MINERAL AQUEOUS FLUID PARTITIONING OF TRACE-ELEMENTS AT 900-1200-DEGREES-C AND 3.0-5.7 GPA - NEW EXPERIMENTAL-DATA FOR GARNET, CLINOPYROXENE, AND RUTILE, AND IMPLICATIONS FOR MANTLE METASOMATISM

In order to constrain the role of fluid phases during metasomatic proc esses 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 f luids and eclogite assemblage minerals (garnet, clinopyroxene, and rut ile) have been determined experimentally at 900-1200 degrees C and 3.0 -5.7 GPa. Using a new experimental technique in which diamond aggregat es are added to the experimental capsule set-up, the fluid was separat ed from the solid residue so that both quenched solute and residual mi nerals could be analysed directly. Trace element concentrations were d etermined in situ by laser ablation microprobe (LAM). The partitioning behaviour is controlled by temperature, pressure, and crystal chemist ry; whereas fluid composition is not as crucial. Neither addition of h ydrochloric acid nor high silica concentrations in the fluid have stro ng 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 Kf show variable solubilities. Low fiel d strength elements (LFSE) and light rare earth elements (LREE) are al ways enriched in the fluid (D-(fluid/Min) > 1). Generally, D-(fluid/Cp x) is positively correlated with temperature only for high field stren gth elements (HFSE), but positively correlated with pressure for all o ther elements. Therefore, the lowest Nb/La is achieved at high pressur es and low temperatures. However, even the highest pressures and lowes t temperatures examined did not exhibit strong negative HFSE anomalies in the fluid. Garnet retains compatible trace elements at 3 GPa and 1 000 degrees 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 tem perature results in a lowered D-(fluid/gt) particularly for Zr, Hf, an d heavy rare earth elements (HREE). At 5 GPa and 900 degrees C a stron g intra-REE fractionation is observed (D-(fluid/gt)Sm/Yb around 100) a nd 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 effectiv ely by rutile, as is the case for rutile/melt partitioning. Fluid/mine ral trace element partitioning has important implications for mantle m etasomatism in subarc regions. A model is proposed in which HFSE deple tions, as observed in island are 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 deple ted by melt extraction (depleted MORE mantle) can produce magmas with trace element patterns similar to those of subduction-related volcanic rocks. Copyright (C) 1998 Elsevier Science Ltd.