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The rare earth elements (REE, from ⁵⁷La to ⁷¹Lu) have been found to be a powerful tool for the evaluation of processes in ultramafic rocks. The partition coefficients of REE between phases of the upper mantle, especially between clinopyroxene and garnet, are temperature dependent. A compositional dependence in rocks with different contents of CaO and Al₂O₃ can also be expected.

The project focuses on the determination of the dependence of empirical partition coefficients of REE between upper mantle phases on temperature, pressure, and composition in absence of melts. To get maximum information on the partition behaviour the elements La, Nd, Sm, Gd, Ho, Yb were selected, because in clinopyroxenes light REE (La to Eu) are preferably substituted, whereas in garnets the heavy REE (Gd to Lu) enter the structure more easily.

The initial experiments were performed on the CaO-free system MgO-Al₂O₃-SiO₂-REE₂O₃ to determine phase stabilities and the maximum REE concentrations in olivine, orthopyroxene, garnet and spinel. This was a prerequisite to optimize conditions for electron probe microanalysis. To avoid line overlaps in the generated X-ray emission spectra the experiments were doped with only one REE each.

The experiments were run at 1400°C, 2.5 GPa and 1200°C, 3.0 GPa. The phases which formed with the highest REE concentrations were a REE-silicate (oxy-apatite) or a garnet solid solution, depending on the stability of the specific REE garnet phase. The REE-silicate phase may contain up to 10 wt% Al₂O₃ and 5 wt% MgO, and in the Al₂O₃-free system up to 9 wt% MgO. This phase subtracted SiO₂ from the bulk composition such that olivine or spinel were formed instead of the expected orthopyroxene. The maximum contents of REE substituted into olivine and spinel range between 0.1 and 0.5 wt% REE₂O₃ and show a systematic increase with increasing atomic number of the REE. For orthopyroxene the contents for Ho₂O₃, Yb₂O₃ and Lu₂O₃ are ca. 1.5 wt% at 1400°C, but ca. 1 wt% at 1200°C. The garnet compositions in the Al₂O₃-bearing system are all solid solutions, whereas the garnet in the Al₂O₃-free system seems to be a pure phase. Therefore the differences between the partition coefficients for REE between garnet and orthopyroxene can have contributions from both temperature and Al₂O₃ content. In one experiment doped with Sm the partition coefficient could be calculated for orthopyroxene coexisting with pyrope (see Fig 3.2-4).
 

Fig. 3.2-4: a) Compositions of synthesized orthopyroxenes and garnets in the section Enstatite - Al2O3 - REE2O3 of the system MgO - Al2O3 - REE2O3 - SiO2 (wt%). Abbreviations: En: Enstatite, Pyr: Pyrope, Mg-Ts: MgAlAlSiO6, REEAlG: REE3Al5O12, MgREEG: Mg2.5REE3Si2.5O12, REEMgMgG: (REE2Mg)Mg2Si3O12.

As a result of the experiments, it is concluded that multiple buffering for SiO₂ and Al₂O₃ in the presence of REE-silicate is needed. Therefore a garnet lherzolite composition (20% olivine, 32% orthopyroxene, 24% clinopyroxene, 24% garnet, all in wt%) was chosen for the current experiments. In the system CaO - MgO - Al₂O₃ - SiO₂, SiO₂ will be buffered by olivine - orthopyroxene, and Al₂O₃ by garnet - orthopyroxene. Two isotherms at 1200°C and 1400°C, 3 GPa are being determined at 10 wt% REE₂O₃ (added as REE-silicate) and will later be determined at lower REE concentration. Additionally compositional effects of Al₂O₃ and CaO are being investigated.