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We report the results of experiments on two natural marine sediments with different carbonate contents (calcareous clay: CO2 = 6·1 wt %; marl: CO2 = 16·2 wt %) at subduction-zone conditions (3 GPa, 750-1200°C). Water (7-15 wt %) was added to the starting materials to simulate the effects of external water addition from within the subducting slab. The onset of melting is at 760°C in water-rich experiments; melt becomes abundant by 800°C. In contrast, the onset of melting in published, water-poor experiments occurs at variable temperatures with the production of significant melt fractions being restricted to more than 900°C (phengite-out). The different solidus temperatures (Tsolidus) can be ascribed to variable fluid XH2O [H2O/(CO2 + H2O)], which, in turn, depends on bulk K2O, H2O and CO2. Partial melts in equilibrium with residual garnet, carbonate, quartz/coesite, epidote, rutile, kyanite, phengite, and clinopyroxene are granitic in composition, with substantial dissolved volatiles. Supersolidus runs always contain both silicate melt and solute-rich fluid, indicating that experimental conditions lie below the second critical endpoint in the granite-H2O-CO2 system. Carbonatite melt coexists with silicate melt and solute-rich fluid above 1100°C in the marl. The persistence of carbonate to high temperature, in equilibrium with CO2-rich hydrous melts, provides a mechanism to both supply CO2 to arc magmas and recycle carbon into the deep Earth. The trace element compositions of the experimental glasses constrain the potential contribution of calcareous sediment to arc magmas. The presence of residual epidote and carbonate confers different trace element characteristics when compared with the trace element signal of Ca-poor marine sediments (e.g. pelagic clays). Notably, epidote retains Th and light rare earth elements, such that some melts derived from calcareous sediments have elevated Ba/Th and U/Th, and low La/SmPUM, thereby resembling fluids conventionally ascribed

Author Posting. © Elsevier B.V., 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Chemical Geology 234 (2006): 179-210, doi:10.1016/j.chemgeo.2006.04.011., Abyssal peridotite from the 15°20’N area of the Mid-Atlantic Ridge show complex geochemical variations among the different sites drilled during ODP Leg 209. Major element compositions indicate variable degrees of melt depletion and refertilization as well as local hydrothermal metasomatism. Strongest evidence for melt-rock interactions are correlated Light Rare Earth Element (LREE) and High Field Strength Element (HFSE) additions at sites 1270 and 1271. In contrast, hydrothermal alteration at Sites 1274, 1272, and 1268 causes LREE mobility associated with minor HFSE variability, reflecting the low solubility of HFSE in aqueous solutions. Site 1274 contains the least-altered, highly refractory, peridotite with strong depletion in LREE and shows a gradual increase in the intensity of isochemical serpentinization; except for the addition of H2O which causes a mass gain of up to 20 g/100 g. The formation of magnetite is reflected in decreasing Fe2+/Fe3+ ratios. This style of alteration is referred to as rock-dominated serpentinization. In contrast, fluid-dominated serpentinization at Site 1268 is characterized by gains in sulfur and development of U-shaped REE pattern with strong positive Eu anomalies which are also characteristic for hot (350 to 400°C) vent-type fluids discharging from black smoker fields. Serpentinites at Site 1268 were overprinted by talc alteration under static conditions due to interaction with high aSiO2 fluids causing the development of smooth, LREE-enriched patterns with pronounced negative Eu anomalies. These results show that hydrothermal fluid-peridotite and fluid-serpentinite interaction processes are an important factor regarding the budget of exchange processes between the lithosphere and the hydrosphere in slow spreading environments., ODP is sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI), Inc.

We report the results of experiments on two natural marine sediments with different carbonate contents (calcareous clay: CO2 = 6·1 wt %; marl: CO2 = 16·2 wt %) at subduction-zone conditions (3 GPa, 750-1200°C). Water (7-15 wt %) was added to the starting materials to simulate the effects of external water addition from within the subducting slab. The onset of melting is at 760°C in water-rich experiments; melt becomes abundant by 800°C. In contrast, the onset of melting in published, water-poor experiments occurs at variable temperatures with the production of significant melt fractions being restricted to more than 900°C (phengite-out). The different solidus temperatures (Tsolidus) can be ascribed to variable fluid XH2O [H2O/(CO2 + H2O)], which, in turn, depends on bulk K2O, H2O and CO2. Partial melts in equilibrium with residual garnet, carbonate, quartz/coesite, epidote, rutile, kyanite, phengite, and clinopyroxene are granitic in composition, with substantial dissolved volatiles. Supersolidus runs always contain both silicate melt and solute-rich fluid, indicating that experimental conditions lie below the second critical endpoint in the granite-H2O-CO2 system. Carbonatite melt coexists with silicate melt and solute-rich fluid above 1100°C in the marl. The persistence of carbonate to high temperature, in equilibrium with CO2-rich hydrous melts, provides a mechanism to both supply CO2 to arc magmas and recycle carbon into the deep Earth. The trace element compositions of the experimental glasses constrain the potential contribution of calcareous sediment to arc magmas. The presence of residual epidote and carbonate confers different trace element characteristics when compared with the trace element signal of Ca-poor marine sediments (e.g. pelagic clays). Notably, epidote retains Th and light rare earth elements, such that some melts derived from calcareous sediments have elevated Ba/Th and U/Th, and low La/SmPUM, thereby resembling fluids conventionally ascribed