Titanite Crystals Research Articles

Revisiting the “C-type adakites” of the Lower Yangtze River Belt, central eastern China: In-situ zircon Hf–O isotope and geochemical constraints

Adakites, or adakitic rocks, in a broad sense, have been used to describe a large range of igneous rocks with the common feature of high Sr/Y and La/Yb ratios that can be achieved though different mechanisms. Among them, the continental, or C-type, adakitic rocks are particularly controversial in terms of their sources and genesis. In this study we revisit both Cu–Au ore-bearing and barren “C-type adakitic rocks” in the Lower Yangtze River Belt (LYRB) of central eastern China, including comprehensive analyses of their in-situ zircon Hf–O isotopes, whole-rock geochemistry and Sr–Nd isotopes. These “C-type adakitic rocks” consist of monzodiorite, granodiorite and quartz monzonite that are classified as shoshonitic to high-K calc-alkaline series according to their chemical compositions. They are characteristically high in potassium (K2O=2.4–4.5%, K2O/Na2O=0.6–1.3), with continental crust-like isotopic compositions, i.e., whole-rock εNd(T)=−3.9 to −7.7, initial 87Sr/86Sr=0.7054–0.7085, zircon εHf(T)=0 to −11, and δ18O=6‰ to 9‰. The ore-bearing and barren rocks are cogenetic. Fractional crystallization of hornblende, titanite, magnetite and apatite played a major role in their chemical variations, with the ore-bearing rocks being more felsic (SiO2=63.3–69.6%) and higher in Sr/Y (41.2–75.6) than the barren rocks (SiO2=57.3–65.0%, Sr/Y=30.4–51.8). All these geochemical and isotopic features, in combination with regional geological data, suggest that the LYRB “C-type adakitic rocks” were unlikely to have been formed by melting of either a thickened and/or delaminated lower continental crust, or an altered oceanic crust as previously thought. These rocks are in general akin in geochemistry and isotopes to the Archean sanukitoids and the Setouchi high-Mg andesites in Japan, and are thus interpreted as being formed by melting of an enriched mantle source metasomatized by dewatering from a delaminated flat-slab. The flat subduction of an oceanic plateau and its subsequent delamination and foundering since the early Mesozoic beneath southeastern China (Li and Li, 2007) thus not only explain the temporal and spatial propagation of widespread Yanshanian igneous rocks regionally since ca. 195Ma, but also the formation of a series of enigmatic “adakitic” rocks in the region, including the LYRB potassium-rich rocks that were inappropriately called the “C-type adakitic rock” by previous workers.

The Evolution of the Peach Spring Giant Magma Body: Evidence from Accessory Mineral Textures and Compositions, Bulk Pumice and Glass Geochemistry, and Rhyolite-MELTS Modeling

The Miocene Peach Spring Tuff is a giant (� 640 km 3 dense rock equivalent) pyroclastic deposit that is extensively exposed in the southwestern USA. Evidence from geochemical and textural analyses of bulk-rocks, glasses, and accessory minerals (zircon, titanite, allanite, chevkinite, magnetite) from outflow and intra-caldera pumice clasts and fiamme, in combination with field observations and rhyolite-MELTS modeling, suggests that the Peach Spring magma body was compositionally and thermally zoned with a basal cumulate, and that it crystallized over millennial timescales before being remobilized by mafic input prior to erupting. Crystal contents, bulk compositions, spatial distributions, and temperatures (recorded by Ti in zircon and Zr in titanite) of pumice clasts and fiamme vary systematically: distal outflow high-silica rhyolites are crystal-poor and document lower temperatures; intra-caldera trachytes are crystal-rich and record higher temperatures. These variations indicate that the Peach Spring magma body was zoned. We interpret the outflow high-silica rhyolites to represent the first portion of the magma body to erupt. Zircon and titanite display core-to-edge reductions in rare earth element (REE) concentration and temperature, suggestive of relatively uninterrupted crystallization as the magma body cooled; crystallization temperature intervals from rhyoliteMELTS are consistent with those recorded by zircon and titanite. Exponential size distributions for accessory minerals and phenocryst textures are consistent with geochemical evidence for a simple cooling and crystallization history. Intra-caldera trachytes and outflow low-silica rhyolites represent the later portion of the magma body to erupt.This magma experienced a late-stage heating event potentially associated with the onset of the eruption.The edges of titanite crystals are enriched in REE and Zr, and zircon edges are enriched in Ti, suggesting higher temperatures during edge crystallization (at least 9008C). Concave-down crystal size distributions and resorption features on phenocrysts are additional signs of heating. Rare trachyandesite enclaves and the presence of mafic to intermediate lavas immediately underlying the Peach Spring Tuff suggest that a mafic magma input may have been the cause of the heating. Evidence further suggests that the intra-caldera trachytes may represent a remobilized cumulate at the base of the magma body, which retained some melt prior to rejuvenation. Bulk pumice and fiamme compositions are very rich in feldspar and accessory mineral phenocrysts, indicative of accumulation of these minerals; high crystal contents (� 35%) and evidence of heating and resorption imply that this magma was even more crystal-rich prior to the heating event. Rhyolite-MELTS simulations suggest that the trachyte magma had roughly 1wt % water, which cannot be totally accounted for by hydrous phases, thus requiring some amount of melt within the cumulate. Kinked magnetite size distributions are interpreted to represent a change from growth-dominated crystallization (larger crystals, shallow slopes) to nucleation-dominated (small crystals, steep slopes) owing to the onset of eruptive decompression. Timescales of magnetite crystallization calculated from these slopes indicate that the Peach Spring magma body crystallized over millennial timescales, and that eruptive decompression began 10 � 1 ^10 2 years prior to eruption.

The morphology of the reaction front of the dissolution-precipitation reaction rutile + wollastonite = titanite in time series experiments at 600 C/400 MPa

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 TiO 2 -CaO-SiO 2 -Na 2 O-HCl-H 2 O. 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 CaSiO 3 + TiO 2 = CaTiSiO 5 . 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.

Silicate–carbonate liquid immiscibility and crystallization of carbonate and K-rich basaltic magma: insights from melt and fluid inclusions

AbstractThis paper reports an investigation of the crystallization products of K-rich silicate and carbonate melts trapped as melt inclusions in clinopyroxene phenocrysts from the Dunkeldyk alkaline igneous complex in the Tajik Republic. Heating experiments on the melt inclusions suggest that the carbonate melt was formed by liquid immiscibility at 1180°C and ∼0.5 GPa. The carbonate-rich inclusions are dominated by Sr-bearing calcite, and rich in incompatible elements. Most of the silicate minerals are SiO2-poor and rich in K, Ba and Ti. Leucite, kalsilite and aegirine are the earliest magmatic minerals. High Ba and Ti contents in the melt resulted in the crystallization of Ba-rich K-feldspar, titanite, perovskite and Ti-bearing garnet, and the rare Ba-Ti silicates fresnoite and delindeite. The last minerals to crystallize from volatile-rich melts and fluids were aegirine, götzenite, K-Ba- and Ca-Sr-bearing zeolites, fluorite and strontium-rich baryte. Interaction of the early minerals with residual melts and fluids produced Ba-rich phlogopite and Sr-rich apatite.

High-resolution P-T-t paths from δ18O zoning in titanite: A snapshot of late-orogenic collapse in the Grenville of New York

Research Article| October 01, 2011 High-resolution P-T-t paths from δ18O zoning in titanite: A snapshot of late-orogenic collapse in the Grenville of New York Chloë E. Bonamici; Chloë E. Bonamici * WiscSIMS, Department of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA *E-mail: bonamici@geology.wisc.edu. Search for other works by this author on: GSW Google Scholar Reinhard Kozdon; Reinhard Kozdon WiscSIMS, Department of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar Takayuki Ushikubo; Takayuki Ushikubo WiscSIMS, Department of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar John W. Valley John W. Valley WiscSIMS, Department of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar Geology (2011) 39 (10): 959–962. https://doi.org/10.1130/G32130.1 Article history received: 01 Feb 2011 rev-recd: 12 May 2011 accepted: 15 May 2011 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Chloë E. Bonamici, Reinhard Kozdon, Takayuki Ushikubo, John W. Valley; High-resolution P-T-t paths from δ18O zoning in titanite: A snapshot of late-orogenic collapse in the Grenville of New York. Geology 2011;; 39 (10): 959–962. doi: https://doi.org/10.1130/G32130.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We characterize oxygen isotope zoning within single titanite crystals from the Carthage-Colton mylonite zone (CCMZ), Adirondack Mountains (New York State, United States), by ion microprobe. Smooth gradients of δ18O, up to 0.6‰ over 90 μm, resulted from diffusive exchange of oxygen during cooling from peak metamorphic temperatures of 650–700 °C. Modeling of the observed profile indicates punctuated cooling rates of 30–60 °C/m.y. along the CCMZ, set within long periods of much slower cooling. These results indicate a previously unrecognized period of rapid cooling along the CCMZ that is interpreted to result from the post-Ottawan collapse of the Grenville mountain belt and exhumation of the central Adirondack Highlands at ca. 1050 Ma. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Cr-doped perovskite and rutile pigments derived from industrial by-products

In order to enhance the use of environment-friendly materials and to stimulate waste recycling, this work has developed brown-maroon inorganic pigments belonging to rutile (TiO 2) and perovskite (CaTiO 3) structures. They were obtained by solid state reaction of industrial residues mixtures: tionite (i.e. insoluble residue of titania slag processing), Cr/Ni plating sludge and marble cutting/sawing sludge, for sake of comparison, analogous formulations were prepared by using reagent-grade precursors. Chromium-doped (0.04 mole of Cr 2O 3) perovskite and rutile pigments were obtained at 1200 °C and their phase composition, microstructure, optical properties and technological behaviour were characterized by XRD, SEM, DRS and application in glazes, respectively. The presence of impurities (e.g. Fe, Ni, Al, Si) in the wastes induced the crystallization of secondary phases, like wollastonite (CaSiO 3) and titanite (CaTiSiO 5), whose occurrence slightly shaded off the final colouration of the waste-based pigments; a further addition of chromium is required to improve their colouring power in transparent glazes. The diffuse reflectance spectroscopy shows that the colour development, in both structures, is due to the partial substitution of titanium by chromium in octahedral coordination.

(U‐Th)/He dating of terrestrial impact structures: The Manicouagan example

The accurate dating of meteorite impact structures on Earth has proven to be challenging. Melt sheets are amenable to high‐precision dating by the U‐Pb and 40Ar/39Ar methods, but many impact events do not produce them, or they are not preserved. In cases where high‐temperature shock metamorphism of the target materials has occurred without widespread melting, these isotopic chronometers may be partially reset and yield dates that are difficult to interpret unambiguously as the age of impact. However, the (U‐Th)/He chronometer is sensitive to thermal resetting and can provide a powerful new tool for dating impactites. We report (U‐Th)/He dates for accessory minerals from the Manicouagan impact structure in Quebec, Canada. Nine zircons from a melt sheet sample yield a weighted mean age of 213.2 ± 5.4 Ma (2SE), indistinguishable from the published 214 ± 1 Ma (2σ) U‐Pb zircon age for the impact. In contrast, five apatites from this sample yield dates between 205.9 ± 6.5 and 162.0 ± 5.3 Ma (2σ), indicating variable postimpact helium loss due to low‐temperature thermal disturbance. Preimpact titanite crystals from a shocked meta‐anorthosite sample yield two dates consistent with the impact age, at 212 ± 27 and 214 ± 13 Ma (2σ), and two younger dates of 189.6 ± 6.9 and 192.2 ± 9.8 Ma (2σ), suggestive of postimpact helium loss. These results indicate that (U‐Th)/He chronometry is a suitable method for dating impact events, although interpretation of the results requires recognition of possible 4He loss related to reheating subsequent to impact.

Deciphering Magma-Mixing Processes Using Zoned Titanite from the Ross of Mull Granite, Scotland

A textural and geochemical investigation of titanite from the Ross of Mull Granite, Scotland, reveals that zoning in trace elements strongly correlates with field evidence of magma-mixing processes within the pluton. Diorite enclaves contain a range of textures indicative of variable states of interaction with the host granite and suggest that a number of discrete mixing events have occurred in the pluton, both in situ and prior to emplacement at the present level. Single titanites within both the enclaves and the host are compositionally zoned and provide a detailed record of magmatic processes. Zoning includes concentric fine-scale oscillatory types and major discontinuities linked to dissolution events associated with changes in titanite stability. The zones reflect growth during changing melt chemistry and correlate with variations in rare earth element and high field strength element contents. Geothermometry using the Zr content of titanite indicates that titanite crystallization occurs at temperatures of c. 750–700°C. Titanites respond to changes in melt composition, oxygen fugacity and temperature caused by mixing and the introduction of fresh diorite magma into the granite host. Mixing processes recorded by the titanites include complete mixing of crystal-free melts, crystal transfer between partially crystallized magmas, melt segregation and transfer from crystalline enclaves and late-stage diffusive exchange. Titanite chemistry responds to local activity adjacent to sites of mixing but is also influenced by more distal magma-mixing events. In this respect titanite is capable of revealing many otherwise hidden events within the magma chamber.

GRENVILLE SKARN TITANITE: POTENTIAL REFERENCE MATERIAL FOR SIMS U-Th-Pb ANALYSIS

We have investigated the homogeneity, chemical composition, structure, degree of radiation damage, and post-formation evolution of titanite crystals from skarns of the Grenville Province of the Canadian Shield using SHRIMP, TIMS, Raman and PL spectroscopy, EBSD, and EPMA–WDS. These results are used to assess the potential of the titanite as Reference Material (RM) for micro-analytical U–Th–Pb age dating. The SHRIMP data show that these megacrysts (5–31g) have concordant U–Pb isotope systematics, 60 to 500 ppm U, 120 to 1200 ppm Th, 206 Pb/ 204 Pb between 500 and 2500, ages of ~1 Ga, and excellent homogeneity at the scale of the analytical volume of the ion probe. The ID–TIMS titanite data for OLT1, OLT2 and TCB show that these crystals are essentially concordant. Data for OLT1 and OLT2 show slight scatter ( i.e. , in excess of that expected from the uncertainty in an individual analysis). For OLT1, one of seven analyses shows Pb loss or, possibly, a younger period of growth. Crystals OLT1 and OLT2 have respective TIMS concordia ages of 1014.8 ± 2.0 Ma (2σ, n = 6, MSWD = 1.8) and 998.0 ± 4.5 Ma (2σ, n = 3, MSWD = 3.3) for domains that have not lost Pb. The TIMS analyses of TCB are tightly clustered and give a concordia age of 1018.1 ± 1.7 Ma (2σ, n = 4, MSWD = 0.92). Raman and PL spectra show a low to moderate degree of accumulated radiation-induced damage in the Grenville Skarn Titanite crystals and uniform internal distributions of this damage. The EDSB contrast images indicate little or no crystallographic misorientation. The EMPA–WDS data show that the outer 50–100 μm of the OLT1 and TCB crystals are enriched in Al and F, and depleted in Fe and Nb, when compared with the interior. In spite of the variation in composition and degree of radiation damage amongst samples, there are no identifiable matrix effects in our SHRIMP data. Some Grenville skarn titanite (GST) crystals have potential as RM for micro-analytical U–Th–Pb age dating. Crystal TCB has excellent homogeneity of U–Th–Pb isotopic composition. Crystals OLT1 and OLT2 have minor TIMS age heterogeneity. However, this heterogeneity is smaller than that of the Khan titanite, our current in-house titanite standard. Careful selection of analysis areas during SIMS, and of chips for TIMS analysis, allows high-quality isotopic data to be obtained from these large crystals of titanite.

Ti-Al zoning of experimentally grown titanite in the system CaO-Al2O3-TiO2-SiO2-NaCl-H2O-(F): Evidence for small-scale fluid heterogeneity

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-Al 2 O 3 -TiO 2 -SiO 2 -NaCl-H 2 O(-CaF 2 ) 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-Al 2 O 3 , optionally powdered CaF 2 , 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.

Petrologia e geocronologia (U/Pb-Sm/Nd) do Granito Passagem, Complexo Granitóide Pensamiento, SW do Cráton Amazônico (MT)

The Passagem granite includes stocks, plugs and dikes located in the Ricardo Franco hill - Vila Bela da Santissima Trindade region -state of Mato Grosso, central Brazil. The Passagem Granite is included in the Paraguá terrane - SW Amazonian Craton. It consists of isotropic monzogranite, sienogranite and more rarely granodiorites with leucocratic dark gray to white color. These rocks range from hypidomorphic inequigranular to xenomorphic texture, fine to medium grained. Biotite is the only primary mafic present as essential phase and characterize an expanded slightly acid sequence formed by a sub-alkaline magmatism of high-potassium calc-alkaline, slightly peraluminous composition from arc magmatic tectonic environment during a post-collisional period. Mechanism of fractional crystallization of plagioclase, biotite, titanite, apatite and zircon associated with simultaneous crustal assimilation are suggested for the evolution of these rocks. The results support the hypothesis of a post-collisional magmatism in the Paraguá terrane at 1284 ± 20 Ma corresponding to the crystallization age of the Passagem granite. This paper propose that Passagem Granite represents as an extension in Brazilian terrane of the Pensamiento Granitoid Complex

CHARACTERIZATION OF FLUX-GROWN TRACE-ELEMENT-DOPED TITANITE USING THE HIGH-MASS-RESOLUTION ION MICROPROBE (SHRIMP-RG)

Crystals of titanite can be readily grown under ambient pressure from a mixture of CaO, TiO 2 and SiO 2 in the presence of molten sodium tetraborate. The crystals produced are euhedral and prismatic, lustrous and transparent, and up to 5 mm in length. Titanite obtained by this method contains approximately 4300 ppm Na and 220 ppm B contributed from the flux. In addition to dopant-free material, titanite containing trace alkali and alkaline earth metals (K, Sr, Ba), transition metals (Sc, Cr, Ni, Y, Zr, Nb, Hf and Ta), rare-earth elements (REE), actinides (Th, U) and p-block elements (F, S, Cl, Ge, Sn and Pb) have been prepared using the same procedure. Back-scattered electron (BSE) imaging accompanied by ion-microprobe (SHRIMP–RG) analysis confirms significant incorporation of selected trace-elements at structural sites. Regardless of some zonation, the large size of the crystals and broad regions of chemical homogeneity make these crystals useful as experimental starting material, and as matrix-matched trace-element standards for a variety of microbeam analytical techniques where amorphous titanite glass, heterogeneous natural titanite or a non-titanite standard may be less than satisfactory. Trace-element-doped synthetic crystals can also provide a convenient proxy for a better understanding of trace-element incorporation in natural titanite. Comparisons with igneous, authigenic and high-temperature metasomatic titanite are examined. The use of high-mass-resolution SIMS also demonstrates the analytical challenges inherent to any in situ mass-spectrometry-based analysis of titanite, owing to the production of difficult-to-resolve molecular interferences. These interferences are dominated by Ca–Ca, Ca–Ti and Ti–Ti dimers that are significant in the mass range of 80–100, affecting all isotopes of Sr and Zr, as well as 89 Y and 93 Nb. Methods do exist for the evaluation of interferences by these dimers and of polyatomic interferences on the LREE.

Zircon and titanite recording 1.5 million years of magma accretion, crystallization and initial cooling in a composite pluton (southern Adamello batholith, northern Italy)

The southern part of the Adamello batholith (the so-called “Re di Castello unit”) is an example of a composite pluton, ranging from gabbro to granodiorite in composition. U–Pb dating of single-zircon crystals from four tonalitic to granodioritic lithologies reveals that zircon crystallization is protracted in all studied lithologies, showing apparent durations of growth between 90 and 700 ka. The youngest zircons crystallized near the solidus and yield identical or slightly older ages than titanite. The formation of these autocrystic zircons is considered to approximate the age of emplacement of the melt and its final crystallization, in contrast to antecrystic zircons present in the same sample, which had formed earlier in the magmatic column or were derived from re-mobilized earlier magma. The autocryst-derived “emplacement” ages range from 42.43 ± 0.09 Ma to 40.90 ± 0.05 Ma, recording 1.5 Ma of intrusion and crystallization history. We anticipate that extended periods of zircon crystallization may be common in silicic rocks, whereas the zircons from residual melts from initially undersaturated mafic liquids should yield far more precise emplacement ages within our present analytical uncertainties of 0.1–0.2% in 206Pb/ 238U age. Decreasing Th/U ratios of dated zircons within one melt batch document the depletion of the residual melt portion in Th due to the contemporaneous crystallization of titanite. Preliminary Hf isotopic compositions of the dated zircon grains suggest that the early stage melts of the southern Re di Castello unit represent hybrid melts with an important crustal component ( ε Hf between − 2.8 and +3.0). Subsequently emplaced melts are more juvenile at ε Hf values at +6.4 to +8.9 and may thus reflect the addition of large volumes of mafic melt to the magmatic system.

Composite wasteform based on SiO 2–PbO–CaO–ZrO 2–TiO 2–(B 2O 3–K 2O) parent glass with zircon as the second component

Many different types of glass and ceramic wasteforms have been investigated for nuclear waste immobilization. This study deals with synthesizing composite wasteforms based on a parent glass belonging to the SiO 2–PbO–CaO–ZrO 2–TiO 2–(B 2O 3–K 2O) system with the use of zircon as a second component. The fabrication involves powder mixing, pressing and pressureless sintering. The processing conditions were investigated so as to achieve the highest density and the best sintering temperature for different amounts of zircon, i.e. 5, 10 and 15 wt%. The sintered products were studied by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM); as well as ICP-MAS for leaching experiments. The most promising composite containing zirkelite and titanite crystals in a lead-rich glassy matrix was obtained at 700 °C for 10 wt% zircon.

Development of TiO 2 white glazes for ceramic tiles

The objective of this work was the use of titania polymorphs (rutile and anatase) in substitution of zirconia as the main component for producing white opacity in ceramic glazes for tile coatings. Initially, total zirconia (12% mass fraction) was replaced in the standard frit by rutile and by anatase in mass fractions of 5%, 10% and 15%, forming two frit groups processed as ceramic glazes. The glaze color was determined by spectrophotometry and the glaze microstructure by XRD and SEM. The results showed the frit containing 10% anatase formed a totally white glaze presenting excellent coating capacity. The frits containing rutile produced yellow opacity. The results of microscopy and X-rays diffraction revealed that opacity was caused by tiny rutile crystals in the anatase frits and by titanite crystals in the rutile frits.

Igneous Layering, Fractional Crystallization and Growth of Granitic Plutons: the Dolbel Batholith in SW Niger

This study reassesses the development of compositional layering during the growth of granitic plutons, with emphasis on fractional crystallization and its interaction with both injection and inflationrelated deformation. The Dolbel batholith (SW Niger) consists of 14, kilometre-sized plutons emplaced by pulsed magma inputs. Each pluton has a coarse-grained core and a peripheral layered series. Rocks consist of albite (An 11), K-feldspar (Or96^99, Ab1^4), quartz, edenite (XMg1⁄4 0 37^0 55), augite (XMg1⁄4 0 65^0 72) and accessories (apatite, titanite and Fe^Ti-oxides). Whole-rock compositions are metaluminous, sodic (K2O/Na2O1⁄4 0 49^0 62) and iron-rich [FeOtot/(FeOtotþMgO)1⁄4 0 65^0 82]. The layering is present as size-graded and modally graded, sub-vertical, rhythmic units. Each unit is composed of three layers, which are, towards the interior: edenite plagioclase (Ca/p), edeniteþ plagioclaseþ augiteþ quartz (Cq), and edeniteþ plagioclaseþ augiteþ quartzþ K-feldspar (Ck). All phases except quartz show zoned microstructures consisting of external intercumulus overgrowths, a central section showing oscillatory zoning and, in the case of amphibole and titanite, complexly zoned cores. Ba and Sr contents of feldspars decrease towards the rims. Plagioclase crystal size distributions are similar in all units, suggesting that each unit experienced a similar thermal history. Edenite, characteristic of the basal Ca/p layer, is the earliest phase to crystallize. Microtextures and phase diagrams suggest that edenite cores may have been brought up with magma batches at the site of emplacement and mechanically segregated along the crystallized wall, whereas outer zones of the same crystals formed in situ.The subsequent Cq layers correspond to cotectic compositions in the Qz^Ab^Or phase diagram at PH2O1⁄4 5 kbar. Each rhythmic unit may therefore correspond to a magma batch and their repetition to crystallization of recurrent magma recharges. Microtextures and chemical variations in major phases allow four main crystallization stages to be distinguished: (1) open-system crystallization in a stirred magma during magma emplacement, involving dissolution and overgrowth (core of edenite and titanite crystals); (2) in situ fractional crystallization in boundary layers (Ca/p and Cq layers); (3) equilibrium ‘en masse’eutectic crystallization (Ck layers); (4) compaction and crystallization of the interstitial liquid in a highly crystallized mush (e.g. feldspar intercumulus overgrowths). It is concluded that the formation of the layered series in the Dolbel plutons corresponds principally to in situ differentiation of successive magma batches. The variable thickness of the Ck layers and the microtextures show that crystallization of a rhythmic unit stops and it is compacted when a new magma batch is injected into the chamber. Therefore, assembly of pulsed magma injections and fractional crystallization are independent, but complementary, processes during pluton construction.

MINERAL CHEMISTRY AND SHRIMP U-Pb GEOCHRONOLOGY OF MESOPROTEROZOIC POLYCRASE-TITANITE VEINS IN THE SULLIVAN Pb-Zn-Ag DEPOSIT, BRITISH COLUMBIA

Small polycrase–titanite veins 0.1–2 mm thick cut the tourmalinite feeder zone in the deep footwall of the Sullivan Pb–Zn– Ag deposit, southeastern British Columbia. Unaltered, euhedral crystals of polycrase and titanite 50–100 μm in diameter are variably replaced by a finer-grained alteration-induced assemblage composed of anhedral polycrase and titanite with local calcite, albite, epidote, allanite, and thorite or uranothorite (or both). Average compositions of the unaltered and altered polycrase, as determined by electron-microprobe analysis, are (Y0.38REE0.49Th0.10Ca0.04Pb0.03Fe0.01U0.01)(Ti1.48Nb0.54W0.04Ta0.02)O6 and (Y0.42REE0.32Th0.15U0.06Ca0.04Pb0.01Fe0.01)(Ti1.57Nb0.44W0.04Ta0.02)Ob, respectively. The unaltered titanite has, in some areas, appreciable F (to 0.15 apfu ), Y (to 0.40 apfu ), and Nb (to 0.13 apfu ). SHRIMP U–Pb geochronology of eight grains of unaltered polycrase yields a weighted 207Pb/206Pb age of 1413 ± 4 Ma (2σ) that is interpreted to be the age of vein formation. This age is 50–60 m.y. younger than the ca . 1470 Ma age of synsedimentary Pb–Zn–Ag mineralization in the Sullivan deposit, which is based on combined geological and geochronological data. SHRIMP ages for altered polycrase and titanite suggest later growth of minerals during the ~1370–1320 Ma East Kootenay and ~1150–1050 Ma Grenvillian orogenies. The 1413 ± 4 Ma age for the unaltered polycrase in the veins records a previously unrecognized post-ore ( 1370 Ma) mineralizing event in the Sullivan deposit and vicinity. The SHRIMP U–Pb age of the polycrase and high concentrations of REE, Y, Ti, Nb, and Th in the veins, together with elevated F in titanite and the absence of associated sulfides, suggest transport of these high-field-strength elements (HFSE) by F-rich and S-poor hydrothermal fluids unrelated to the fluids that formed the older Fe–Pb–Zn–Ag sulfide ores of the Sullivan deposit. Fluids containing abundant REE, HFSE, and F may have been derived from a geochemically specialized magma such as those that form alkaline granites, pegmatites, or carbonatites. In an alternative model, preferred here, these fluids were associated with a rift-related, crustal metasomatic event in the region. Determination of a Mesoproterozoic age for the polycrase–titanite veins establishes the first known occurrence of pre-Grenvillian REE-rich mineralization in the Belt–Purcell basin.

Oxygen self-diffusion “fast-paths” in titanite single crystals and a general method for deconvolving self-diffusion profiles with “tails”

Like most other minerals, titanite rarely if ever forms perfect crystals. In addition to the point defects that might affect lattice diffusion, there may be extended line- or planar defects along which fast diffusion could occur. During the course of an experimental study of oxygen lattice diffusion in titanite, we found that almost all of the 18O uptake profiles produced in natural titanite crystals departed from the complementary error function solution expected for simple lattice diffusion with a constant surface concentration. Instead, they exhibited “tails” extending deeper into the samples than expected for simple lattice diffusion. The purpose of this contribution is to report on these features—described as “fast-paths” for oxygen diffusion—and outline a method for coping with them in extracting information from diffusion profiles. For both dry and hydrothermal experiments in which the “fast paths” are observed, 18O was used as the diffusant. In dry experiments, the source material was 18O-enriched SiO 2 powder, while 18O-enriched water was used for the hydrothermal experiments. Diffusive uptake profiles of 18O were measured in all cases by nuclear reaction analysis (NRA) using the 18O (p,α) 15N reaction [see Zhang X. Y., Cherniak D. J., and Watson E. B. (2006) Oxygen diffusion in titanite: lattice and fast-path diffusion in single crystals. Chem. Geol. 235 105–123]. In our experiments, different sizes of “tails” (with varying 18O concentrations) were observed. Theoretically, under the same temperature and pressure conditions, the sizes of tails should be affected by two factors: the diffusion duration and the defect density. For the same experiment duration, the higher the defect density, the larger the “tail”; for the same defect densities, the longer the diffusion duration, the larger the “tail.” The diffusion “tails” could be a result of either planar defects or one-dimensional “pipe” diffusion. AFM imaging of HF etched titanite surfaces confirmed that the etched features might be caused by either parallel planar defects or parallel pipe defects, but could not differentiate between these possibilities. Through theoretical calculations simulating the tailed diffusion profiles using reasonable assumptions of lattice diffusivities and fast-path diffusivities, and comparing these with tail features measured in our samples, it can be concluded that the “tails” observed in our experiments are caused by planar defects rather than pipe defects. A new method was developed for separating the “fast-path” contribution from the overall composite diffusion profile consisting of both “fast-path” and lattice diffusion. Through this process, the lattice diffusion coefficient could be determined, which is required to analyze the tail. The oxygen diffusion rates in the fast-paths were obtained by traditional graphical analysis methods, using the Whipple–Le Claire equation (for 2-D defects) assuming that the width of the fast-path is 1 nm. Two Arrhenius relations were obtained for the fast-path diffusion phenomenon, one for experiments under dry conditions, and the other for hydrothermal conditions: D dry = 4.03 × 10 - 2 ( m 2 / s ) exp ( - 313 ± 22 ) ( kJ / mol ) / RT D wet = 3.48 × 10 - 7 ( m 2 / s ) exp ( - 219 ± 39 ) ( kJ / mol ) / RT Along with the lattice diffusivity, the presence and 3-D distribution of any fast-paths—and the diffusivities in these paths—are important to the bulk closure properties of single crystals. For titanites, AFM imaging showed that the fast-paths may not be interconnected at a length-scale comparable with the crystal dimension, so they may not have a significant effect on bulk closure properties.

Geochemistry of the Enjerd Skarn and Its Association with Copper Mineralization, Northwestern Iran

The Enjerd skarn is located 28 km northwest of Ahar in East Azarbaijan Province, northwestern Iran. Based on mineral composition, paragenesis, and mineral relationships, two stages of hydrothermal alteration and related mineralization have been identified. Stage I was produced by early pulses of Fe-rich, magmatically derived fluids coincident with potassic alteration of the main stock. This event caused the formation of oscillatory zoned garnets, followed by pyroxene (diopside-heden-bergite) + wollastonite + magnetite and rare MoS at high temperatures (∼445°C). Molybdenite was the only sulfide mineral deposited at this stage. Stage II occurred at a lower temperature (∼360°C), and is characterized by tremolite-actinolite, epidote, quartz, calcite, apatite, titanite, and hematite crystallization and by copper deposition. Copper mineralization was due to decreasing fO2 and increasing pH as temperatures fell below 400°C. Fluctuations in the Al+3/Fe+3 ratio of hydrothermal fluids at the Enjerd skarn resulted in complex oscillatory zoning in the garnets from andradite100 to andradite76-grossular24. Individual zones are typically composed either of near-end-member andradite or andradite containing approximately 20 mole % grossular. Introduction of fresh batches of Fe-rich fluid caused crystallization of near-end-member andradite garnet because the system was buffered by the fluid. The Al in the system was derived from Cretaceous marls and limestone sequences, because the Al available from granitoid magmatic solutions was very low and almost constant.

Oxygen diffusion in titanite: Lattice diffusion and fast-path diffusion in single crystals

Oxygen diffusion in natural and synthetic single-crystal titanite was characterized under both dry and water-present conditions. For the dry experiments, pre-polished titanite samples were packed in 18O-enriched quartz powder inside Ag–Pd capsules, along with a fayalite–magnetite–quartz (FMQ) buffer assemblage maintained physically separate by Ag–Pd strips. The sealed Ag–Pd capsules were themselves sealed inside evacuated silica glass tubes and run at 700–1050 °C and atmospheric pressure for durations ranging from 1 h to several weeks. The hydrothermal experiments were conducted by encapsulating polished titanite crystals with 18O enriched water and running at 700–900 °C and 10–160 MPa in standard cold-seal pressure vessels for durations of 1 day to several weeks. Diffusive uptake profiles of 18O were measured in all cases by nuclear reaction analysis (NRA) using the 18O (p,α) 15N reaction. For the experiments on natural crystals, under both dry and hydrothermal conditions, two mechanisms could be recognized as responsible for oxygen diffusion. The diffusion profiles showed two segments: a steep one close to the initial surface attributed to self diffusion in the titanite lattice; and a “tail” reaching deep into the sample attributable to diffusion in a “fast path” such as planar defects or pipes. For the experiments on synthetic crystals, lattice diffusion only is apparent in crystals with euhedral morphology, while both mechanisms operate in crystals lacking euhedral morphology. For the dry experiments, the following Arrhenius relation was obtained: D dry lattice = 3.03 × 10 − 8 exp (− 276 ± 16 kJmol − 1 /RT) m 2/s Under wet conditions at P H 2O = 100 MPa, Oxygen diffusion conforms to the following Arrehenius relation: D wet lattice = 2.05 × 10 − 12 exp (− 180 ± 39 kJmol − 1 /RT) m 2/s Diffusive anisotropy was explored only at hydrothermal conditions, with little evidence of diffusive anisotropy observed. Oxygen diffusivity shows no dependence on water pressure 800 °C ( P H 2O = 10–160 MPa) or 880 °C ( P H 2O = 10–100 MPa). However, like many other silicates, titanite shows a lower activation energy for oxygen diffusion in the presence of H 2O than under dry conditions; therefore the difference between the “dry” and “wet” diffusivities increases as temperature decreases below the range of this study. For example, at 500 °C, dry diffusion is almost 2.5 orders of magnitude slower than wet diffusion. Accordingly, the retentivity of oxygen isotope signatures will be quite different between dry and wet systems at geologically interesting conditions. For most cases, wet diffusion results may be the appropriate choice for modeling natural systems.