Island arc volcanic rocks contain low amounts of high field strength elements, but are enriched in LREE relative to MORB. Of several hypotheses put forward to explain this the selective enrichment of the mantle wedge with LILE and REE but
not HFSE by aqueous fluids from the subducting slab is a popular explanation. However, our knowledge of trace element
partitioning between solid mantle phases and fluids is at present only poorly constrained by experimental work. In this study, trace element partitioning between aqueous fluids and solid mantle phases is being investigated using diamond aggregates to trap fluid solutes during quenching from high pressure-temperature conditions. Starting materials are synthetic glasses doped with Sm,Yb and Hf. Experiments were carried out in a belt apparatus at 3 to 5 GPa and 1000 to 1300oC and residues consist of equal amounts of Gt, Cpx and Opx plus minor amounts of olivine. Trace elements in traps and residual solids were analysed by laser ablation microprobe (LAM). Element concentrations in the trap were internally standardized assuming DCa(fluid/solid) = 0.2. Therefore, all D-values for trace elements can only be regarded as relative values. Sm and Yb show increasing D(fluid/solid) with increasing pressure between 3 and 4 GPa but then no further change till 5 GPa. The D Sm/D Yb ratio remains constant.
Table 1: D(fluid/solid) at 1000oC; solids contain Gt+Cpx+Opx+Ol
DSm D Yb D Hf
30 kbar 0.20 0.006 0.25
40 kbar 0.50 0.017 0.25
Hf partitioning seems to be pressure independent (Table 1). However, Hf becomes oversaturated at levels above 400 ppm in the solid phases and a stable Hf-bearing phase forms. Although undoped, Sr and Ba are measurable and partition strongly into the fluid. The addition of 1M HCl to the experimental charge increases REE- solubility by one order of magnitude but has little effect on Hf-partitioning. Thus, the results so far suggest that a water-rich fluid is less effective in producing a relative depletion of HFSE than a highly chlorine solution.