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.

海洋岩石圈在中山山脊上的形成和演变是我们行星化学分化和物理进化的主要过程之一。大约60.000公里长的山脊长山链与90％的地球火山和地球释放的25％有关。尽管如此重要，但岩浆积聚方式的主要问题，较低地壳的冷却方式或地震层2-3边界的地质意义的方式仍然很少了解。为了解决与快速扩张中山山脊系统的地球动力学有关的关键问题，IODP钻探活动“超快速扩散地壳”（Leg 206，Expeditions，309，312，335）在Site 1256发起。站点1256位于1500万年历史的地壳上，在超快海洋传播的一集（> 200 mm/yr全率）中形成。这是这项运动的主要成就之一，在IODP中，第一次通过熔岩的完整地壳通过板堤钻到最高的Gabbros，这是一个完整的部分。最低板堤的一个特征以及1256钻芯的Gabbroic部分的特征是丰富的长英质岩石，它们在组成，增强模式以及与各种背景岩石的关系上显示出显着差异。它们提供了理解在快速膨胀的中洋山脊系统中轴向熔体镜头顶部的岩浆，变质和水热过程之间的复杂和亲密耦合的关键。为了理解岩浆过程中进化熔体形成的不同机制，从MORB分化到同化以前的水热改变材料的同化 - 融化及其在污染Morb中的作用，我们试图开始对这些岩石的地球化学和岩石的系统投资。为此，主要和痕量元素地球化学方法将应用于块状岩石以及矿物相。结果将使我们能够揭开所涉及的主要岩浆末端过程，以及它们作为原发性医疗的污染物的作用。在项目的第二部分中，我们计划采用实验技术 - 结晶和部分熔化实验，使用现场1256中回收的天然系统进行测试，确认或修改我们的地球化学和质学结果所得出的那些模型。两种方法的结合都将提供对复杂干扰岩浆，变质和水热过程的考虑，并在快速播放的中洋山脊系统中轴向熔体镜头的顶部正在进行。有关此的详细信息现在已经众所周知，IODP钻探的Site1256的Dike-Gabbro过渡的非凡岩石为改善我们的理解提供了关键。

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