Results of high-pressure experiments on samples of hydrated mantle rocks show that the serpentine mineral antigorite is stable to approximately 720 degrees C at 2 gigapascals, to approximately 690 degrees C at 3 gigapascals, and to approximately 620 degrees C at 5 gigapascals. The breakdown of antigorite to forsterite plus enstatite under these conditions produces 13 percent H(2)O by weight to depths of 150 to 200 kilometers in subduction zones. This H(2)O is in an ideal position for ascent into the hotter, overlying mantle where it can cause partial melting in the source region for calc-alkaline magmas at a depth of 100 to 130 kilometers and a temperature of approximately 1300 degrees C. The breakdown of antigorite in hydrated mantle produces an order of magnitude more H(2)O than does the dehydration of altered oceanic crust.

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>0•3 GPa and cooling to >500°C, consistent with the pre-The Braccia gabbro of Val Malenco, Italian Alps, intruded 275 My servation of the olivine + tremolite + talc assemblage in ultramafic ago during Early Permian lithospheric extension. The intrusion rocks. Estimated uplift rates of 1-2 mm/year indicate a 15-30 My took place along the crust-mantle transition zone and caused exhumation related to Jurassic rifting. The two-stage retrograde granulite metamorphism of lower-crustal and upper-mantle rocks.path of the Malenco granulites separated by >50 My suggests that The magmatic crystallization of the gabbro was outlasted by ductile Permian extension and Jurassic rifting are two independent tectonic deformation, which is also observed in the other rocks of the processes. The presence of hydrous, Cl-rich minerals at 600 ± crust-mantle transition. Two stages of retrograde metamorphism 50°C and 0•8 ± 0•1 GPa requires input of externally derived followed. Mineral parageneses in garnet-kyanite gneiss, metagabbro, fluids at the base of 30 km thick continental crust into previously and metaperidotite record a first stage of near-isobaric cooling under dry granulites at the onset of Jurassic rifting. These fluids were anhydrous conditions. The stabilized crust-mantle transition then generated by dehydration of continental crust juxtaposed during persisted over a period of about 50 My into the Late Triassic.rifting with the hot, exhuming granulite complex along a active Exhumation of the crust-mantle complex began with the onset of shear zone. continental rifting during Early Jurassic. This stage of retrograde metamorphism is recorded by near-isothermal decompression and partial hydration of the granulitic mineral assemblages. The whole crust-to-mantle complex was then exposed in the Tethyan ocean KEY WORDS: subcontinental lithospheric mantle; granulite; continental near its Adriatic margin. The magmatic assemblage of the Braccia rifting; retrograde metamorphism; Malenco gabbro formed at 1-1•2 GPa and 1150-1250°C. Microstructures show that the gabbroic rocks evolved from olivine gabbros through spinel to garnet granulite whereas the peridotites recrystallized within

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The prograde, high pressure, transition from antigorite serpentinite to enstatite-olivine rock occurs along a tectonically undisturbed pro®le at Cerro del Almirez, SE Spain. The reactant assemblage is antigorite + olivine with tremolite rimming precursor diopside. The product assemblage of tremolite + chlorite + enstatite + olivine has a spinifex-like texture with arborescent or radiating olivine elongated parallel to [001] and with radially grown enstatite. Product enstatite is very poor in Al 2 O 3 . Due to numerous oriented submicroscopic inclusions of chromian magnetite, product olivine has a brownish pleochroism and a bulk chromium content similar to precursor antigorite. Titanian clinohumite with a ¯uorine content of 0.45±0.50 wt% persisted beyond the breakdown of antigorite. The partitioning of iron and magnesium amongst the silicate phases is almost identical to that at lower pressures. Average K d values Mn/Mg and Ni/Mg are 0.17 and 0.70 for antigorite-olivine pairs and 1.83 and 0.22 for orthopyroxene-olivine pairs, respectively. These data are useful in discriminating generations of olivine grown on each other. From the ®eld data a phase diagram topology for a portion of the system CaO-MgO-SiO 2 -H 2 O is derived. This topology forms the basis for extrapolations into inaccessible P-T regions.