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Trace element diffusion in rhyolitic melts

¹ Institut für Mineralogie, Universität Hannover, Callinstr. 3, D-30167 Hannover, Germany
² Hamburger Synchrotron-Strahlungslabor HASYLAB at Deutsches Elektronen-Synchrotron DESY, Notkestraße 85, D-22607 Hamburg, Germany
³ GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany

^* Corresponding author, e-mail: m.hahn{at}mineralogie.uni-hannover.de http://www.min.uni-hannover.de

Two microbeam techniques, synchrotron radiation X-ray fluorescence micro-analysis (µ-SRXRF) and secondary ion mass spectrometry (SIMS) are compared for analyzing diffusion profiles of trace elements in two hydrous rhyolitic glasses (1.87 and 5.00wt% H₂O). In order to verify the results, laser ablation coupled to inductively coupled plasma optical emission (LA-ICP-OES) has been used on one sample. Samples were produced by diffusion couple experiments performed in an internally heated gas pressure vessel at 1200 °C and 500MPa. One half of each couple was doped with 24 trace elements representing different geochemical groups: low field strength elements (Rb, Sr, Ba), transition metals (Cr, Co, Ni, Cu, Zn), rare earth elements (La, Ce, Nd, Sm, Eu, Gd, Er, Yb) + Y, high field strength elements (V, Zr, Nb, Hf, Ta) and main group elements (Ge, Sn).

Several profiles were measured with both µ-SRXRF and SIMS on both samples. In principle, concentrations of all elements can be extracted simultaneously from a single SRXRF spectrum. However, some trace elements could not be reliably quantified with our analytical system: Ta and Pb (used for detector collimator material), Ti, V (low energy of K), Co (K-peak overlapping with Fe K_β-peak) and Cr, Ni, Cu, Zn (overlapping with 1-lines of REEs). In contrast, SIMS analyses measure each element sequentially. Hence, not all elements of the large total set of trace elements could be analyzed in a single run. Some elements requiring a high mass resolution (NaSi interfering with V, CaO interfering with Ni) or having low yields (Sn) were not profiled.

Multiple diffusivities derived from µ-SRXRF and SIMS profiles are in very good agreement for most elements. In general, the trace element diffusivity decreases with increasing valence state, e.g. in sample D22 containing 1.87wt% H₂O from log D=-10.80 for the monovalent Rb to log D=-13.34 for the tetravalent Zr (Din m²/s). By increasing the water content in sample D18 to 5.00wt%, diffusion coefficients increase approximately by one order of magnitude for all elements studied.

Key-words: synchrotron radiation X-ray fluorescence microanalysis, SIMS, trace element diffusion, rhyolite.