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3.4 e. Measurements of fluid/melt partition coefficients by Laser-Ablation-ICP-MS (B. Schäfer and D.B. Dingwell, in collaboration with D. Günther, R. Frischknecht and H. Cousin/Zürich)

Element partitioning between a fluid phase and melt has been investigated in a large number of studies employing mostly bulk analytical techniques to determine the compositions of the phases. In our experiments, the fluid phase coexisting with the melt is encapsulated at elevated temperature and pressure in vesicles in the melt and then quenched as fluid inclusions in the glass. Glass and inclusions are analysed using the laser-ablation-ICP-MS technique. In this method the laser drills through the glass matrix towards the fluid inclusion. Thus within one analysis glass immediately next to the fluid inclusion and the inclusion itself are sampled. Analyses yield transient signals for those elements contained in the inclusion.

Experiments have been performed with doped haplogranitic glass powder (doping with 20 elements ranging in atomic number from Li to W, each at ˜ 1000 ppm) and 5 wt% NaCl-solution. The syntheses have been carried out in TZM vessels at 2 kbars and 1050°C. Analyses were performed using the ELAN 6000/Perkin Elmer ICP-MS in combination with a 193 nm ArF excimer laser at the Institut für Isotopengeologie und Mineralische Rohstoffe, ETH Zürich. NIST SRM 610 glass and halite were used as external standards for the glass matrix and the fluid inclusions, respectively. Si, determined by electron microprobe, and Na or Cl, calculated from microthermometric measurements, served as internal standards for the glass and the inclusions, respectively. To correct for the matrix contribution to the fluid inclusion signal, signal integration requires at least one highly compatible matrix element. We apply a correction based on the highly compatible matrix element Si. Figure 3.4-8 shows the spectra of two fluid inclusion analyses for selected elements. For elements which are only contained in the inclusion (but not in the surrounding matrix) detection limits can be 1 ppm or even less. Calculated detection limits for Nb, Tb, Hf, and Ta in such a case are usually ≤ 1-5 ppm for the investigated inclusions. The additional correction for the matrix contribution increases these values by at least a factor of 10. Thus expected concentrations of a few ppm in the fluid phase for matrix compatible elements, such as Nb and Hf, cannot be detected in the inclusions of our samples.

Fig. 3.4-8a: Typical laser ablation ICP-MS analysis spectra showing (a) transient signals for Cl, Na, K, I, Br, Mo and W during inclusion ablation and (b - see next page) the matrix compatible elements Nb, Nd, Ta, Tb, La and Hf, which are not detected in the inclusion.
Fig. 3.4-8b: Figure caption see Figure 3.4-8a.

To investigate the influence of the halogens Br and I on element partitioning, further experiments were carried out using a glass powder doped with Cs, Nb, Mo, and W at 1000 ppm and aqueous solutions containing a mixture of salts (NaCl, KI, KBr) at different concentration levels up to a total salinity of about 20 wt%. Experiments were performed in rapid-quench vessels at 850°C and 2 kbars. The laser-ablation-ICP-MS analyses were calibrated externally using NIST SRM 610 and Br- and I- bearing glasses, respectively. Cl, Br, and I in the fluid inclusion analyses were calibrated using a standard solution. Calculated distribution coefficients Dfluid/melt as a function of Br concentration are shown in Figure 3.4-9. For the major elements Na and K, as well as for the trace elements Cs, Mo, and W, no effect of Br or I on the partition behavior was found.

Fig. 3.4-9: Calculated distribution coefficients as a function of Br concentration. In comparison with initial experiments with "pure" NaCl solutions no effect of Br on the partition behavior can be detected.

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