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Antonio Simonetti – One of the best experts on this subject based on the ideXlab platform.

  • combined u pb geochronology and sr nd isotope analysis of the ice river perovskite standard with implications for kimberlite and Alkaline Rock petrogenesis
    Chemical Geology, 2012
    Co-Authors: Sebastian Tappe, Antonio Simonetti

    Abstract:

    Abstract Growing interest in the mineral perovskite (CaTiO 3 ) as a U–Pb chronometer and archive of near-primary Sr and Nd isotope compositions in magmatic systems highlights the need for well-characterized mineral standards. Based on a conventionally determined, high-precision Sr and Nd isotope data set, we propose perovskite from the Alkaline Ice River intrusion as a natural reference material for Sr and Nd isotope ratio determinations. Ice River perovskite has mean present-day 87 Sr/ 86 Sr (TIMS) and 143 Nd/ 144 Nd (ID-MC-ICP-MS) ratios of 0.702838 ± 51 and 0.512581 ± 32, respectively (2-sigma uncertainties). The TIMS 206 Pb/ 238 U age of 361.7 ± 1.0 Ma (2-sigma), determined on the same crystal fragments as the Sr and Nd isotope compositions, falls within the accepted range of 355–372 Ma for the Ice River intrusion. Although there is a ~ 2.2% difference between the ages of Ice River perovskite (~ 355–363 Ma) reported in various U–Pb studies (determined by TIMS, SIMS, and LA-MC-ICP-MS methods), the material investigated here has the currently best-calibrated U–Pb systematics for perovskite. We therefore recommend the 206 Pb/ 238 U age of 361.7 ± 1.0 Ma as a reference value for use in geochronology studies. Perovskite is a prime target for petrogenetic studies of kimberlites and related Alkaline Rocks, because it preserves primary magmatic signatures. The ability to combine important petrogenetic information with high-resolution U–Pb emplacement ages at mineral scale will be instrumental for an improved understanding of deep magmatism beneath cratons and areas of rifted continental lithosphere.

  • Combined U–Pb geochronology and Sr–Nd isotope analysis of the Ice River perovskite standard, with implications for kimberlite and Alkaline Rock petrogenesis
    Chemical Geology, 2012
    Co-Authors: Sebastian Tappe, Antonio Simonetti

    Abstract:

    Abstract Growing interest in the mineral perovskite (CaTiO 3 ) as a U–Pb chronometer and archive of near-primary Sr and Nd isotope compositions in magmatic systems highlights the need for well-characterized mineral standards. Based on a conventionally determined, high-precision Sr and Nd isotope data set, we propose perovskite from the Alkaline Ice River intrusion as a natural reference material for Sr and Nd isotope ratio determinations. Ice River perovskite has mean present-day 87 Sr/ 86 Sr (TIMS) and 143 Nd/ 144 Nd (ID-MC-ICP-MS) ratios of 0.702838 ± 51 and 0.512581 ± 32, respectively (2-sigma uncertainties). The TIMS 206 Pb/ 238 U age of 361.7 ± 1.0 Ma (2-sigma), determined on the same crystal fragments as the Sr and Nd isotope compositions, falls within the accepted range of 355–372 Ma for the Ice River intrusion. Although there is a ~ 2.2% difference between the ages of Ice River perovskite (~ 355–363 Ma) reported in various U–Pb studies (determined by TIMS, SIMS, and LA-MC-ICP-MS methods), the material investigated here has the currently best-calibrated U–Pb systematics for perovskite. We therefore recommend the 206 Pb/ 238 U age of 361.7 ± 1.0 Ma as a reference value for use in geochronology studies. Perovskite is a prime target for petrogenetic studies of kimberlites and related Alkaline Rocks, because it preserves primary magmatic signatures. The ability to combine important petrogenetic information with high-resolution U–Pb emplacement ages at mineral scale will be instrumental for an improved understanding of deep magmatism beneath cratons and areas of rifted continental lithosphere.

Andreas K. Kronenberg – One of the best experts on this subject based on the ideXlab platform.

  • Fluid‐Rock reaction weakening of fault zones
    Journal of Geophysical Research, 1995
    Co-Authors: Robert P. Wintsch, R. Christoffersen, Andreas K. Kronenberg

    Abstract:

    The presence of weak phyllosilicates may explain the low shear strengths of fault zones if they define well-developed fabrics. The growth of phyllosilicates is favored in meteoric water-dominated granitic fault systems, where mineral-aqueous fluid equilibria predict that modal phyllosilicate will increase via feldspar replacement reactions. In deeper, more Alkaline, Rock-dominated regimes, the reactions reverse, and feldspars tend to replace phyllosilicates. In Mg-rich mafic Rocks, however, phyllosilicates (chlorite, biotite) can replace stronger framework and chain silicates in both shallower (

  • fluid Rock reaction weakening of fault zones
    Journal of Geophysical Research, 1995
    Co-Authors: Robert P. Wintsch, R. Christoffersen, Andreas K. Kronenberg

    Abstract:

    The presence of weak phyllosilicates may explain the low shear strengths of fault zones if they define well-developed fabrics. The growth of phyllosilicates is favored in meteoric water-dominated granitic fault systems, where mineral-aqueous fluid equilibria predict that modal phyllosilicate will increase via feldspar replacement reactions. In deeper, more Alkaline, Rock-dominated regimes, the reactions reverse, and feldspars tend to replace phyllosilicates. In Mg-rich mafic Rocks, however, phyllosilicates (chlorite, biotite) can replace stronger framework and chain silicates in both shallower (<{approximately}10 km) meteoric H{sub 2}O-dominated and in deeper, Alkaline, Rock-dominated regimes. Where these phyllosilicates precipitate in active fault zones, they contribute directly to reaction softening. Because low-temperature deformation of phyllosilicates is not governed by frictional processes alone but can occur by pressure-independent dislocation glide, the strength of phyllosilicate-rich fault Rocks can be low at all depths. Low strain rate creep during interseismic periods can align phyllosilicate grains in foliated gouge and phyllonites. Where preferred orientations are strong and contiguity of phyllosilicates is large, strengths of Rocks within fault zones may approach minimum strengths defined by single phyllosilicate crystals. Fault zones containing localized high concentrations of phyllosilicates with strong preferred orientations in well-defined folia can exhibit aseismic slip, especially where mafic Mg-rich Rocksmore » occur along the fault (like parts of the San Andreas Fault). 104 refs., 6 figs., 1 tab.« less

Peter Dulski – One of the best experts on this subject based on the ideXlab platform.

  • Carbonatite diversity in the Central Andes: the Ayopaya Alkaline province, Bolivia
    Contributions to Mineralogy and Petrology, 2004
    Co-Authors: Frank Schultz, Bernd Lehmann, Sohrab Tawackoli, Reinhard Rössling, Boris Belyatsky, Peter Dulski

    Abstract:

    The Ayopaya province in the eastern Andes of Bolivia, 100 km NW of Cochabamba, hosts a Cretaceous Alkaline Rock series within a Palaeozoic sedimentary sequence. The Alkaline Rock association comprises nepheline-syenitic/foyaitic to ijolitic intrusions, carbonatite, kimberlite, melilititic, nephelinitic to basanitic dykes and diatremes, and a variety of Alkaline dykes. The carbonatites display a wide petrographic and geochemical spectrum. The Cerro Sapo area hosts a small calciocarbonatite intrusion and a multitude of ferrocarbonatitic dykes and lenses in association with a nepheline-syenitic stock. The stock is crosscut by a spectacular REE-Sr-Th-rich sodalite-ankerite-baryte dyke system. The nearby Chiaracke complex represents a magnesiocarbonatite intrusion with no evidence for a relationship to igneous silicate Rocks. The magnesiocarbonatite (Σ REE up to 1.3 wt%) shows strong HREE depletion, i.e. unusually high La/Yb ratios (520–1,500). Calciocarbonatites (Σ REE up to 0.5 wt%) have a flatter REE distribution pattern (La/Yb 95–160) and higher Nb and Zr contents. The sodalite-ankerite-baryte dyke system shows geochemical enrichment features, particularly in Na, Ba, Cl, Sr, REE, which are similar to the unusual natrocarbonatitic lavas of the recent volcano of Oldoinyo Lengai, Tanzania. The Cerro Sapo complex may be regarded as an intrusive equivalent of natrocarbonatitic volcanism, and provides an example for carbonatite genesis by late-stage crystal fractionation and liquid immiscibility. The magnesiocarbonatite intrusion of Chiaracke, on the other hand, appears to result from a primary carbonatitic mantle melt. Deep seated mantle magmatism/metasomatism is also expressed by the occurrence of a kimberlite dyke. Neodymium and strontium isotope data (ɛ_Nd 1.4–5.4, ^87Sr/^86 Sr