Experimental Study of Slab-Mantle Geochemical Exchange in Subduction Zones

Aqueous fluids derived from subducting oceanic crust play an important role in the material transport leading to the production or arc lavas, and in the long-term chemical evolution of the Earth's mantle and crust. In order to determine the geochemical evolution of both the subducting slab and the overlying mantle wedge, a series of dehydration/hydration experiments was carried out at conditions of 0.8-4.0 GPa and 650-900° C appropriate for subduction zones. Blueschist facies rocks/minerals, and olivine (Fo₉₀) were used for starting materials, as analogue materials of slab and mantle, respectively. Finely ground metabasalt (H₂O = 5.9 wt%) and glaucophane (H₂O = 2.3 wt%) were separately sealed in gold capsules with an olivine grain (1mm diameter), and then run in a piston-cylinder apparatus. Polished sections of run products were observed and analyzed for major element compositions with an electron micro-probe. Trace elements of selected run-products were determined using an ion probe (Cameca-5f). At subsolidus conditions, the metabasalt was transformed into amphibolite-facies mineral assemblages containing Mg-ilmenite at <1.5 GPa, and eclogite-facies assemblages containing rutile at >1.5 GPa. Glaucophane was transformed into the mineral assemblage of Na-Cpx, Opx +/- garnet. Garnets formed in the slab portion show low-LREE/HREE and higher-HREE contents when compared with the starting materials. In all subsolidus experiments, Al-rich silicate glasses, which could be quenched aqueous fluids, were observed between mineral grain boundaries in the slab portions. The fluids at 3.0 GPa show high-LREE/HREE, and higher-LILE and lower-HREE contents. In contrast, the quenched fluids for <1.5 GPa, which did not coexist with garnet, do not show strong depletion in HREE. Negative Nb anomalies were observed in the quenched fluids in the metabasalt experiments, but this anomaly does not appear to exist in the glaucophane experiments. The behavior of the HREE and HFSE is consistent with the existence of garnet and Ti-oxides (rutile and ilmenite) in the slab portion of the experiments. The fluids should therefore be enriched in SiO₂, LILE and LREE. Mineral zones were observed on olivine grains near the initial olivine-slab interface. These reaction zones consisted of talc and enstatite layers at < 800° C, and an enstatite layer only at > 800° C. Because the enstatite layers are strongly LILE and LREE-enriched compared with the primary olivine, the reacted layers are thought to have been formed by chemical interaction between olivine and SiO₂-riched aqueous fluids which were capable of also delivering these other trace elements. As a consequence of this transfer, the chemical compositions of the descending slab and the overlying mantle could be considerably depleted and enriched, respectively, in SiO₂, LILE and LREE during the process of subduction.