Experimental determination of partition coefficients for Rb,Sr and Ba between alkali feldspar and silicate liquid

Partitioning of Rb, Sr and Ba between alkali feldspar and a synthetic granitic melt has been determined at 8 kb and 720 to 780°C for a single quaternary granite composition. The results suggest that Henry's law is obeyed by Rb up to ~0.8 wt.% Rb₂O in both the liquid and in the alkali feldspar. The measured D values for Rb range from 0.77 to 1.1. For Ba, Henry's Law is obeyed up to ~0.6 wt.% BaO in the liquid and ~5 wt.% BaO in the alkali feldspar. D values for Ba range from 6.4 to 14. For Sr there is only a crude relationship between concentration in the liquid and concentration in the alkali feldspar at concentrations greater than ~0.6 wt.% SrO in the liquid and ~0.4 wt.% SrO in the alkali feldspar. D values for Sr range from 1.2 to 5.0. Partitioning of Sr is apparently sensitive to the concentration of Ba in the system and this partly explains the failure of Sr to obey Henry's Law.Linear least-squares fits to the partitioning data as a function of temperature suggest inverse correlation between temperature and D values. Rb shows only a slight temperature effect whereas Ba and Sr appear to be rather strongly affected by temperature, but the temperature range examined here is small compared to the scatter in the data making these trends relatively uncertain. Other factors that appear to affect partitioning, especially of Sr, are growth rate, development of sector zoning and Or content of the alkali feldspar. These factors severely limit the use of partitioning of these elements in alkali feldspar as geothermometers.The technique for measuring growth rates utilized here combined with measurement of trace element depletion in diffusion boundary layers adjacent to the alkali feldspar crystals makes it possible to estimate diffusivities for Ba and Sr. These estimates suggest a difference of 2 orders of magnitude between diffusivities for Ba and Sr in a vapor-saturated melt and those measured by HOFMANN and MAGARITZ (1976) for a dry obsidian glass.