Fluid assisted mineral replacement processes

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Mineral replacement reactions in the presence of a fluid phase are a wide spread phenomenon in many rocks. We choose to study the reaction rutile (TiO2) + fluid (Ca2+ aq + Si4+ aq) = titantite (CaTiSiO5) both, in natural examples and by experimental methods. Our goal is to explain the trace element signature and zoning pattern of titanite grown under such conditions, with its applications to geothermobarometry and isotope determination. Typical natural examples are found in high-pressure rocks during uplift and hydration. Although the reaction is a dissolution-precipitation reaction, it is not volume-conserving with the formation of pseudomorphs, but largely isochemical for Ti. The investigated samples of titanite reaction rims show rutile relicts of different size and strong enrichment with a typical diffusion profile of HFS trace elements into rutile. Titanite is irregularly zoned in Al and Sr. Our working hypothesis is that different element mobilities are responsible for the major and trace element variation. The reaction is simulated experimentally at 400 MPa/600°C in a setup, where the Ti-source is physically separated in a perforated inner capsule from the Ca and Si (+Al, +trace elements) source in an outer capsule. Element transport takes place in a NaCl (±F) solution. The Tisource is natural rutile with high ‘water’ contents. The reactions starts immediately with the formation of numerous titanite crystals which cover the rutile surface already after one day run time; reactions proceeds mainly with coarsening of the reaction rim (runs up to 107 days). We will continue our experiments with synthetic (pure and doped with HFSE, Cr and other elements) melt grown rutile and will explore the influence of H in the nominally anhydrous rutile on the reaction.