Homuth B.,Goethe University Frankfurt |
Lobl U.,Goethe University Frankfurt |
Batte A.G.,Makerere University |
Link K.,Johannes Gutenberg University Mainz |
And 3 more authors.
International Journal of Earth Sciences | Year: 2014
Shear-wave splitting measurements from local and teleseismic earthquakes are used to investigate the seismic anisotropy in the upper mantle beneath the Rwenzori region of the East African Rift system. At most stations, shear-wave splitting parameters obtained from individual earthquakes exhibit only minor variations with backazimuth. We therefore employ a joint inversion of SKS waveforms to derive hypothetical one-layer parameters. The corresponding fast polarizations are generally rift parallel and the average delay time is about 1 s. Shear phases from local events within the crust are characterized by an average delay time of 0.04 s. Delay times from local mantle earthquakes are in the range of 0.2 s. This observation suggests that the dominant source region for seismic anisotropy beneath the rift is located within the mantle. We use finite-frequency waveform modeling to test different models of anisotropy within the lithosphere/asthenosphere system of the rift. The results show that the rift-parallel fast polarizations are consistent with horizontal transverse isotropy (HTI anisotropy) caused by rift-parallel magmatic intrusions or lenses located within the lithospheric mantle-as it would be expected during the early stages of continental rifting. Furthermore, the short-scale spatial variations in the fast polarizations observed in the southern part of the study area can be explained by effects due to sedimentary basins of low isotropic velocity in combination with a shift in the orientation of anisotropic fabrics in the upper mantle. A uniform anisotropic layer in relation to large-scale asthenospheric mantle flow is less consistent with the observed splitting parameters. © 2014 Springer-Verlag Berlin Heidelberg.
Malsy A.-K.,Gubelin Gem Laboratory Ltd. |
Malsy A.-K.,University of Bern |
Armbruster T.,University of Bern
European Journal of Mineralogy | Year: 2012
Synthetic alexandrite crystals grown by various methods (flux, Czochralski, zone melting) and originating from different manufacturers were investigated by standard gemmological testing and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). All samples investigated have (Cr 3+ + V 3+), or (Cr 3+ + Fe 3+), or Cr 3+ only as colour dopant. The presence of trace contents of iridium ± platinum confirm Russian Czochralski and flux ''Cr+Fe-type'' samples to be synthesized from commonly used Ir crucibles. Other samples, i.e. Russian flux ''Cr+V-type'', Kyocera Czochralski and Seiko zone-melting synthetics, have no Ir, but stand out by consistently present Mo contents. Thus, it can be assumed that Mo containers instead of Pt or Ir crucibles were used. Traces of molybdenum have also been observed in Russian flux ''Cr+Fe-type'' samples. However, the strongly variable contents of Mo, Bi, andWimply the use of a Bi 2O 3-MoO 3-WO 3 flux. These synthetic crystals further show additives such as B, Ga, Ge, and Sn in considerable concentrations. Other randomly encountered ''impurities'' are Sc, Mn, Co and Zr. Observed inclusion features are mostly in accordance with the respective synthesis method. Flux-grown crystals show inclusions of flux residues of varying texture whereas Czochralski crystals characteristically display curved growth structures. Crystals produced by zone melting (Seiko) typically have swirled or undulating growth structures. Samples received as Tairus zone-melting alexandrites do strongly differ from the Seiko products by their Mo content and different inclusions features. The manufacturing method of these samples is therefore doubted. A proper distinction of natural alexandrite from melt-grown synthetic alexandrite can be done on the basis of infrared features in the range from 2500 to 4000 cm -1. © 2011 E. Schweizerbart'sche Verlagsbuchhandlung.
Widmer R.,University of Bern |
Malsy A.-K.,Gubelin Gem Laboratory Ltd. |
Armbruster T.,University of Bern
Physics and Chemistry of Minerals | Year: 2015
A red spinel, MgAl2O4, from Burma (Myanmar) containing as chromophores ca. 0.5 wt% of each Cr2O3 and V2O3, was sequentially heated for at least 72 h at temperatures ranging from 600 °C to 1,100 °C. The untreated and quenched samples were examined with single-crystal X-ray diffraction (XRD), Raman spectroscopy and photoluminescence spectroscopy. XRD results display a linear decrease of the cell parameter a and a continuous shift of the oxygen coordinate u, u, u at 3 m toward lower values with increasing temperature and associated Mg, Al disorder: T(Mg1-xAlx)M(Al2-xMgx)O4. The natural spinel has x = 0.157(2) and reaches x = 0.286(4) after quenching from 1,100 °C. In its natural state, M–O and T–O distances are 1.9226(2) and 1.9361(4) Å. With increasing inversion of Mg from the tetrahedrally coordinated T to the octahedrally coordinated M site, M–O distances increase at 1,100 °C to 1.9333(4) Å and T–O distances decrease to 1.9130(8) Å. The crossover temperature, at which T–O and M–O distances become equal (i.e., 1.927 Å), is found to be at 650 °C and corresponds to an inversion parameter x = 0.208(3). With increasing heat treatment, Raman spectra of quenched samples become significantly broadened and a peak characteristic for Mg, Al disorder at 721 cm−1 firstly appears for a crystal quenched from 800 °C with x = 0.248(4). At room temperature, photoluminescence spectra are dominated by a strong R line at 684.5 nm accompanied by poorly resolved N lines: N1 (687 nm), N2 (688 nm), and N3 (689 nm). N lines are caused by different Mg, Al environments of Cr3+. With increasing inversion parameter (x), the R line decreases in intensity and the N lines become prominent leading to strongly broadened peaks with a maximum shifted toward higher wave lengths (687.5 nm at 1,100 °C). Criteria for the detection of heat treatment on gemstone spinel applicable to gemological routine examination are provided. Extrapolation of u,a, and bond lengths from heat-treated Burma spinel toward the natural crystal suggests a retrograde “closing temperature” of ca. 400 ± 100 °C at which Mg, Al disorder was frozen. © 2014, Springer-Verlag Berlin Heidelberg.
Pettke T.,University of Bern |
Oberli F.,ETH Zurich |
Audetat A.,University of Bayreuth |
Simon A.C.,University of Nevada, Las Vegas |
And 2 more authors.
Ore Geology Reviews | Year: 2012
Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has become a most powerful technique for the elemental analysis of individual, polyphase inclusions completely enclosed in minerals, be they solid or a solid-liquid-gas mixture at the time of measurement. Simultaneous, accurate quantification of major to ultra-trace element concentrations from Li to U by well-controlled ablation of the entire fluid or melt inclusion content and successful use of largely matrix-independent external calibration protocols are unique features of this method.This contribution reviews fluid inclusion fundamentals relevant for their bulk analysis by LA-ICP-MS and discusses key aspects of the analytical protocol. Emphasis is on figures of merit (precision, accuracy) obtained from the analysis of individual inclusions and fluid inclusion assemblages, and procedures and technical developments to improving data quality are elaborated. A new equation for the calculation of detection limits for LA-ICP-MS analysis is presented, which closely follows IUPAC conventions.Applications are reviewed with emphasis on the use of synthetic fluid inclusions in constraining metal solubility and distribution between co-existing phases. New data for natural bismuth "fluid" inclusions document the seamless transition to melt inclusion analysis by LA-ICP-MS, thus highlighting the fact that the procedures presented here are generally applicable to the analysis of inclusions in complex host minerals.Isotope ratio analysis of individual fluid inclusions by multicollector ICP-MS (MC-ICP-MS) is a recent development that requires fast transient signals to be accurately recorded by instrumentation designed for high-precision static measurements of long-lasting stable ion beams. We address the general principles based on Pb isotopes and review a first application to the Bingham Canyon porphyry Cu-Au±Mo deposit. A pilot study using about 50 synthetic fluid inclusions containing SRM 987 Sr and variable NaCl, Ca, and Rb concentrations demonstrates that accurate 87Sr/ 86Sr isotope ratios can be obtained on an individual Rb-poor fluid inclusion, at absolute 2 σ precisions of 0.0003 to 0.002. A residual trend in 87Sr/ 86Sr as a function of the Rb/Sr abundance ratio in the fluid inclusions suggests that interference correction of 87Rb on mass 87 assuming identical mass bias coefficients for the two elements may be inaccurate; however, the offset can be accurately corrected for by regressing the data to zero 87Rb.The versatility and detection power of LA-ICP-MS makes this technique the method of choice for solute abundance and isotope ratio analysis of individual fluid inclusions. Significant future progress can be achieved by improvements in ion production, transmission and data recording efficiency and by further improving control on inclusion ablation by pulsed laser beams. Data quantification strategies may also have to be further refined to keep pace with instrumental progress and innovation. © 2011 Elsevier B.V.
Malsy A.,University of Bern |
Malsy A.,Gubelin Gem Laboratory Ltd. |
Klemm L.,Gubelin Gem Laboratory Ltd.
Chimia | Year: 2010
Gem spinel deposits in Myanmar, Vietnam and Tajikistan have their formation in association with Himalayan orogenesis. Gem-quality orange, pink, red and purple spinels from deposits at Mogok (Myanmar), Luc Yen (Vietnam), and Kuh-i-Lal (Tajikistan) have been investigated by 'standard' gemological testing and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Microscopic examination yielded apatite and calcite inclusions together with octahedral negative crystals to be most frequently present in Mogok spinels. The presence of dislocation systems and titanite inclusions are indicative inclusion features for spinels from Luc Yen. Trace elements such as Ti, Fe, Ni, Zn, Zr and Sn differ slightly in spinels from the sources investigated. A distinction of spinels from these deposits is therefore possible by trace element chemistry. This is especially helpful for gem spinels, which often show few inclusions or completely lack inclusion features. © Schweizerische Chemische Gesellschaft.