Serpentinized peridotites outcrop over large domains in modern oceans (Cannat et al., 1995) and can contain 10-13% H2O by weight, which is 6 to 10 times the water content of hydrothermally altered mafic oceanic crust. Water is fixed in hydrous phases such as talc, chlorite and more diffusely serpentine, which may survive to considerable depths during subduction of cold oceanic lithosphere (Ulmer and Trommsdorff, 1995; Scambelluri et al., 1995). These features candidate serpentinites as the most effective
transporters of water into the mantle in cold subduction
environments. At some stage of burial, serpentinites may partially dehydrate originating metamorphic fluids. In such cases the textural and compositional relations between seafloor assemblages and high-pressure minerals + fluids are critical to assess the processes of fluid release and the origin of subduction-zone fluids. These aspects have been investigated in the Erro-Tobbio peridotite (Italian Western Alps), a slice of subcontinental mantle that was early exposed to the seafloor of the Mesozoic Ligurian-Piedmontese Tethys and was later involved in Alpine subduction-zone metamorphism. It records a stage of seafloor hydration followed by partial dewatering and veining at eclogite-facies (25 Kbar and 550-600oC) (Scambelluri et al., 1995). Our study shows that the composition of fluids released was controlled by the host ultramafic rocks, and demonstrates a link between the breakdown of seafloor mineral assemblages and the genesis of the eclogite-facies fluid.
Early Cl-bearing serpentine and brucite, and Cl-bearing K-rich phyllosilicate overprint the mantle mineral
assemblage and derive from interaction with seawater during exposure to the ocean floor. Such phases occur as relics within the high-pressure assemblage (olivine + titanian clinohumite + chlorite + diopside + magnetite + antigorite) developed during eclogitization of the hydrous peridotite.
At this stage, partial peridotite dehydration causes fluid mobilization and mineral deposition within diffuse vein systems. Host rocks and veins display the same mineralogy, although the modal proportions of the vein filling minerals are variable from place to place. Vein diopside analyzed by ion probe (CSCC, university of Pavia, Italy) display LREE depleted patterns, and M to HREE absolute concentrations up to 10-20 x C1. Such compositions are quite similar to those of clinopiroxene forming the host ultramafic rocks. All these features suggest a local scale derivation of the vein filling material and a strong control of the host rocks on the solute content of the vein fluid. The high pressure phases are devoid of chlorine and alkalies characterizing the seafloor assemblage, and are coeval with primary fluid inclusions trapped in the vein minerals. The inclusions are multiphase (liquid + vapour + salt phase + ilmenite and magnetite) and display chlorine-rich alkaline compositions with up to 50% by weight Cl, Na, K, Mg and Fe. High chlorinity of the vein fluid is a strong evidence that devolatilization fluids did not escape the system, but were internally recycled and interacted with rocks to produce the high-pressure hydrous minerals.
Our results demonstrate that the Erro-Tobbio serpentinized peridotite internally produced and buffered an high-pressure aqueous fluid that became extremely enriched in solute. The fluid was produced during breakdown of early hydrous seafloor assemblages containing chlorine and potassium. Partitioning of chlorine and alkalies into the fluid allowed inheritance by the eclogite-facies brine of the hydrothermal signature acquired during seafloor alteration of the peridotite precursor. These features underscore eclogite-facies recycling of seawater from hydrous ultramafic systems, which therefore behave as large-scale carriers of remarkable amounts of seawater into the mantle. Antigorite stability during eclogite-facies metamorphism and partial dewatering of hydrous ultramafic rocks, allows survival of serpentinite residua after production of the observed high-pressure brine. In the Alps only a small part of subducted serpentinite and ophiolites were exhumed to the surface. This suggests that rheologically weak hydrous domains
can be present in the root zone of orogens deriving from
subduction of cold oceanic lithosphere.
Cannat, M., Mevel, C., Mala, M., Deplus, C., Durand, C., Gente, P., Agrinier, P., Belarouchy, A., Dubuisson, G., Humler, E. & Reynolds, J. Geology 23, 49-52 (1995).
Scambelluri, M., Muentener, O., Hermann, J., Piccardo, G.B. & Trommsdorff, V., Geology 23, 459-462 (1995).
Ulmer, P. & Trommsdorff, V., Science 268, 858-861 (1995).