Trace element diffusion and element partitioning between garnet and andesite melt using synchrotron X-ray fluorescence microanalysis (-SRXRF)

Abstract

Synchrotron radiation X-ray fluorescence microanalysis (μ-SRXRF) was applied to products of experimental geochemistry to determine (1) trace element diffusivities in andesite melts and (2) trace element partitioning behaviour between garnet and melt. To achieve sufficient spatial resolution, non-focusing and focusing glass capillaries reduced the incoming synchrotron beam down to sizes of 20 and 2.7 μm, respectively. (1) Diffusion couples of trace element-doped and undoped andesite melts were prepared in internally heated pressure vessels. A special sample setup allowed the pencil-shaped synchrotron beam to irradiate volume elements showing identical diffusion behaviour. Eighteen trace elements were measured simultaneously and quickly, resulting in diffusion profiles well suited for evaluating diffusion coefficients. (2) Garnet and andesitic melt were synthesized and equilibrated in a piston cylinder apparatus. The garnets were exceptionally large due to specially designed dehydration-melting experiments with monazite as a trace element source. Coexisting garnets and melt were analyzed with μ-SRXRF, and new distribution coefficients for Sr (0.126), Y (5.27), Zr (0.533), La (0.014), Ce (0.020), Nd (0.245), Sm (1.21), Eu (1.18), Gd (5.29), Yb (52.5), and Lu (76) were determined. However, the general use of μ-SRXRF for experimental partitioning studies is limited due to the relatively poor spatial resolution caused by the penetrating character of the synchrotron beam and due to limited count rates at high energies.