Phase diagrams of hydrous mid-ocean ridge (MOR) basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones. A synthesis of our experimentally determined phase relations of chlorite, lawsonite, epidote-zoisite, amphibole, paragonite, chloritoid, talc, and phengite in basalts and of phase relations from the literature of serpentine, talc, chlorite, amphibole, and phase A in ultramafics permits calculation of H 2 O contents in hydrous phase assemblages that occur in natural compositions. This yields the information necessary to calculate water budgets for descending slabs. Starting from low-grade blueschist conditions (10-20 km depth) with H 2 O contents between 5 and 6 wt% for hydrated oceanic crust, complete dehydration is achieved between 70 and >300 km depth as a function of individual slab geotherms. Hydrous phases which decompose at depth below volcanic arcs are lawsonite, zoisite, chloritoid, and talc (š phengite) in mafic compositions and chlorite and serpentine in peridotite. Approximately 15-35% of the initially subducted H 2 O are released below volcanic arcs. The contribution of amphibole dehydration to the water budget is small (5-20%) and occurs at relatively shallow depth (65-90 km). In any predicted thermal structure, dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab. Such a dehydration characteristic is incompatible with 'single phase dehydration models' which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts. As a consequence of continuously progressing dehydration, water ascending from the slab will be generally available to depth of ca. 150-200 km. The fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge. We propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas. Thermal models show that such an isotherm (ca. 1300ºC) locates below volcanic fronts, slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts.

Fluids and melts liberated from subducting oceanic crust recycle lithophile elements back into the mantle wedge, facilitate melting and ultimately lead to prolific subduction-zone arc volcanism. The nature and composition of the mobile phases generated in the subducting slab at high pressures have, however, remained largely unknown. Here we report direct LA-ICPMS measurements of the composition of fluids and melts equilibrated with a basaltic eclogite at pressures equivalent to depths in the Earth of 120-180 km and temperatures of 700-1,200 degrees C. The resultant liquid/mineral partition coefficients constrain the recycling rates of key elements. The dichotomy of dehydration versus melting at 120 km depth is expressed through contrasting behaviour of many trace elements (U/Th, Sr, Ba, Be and the light rare-earth elements). At pressures equivalent to 180 km depth, however, a supercritical liquid with melt-like solubilities for the investigated trace elements is observed, even at low temperatures. This mobilizes most of the key trace elements (except the heavy rare-earth elements, Y and Sc) and thus limits fluid-phase transfer of geochemical signatures in subduction zones to pressures less than 6 GPa.

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Very low seismic velocity anomalies in the Earth's mantle may reflect small amounts of melt present in the peridotite matrix, and the onset of melting in the Earth's upper mantle is likely to be triggered by the presence of small amounts of carbonate. Such carbonates stem from subducted oceanic lithosphere in part buried to depths below the 660-kilometre discontinuity and remixed into the mantle. Here we demonstrate that carbonate-induced melting may occur in deeply subducted lithosphere at near-adiabatic temperatures in the Earth's transition zone and lower mantle. We show experimentally that these carbonatite melts are unstable when infiltrating ambient mantle and are reduced to immobile diamond when recycled at depths greater than ∼250 kilometres, where mantle redox conditions are determined by the presence of an (Fe,Ni) metal phase. This 'redox freezing' process leads to diamond-enriched mantle domains in which the Fe(0), resulting from Fe(2+) disproportionation in perovskites and garnet, is consumed but the Fe(3+) preserved. When such carbon-enriched mantle heterogeneities become part of the upwelling mantle, diamond will inevitably react with the Fe(3+) leading to true carbonatite redox melting at ∼660 and ∼250 kilometres depth to form deep-seated melts in the Earth's mantle.

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Phase relationships in natural andesitic and synthetic basaltic systems were experimentally investigated from 2.2 to 7.7 GPa, and 550°C to 950°C, in the presence of an aqueous fluid, in order to determine the stability of hydrous phases in natural subducted crustal material and to constrain reactions resulting in the release of water from subduction zones to the mantle wedge. Water reservoirs in subducted oceanic crust at depths exceeding the amphibole stability field (>70–80 km) are lawsonite (11 wt % H2O), Mg‐chloritoid (8 wt %), talc (5 wt %), and zoisite‐clinozoisite (2 wt %) in basaltic rocks; and lawsonite, zoisite‐clinozoisite, phengite (4 wt %) and staurolite (2 wt %) in andesitic compositions. The thermal stability of lawsonite at 6.0 GPa extends to ≈800°C and 870°C in basaltic and andesitic compositions, respectively. At pressures above amphibole‐out (2.3–2.5 GPa) lawsonite reacts through continuous reactions with steep positive dP/dT slopes to zoisite‐clinozoisite (until 3.0–3.2 GPa), and at higher pressures (to more than 7.7 GPa) to assemblages containing garnet + clinopyroxene and garnet + clinopyroxene + kyanite in basaltic and andesitic compositions, respectively. On the contrary, the breakdown of zoisite‐clinozoisite is mainly pressure‐sensitive. Phengite represents the hydrous phase with the largest stability field encountered in this study. In andesite, phengite is stable to more than 7.7 GPa and more than 920°C. Talc and staurolite contribute in minor amounts to the water balance in basaltic and andesitic rock compositions. A model for water release from the subducted slab is developed combining thermal models for subduction zones with the experimentally determined phase relationships. Up to 1 wt % and 2 wt % H2O in basaltic and andesitic rocks, respectively, can be stored to depths beyond 200 km in cold subduction zones, mainly by lawsonite and phengite. Dehydration rates are high until amphibole‐out, and relatively low at greater depths. The amphibole‐out reactions are found to release a significant amount of water in a depth interval of several kilometers, however, they do not represent a discrete pulse of fluid and do not completely dehydrate the descending slab. Fluid release at depths greater than 200 km through phengite and progressive lawsonite breakdown would hydrate the overlying mantle, causing the generation of amphibole or phlogopite peridotite. At higher geothermal gradients, epidote/zoisite contributes to fluid flux to the mantle wedge at 100–120 km depth. The extensive stability field of phengite may greatly enhance the role of sediments and the small amount of potassium in mafic compositions for the fluid budget in subduction zones at increasing depth.

Key Words phase diagram, experiment, peridotite, basalt, sediment s Abstract The subducted lithosphere is composed of a complex pattern of chemical systems that undergo continuous and discontinuous phase transformation, through pressure and temperature variations. Volatile recycling plays a major geodynamic role in triggering mass transfer, melting, and volcanism. Although buoyancy forces are controlled by modal amounts of the most abundant phases, usually volatile-free, petrogenesis and chemical differentiation are controlled by the occurrence of minor phases, most of them volatile-bearing. Devolatilization of the subducted lithosphere is a continuous process distributed over more than 300 km of the slab-mantle interface. Melting of the subducted crust, if any, along sufficiently hot P-T paths, is governed by fluid-absent reactions, even though the difference between fluid and melt vanishes at pressures above the second critical end point. The density distribution at a depth of 660 km suggests episodic penetration in space and time of subducted slabs into the lower mantle and sinking down to the D region at the core-mantle boundary.

The evolution of the martian core is widely assumed to mirror the characteristics observed for Earth's core. Data from experiments performed on iron-sulfur and iron-nickel-sulfur systems at pressures corresponding to the center of Mars indicate that its core is presently completely liquid and that it will not form an outwardly crystallizing iron-rich inner core, as does Earth. Instead, planetary cooling will lead to core crystallization following either a "snowing-core" model, whereby iron-rich solids nucleate in the outer portions of the core and sink toward the center, or a "sulfide inner-core" model, where an iron-sulfide phase crystallizes to form a solid inner core.