Werner E. Halter

Quantitative multi-element analysis of minerals, fluid and melt inclusions by laser-ablation inductively-coupled-plasma mass-spectrometry

Laser-ablation ICPMS has become widely accessible as a powerful and efficient multi-element microanalytical technique. One of its key strengths is the ability to analyse a wide concentration range from major (tens of wt.%) to trace (ng/g) levels in minerals and their microscopic inclusions. An ArF excimer laser system (λ = 193 nm) with imaging optics for controlled UV ablation and simultaneous petrographic viewing was designed specifically for representative sampling and quantitative multi-element analysis of microscopic fluid, melt and mineral inclusions beneath the sample surface. After a review of the requirements and recent technical developments, results are presented which together document the reliability and reproducibility of quantitative microanalysis of complex samples such as zoned crystals or fluid and melt inclusions in various host minerals. Analytical errors due to elemental fractionation are reduced to the typical precision achieved by quadrupole LA-ICPMS in multi-element mode (2–5% RSD). This progress is largely due to the small size of aerosol particles generated by the optimized UV optical system. Depth profiling yields representative and accurate concentration results at a resolution of ∼0.1 μm perpendicular to the ablation surface. Ablation is largely matrix-insensitive for different elements, such that silicate and borate glasses, silicates and oxide minerals, or direct liquid ablation can be used interchangeably for external standardization of any homogeneous or heterogeneous material. The absolute ablation rate is material dependent, however, so that quantitative LA-ICPMS analysis requires an internal standard (i.e., an independent constraint such as the absolute concentration of one element). Our approach to quantifying fluid and melt inclusion compositions is described in detail. Experiments with synthetic fluid inclusions show that accurate results are obtained by combining the LA-ICPMS analysis of element concentration ratios with a microthermometric measurement of the NaCl equivalent concentration and an empirical description of the effect of major cations on the final melting temperatures of ice, hydrohalite or halite. Expected calibration errors for NaCl-H2O-dominated fluids are smaller than the typical analytical scatter within an assemblage of simultaneously trapped fluid inclusions. Analytical precision is limited by representative ablation of all phases in heterogeneous inclusions and the integration of transient ICPMS signals, to typically ±10 to 20% RSD. Element concentrations in devitrified and even coarsely crystallized silicate melt inclusions can be reconstituted from LA-ICPMS signals. Deconvolution of inclusion and host signals with internal standardization automatically corrects for sidewall crystallization after melt entrapment at high temperature. A test using melt inclusions in a midocean ridge basalt, a summary of published geochemical studies and a new application to REE analysis of coexisting fluids and mineral phases in carbonatite-related veins illustrate the versatility and some of the strengths and limitations of LA-ICPMS, in comparison with other microanalytical techniques.

Major to trace element analysis of melt inclusions by laser-ablation ICP-MS : Methods of quantification

Current techniques for the quantification of melt inclusion chemistry require that inclusions are compositionally homogeneous and that post-entrapment devitrification or crystallization onto the inclusion walls could be reversed by appropriate re-melting. Laser-ablation ICP-MS provides a technique by which single heterogeneous inclusions can be analysed, thus avoiding the above prerequisites. Because host mineral is ablated with the inclusion, quantification of the melt composition necessitates deconvolution of the mixed signal by an internal standard. This can be obtained in various ways, including: (1) a fixed, pre-determined, concentration of a given element in the melt; (2) whole rock differentiation trends in a given igneous suite; (3) a constant, measured, distribution coefficient between the host and the inclusion melt and (4) determination of the volume ratios between the inclusion and total ablated volume. These four approaches were tested on a large set of cogenetic inclusions from a single plagioclase crystal in a rhyodacitic intrusion. Results suggest that quantification through whole rock differentiation trends is the most widely applicable, the most accurate and the least time-consuming technique, provided that the resulting data are critically interpreted with regard to the underlying assumptions. Uncertainties on the calculated element concentrations in the inclusions depend on the mass ratio between the melt inclusion and the host for a given ablation. They are of the order of 10% if the melt inclusion contributes more than 20% to the bulk analytical signal of a particular element. Calculated limits of detection for spherical 10 μm melt inclusions are of the order of a few ppm for elements strongly enriched in the melt relative to the host crystal. Concentrations in the melt inclusions can be determined even for elements enriched in the host mineral, but in this case uncertainties and calculated limits of detection increase with the concentration in the host. The uncertainty on the melt composition from a set of cogenetic inclusions can be commonly decreased by calculating of an uncertainty-weighted average of the concentration and their uncertainty.

Amorphous materials: Properties, structure, and durability: Quantitative Raman spectroscopy: Speciation of Na-silicate glasses and melts

Abstract In situ, high-temperature Raman spectroscopy was used to study the Qn speciation in binary Nasilicate glasses and melts. Over 300 Raman spectra in the compositional range from 25 to 40 mol% Na2O were collected at room and high temperatures between 800 and 1200 K. Quantitative information on the relative abundances of species in melts was obtained from the Raman spectra through a quantification procedure that does not require any a priori assumptions about the line shapes or external calibration of the Raman scattering efficiencies for the various Qn species. The ΔH° associated with the speciation reaction 2Q3 = Q4 + Q2 was found to be 20.3 ± 7.9 kJ/mol. For a given temperature, the speciation is more disordered in sodium than in potassium silicate melts. Because of the smaller temperature dependence of the speciation in the sodium silicate system, the difference in the speciation for the sodium and potassium silicate system decreases with increasing temperature. In addition to the speciation data, the partial Raman spectra for the different species were obtained. The experimentally observed variation of the partial Raman spectra with temperature, and, to a minor extent, with composition, should stimulate future theoretical studies on the vibrational properties of silicate glasses and melts.

Bilevel Optimization of Multi-Component Chemical Systems Using Particle Swarm Optimization

In this paper we study a real-world optimization problem in mineralogy which contains a large number of parameters and several objective functions. The problem has been described through a thermodynamic model for which the parameters have to be quantified. Due to non-linear chemical reactions and the characteristics of the problem, we developed a bilevel optimization approach in which the parameters that need to be determined can be split in two levels. The lower level contains three non linear equations and two linear charge and mass balance constraints. We use linear multi-objective particle swarm optimization (LMO PSO) to solve this problem for a general case. We then use the results in the upper level. This bilevel optimization has been used to find the thermodynamic parameters of the Na2O-SiO2 system. The results are analytically analyzed and compared with the reference data. This shows that the thermodynamic model is accurate and that the termination of thermodynamic properties using a bilevel optimization method based on PSO algorithms is reliable and efficient.

Auf der Spur riesiger Erzlagerstätten

Die weltweit reichsten Quellen von Kupfer, Molybdan, Wolfram, Zinn, und ein wesentlicher Anteil an Gold stammen aus porphyrischen Erzlagerstatten wie die riesige Kupfer-Gold-Mine von Bajo de la Alumbrera in Argentinien. Hier werden Erzreserven von 800 Millionen Tonnen mit Gehalten von rund 0, 6 Gewichtsprozent Kupfer und 0, 6 g/t Gold im Tagebau gewonnen. Vorstellungen zur Bildung dieser Metallvorkommen gehen davon aus, dass in 5—10 Kilometern Tiefe in vulkanisch aktiven Gebieten Magmakammern existieren.