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%的海洋地壳，也就是- 30%的地球表面，是在洋中脊以快速到超高速的扩张速率（全速速度约80毫米/年）形成的。Cocos板块（中太平洋东部）的IODP 1256站点代表了迄今为止钻探的最完整的在超快速扩张中心形成的海洋地壳原位剖面。IODP探险队309和312的科学参与者目前正在调查从更深的部分（低于750兆立方英尺）回收的物质，主要是侵入性的。然而，迄今为止，相对较少的地球化学数据存在于上部（在OOP Leg 206期间钻探），由-250 m的海洋沉积物和-500 m的熔岩流组成。上部750米占1256站点总钻井深度的- 50%。本课程组在项目中对Exp. 309/312样品进行了Sr、Nd、Hf、Pb同位素（DS）的研究，包括低温熔岩岩脉蚀变带和岩脉-辉长岩过渡带。为了完整、详细地记录1256站点整个海洋地壳的Sr-Nd-Hf-Pb （DS）同位素及相关微量元素的变化，我们拟对206站点沉积物和熔岩的井下地球化学变化进行研究。这些数据将为以下问题提供重要的新见解：1)超快速扩张脊（东太平洋隆起）岩浆成因；2)随后蚀变的影响和时间，包括以超快扩张速度形成的海洋地壳中的元素通量（与较慢的扩张速度相比）；3)海水化学演化；4)俯冲带的元素旋回；5)洋壳对地幔柱源的贡献。拟议的项目是我们目前研究的重要延伸，并很好地补充了国际Site 1256 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.