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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]

The isotopic composition of mafic small-volume intra-plate magmatism constrains the compositions of the sub-continental mantle sources. The Nd, Pb, and Sr isotope signatures of widespread late Mesozoic to Quaternary intra-plate magmatism in NE Africa (Sudan, South Egypt) are surprisingly uniform and indicate the presence of a high-μ (μ = 238U/204Pb) source in the mantle. The rocks are characterized by small ranges in the initial isotopic composition of Nd, Pb, and Sr and most samples fall within ε Nd ca. 3–6, 206Pb/204Pb ca. 19.5–20.5, 207Pb/204Pb ca. 15.63–15.73, 208Pb/204Pb ca. 39–40 and 87Sr/86Sr ca. 0.7028–0.7034. We interpret this reservoir as lithospheric mantle that formed beneath the Pan-African orogens and magmatic arcs from asthenospheric mantle, which was enriched in trace elements (U, Th, and light REE). Combining our new data set with published data of intra-plate magmatic rocks from the Arabian plate indicates two compositionally different domains of lithospheric mantle in NE-Africa–Arabia. The two domains are spatially related to the subdivision of the Pan-African orogen into a western section dominated by reworked cratonic basement (NE-Africa; high-μ lithospheric mantle) and an eastern section dominated by juvenile Pan-African basement (easternmost NE-Africa and Arabia; moderate μ lithospheric mantle). The compositions of the Pan-African lithospheric mantle and the MORB-type mantle of the Red Sea and Gulf of Aden spreading centers could explain the Nd–Pb-Sr isotopic compositions of the most pristine Afar flood basalts in Yemen and Ethiopia by mixtures of the isotopic composition of regional lithospheric and asthenospheric sources. [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]