Oxygen and carbon isotope ratios of silicates and carbonates from exhumed peridotites in the Iberian Abyssal Plain

Legs 173 and 149 of the Ocean Drilling Program profiled a zone of exhumed mantle peridotite at the ocean-continent transition (OCT) beneath the Iberia Abyssal Plain. The zone of exhumed peridotite appears to be tens of kilometers wide and is situated between blocks of continental crust and the first products of ocean accretion. Exhumed peridotite is 95-100% serpentinised to probable depths of 2-3 km. Down core oxygen isotope profiles of serpentinised peridotite at Sites 1068 and 1070 (Leg 173) show evidence for two fluid infiltration events. The earlier event involved pervasive infiltration of comparatively warm (>175°C) sea water and accompanied serpentinisation. The later event involved structurally focused infiltration of comparatively cool (650-150°C) sea water and accompanied active mantle exhumation. We therefore conclude that the uppermost mantle was serpentinised before it was exhumed at the Iberian OCT. Implicit to this conclusion is that a sizeable region of serpentinised mantle existed directly beneath thinned but intact continental crust. Serpentinite has comparatively low density, low frictional strength and low permeability. The presence of such a "soft" layer may have localised deformation and consequently promoted detachment-style exhumation of the uppermost mantle. The low permeability of a serpentinite 'cap' layer might help to explain the lack of observed melt at the Iberian OCT.

(Table 1) Chemical composition of Fe-rich layered silicates from the East Korean Rise, Japan Sea

Mineral and chemical compositions of highly ferruginous layered silicates (HLS) of glauconite sands occurred on the East Korean Rise outside volcanic structures and on an unnamed volcano and the Chentsov Volcano have been studied. The use of cluster and discriminant analyses has resulted to more objectively distinguished groups among HLS, and the use of factor analysis - to illustrate correlations between chemical elements in different groups. It has been found that green mineral assemblages of the East Korean Rise are heterogeneous in terms of morphology, composition and origin, and their formation is a complex multistage process including both neoformation and degradation.

Chemical composition of layer silicates from the Sado Ridge, Sea of Japan

Detailed mineralogical investigations of high-Fe layer silicates from loose sediments (glauconite sands) of the Sado Ridge revealed that green aggregates found on submarine rises of the Japan Sea floor have different genesis. It was demonstrated that round dark green grains approximate micas in composition. Primary volcanic rocks presumably have undergone extensive secondary alterations and then were disintegrated. Their disintegration products (protoceladonite) filling pores were redeposited and buried in sediments for a long time. Angular green grains mainly represented by smectite also formed at lower temperatures during disintegration of altered volcanosedimentary rocks. These younger grains had no prolonged exposure. Pseudomorphs of siliceous microplankton consist of both hydromica and smectites. They are presumably authigenic products formed with participation of microorganisms or electrostatic processes (spherical shape), or their combination. The formation mechanism of minerals filling cavities in pyroclastics is not entirely clear.