Fluid assisted mineral replacement processes
Final Report Abstract
Experiments for the fluid-assisted replacement of plagioclase were performed in cold seal hydrothermal vessels at 4 kbar and 200-800°C. A perforated inner gold capsule was filled with cleavage fragments of natural albite from three different localities, whereas the shut outer capsule was filled with γ-Al2O3 and an aqueous brine of varying salinity. Reaction products were investigated using scanning electron microscopy, high-resolution transmission electron microscopy, X-ray powder diffraction, and cathodoluminescence petrography. Changes in major element compositions were determined using EMP and La-ICPMS analysis. At high salinities sodalite is the stable reaction product over the whole temperature range whereas nepheline occurs at low bulk salinities of the fluid. The transformation of albite always starts along its grain margins and, if present, at cracks or albite twin lamellae. Along the reaction front sodalite crystallizes as very fine grained, strongly porous grains forming polycrystalline aggregates with randomly oriented crystallites displaying no detectable crystallographic relationship with albite. Secondary sodalite may contain fluid inclusions with trapped NaCl-rich brine, demonstrating that the interconnected microporosity provides excellent pathways for fluid-assisted material transport. In many of the experimental runs, a zone of porous amorphous material is developed at the reaction front, which occurs at temperatures of up to of 750°C as several nanometres to 350 μm wide reaction zone around albite. When compared to the latter, the amorphous layer is uniformly depleted in Na and Al and enriched in Si and H. This change in composition corresponds with the abrupt termination of the crystalline feldspar structure. The presence of sodalite as micro- to nanometre-sized, euhedral crystals within the amorphous zone is indicative of its in situ nucleation and growth. Remarkably, both product sodalite and the amorphous material largely inherit the trace element budget of the respective ancestor albite, displaying almost indistinguishable trace element patterns, hence indicating that at least part of the trace elements remain fixed during the reaction process. Remarkably, the replacement textures and chemical signatures of the run products of the experiments are very similar to those of natural rock samples from NW Namibia. The observed reaction textures indicate an interfacial dissolution-reprecipitation mechanism. The sharp chemical and structural interface between albite and the amorphous layer and the lack of albite structure relicts in the amorphous material strongly suggest that the latter represents a (local) thermodynamically stable secondary phase rather than a leached surface layer of albite. The fact that sodalite crystallises from the Si-rich amorphous matrix demonstrates that both the sodalite reaction rim and the amorphous material allow for fluid-assisted material transport between the crystalline albite (release of Si, Al) and the bulk fluid (H2O, Na, Cl). Results of our study bear important implications with respect to mineral replacement in the presence of a fluid phase, especially regarding the interpretation of trace element patterns of the product phases.
Publications
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(2006): Subsolidus conversion of plagioclase to sodalite. EMPG XI Bristol, Programme Volume: 15
Drüppel, K., Franke, H., Lehmann, C., Spiekermann, G., Thiele, M.
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(2007): Metasomatic transformation of plagioclase to sodalite: comparison between nature and experiment. Frontiers in Geosciences 2007
Drüppel, K., Wirth, R.
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(2008): Mesoproterozoic ferrocarbonatite magmatism and related metasomatic processes, Namibia. International Geological Congress (IGC) Oslo, 2008
Drüppel K., Littmann S., Romer R.L.
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(2009): Fluid assisted replacement of feldspar – structure of the reaction interface and the role of amorphous material formation. DMG meeting Halle, 2009, Hallesches Jahrbuch für Geowissenschaften 31: 44
Drüppel K., Tommaseo C.
