ALBERT

Description: Hole 651 A, drilled during ODP leg 107 in the Tyrrhenian Sea, bottomed in a 30-m-thick section of serpentinized peridotite. Site 651 was drilled on the eastern flank of a north-south trending basement high located along the axis of the Vavilov basin. Peridotite is overlain by a 136-m-thick basement section consisting of two distinct basalt units separated by a dolerite-albitite intrusive unit. Peridotite has a tectonitic texture and probably derived from the upper mantle; it was affected by a complex history of ductile and brittle deformation. High temperature (〉700°C) hydrous metasomatism, which probably occurred at upper mantle levels and produced tschermakitic and Mg-hornblende, tremolite, and possibly chlorite, affected the peridotite. Lower temperature metamorphic events followed, which gave rise to talcquartz ± chlorite assemblages and to serpentinization. A relict primary assemblage consists of olivine, variable amounts of orthopyroxene, and spinel, implying that the rocks were originally harzburgite and dunite. Both bulk rock and relict primary mineral chemistry suggest that the peridotite is highly depleted, and chemically distinct from mantlederived peridotites from circum-Tyrrhenian ophiolites (North Apennine, Corsica), from passive margins and from the ocean basins. ODP Site 651 peridotite appears to have affinity with modern (Mariana, Tonga, and Puerto Rico Trenches) and ophiolitic (Troodos, Vourinos) arc-subduction related peridotites. It may represent a refractory upper mantle sliver which was affected by island arc/subduction systems inferred to have migrated southeast from Sardinia during the opening of the Tyrrhenian Sea.