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    The role of grain boundaries and transient porosity in rocks as fluid pathways for reaction front propagation

    Access Status
    Fulltext not available
    Authors
    Jonas, L.
    John, T.
    King, H.
    Geisler, T.
    Putnis, Andrew
    Date
    2014
    Type
    Journal Article
    
    Metadata
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    Citation
    Jonas, L. and John, T. and King, H. and Geisler, T. and Putnis, A. 2014. The role of grain boundaries and transient porosity in rocks as fluid pathways for reaction front propagation. Earth and Planetary Science Letters. 386: pp. 64-74.
    Source Title
    Earth and Planetary Science Letters
    DOI
    10.1016/j.epsl.2013.10.050
    ISSN
    0012-821X
    URI
    http://hdl.handle.net/20.500.11937/41122
    Collection
    • Curtin Research Publications
    Abstract

    The pseudomorphic replacement of Carrara marble by calcium phosphates was used as a model system in order to study the influence of different fluid pathways for reaction front propagation induced by fluid–rock interaction. In this model, grain boundaries present in the rock as well as the transient porosity structures developing throughout the replacement reaction enable the reaction front to progress further into the rock as well as to the center of each single grain until transformation is complete. Hydrothermal treatment of the marble using phosphate bearing solutions led to the formation of hydroxylapatite and β -TCP; the formation of the latter phase was probably promoted by the presence of ∼0.6 wt.%∼0.6 wt.% Mg in the parent carbonate phase. Completely transformed single grains show a distinctive zoning, both in composition and texture. Whereas areas next to the grain boundary consist of nearly pure hydroxylapatite and show a coarse porosity, areas close to the center of the single grains show a high amount of β-TCP and a very fine porous microstructure. If fluorine was added as an additional solution component, up to 3 wt.% of F were incorporated into the product apatite and the formation of β-TCP was avoided.The use of the isotope 18O as a chronometer for the replacement reaction makes it possible to reconstruct the chronological development of the calcium phosphate reaction front. Raman analysis revealed that the incorporation of 18O in the PO4 tetrahedron of hydroxylapatite results in the development of distinct profiles in the calcium phosphate reaction front perpendicular to the grain boundaries of the marble. Through the use of the 18O chronometer, it is possible to estimate and compare the time effectiveness of the different fluid pathways in this model system. The results demonstrate that the grain boundaries are an effective pathway enabling the fluid to penetrate the rock more than one order of magnitude faster compared to the newly developing channel-like porosity structures, which act as pathways towards the center of single mineral grains. Thus, after only short reaction durations, it may be possible for the fluid to progress relatively large distances along the grain boundaries without developing broad reaction fronts along the path.

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