Sediment melts at sub-arc depths: An experimental study

The phase and melting relations in subducted pelites have been investigated xperimentally at conditions relevant for slabs at sub-arc depths (T = 600-1050°C, P = 2·5-4 GPa). The fluid-present experiments produced a dominant paragenesis consisting of arnet-phengite-clinopyroxene-coesite-kyanite that coexists with a fluid phase at run conditions. Garnet contains detectable amounts of Na₂O (up to 0·5 wt%), P₂O₅ (up to 0·56 wt%), and TiO₂ (up to 0·9 wt%) in all experiments. Phengite is stable up to 1000°C at 4·5 GPa and is characterized by high TiO₂ contents of up to 2 wt%. The solidus has been determined at 700°C, 2·5 GPa and is situated between 700 and 750°C at 3·5 GPa. At 800°C, 4·5 GPa glass was present in the experiments, indicating that at such conditions a hydrous melt is stable. In contrast, at 700°C, 3·5 and 4·5 GPa, a solute-rich, non-quenchable aqueous fluid was present. This indicates that the solidus is steeply sloping in P-T space. Fluid-present (vapour undersaturated) partial melting of the pelites occurs according to a generalized reaction phengite + omphacite + coesite + fluid + melt + garnet. The H₂O content of the produced melt decreases with increasing temperature. The K₂O content of the melt is buffered by phengite and increases with increasing temperature from 2·5 to 10 wt%, whereas Na₂ O decreases from 7 to 2·3 wt%. Hence, the melt compositions change from trondhjemitic to granitic with increasing temperature. The K₂O/H₂O increases strongly as a function of temperature and nature of the fluid phase. It is 0·0004-0·002 in the aqueous fluid, and then increases gradually from about 0·1 at 750-800°C to about 1 at 1000°C in the hydrous melt. This provides evidence that hydrous melts are needed for efficient extraction of K and other large ion lithophile elements from subducted sediments. Primitive subduction-related magmas typically have K₂O/H₂O of ∼0·1-0·4, indicating that hydrous melts rather than aqueous fluids are responsible for large ion lithophile element transfer in subduction zones and that top-slab temperatures at sub-arc depths are likely to be 700-900°C.