Amphibole and phlogopite in “hybrid” metasomatic bands monitor trace element transfer at the slab-mantle interface (Eastern Alps, Italy). DOI: 10.1016/j.lithos.2010.02.011

Ultramafic rocks in the Mt. Hochwart HP mélange (Eastern Italian Alps) preserve a series of metasomatic mineral zones generated by infiltration of hydrous fluids at T of 660-700°C and P<1.2 GPa, which occurred at the gneiss-peridotite interface. We present the results of in situ LA ICP-MS trace element analysis of minerals from contact lithologies (metasomatic zones and phlogopitite). Trace element composition of hydrous phases (phlogopite and amphibole) in different metasomatic zones indicate mobility of many elements, including elements such as Ta, which are considered to show scarce mobility in fluids. Trace element composition of accessory minerals such as apatite in the phlogopite-rich zone suggests that the trace element signature of subduction zone fluids may be fractionated in this zone. The progressive depletion in some trace elements (LREE and LILE) and the enrichment in Li from the gneiss towards the peridotite suggest a strong influence of bulk composition on the trace element budget of hydrous minerals. Since these metasomatic zones represent geochemical processes occurring at the slab-mantle interface, we can infer that metasomatic reactions between slab derived fluids and ultramafic mantle wedge will follow a specific series of reactions, creating mineral zones similar to those observed in this study. We stress the importance of phlogopite crystallization at the slab-mantle interface as an efficient mechanism to filter LILE from the fluid. Our results further favour the evidence that the primary composition of subduction zone fluids reaching the source region of arc magmas is substantially modified by metasomatic reactions occurring in the mantle wedge.