Partition coefficients for Rb, Ba, Th, Ta, Nb, La, Ce, Sr, Nd, Hf, Zr, Sm, Eu, Gd, Li, Y, Yb and Ga between amphibole and hydrous basaltic to andesitic melts were determined in order to get some information on the effect of amphibole in trace element fractionation processes in subduction related magmas.
Experiments were conducted in an internally heated pressure vessel at 5 kbar and 940-980 °C. Starting materials were prepared as glasses from synthetic oxides and carbonates doped with the interesting trace elements at 500 ppm levels each. Bulk compositions are analogous to a natural basaltic andesite and an andesite. Grounded glass was welded into Ag-Pd capsules with weighed quantities of water. Experiments yielded pargasitic amphiboles about 100 µm in size. Amphibole/melt pairs were analyzed by electronbeam-microprobe and by secondary ion mass spectrometry (SIMS).
Trace element partitioning between amphibole and basaltic andesite and amphibole and andesite is very similar to each other in our experiments. Sm, Eu, Gd and Yb seem to behave compatible for amphibole. They have D-values in the range of 1.6-2.2. Hf (D~1.2) and Y (D=1.9-2.1) also show a compatible behaviour for amphibole.
Compared with the D-values published by Brenan et al. (1995) most of our D-values are higher (Hf, Zr and Sm nearly 3 times, Nb, Ta and Ce ~2 times, Yb and Nd ~1,5 times and at least Th ~1-2 times). D-values for Ba and Sr are similar to the data published by Brenan et al. (1995), whereas DRb is 3 times lower in our experiments.
If the data set from this work is compared with that of La Tourette et al. (1995). a shift to higher D-values, with exception of Rb (4 times lower) and Sr (nearly the same) is observed, too.
Partition coefficients determined in this work favour amphibole as a major candidate for the relative depletion of HFSE compared to LILE and LREE in subduction related basaltic melts because of relative high D-values for Ta, Nb, Hf, Zr and Y in comparison with D-values for LILE Rb, Ba and Th as well as LREE La and Ce (see Table 1).
Brenan, J.M, Shaw, H.F., Ryerson, F.J. & Phinney, D.L., Earth planet. Sci. Lett. 135, 1-11 (1995).
La Tourrette, T., Richard, R.L. & Holloway, J.R., Earth Planet. Sci. Lett. 135, 13-30 (1995).
Basaltic
Andesite Andesite
Rb 0.052 0.055
Ba 0.140 0.132
Th 0.039 0.035
Ta 0.548 0.457
Nb 0.506 0.500
La 0.232 0.251
Ce 0.423 0.461
Sr 0.284 0.322
Nd 1.000 1.120
Hf 1.242 1.205
Zr 0.941 0.772
Sm 1.650 1.850
Eu 1.748 1.977
Gd 2.120 2.221
Li 0.102 0.121
Y 1.902 2.094
Yb 1.653 1.774
Ga 1.427 1.530
Table 1: Average amphibole/melt partition coefficients determined in this work (standard deviation ~ ±10% rel.).