Sediment melts at sub-arc depths: An experimental study

The phase and melting relations in subducted pelites have been investigated experimentally at conditions relevant for slabs at sub-arc depths (T 600-1050 degrees C, P 2.5-4.5 GPa). The fluid-present experiments produced a dominant paragenesis consisting of garnetphengiteclinopyroxenecoesitekyanite that coexists with a fluid phase at run conditions. Garnet contains detectable amounts of Na2O (up to 05 wt%), P2O5 (up to 056 wt%), and TiO2 (up to 09 wt) in all experiments. Phengite is stable up to 1000 degrees C at 4.5 GPa and is characterized by high TiO2 contents of up to 2 wt%. The solidus has been determined at 700 degrees C, 25 GPa and is situated between 700 and 750 degrees C at 3.5 GPa. At 800 degrees C, 4.5 GPa glass was present in the experiments, indicating that at such conditions a hydrous melt is stable. In contrast, at 700 degrees 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 25 to 10 wt, whereas Na2O decreases from 7 to 23 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 degrees C to about 1 at 1000 degrees 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 similar to 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 degrees C.