Halogen concentrations and stable Cl isotopes in apatite as a fluid probe: mapping regional-scale fluid pulses by Cl-isotopes
Final Report Abstract
The project combines field- and experimentally-derived data. It is on one hand a detailed microanalytical, petrological and geochemical investigation of highly metasomatized rocks and on the other hand an experimental study on apatite-related replacement processes and the associated distribution of halogens between fluid and solid. This twofold approach was necessary establish apatite as a fluid probe for halogens and to use natural metasomatized rocks from the Bamble sector as a first application. To understand the role of halogens during the metasomatism of metagabbros from Bamble area, whole rock and mineral separates (i.e., amphibole, biotite, scapolite) from samples showing different degrees of alteration were analyzed for halogens and Cl stable isotopes. Due to the hydrophilic character of halogens, their concentrations and compositions represent the fluid phase rather than that of the precursor solids. Accordingly, the halogen incorporation into newly formed alteration minerals (i.e., amphibole, scapolite, biotite) was used to quantify the fluid chemistry and compositional changes during ongoing fluid-rock interaction. Calculated salinities of coexisting fluid indicate a NaCl concentration of 48 wt.% for the initial fluid, which evolved during fluid-rock interaction up to halite saturation (73 wt.% NaCl). The observed variations in δ37Cl values are interpreted to be the result of a Rayleigh fractionation process, which further suggests that the fluid and the halogens were completely consumed during alteration of the gabbro. To investigate the distribution behavior of halogens between aqueous fluids and apatite, cold-seal-pressure-vessel experiments were performed. The P-T range, compositions of fluid and apatite were chosen to mimic the conditions of the Bamble sector. The experimental data allow an estimation of the Dapatite-fluid values for halogens based on a lattice strain model, which displays a sequence with DF of ~120, DOH of ~100, DCl of ~2.3 DBr ~0.045, and DI ~0.0025. Hydrothermally altered apatite of the Bamble sample set was studied in-situ by electron microscopy and Secondary Ion Mass Spectrometry (SIMS). We found different features in halogen composition of replaced apatite: i) F zonation with high F concentrations along cracks and rims of individual apatite grains; ii) overall decreasing F concentrations with increasing distance to the shear zone; iii) constant Cl concentrations throughout the alteration sequence; iv) increasing Br concentrations with increasing distance to the shear zone. We interpret all these observed compositional features to be the result of a continuous evolution of the fluid during fluid-rock interaction. Due to its high compatibility, F from the infiltrating fluid is incorporated early into recrystallized apatite (close to shear zone and rims of individual apatite grains). In contrast, Br as an incompatible halogen becomes enriched in the fluid and is highest in the most evolved fluid. Using the experimental partition data, we calculated F concentrations of the evolving fluid to decrease from 60 to <1 µg/g and Br to increase from ~1500 to ~ 2500 µg/g; I concentrations of the fluid are constant in the order of 300µg/g. Although, Cl is expected to show a similar behavior as Br, replaced apatite has constant Cl concentrations throughout the alteration sequence (~1 wt.%), which is likely the result of a rather constant Cl activity in the fluid. Chlorine stable isotope values of individual apatite grains are heterogeneous (-1.2‰ to +3.7‰). High δ37Cl values correlates with OH-rich zones of replaced apatite, whereas low δ37Cl values are measured in F-rich zones of replaced apatite and in Cl-apatite of probably magmatic origin. Though apatite δ37Cl values follow the general bulk trend, the individual δ37Cl signature seems to reflect the highly localized composition of interfacial fluid at the reaction front. Our findings suggest that apatite can be used as a fluid probe for F, Br and I to detect a compositional evolution of the fluid, which can be quantified by using experimentally derived partition coefficients. Partitioning of Cl and Cl stable isotopes between highly saline fluids and apatite is complex and likely controlled by more unknown factors than just the Cl concentration.
Publications
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(2015) Apatite as probe for the halogen composition of metamorphic fluids (Bamble Sector, SE Norway). Contrib Mineral Petrol (Contributions to Mineralogy and Petrology) 170 (4)
Kusebauch, Christof; John, Timm; Whitehouse, Martin J.; Engvik, Ane K.
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(2015) Distribution of halogens between fluid and apatite during fluid-mediated replacement processes. Geochimica et Cosmochimica Acta 170 225–246
Kusebauch, Christof; John, Timm; Whitehouse, Martin J.; Klemme, Stephan; Putnis, Andrew
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(2013). Synthesis of trace element bearing single crystals of Chlor-Apatite (Ca5(PO4)3Cl) using the flux growth method. Chemistry Central Journal, 7
Klemme, S., John, T., Wessels, M., Kusebauch, C., Berndt, J., Rohrbach, A., Schmid-Beurmann, P.
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(2013). Influence of temperature and Cl on the hydrothermal replacement of calcite by apatite and the development of porous microstructures. American Mineralogist, 98: 1516-1525
Jonas, L., John, T., Putnis, A.
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(2014). The role of grain boundaries and transient porosity in rocks as fluid pathways for reaction front propagation. Earth and Planetary Science Letters, 386: 64-74
Jonas, L., John, T., King, H.E., Geisler, T., Putnis, A.
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(2015). Halogen element and stable chlorine isotope fractionation caused by fluid-rock interaction (Bamble sector SE Norway). Journal of Petrology, 56: 299-324
Kusebauch, C., John, T., Barnes, J., Klügel, A., Austrheim, H.
