(Table 1) Lead isotope ratios of zircons from sample 122 of ODP Hole 153-922A@en

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.根据板块构造理论（plate tectonics），洋中脊（mid-ocean ridges，大洋板块的形成区域）附近所发现的岩石应当相对年轻，最多仅数百万年（Myr）历史。本研究报道了在凯恩断裂带（Kane fracture zone）附近大西洋中脊（Mid-Atlantic Ridge）下方出露辉长岩（gabbros）中钻探获取的锆石（zircons）的发现，这些锆石的年龄分别约为330百万年和1600百万年（坎纳特与大洋钻探计划（Ocean Drilling Program, ODP）船上科学考察队，1995；沃纳，1997，doi:10.2973/odp.proc.sr.153.031.1997；沃纳与派洛特，1997，doi:10.2973/odp.proc.sr.153.018.1997；派洛特等人，1996）。年龄测定采用207Pb/206Pb蒸发法（207Pb/206Pb evaporation method，科伯，1987，doi:10.1007/BF00375526）完成，并通过传统铀-铅定年法（U–Pb dating）与离子微探针（ion microprobe，高灵敏度离子微探针Sensitive High Resolution Ion Microprobe, SHRIMP）分析进行了验证。针对这些异常古老的锆石的成因，我们提出两种合理的解释：其一，在大西洋张开过程中，剪切地壳物质（sheared crustal material）或拆沉大陆岩石圈（delaminated continental lithosphere）会沉降至洋脊轴两侧的浅部地幔（shallow mantle）中形成的小型滚状环流单元（roll-like circulation cells）内，随后这些物质通过该环流单元被搬运至洋脊轴部；其二，自大西洋盆地张开以来，大陆地壳物质便通过一系列转换断层迁移（transform migrations）与洋脊跃迁（ridge jumps）过程被捕获于凯恩断裂带中，其中部分物质随后沿洋脊轴部迁移（博纳蒂与克莱恩，1982，doi:10.1038/300343a0；凯佩日斯卡斯与德米季耶夫，1992）。