Accessory phase control on the trace element signature of sediment melts in subduction zones

Phase and melting relations in deeply subducted sediments, the main reservoir of large ion lithophile elements (LILE) in subduction zones, are of first order importance to understand the general trace element signature of arc lavas. Here we present results from an experimental study on trace element liberation from metapelites in the range of 2.5-4.5 GPa and 750-1050 °C, i.e., conditions relevant for the slab at sub-arc depth. Apart from the common phases garnet, clinopyroxene, quartz/coesite and phengite, accessory phases such as rutile, apatite, allanite, monazite and zircon occur in the run products. The hydrous melts produced during wet melting of sediments have been analysed with laser ablation ICP-MS. The minor phase phengite is the main host of LILE in the residue and exerts a strong control on Rb and K in the coexisting melt. The presence of accessory phases profoundly influences the trace element characteristics of the hydrous melts because these phases control several trace elements (Ti by rutile, P by apatite, Zr by zircon, Th and light rare earth elements (Th, LREE) by monazite/allanite). The concentration of these elements in the hydrous melts is primarily a function of temperature and is independent of the amounts of the mineral and melt present. The saturation levels for these elements are used to predict that, in a common subducted sediment, accessory phases are likely to persist up to 850-900 °C at up to 50% melting. Trace element concentrations in hydrous melts are at least one order of magnitude higher than in aqueous fluids. Therefore, sediment melts provide an efficient way of extracting incompatible elements from the subducted slab. Accessory phases also influence the fractionation of elements with similar geochemical behaviour such as Th/La, Th/U, Ce/Pb (allanite/monazite), Nb/Ta (rutile) and Zr/Hf (zircon). Moreover, phengite is able to retain Rb while Cs is incompatible. These accessory phases therefore control characteristic element fractionation in the subduction component of arc magmas.