✍ Scribed by Wendy J. Harrison; Bernard J. Wood
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The partitioning of samarium and thulium between garnets and melts in the systems Mg3AI2-Si3012-H20 and Ca3A12Si3012-H20 has been studied as a function of REE concentration in the garnets at 30 kbar pressure. Synthesis experiments of variable time under constant P, T conditions indicate that garnet initially crystallizes rapidly to produce apparent values of Dsm (Dsm = concentration of Sm in garnet/concentration of Sm in liquid) which are too large in the case of pyrope and too small in the case of grossular. As the experiment proceeds, Sm diffuses out of or into the garnet and the equilibrium value of Dsm is approached. Approximate values of diffusion coefficients for Sm in pyrope garnet obtained by this method are 6x10 13 cm 2 s 1 at 1,300 ~ and 2x10-12cm zs 1 at 1,500 ~ and for grossular, 8.3 • 10-12 cm 2 s 1 at 1,200 ~ C and 4.6x 10-tl cm z s-1 at 1,300 ~
The equilibrium values of Dsm have been reversed by experiments with Sm-free pyrope and Sm-bearing glass, and with Sinbearing grossular and Sm-free glass.
Between 12 ppm and 1,000 ppm Sm in pyrope at 1,300 ~ C and between 80 ppm and >2 wt.% Tm in pyrope at 1,500 ~ C, partition coefficients are constant and independent of REE concentration. Above 100 ppm of Sm in garnet at 1,500 ~ C, partition coefficients are independent of Sm concentration. At lower concentrations, however, Dsm is dependent upon the Sm content of the garnet. The two regions may be interpreted in terms of charge-balanced substitution of Sm3A150 t 2 in the garnet at high Sm concentrations and defect equilibria involving cation vacancies at low concentrations. At very low REE concentrations (< 1 ppm Tm in grossular at 1,300 ~ C) DREE garnet/liquid again becomes constant with an apparent
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