Experimental Study of the Slab-mantle Interaction and Implications for the Formation of Titanoclinohumite at Deep Subduction Zone

In order to examine the behavior of elements under the control of aqueous fluid through slab-mantle interactions, a series of high-pressure and high-temperature experiments was carried out at subsolidus temperature, 850°C. The starting materials consist of two portions: metabasite from blueschist as a slab material and olivine as a mantle material. The metabasite was directly placed on the olivine and sealed within an Au tube. At 3 and 6 GPa corresponding to 100 and 200 km in depth, respectively, the slab material was changed from blueschist to rutile-bearing eclogite. Observed phases in the mantle side were primary olivine and enstatite formed by the reaction between olivine and Si0₂-rich aqueous fluid from the slab side. At 8 GPa, the slab material was further transformed to rutile-free eclogite, and phases occurred in the mantle side were titanoclinohumite in addition to olivine and enstatite. The titanoclinohumite was formed by the reaction between olivine and Ti-bearing aqueous fluid which was supplied from the slab side. The titanoclinohumite is essentially identical in chemical composition to those principally occurring in kimberlite xenoliths. Because the Ti-phase is a host reservoir of HFSE, the formation of titanoclinohumite at 8 GPa corresponding to 250 km in depth suggests that the aqueous fluid can be a carrier of HFSE from the subducting slab to the upper mantle, and thus the source region of kimberlite magma could be impregnated by the interaction between mantle peridotite and the HFSE-bearing aqueous fluid from the subducting slab.