PALEOZOIC AND PROTEROZOIC ZIRCONS FROM THE MID-ATLANTIC RIDGE

According to the theory of plate tectonics, rocks found in the vicinit y of mid-ocean ridges-where oceanic plates are created-should be relat ively young (at most several Myr old). Here we report the discovery of zircons with ages of about 330 and 1,600 Myr that were drilled from e xposed gabbros beneath the Mid-Atlantic Ridge near the Kane fracture z one(1-4). Age determinations were made using the Pb-207/Pb-206 evapora tion method(5) and confirmed with conventional U-pb dating and ion mic roprobe (SHRIMP) analysis. Fire suggest two plausible explanations for the origin of these unusually old zircons. During the opening of the Atlantic, sheared crustal material or delaminated continental lithosph ere sank into small roll-like circulation cells(6,7) that developed in the shallow mantle at each side of the ridge axis and the material wa s then transported through these cells to the ridge axis. Alternativel y, material from the continental crust has been trapped within the Kan e fracture zone since the opening of the Atlantic Ocean basin through a series of transform migrations and ridge jumps(8,9), with portions o f this material subsequently migrating down the ridge axis.