According to the theory of plate tectonics, rocks found in the vicinity of mid-ocean ridges - where oceanic plates are created - should be relatively 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 exposed gabbros beneath the Mid-Atlantic Ridge near the Kane fracture zone (Cannat and ODP Shipboard Scientific Party, 1995; Werner, 1997, doi:10.2973/odp.proc.sr.153.031.1997; Werner and Pilot, 1997, doi:10.2973/odp.proc.sr.153.018.1997; Pilot et al., 1996). Age determinations were made using the 207Pb/206Pb evaporation method (Kober, 1987, doi:10.1007/BF00375526) and confirmed with conventional U–Pb dating and ion microprobe (SHRIMP) analysis. We suggest two plausible explanations for the origin of these unusually old zircons. During the opening of the Atlantic, sheared crustal material or delaminated continental lithosphere sank into small roll-like circulation cells (Nocolas, 1995; Rabinowicz et al., 1984, doi:10.1016/0012-821X(84)90042-6) that developed in the shallow mantle at each side of the ridge axis and the material was then transported through these cells to the ridge axis. Alternatively, material from the continental crust has been trapped within the Kane fracture zone since the opening of the Atlantic Ocean basin through a series of transform migrations and ridge jumps (Bonatti and Crane, 1982, doi:10.1038/300343a0; Kepezhinskas and Dmitiev, 1992), with portions of this material subsequently migrating down the ridge axis.

Supplement to: Pilot, Joachim; Werner, Carl-Dietrich; Haubrich, Frank; Baumann, Nils (1998): Palaeozoic and Proterozoic zircons from the Mid-Atlantic Ridge. Nature, 393(6686), 676-679