Assimilation, contamination and felsic melt generation at the top of the axial melt lens at fast-spreading oceanic crust: Combined geochemical and experimental investigations at the dike/gabbro transition from IODP Site 1256 (East Pacific Rise)

大洋中脊大洋岩石圈的形成和演化是地球化学分异和物理演化的主导过程之一。约60.000公里长的大洋中脊山脉与地球90%的S火山活动和约25%的地球热释放有关。尽管如此重要，但关于岩浆吸积方式、下地壳如何热液冷却或地震层2-3边界的地质意义等重大问题仍然知之甚少。为了解决与快速扩张的大洋中脊系统的地球动力学有关的关键问题，在1256号站点发起了IODP钻探运动“超快扩张地壳”(206段，第309、312、335次探险)。1256号站点位于东太平洋隆起形成的1500万年前的地壳上，当时发生了一次超快的海洋扩张(&gt；200毫米/年)。这项运动的主要成果之一是，在IODP第一次钻取了完整的地壳，从熔岩穿过片状岩脉进入最上层的辉长岩。最下面的席状岩脉以及1256钻芯的辉长岩段的一个特征是富含长英质岩石，这些岩石在成分、侵位方式以及与各种背景岩的关系上表现出显著的差异。它们为理解快速扩张的大洋中脊系统轴向熔融透镜顶部的岩浆、变质和热液过程之间的复杂而密切的耦合提供了关键。为了了解从MORB分异到先前热液蚀变物质同化-部分熔融的岩浆过程谱内演化熔体形成的不同机制及其在MORB污染中的作用，我们试图开始对这些岩石的地球化学和岩石学进行系统的研究。为此，将对块状岩石和矿物相应用主量元素和微量元素地球化学方法。这一结果将使我们能够解开所涉及的主要岩浆端元过程，以及它们作为原生MORB的污染者的作用。在项目的第二部分，我们计划应用实验技术--使用在1256号场地恢复的自然系统进行的结晶和部分熔融实验--以测试、确认或修改根据我们的地球化学和岩石学结果得出的模型。这两种方法的结合将提供相当大的洞察力，了解在快速扩张的大洋中脊系统的轴向熔融透镜顶部正在进行的复杂的干扰岩浆、变质和热液过程。到目前为止，关于这一点的细节还知之甚少，IODP钻探的1256号站位的岩脉-辉长岩过渡的特殊岩石为我们提供了提高认识的关键。

The formation and evolution of oceanic lithosphere at mid-ocean ridges is one of the dominant processes in the chemical differentiation and physical evolution of our planet. The about 60.000 km long mountain chain of mid-ocean ridges is associated with 90% of Earth´s volcanism and about 25 % of heat release of the Earth. Although so important, major issues on the mode of magmatic accretion, the way how the lower crust is hydrothermal cooled, or the geological meaning of the seismic Layer 2-3 boundary are still poorly understood. To solve critical questions related to the geodynamics of fast-spreading mid-ocean ridge systems, the IODP drilling campaign "Superfast Spreading Crust" (Leg 206, Expeditions 309, 312, 335) at Site 1256 was initiated. Site 1256 is located on 15 million year old crust formed at the East Pacific Rise during an episode of superfast ocean spreading (>200 mm/yr full rate). It is one of the major achievements of this campaign that, for the first time in IODP, a complete section through intact crust from lavas via sheeted dikes into the uppermost gabbros was drilled. One characteristic of the lowermost sheeted dikes as well as in the gabbroic section of the 1256 drill core, is the abundance of felsic rocks which show significant differences in compositions, modes of emplacement, and relations to the various background rocks. They provide the key for understanding the complex and intimate coupling between magmatic, metamorphic and hydrothermal processes at the top of the axial melt lens in fast-spreading mid-ocean ridge systems. In order to understand the different mechanisms of evolved melt formation within a spectrum of magmatic processes from MORB differentiation to assimilation-partial melting of previously hydrothermally altered material and its role in contaminating MORB, we attempt to start a systematic investigation on the geochemistry and petrology of these rocks. For this, major and trace element geochemical methods will be applied to bulk rocks as well as to mineral phases. The results will enable us to unravel the principal magmatic endmember processes involved, and their role as contaminator for primary MORB. In a second part of the project we plan to apply experimental techniques - crystallization and partial melting experiments using natural systems recovered at Site 1256 - in order to test, confirm, or modify those models derived by our geochemical and petrological results. The combination of both approaches will offer considerable insight into the complex interfering magmatic, metamorphic, and hydrothermal processes ongoing at the top of axial melt lens in fast-spreading mid-ocean ridge systems. Details on this are poorly known up to now, and the exceptional rocks of the dike-gabbro transition at Site1256 drilled by IODP provide the key for improving our understanding.