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Laura Jonas, Timm John, Helen E. King, Thorsten Geisler & Andrew Putnis

The role of grain boundaries and transient porosity in rocks as fluid pathways for reaction front propagation . Laura Jonas, Timm John, Helen E. King, Thorsten Geisler & Andrew Putnis Institut für Mineralogie , University of Münster , Germany .

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Laura Jonas, Timm John, Helen E. King, Thorsten Geisler & Andrew Putnis

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  1. The role of grain boundaries and transient porosity in rocks as fluid pathways for reaction front propagation Laura Jonas, Timm John, Helen E. King, Thorsten Geisler & Andrew Putnis InstitutfürMineralogie, University of Münster, Germany

  2. During fluid-rock interaction (metasomatism, metamorphismetc.) ...... • ........ how do thefluidsmovethroughlowpermeability rock ? • Alonggrainboundariesandotherpre-existingpathways • Hydraulicfractures • Porositygeneratedbythereactions

  3. During fluid-rock interaction (metasomatism, metamorphismetc.) ...... • ........ how do thefluidsmovethroughlowpermeability rock ? • Alonggrainboundariesandotherpre-existingpathways • Hydraulicfractures • Porositygeneratedbythereactions

  4. Howisporositygeneratedduringmineralreplacementreactions. Interface-coupleddissolution-precipitation

  5. Interface-coupled dissolution-precipitation crystal a c b d e • Dissolution of even a few monolayers of the parent crystal may result in an interfacial fluid layer which is supersaturated with respect to another phase • (b) This product phase may nucleate on the surface of the parent Putnis & Putnis, J.Solid State Chem. 180, 1783 (2007)

  6. Interface-coupled dissolution-precipitation crystal a c b d e (c) The porosity generated depends on both the solid molar volume change and the relative solubilitiesof parent and product in the fluid (d-e) A reaction/replacement interface moves through the parent crystal

  7. The model example: The experimental replacement of Carrara marble by apatite 5CaCO3 + 3HPO42- + H2O= Ca5(PO4)3(OH) + 3CO32- + 2H2CO3

  8. Cross section of a cube of Carrara marble reacted with phosphate solution at 1500C for 1 week Calcite is partially replaced by apatite

  9. calcite is being replaced by apatite, Ca5(PO4)3(OH) Experiments with polycrystalline Carrara marble with a hydrothermal fluid containing phosphate ions. The dissolution of the calcite results in a Ca-P bearing fluid, from which apatite nucleates. The dissolution and precipitation are coupled so that the parent phase is pseudomorphically replaced by the product.

  10. porous apatite replacing calcite Jonas et al. (2014) EPSL

  11. Using18O as a tracerforthe fluid transportandreplacementmechanism First, equilibrationofthephosphate in solutionwith H218O HP16O42- +H218O → HP18O42- +H216O Experiment : Dilute an equalvolumeof (NH4)2HPO4solutionwith H218O The isotope equilibrationisveryslowatroom T

  12. BUT Atthetemperatureofthe experimental runsforthereplacementreaction (1500C) theexchange rate in the fluid iscomparabletothe rate ofthereplacementreaction. Kasioptas et al. GCA (2011)

  13. Using Raman spectroscopytodetectandmeasure18O in PO4.

  14. Cross-sectionthroughpartof a Carrara marblecube after 5 daysat 1500C in phosphatesolution

  15. 4 3 2 1

  16. 4 3 2 1

  17. 18O content in theapatite This methodprovides an internalchronometerforthe time ofthereplacementateachmeasuredpoint in theapatite 1 2 3 4 18O exchange in thesolution

  18. 18O content in theapatite Duringthe same periodthatthe fluid penetrated 200 µm alongthegrainboundary, thereaction front penetrated 10 µm normal tothegrainboundary 1 2 3 4 18O exchange in thesolution

  19. EPSL 386, 64-74 (2014)

  20. Acknowledgements: European Union Network project: “Delta-Min” (Mechanisms of Mineral Replacement Reactions) Deutsche Forschungsgemeinschaft (German Research Foundation)

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