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)

快速扩张的洋壳轴向熔体透镜体顶部的同化、污染和长英质熔体生成：对 IODP 站点 1256（东太平洋海隆）的岩脉/辉长岩转变进行地球化学和实验联合研究

• 批准号: 224563012
• 负责人: Professor Dr. Jürgen Koepke
• 金额: --
• 依托单位: Institut für Mineralogie
• 依托单位国家: 德国
• 项目类别: Infrastructure Priority Programmes
• 财政年份: 2012
• 资助国家: 德国
• 起止时间: 2011-12-31 至 2015-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/224563012?language=en
• 关键词: Assimilation; contamination; felsic; melt; generation

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.

大洋中脊海洋岩石圈的形成和演化是地球化学分异和物理演化的主要过程之一，大约 60,000 公里长的大洋中脊山脉与地球 90% 的面积有关。火山活动和地球约 25% 的热量释放虽然如此重要，但主要问题在于岩浆增生的模式、下地壳如何被热液冷却的方式或地壳的热液冷却方式。地震层 2-3 边界的地质意义仍然知之甚少，为了解决与快速扩张的洋中脊系统的地球动力学相关的关键问题，IODP 钻探活动“超快速扩张地壳”（第 206 段，探险 309、312）。 , 335) 于 1256 号站点启动 1256 号站点位于东太平洋形成的 1500 万年前的地壳上。超高速海洋扩张期间的上升（> 200 毫米/年全速率） 这次活动的主要成就之一是，在 IODP 中首次完整地从熔岩通过片状堤坝穿过完整的地壳。最上面的片状岩脉以及 1256 钻芯的辉长岩部分的一个特征是长英质岩石的丰度，这些岩石在成分和形态上表现出显着的差异。它们为理解快速扩张的洋中脊系统中轴向熔融透镜体顶部的岩浆、变质和热液过程之间复杂而密切的耦合提供了关键。为了了解从 MORB 分化到同化 - 先前热液物质的部分熔融的一系列岩浆过程中演化熔体形成的不同机制及其在污染 MORB 中的作用，我们尝试开始对 MORB 的系统研究为此，主量元素和微量元素地球化学方法将应用于块状岩石以及矿物相，其结果将使我们能够揭示所涉及的主要岩浆端元过程及其作为原生污染物的作用。在该项目的第二部分中，我们计划应用实验技术——使用在 1256 号站点恢复的自然系统进行结晶和部分熔融实验——以测试、确认或修改我们的地球化学和物理模型得出的模型。两种方法的结合将提供对快速扩张的洋中脊系统中轴向熔融透镜体顶部发生的复杂干扰岩浆、变质和热液过程的深入了解。 ，以及 IODP 钻探的 Site1256 岩脉-辉长岩过渡区的特殊岩石，为我们加深了解提供了关键。

项目成果

Apatite in the dike-gabbro transition zone of mid-ocean ridge: Evidence for brine assimilation by axial melt lens

• DOI: 10.2138/am-2017-5906
• 发表时间: 2017-03
• 期刊: American Mineralogist
• 影响因子: 3.1
• 作者: Chao Zhang;J. Koepke;M. Albrecht;I. Horn;F. Holtz

Trace elements in anatectic products at the roof of mid-ocean ridge magma chambers: An experimental study

• DOI: 10.1016/j.chemgeo.2017.03.004
• 发表时间: 2017-05
• 期刊: Chemical Geology
• 影响因子: 3.9
• 作者: M. Erdmann;L. Fischer;E. Deloule;J. Koepke

Felsic Plutonic Rocks from IODP Hole 1256D, Eastern Pacific: Implications for the Nature of the Axial Melt Lens at Fast-Spreading Mid-Ocean Ridges

• DOI: 10.1093/petrology/egx064
• 发表时间: 2017-08
• 期刊: Journal of Petrology
• 影响因子: 3.9
• 作者: Chao Zhang;J. Koepke;L. France;M. Godard

A Practical Method for Accurate Measurement of Trace Level Fluorine in Mg‐ and Fe‐Bearing Minerals and Glasses Using Electron Probe Microanalysis

• DOI: 10.1111/j.1751-908x.2015.00390.x
• 发表时间: 2016-09
• 期刊: Geostandards and Geoanalytical Research
• 影响因子: 3.8
• 作者: C. Zhang;J. Koepke;Lian-Xun Wang;Paul Eric Wolff;S. Wilke;A. Stechern;R. Almeev;F. Holtz