Search

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]

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]

We performed hydration experiments of pure and Nb-, Cr-, and V-doped synthetic dry (H2O < 3 ppm) single rutile crystals. They were equilibrated with pure H2O in hydrothermal experiments at constant conditions of 600 °C, 400 MPa, and fO2 near the Ni-NiO buffer, run time between ~25 min and 14 days. Slabs cut parallel to (110) of the reacted single crystals (1 to 2 mm³) were analyzed for H+ by FTIR. Hydration occurs almost spontaneously and the H2O-equivalent is uniformly distributed in the samples, but depends extremely on trace element contents. In pure rutile, the average H2O-content is 314 ± 50 ppm, the saturation level at these conditions. Rutile doped with 500 ppm Nb has a lower average H2O content of ~235 ppm, rutile with 2000 ppm Cr has ~900 ppm H2O, and rutile with 2000 ppm V does not incorporate H2O. During stepwise heating at atmospheric pressure of a reacted Nb-doped rutile, H+ is quickly released between 450 and 550 °C. UV-VIS spectra of unreacted colorless and reacted deep blue pure rutile show the rutile-characteristic sharp absorption edge in the UV spectra. The reacted rutile has a broad absorption band at 6500 cm-1 wavenumber attributed to intervalence charge transfer transition Ti3++Ti4+ → Ti4++Ti3+. The reduction of Ti4+ to Ti3+ is charge balanced by the incorporation of H+. The Nb-doped rutile changed its color from light greenish-blue (untreated) to deep blue. In the untreated Nb rutile, the UV-VIS absorption band at 6500 cm-1 indicates that Nb5+ is charge balanced by Ti3+. In the reacted Nb-rutile the absorption band is more intense, but compared with the pure rutile, H+ incorporation is lower by the equivalent of Ti3+ reduced in the untreated rutile. Reacted Cr-rutile almost retains its brownish-orange color, but the spectrum shows a prominent Ti3+/Ti4+ IVCT band at ~6400 cm-1 with moderate intensity considering the high-H2O contents of ~900 ppm. The high-H+ contents are best explained by the reduction of Cr4+ to Cr2+. The UV-VIS spectra of the dark-blue to opaque V-doped rutile show a very strong absorption toward low energies, which is likely caused by reduction of Ti4+ to Ti3+ for charge balance of V5+. This forms a deep narrow window of transmittance in the range 25 000--20 000 cm-1, which causes the dark-blue color. To explore the possible use of H-in-rutile as a geohygrometer, geothermobarometer, and oxybarometer, we measured the H+ content in a natural rutile crystal from a retrograded eclogite with a zoned trace element (Fe, Nb, and Zr) content. The crystal reveals a slight correlation between the variable H2O (~200 to 900 ppm) and its trace element concentrations. The observations indicate that the preservation of H+ contents in this natural rutile is a complicated interplay of diffusive reequlibration of fast H+, slower Fe and very slow other trace elements. An interpretation of the H2O contents of the natural crystal in terms of fO2 or aH2O is not possible. [ABSTRACT FROM AUTHOR]

A new type of sector zoning, with an hourglass shape, has been identified in metamorphic tourmalines that formed under a wide variety of physical and chemical conditions. The two sectors in the c-direction are not equivalent due to asymmetry in the crystal structure of tourmaline along the c-axis. The c+ sector is characterized by low concentrations of Ti, Ca, Mg, and Na, although Al is high, and has a pale (commonly blue or pale-green) color. Conversely, the c- sector is low in Mg and Al, and high in Ca, Fe, and Ti (the latter two causing the dark-brown color of this sector). The a-sector has intermediate characteristics and probably approximates a sector-free tourmaline. Thin sectioning of these sector-zoned tourmalines perpendicular to the c-axis can produce three types of apparent radial zoning patterns: blue-green cores, dark-brown cores, or no distinct cores. These apparent cores will further vary in relative diameter depending on the sectioning level. Furthermore, "core" boundaries can be straight or ragged depending on whether the relative growth speeds for the different faces was constant or variable. These textures have been used to argue for a prograde or detrital origin of tourmaline cores. However, sector zoning is a more appealing explanation for most of these textures, and can further explain the textural resemblance among metamorphic tourmalines from highly variable bulk-rock composition, metamorphic history, and mineral paragenesis. The sector zoning that is described here develops by preferential uptake of elements on the r growth plane, resulting from a combined effect of differences in surface charge and morphology of this plane in the c+ and c- directions. This leads to the preferential incorporation of more positively charged elements in the c- direction, and a preference for a vacant X-site in the c+ direction. Because the compositional differences among the sectors are pronounced in both major and trace elements and in the same order of magnitude as growth zoning variability, the presence of sector zoning must be established and taken into account when making inferences from tourmaline chemistry. [ABSTRACT FROM AUTHOR]