High-precision Pb (double spike) and Sr-Nd-Hf-isotopoc record of upper oceanik crust at Leg 206 (Site 1256, Eastern Central Pacific)

现今约50%的洋壳，也就是约30%的地球表面，是在洋中脊以快速到超快速的扩张速度（> 80毫米/年）形成的。位于科科斯板块（中太平洋东部）的IODP 1256号站点代表了迄今为止钻探的最完整的海洋地壳原位剖面，该剖面形成于超快速扩张中心。IODP第309和312号考察队的科学参与者目前正在调查从更深的，主要是侵入性的，洞的一部分（低于750 mbsf）回收的物质。然而，到目前为止，上部（在OOP第206航次期间钻探）的地球化学数据相对较少，其中包括-250 m的海洋沉积物和-500 m的熔岩流。上部750 m代表1256号现场钻探总深度的约50%。在一个正在进行的项目过程中，我们的小组正在研究的Sr，Nd，Hf和Pb（双穗法，DS）同位素变化的实验。309/312材料，包括低温至高温熔岩岩脉蚀变和岩脉辉长岩过渡带。在这里，我们建议检查的206腿沉积物和熔岩的井下地球化学变化，以产生一个完整的和详细的记录的Sr-Nd-Hf-Pb（DS）同位素和相关的微量元素的变化，整个大洋地壳在站点1256。这些数据将为以下方面提供重要的新见解：1）超快速扩张脊（东太平洋隆起）的岩浆成因; 2）后续蚀变的影响和时间，包括以超快速扩张速率（与较慢速率相比）形成的洋壳中的元素通量; 3）海水的化学演化; 4）俯冲带中的元素循环; 5）洋壳对地幔柱来源的贡献。拟议的项目是我们目前研究的一个重要延伸，并很好地补充了国际1256站点IODP科学界的研究活动。

About 50 % of the present-day ocean crust and thus -30 % of the Earth's surface was formed at mid-ocean ridges at fast to superfast spreading rates (> 80 mm/y full rate). IODP Site 1256 in the Cocos Plate (Eastern Central Pacific) represents the most complete in situ section of ocean crust formed at a superfast spreading center drilled thus far. Scientific participants of IODP Expeditions 309 and 312 are currently investigating the recovered material from the deeper, mainly intrusive, part of the hole (below 750 mbsf). Relatively few geochemical data, however, exist thus far for the upper part (drilled during OOP Leg 206) consisting of -250 m of marine sediments and -500 m of lava flows. The upper 750 m represent -50 % of the total depth drilled at Site 1256. In the course of an ongoing project, our group is investigating the Sr, Nd, Hf and Pb (with the double spike method, DS) isotopic variation in the Exp. 309/312 material, including the low- to high-temperature lava-dike alteration and the dike-gabbro transition zones. Here we propose to examine the down-hole geochemical variation of Leg 206 sediments and lavas, in order to generate a complete and detailed record of the Sr-Nd-Hf-Pb (DS) isotopic and relevant trace element variations of the entire oceanic crust drilled at Site 1256. These data will provide important new insights into 1) magma genesis at superfast spreading ridges (East Pacific Rise); 2) the effects and timing of subsequent alteration including element fluxes in oceanic crust formed at superfast spreading rates (compared to slower rates); 3) the chemical evolution of seawater; 4) element cycles in subduction zones, and 5) the contribution of ocean crust to mantle plume sources. The proposed project is an important extension of our current studies and complements well the research activities within the international Site 1256 IODP scientific community.