Melting of pelitic sediments at subarc depths: 1. Flux vs. fluid-absent melting and a parameterization of melt productivity

• Subarc pressure experiments to define the difference between fluid-present, fluid-absent, with or without CO2 sediment melting
• Melt compositions at very low melt fractions
• Equations for fluid-saturated and fluid-absent solidi of pelitic metasediments
• Equations for melt fraction as a function of temperature, pressure and volatile content based on all available experiments

To evaluate the potential of subducted sediments to contribute melts for mantle metasomatism in the subarc region we have experimentally studied and reviewed the melting behavior of pelites at subarc depths. Our experimental data set completes previous studies such that all combinations of fluid-saturated or -absent melting with or without H2O and CO2 are now investigated. Experiments were conducted at 3–4.5 GPa, 750–1200 °C with varying proportions of H2O (0.7–4.4 wt.%) and CO2 (0–4 wt.%), the maximum amount of H2O stored away in phengite being ~ 0.9 wt.% For all pelites, the onset of melting is controlled by an assemblage of phengite + jadeitic cpx + quartz/coesite ± fluid. The wet solidus locates at 715 to 830 °C, 3 to 4.5 GPa, increasing by ~ 30 °C with CO2. The fluid-absent pelite solidus is at 890 to 1040 °C, 3 to 4.5 GPa, decreasing little with CO2. To 5 GPa, quartz-saturated pelites yield highly siliceous meta- to slightly peraluminous granites.To all available experiments we fitted (i) 3rd order polynomials to describe the fluid-saturated and fluid-absent solidi, and (ii) equations to parameterize melt productivity in sediments as a function of temperature, pressure, bulk H2O- and CO2-contents (valid for 2–5 GPa). For H2O-contents leading to fluid-saturation at subsolidus conditions, melt productivity at the solidus increases with pressure and depends strongly on the amount of H2O available: experimental and calculated melt fractions (F) are 2–17 and 23–38 wt.% at 2.5–5 GPa, 2 and 15 wt.% bulk H2O, respectively. As calcite remains stable above the fluid-saturated solidus, partial melting does not lead to complete decarbonation. Near the solidus, the silicate melt may only transport 0.05 to 0.6 wt.% bulk CO2. Fluid-absent melting yields 15–30% melt at the solidus with phengite melting out immediately above the solidus.Warm thermal subduction models predict temperatures that just cross the wet solidus in hot subduction zones but not in intermediate to cold ones. The only realistic scenario for extracting sediment melts from subducting slabs is flux melting where fluid is added from metamorphic reactions in underlying slab lithologies. With the meta-basaltic layer being fully dehydrated when sediments reach melting temperatures, the only viable mechanism for H2O delivery is serpentine breakdown in the subducting oceanic peridotite. However, if sediments melt in the descending slab, the same slab will almost completely dehydrate. We thus conclude that sediment melting in descending slabs cannot be a general process as otherwise the Earth interior would dry out on geological time scales. Alternatives are cold sediment diapirs rising into the mantle wedge or trace element transfer by supercritical liquids at > 5 GPa, the latter propelled by partial peridotite dehydration.