Synchronous solid-state diffusion, dissolution-reprecipitation, and recrystallization leading to isotopic resetting: insights from chalcopyrite replacement by copper sulfides

Alok Chaudhari, Joël Brugger, Rahul Ram, Priyadarshi Chowdhury, Barbara Etschmann, Paul Guagliardo, Fang Xia, Allan Pring, Gediminas Gervinskas, Amelia Liu, Andrew Frierdich

Research output: Contribution to journalArticleResearchpeer-review

Abstract

Reactions among minerals occur via solid-state diffusion or fluid-induced, interface-coupled dissolution-reprecipitation (ICDR). Both mechanisms can coexist under conditions where the rates of both processes are similar, depending mainly on the nature of the mineral, temperature, and fluid composition. To clarify the synergies between these reaction mechanisms, we investigated the replacement and recrystallization reactions of chalcopyrite in the presence of a 65Cu-enriched aqueous fluid. The replacement of chalcopyrite by secondary copper sulfides follows a paragenetic sequence, whereby chalcopyrite is initially replaced by covellite, and then by digenite, with some of the digenite replacing covellite in longer duration experiments. The replacement reactions proceed via ICDR along with solid-state diffusion of 65Cu from the fluid into product minerals, with both mechanisms occurring at similar rates. Over time, chalcopyrite is completely replaced, and the secondary copper sulfides attain isotopic equilibrium with the fluid. Depending on temperature, thermal history, and fluid-mineral ratio, the reaction products may preserve kinetic or equilibrium signatures. We have identified a new geochemical process of ‘porosity-aided recrystallization’, which may result in a re-equilibration or perturbation of mineral isotopic/trace element compositions due to exchanges between the mineral and fluid facilitated by extensive micro- to nano-scale porosity created by ICDR reactions. Porosity-aided recrystallization will affect any mineral-fluid system where mass transfers via solid-state diffusion and ICDR operate at similar rates, causing rapid (days-weeks) ‘resetting’ of the original isotopic abundances and trace element contents of the affected minerals. In copper sulfides, porosity-aided recrystallization occurs at lower temperatures (<300 °C) due to fast cation diffusion rates (log10D ≈ –13.6, 300 °C), but also takes place across other mineral systems where diffusion rates are faster at high temperatures. The open-system isotopic and trace element exchanges associated with porosity-aided recrystallization could lead to erroneous petrological interpretations, especially where these markers are used as a proxy for reconstructing geological evolution.

Original languageEnglish
Pages (from-to)48-68
Number of pages21
JournalGeochimica et Cosmochimica Acta
Volume331
DOIs
Publication statusPublished - 15 Aug 2022

Keywords

  • Chalcopyrite
  • Copper isotopes
  • Copper sulfides
  • Diffusion
  • Interface coupled dissolution reprecipitation
  • Isotope tracer
  • Porosity-aided recrystallization

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