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Trace element distribution in titanite overgrowths on rutile has been investigated experimentally at 600 °C, 400 MPa and f near NiNiO buffer. Compositionally homogenous Cr- or Nb-doped synthetic rutile single crystals or Nb-containing natural rutile crystals were the source of Cr, Nb and Ti to synthesize titanite using the double-capsule technique. All element exchange with the source of Si, Ca and Al occurred via a NaCl-HO fluid. Titanite forms quickly and exclusively around the rutile crystals. The titanite overgrowth separates rutile from the bulk fluid, and all elements from rutile dissolution have to pass through the titanite rim. Trace element concentrations in titanite show a considerable scatter in experiments with and without Al, although the average concentrations of Cr or Nb of titanite around compositionally homogeneous synthetic rutile approach the expected values for closed system conditions. Variability of Al with Cr or Nb in the titanite is not correlated. The Al zoning is irregular and patchy, and also the distribution of trace elements does not show systematic trends in the spatial distribution. In experiments using zoned natural rutile, the concentrations of Nb in titanite are related to the Nb zoning in rutile, but the contents also vary unsystematically. Under the controlled conditions of the experiment, the explanation for the strongly irregular spatial distribution is most likely due to variations in elemental concentrations during transport from the rutile along the titanite grain boundaries. The transport pathway is complex because grain boundary migration is important during titanite growth. Such irregular element distribution is also found in a natural sample of titanite overgrowth on rutile from an eclogite with retrograde overprint in the amphibolite facies. Transport of Ti and trace elements was focused on grain boundaries and shielded from the rutile as a source of these elements. We conclude that this type of zoning is not related to changes in P-T or composition in an open system, but solely controlled by transport in and through the titanite rim. [ABSTRACT FROM AUTHOR]

Fluid-assisted mass transport reactions by dissolution-precipitation, where a precursor mineral reacts with a fluid, play an important role in metamorphism and metasomatism. We investigated titanite growth on rutile in time series experiments between one and 107 days at constant P-T conditions of 600 °C and 400 MPa in the system TiO2-CaO-SiO2-Na2O-HCl-H2O. A two-capsule assemblage allows for transport of Ca and Si from dissolving wollastonite to dissolving rutile, the Ti-source, in a NaCl-bearing aqueous fluid, according to the general reaction CaSiO3 + TiO2 = CaTiSiO5. Complete overgrowth of rutile by titanite occurred after just one day of experiment. Fine-grained lozenge-shaped titanite crystals of short-time runs (up to 14 days) reorganize to larger predominantly prismatic crystals after >14 days. After investigation by scanning electron microscopy, the titanite overgrowth was removed from the rutile by hydrofluoric acid, to provide a three-dimensional view of the dissolution-reaction front on the rutile surface. The morphology of the rutile surface is dominated by humps or ridges beneath the central region of a titanite crystal and valleys at the grain boundaries between adjacent titanite crystals. The dissolution pattern on the rutile surface mimics the titanite overgrowth and changes with changing grain size and shape of the titanite with longer run times. The preferred dissolution of rutile in the valleys is clearly linked to the position of the titanite grain boundaries, which served as pathways for fluid-assisted element transport. Rutile-titanite and titanite-titanite boundaries show a significant porosity in transmission electron microscopy images of foils prepared by focused ion beam milling. The large-scale dissolution pattern on the rutile surface is independent of the crystallographic orientation of the rutile and entirely dominated by the arrangement of titanite crystals in the overgrowth. Dissolution features on a scale smaller than ~1 µm are dominated by stepwise dissolution and etch-pits following the crystallographic orientation of the rutile. Similar observations were made in experiments with an additional Al-source, although these experiments result in a different overgrowth pattern; i.e., an exposed rutile surface is always present and solitary titanite crystals are accompanied by partial overgrowths. Quantitative characterization of the surface morphology by white-light interference microscopy demonstrates that, with increasing grain size of titanite, dissolution of rutile is strongly enhanced at the titanite grain boundaries. Natural examples of titanite overgrowths on rutile show the same relations between element pathways, arrangement of titanite crystals and 3D dissolution pattern on rutile as in the experimental systems. We conclude that the transport of Ti away from the rutile and of Ca + Si into the reaction rim occurred in a grain boundary fluid, the composition of which must have been strongly different from the composition of the bulk fluid in the experiment, as well as in the natural system. The reaction progress depends on the availability of a fluid, and relicts of rutile in titanite indicate restricted availability of fluid in the natural system (e.g., a fluid pulse that was consumed by the reactions). The reaction examined here can serve as a proxy for other reactions of the conversion of oxide minerals (e.g., spinel or corundum) into silicates. [ABSTRACT FROM AUTHOR]

Titanite growth at the expense of rutile during retrograde hydration of eclogite into amphibolite is a common phenomenon. We investigated an amphibolite sample from the Tromsø eclogite facies terrain in Northern Norway to gain insight into the trace element distribution between rutile and titanite during incomplete resorption of the rutile by titanite. Patchy compositional zoning of Al, Ti, and F in titanite relates to the presence of a fluid with variable Ti/Al and/or F during its growth. Laser ablation ICP–MS and electron microprobe data for high field strength elements (HFSE: Nb, Zr, Ta, and Hf) of rutile resorbed by titanite indicate a pronounced enrichment of these elements in the rim of a large single rutile crystal (~8 mm) and a systematic decrease towards uniform HFSE contents in the large core. HFSE contents of smaller rutile grains (~0.5 mm) and rutile inclusions (<100 μm) in the titanite overgrowth are similar or higher than in the rims of large rutile crystals. Element profiles from the rim inward demonstrate that HFSE enrichment in rutile is controlled by diffusion. HFSE ratios in diffusion-altered rutile show systematic variations compared with the uniform core composition of the large rutile. Modelling of Zr and Nb diffusion in rutile indicates that diffusion coefficients in rutile in fluid-dominated natural systems must be considerably higher than those determined experimentally at 1 bar in dry systems. Variations of HFSE contents in the newly formed titanite show no systematic spatial distribution. HFSE ratios in titanite and the rims of rutile are different, indicating different solid/fluid distribution coefficients in these minerals. Element fractionation by diffusion into the relict rutile and during fluid-mediated growth of new titanite could substantially change the HFSE budget of these minerals and could affect their use for geochemical tracing and other applications, such as Zr-based geothermobarometry. [ABSTRACT FROM AUTHOR]

Chemical zoning according to the substitution Al + (OH,F) = Ti + O is a common phenomenon in natural titanite and can be used to reconstruct its growth conditions. We synthesized titanite, which has a zoning pattern similar to that found in natural titanite that grew at the expense of rutile, visible in element distribution maps as a patchy irregular pattern. Run conditions in the system CaO-Al2O3-TiO2-SiO2-NaCl-H2O(-CaF2) were 600 °C, 0.4 GPa, 1 to 107 days duration. Natural rutile crystals were placed in a perforated Pt-tube within an outer Au-capsule containing wollastonite, γ-Al2O3, optionally powdered CaF2, and a NaCl brine, to simulate conditions, where rutile is transformed into titanite by a hydrous Ca-Si-Al(-F) fluid, driven by a chemical potential gradient between inner and outer capsule. Spontaneous nucleation of titanite is restricted to a small number of crystals on the rutile surface in the inner capsule. Growth proceeds from sparse isolated titanite crystals after 1 day to assemblages of several 100 µm large crystals in a reaction rim in the long-time runs. Titanite is strongly zoned in Al-Ti and shows up to ~0.5 Al per formula unit (pfu) in experiments containing F; without F, substitution is limited to ~0.25 Al pfu. The range of the Ti-Al exchange is already large in short run times and the same heterogeneity is observed in long-time runs. The Al-Ti distribution (with and without F) in compositional domains (up to several tens of micrometers) is patchy and irregular. No relation of the zoning with time (core-rim) or preferred growth directions was observed. In the case of fluid-mediated growth, mineral growth zoning represents a disequilibrium at supersaturated conditions. The concentration of Ti in the fluid is likely the limiting factor for titanite growth considering the low solubility compared to that of Al, Ca, and Si. Titanium is not transported in significant amounts into the outer capsule, indicating low mobility, in contrast to Al, Ca, and Si, which are transported into the inner capsule. Assuming buffered Al concentration from dissolution of the Al-source, we speculate that the Ti/Al of the fluid at the precipitation site was controlled by the local Ti concentration, which varies over the distance from the rutile surface to the growing titanite crystal and produces the irregular patchy zoning. [ABSTRACT FROM AUTHOR]