Abstract:
In order to constrain the role of fluid phases during metasomatic processes in the upper mantle, trace element partition coefficients for Ba, Sr, Pb, Nb, Ta, Zr, Hf, Ti, La, Ce, Sm, Tb, and Yb between aqueous fluids and eclogite assemblage minerals (garnet, clinopyroxene, and rutile) have been determined experimentally at 900-1200°C and 3.0-5.7 GPa. Using a new experimental technique in which diamond aggregates are added to the experimental capsule set-up, the fluid was separated from the solid residue so that both quenched solute and residual minerals could be analysed directly. Trace element concentrations were determined in situ by laser ablation microprobe (LAM).The partitioning behaviour is controlled by temperature, pressure, and crystal chemistry; whereas fluid composition is not as crucial. Neither addition of hydrochloric acid nor high silica concentrations in the fluid have strong effects on trace element partitioning. Results indicate that in the presence of garnet or clinopyroxene, Nb and Ta are highly soluble in aqueous fluids, whereas Zr and Hf show variable solubilities. Low field strength elements (LFSE) and light rare earth elements (LREE) are always enriched in the fluid (D(fluid/Min) > 1). Generally, D(fluid/Cpx) is positively correlated with temperature only for high field strength elements (HFSE), but positively correlated with pressure for all other elements. Therefore, the lowest Nb/La is achieved at high pressures and low temperatures. However, even the highest pressures and lowest temperatures examined did not exhibit strong negative HFSE anomalies in the fluid. Garnet retains compatible trace elements at 3 GPa and 1000°C much more effectively (D(fluid/gt)Yb= 0.002) than at 5.7 GPa at the same temperature (D(fluid/gt)Yb = 0.04). Decreasing temperature results in a lowered D(fluid/gt), particularly for Zr, Hf, and heavy rare earth elements (HREE). At 5 GPa and 900°C a strong intra-REE fractionation is observed (D(fluid/gt)Sm/Yb around 100) and significantly negative anomalies for Hf and Zr, but not for Nb and Ta, are developed. Only residual rutile fractionates all HFSE from all other trace elements. Tantalum and niobium are retained most effectively by rutile, as is the case for rutile/melt partitioning.Fluid/mineral trace element partitioning has important implications for mantle metasomatism in subarc regions. A model is proposed in which HFSE depletions, as observed in island arc volcanic rocks, could originate from a selective enrichment of the mantle wedge in LFSE and LREE by aqueous fluids derived from a rutile-bearing subducted slab. It is shown that melting of the enriched mantle wedge, which had previously been depleted by melt extraction (depleted MORB mantle) can produce magmas with trace element patterns similar to those of subduction-related volcanic rocks.