournal of Metamorphic Geology
    Volume19
    Issue number5
    DOIs
    Publication statusPublished - Jan 1 2001

    Keywords

    • Archaean
    • Greenstone
    • Hydrothermal metamorphism/ alteration
    • Pilbara craton

    ASJC Scopus subject areas

    • Geology
    • Geochemistry and Petrology

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