TY - JOUR

T1 - Trace element diffusion and viscous flow in potassium-rich trachytic and phonolitic melts

AU - Behrens, Harald

AU - Hahn, Matthias

N1 - Funding Information: This work was supported by the German Science Foundation (DFG grant BE1720/6 and BE1720/11). We thank Karen Rickers and Gerald Falkenberg for technical and scientific support at Hasylab, Hamburg. Special thanks also to Lothar Borchers at Medical University of Hannover (MHH) for helping us in recording X-ray images of viscosity samples. We thank J. Mungall and D. Baker for carefully reviewing the paper. Improvement of writing by K. Norton and editiorial corrections by Andrew Berry are acknowledged.

PY - 2009/2/15

Y1 - 2009/2/15

N2 - Trace element diffusion was experimentally investigated in nominally dry (0.03-0.10 wt.% H2O) and hydrous (1.13-1.86 wt.% H2O) melts of trachytic and phonolitic composition at temperatures from 1050 to 1250 °C and a pressure of 500 MPa. Experiments with a large set of trace elements (Rb, Sr, Ba, Cr, Ni, Cu, Zn, Y, La, Nd, Sm, Eu, Gd, Yb, Zr, Nb, Hf, and Sn) were performed in an internally heated gas pressure vessel using the diffusion triple technique. In doing so, two diffusion couples are combined in a single experiment in order to avoid problems in deconvolution of X-ray fluorescence spectra due to peak overlapping. Profiles of all trace elements were simultaneously analyzed by synchrotron radiation X-ray fluorescence microanalysis (μ-SRXRF). Most of the diffusion profiles can be fitted well assuming constant diffusivity. The diffusion data recorded in a single experiment cover a range of 1.4 log units to 2.9 log units. While the low field strength elements (LFSEs) Rb, Sr and Ba are always the fastest elements, the high field strength elements (HFSEs) Zr, Nb and Hf are the slowest elements. At constant temperature and water content the trace element diffusivities increase with increasing fraction of network modifying cations from rhyolite over trachyte to phonolite. For a given composition the diffusivities of the rare earth elements (REEs) decrease slightly with increasing atomic number except for Eu which diffuses faster than the other REEs. This finding can be explained by a significant fracton of europium being in the divalent state while the other REEs are present only in the trivalent state in silicate melts. The diffusivity of Eu and other redox sensitive cations (Cr, Sn) depends strongly on Fe2+/Fetotal of the melt. At reducing conditions these three elements are accelerated compared to univalent elements. Diffusion of all trace elements is strongly enhanced by dissolved water. In general, the effect is larger for HFSEs than for LFSEs. For instance, at 1150 °C the diffusivity of Zr increases by 1.4 orders of magnitude, but that of Sr only by 0.5 orders of magnitude when adding 1.7 wt.% of H2O to dry phonolite. To test viscosity-diffusivity relationships, we have performed viscosity experiments with the same melt compositions using the falling sphere method. The viscosity of phonolite and trachyte melts can be correlated with the Zr diffusivity by the Eyring relationship when using a jumping distance of 0.503 ± 0.067 nm. The empirical model of Mungall [Mungall, J.E., 2002b. Empirical models relating viscosity and tracer diffusion in magmatic silicate melts. Geochim. Cosmochim. Acta, 66, 125-143.] reproduces the diffusivity data within 0.8 log units for HFSEs and within 0.6 log units for intermediate field strength elements (IFSEs) when using the viscosity data as an input.

AB - Trace element diffusion was experimentally investigated in nominally dry (0.03-0.10 wt.% H2O) and hydrous (1.13-1.86 wt.% H2O) melts of trachytic and phonolitic composition at temperatures from 1050 to 1250 °C and a pressure of 500 MPa. Experiments with a large set of trace elements (Rb, Sr, Ba, Cr, Ni, Cu, Zn, Y, La, Nd, Sm, Eu, Gd, Yb, Zr, Nb, Hf, and Sn) were performed in an internally heated gas pressure vessel using the diffusion triple technique. In doing so, two diffusion couples are combined in a single experiment in order to avoid problems in deconvolution of X-ray fluorescence spectra due to peak overlapping. Profiles of all trace elements were simultaneously analyzed by synchrotron radiation X-ray fluorescence microanalysis (μ-SRXRF). Most of the diffusion profiles can be fitted well assuming constant diffusivity. The diffusion data recorded in a single experiment cover a range of 1.4 log units to 2.9 log units. While the low field strength elements (LFSEs) Rb, Sr and Ba are always the fastest elements, the high field strength elements (HFSEs) Zr, Nb and Hf are the slowest elements. At constant temperature and water content the trace element diffusivities increase with increasing fraction of network modifying cations from rhyolite over trachyte to phonolite. For a given composition the diffusivities of the rare earth elements (REEs) decrease slightly with increasing atomic number except for Eu which diffuses faster than the other REEs. This finding can be explained by a significant fracton of europium being in the divalent state while the other REEs are present only in the trivalent state in silicate melts. The diffusivity of Eu and other redox sensitive cations (Cr, Sn) depends strongly on Fe2+/Fetotal of the melt. At reducing conditions these three elements are accelerated compared to univalent elements. Diffusion of all trace elements is strongly enhanced by dissolved water. In general, the effect is larger for HFSEs than for LFSEs. For instance, at 1150 °C the diffusivity of Zr increases by 1.4 orders of magnitude, but that of Sr only by 0.5 orders of magnitude when adding 1.7 wt.% of H2O to dry phonolite. To test viscosity-diffusivity relationships, we have performed viscosity experiments with the same melt compositions using the falling sphere method. The viscosity of phonolite and trachyte melts can be correlated with the Zr diffusivity by the Eyring relationship when using a jumping distance of 0.503 ± 0.067 nm. The empirical model of Mungall [Mungall, J.E., 2002b. Empirical models relating viscosity and tracer diffusion in magmatic silicate melts. Geochim. Cosmochim. Acta, 66, 125-143.] reproduces the diffusivity data within 0.8 log units for HFSEs and within 0.6 log units for intermediate field strength elements (IFSEs) when using the viscosity data as an input.

KW - Hydrous phonolite melt

KW - Hydrous trachyte melt

KW - Synchrotron X-ray fluorescence microanalysis

KW - Trace element diffusion

KW - Viscosity

UR - http://www.scopus.com/inward/record.url?scp=58649098164&partnerID=8YFLogxK

U2 - 10.1016/j.chemgeo.2008.10.014

DO - 10.1016/j.chemgeo.2008.10.014

M3 - Article

AN - SCOPUS:58649098164

VL - 259

SP - 63

EP - 77

JO - Chemical geology

JF - Chemical geology

SN - 0009-2541

IS - 1-2

ER -