Differentiating marine vs hydrothermal processes in Devonian carbonatemounds using rare earth elements (Kess Kess mounds, Anti-Atlas, Morocco)

Fulvio Franchi, Axel Hofmann, Barbara Cavalazzi, Allan Wilson, Roberto Barbieri

    Research output: Contribution to journalArticle

    17 Citations (Scopus)

    Abstract

    Petrological and geochemical analyses were carried out on Early Devonian Kess Kess mound limestones of the Seheb el Rhassel Group exposed in the Hamar Laghdad Ridge (Tafilalt Platform, Eastern Anti-Atlas, Morocco) in order to evaluate marine vs hydrothermal processes for the origin of the mounds. Hydrothermal fluid circulation affected limestone deposition resulting in the formation of a plumbing system preserved in the mound facies and inter-mound facies as cavities, veins, and dykes.Shale-normalized rare earth element (REE) patterns for limestones display pronounced light REE depletion compared to middle REE (average PrSN/DySN=0.46) and heavy REE (average PrSN/YbSN=0.55), a super-chondritic Y/Ho ratio (up to 87) and positive La anomaly consistent with precipitation from normal marine seawater. Within our data set the Ce anomaly, as well as U and other trace element concentrations vary with facies providing evidence for variable redox conditions. The presence of positive Ce anomaly in carbonates of the plumbing system is consistent with precipitation under anoxic condition. Samples from mound facies and quartz veinlets in the upper part of the group show REE patterns consistent with precipitation from hydrothermal fluids.Early Devonian hypothetical seawater REE patterns were calculated from samples showing normal marine REE patterns (i.e. inter-mound facies) and from samples with weaker marine signature (i.e. fossiliferous mound limestones and plumbing system deposits). Hypothetical patterns are slightly enriched in σREE compared to modern open ocean seawater.This study reveals that REE analysis is a powerful tool for understanding polygenetic carbonate systems. It sheds light into the genesis of the Kess Kess mounds by reconstructing fluid pathways and palaeo-redox conditions.

    Original languageEnglish
    Pages (from-to)69-86
    Number of pages18
    JournalChemical Geology
    Volume409
    DOIs
    Publication statusPublished - Aug 1 2015

    Fingerprint

    Rare earth elements
    atlas
    rare earth element
    Calcium Carbonate
    Plumbing
    Seawater
    limestone
    Carbonates
    redox conditions
    seawater
    hydrothermal fluid
    anomaly
    Fluids
    carbonate system
    Quartz
    Trace Elements
    Shale
    open ocean
    anoxic conditions
    shale

    All Science Journal Classification (ASJC) codes

    • Geochemistry and Petrology
    • Geology

    Cite this

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    title = "Differentiating marine vs hydrothermal processes in Devonian carbonatemounds using rare earth elements (Kess Kess mounds, Anti-Atlas, Morocco)",
    abstract = "Petrological and geochemical analyses were carried out on Early Devonian Kess Kess mound limestones of the Seheb el Rhassel Group exposed in the Hamar Laghdad Ridge (Tafilalt Platform, Eastern Anti-Atlas, Morocco) in order to evaluate marine vs hydrothermal processes for the origin of the mounds. Hydrothermal fluid circulation affected limestone deposition resulting in the formation of a plumbing system preserved in the mound facies and inter-mound facies as cavities, veins, and dykes.Shale-normalized rare earth element (REE) patterns for limestones display pronounced light REE depletion compared to middle REE (average PrSN/DySN=0.46) and heavy REE (average PrSN/YbSN=0.55), a super-chondritic Y/Ho ratio (up to 87) and positive La anomaly consistent with precipitation from normal marine seawater. Within our data set the Ce anomaly, as well as U and other trace element concentrations vary with facies providing evidence for variable redox conditions. The presence of positive Ce anomaly in carbonates of the plumbing system is consistent with precipitation under anoxic condition. Samples from mound facies and quartz veinlets in the upper part of the group show REE patterns consistent with precipitation from hydrothermal fluids.Early Devonian hypothetical seawater REE patterns were calculated from samples showing normal marine REE patterns (i.e. inter-mound facies) and from samples with weaker marine signature (i.e. fossiliferous mound limestones and plumbing system deposits). Hypothetical patterns are slightly enriched in σREE compared to modern open ocean seawater.This study reveals that REE analysis is a powerful tool for understanding polygenetic carbonate systems. It sheds light into the genesis of the Kess Kess mounds by reconstructing fluid pathways and palaeo-redox conditions.",
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    Differentiating marine vs hydrothermal processes in Devonian carbonatemounds using rare earth elements (Kess Kess mounds, Anti-Atlas, Morocco). / Franchi, Fulvio; Hofmann, Axel; Cavalazzi, Barbara; Wilson, Allan; Barbieri, Roberto.

    In: Chemical Geology, Vol. 409, 01.08.2015, p. 69-86.

    Research output: Contribution to journalArticle

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