The Gabbro/Dike transition in oceanic crust generated at slow- and ultraslow-spreading ocean ridge systems

慢速和超慢速扩张的洋脊系统产生的洋壳中的辉长岩/岩脉转变

• 批准号: 227716517
• 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: Research Grants
• 财政年份: 2012
• 资助国家: 德国
• 起止时间: 2011-12-31 至 2014-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/227716517?language=en
• 关键词: Gabbro; Dike; transition; oceanic; crust

The gabbro/dike transition of the oceanic crust is one of the most important interfaces of our planet, where magmatic, metamorphic and hydrothermal processes interfere. At fast-spreading ridges, where the crust is generally accepted to be layered and uniform, the gabbro/dike transition is characterized by a decametre-thick horizon made of hornfels formed under metamorphic conditions up to the granulite facies. This corresponds to the conductive boundary layer (CBL), a thin (<100 m), hot (>650 °C), impermeable zone predicted by theoretical models, which is sandwiched between the axial magma chamber and the sheeted dikes and across which the heat of the axial magma chamber is exchanged. In contrast, crust formed at slow- and ultraslow-spreading ridges is highly variable, ranging from emplacement of the mantle directly to the seafloor over broad regions, to local highly magmatic spreading that forms layered crust broadly similar to that from fast-spreading ridges. Due to the lack of systematic investigations, the nature of the gabbro/dike transition of slow- and ultraslow-spreading ridge systems is only poorly constrained. It is the aim of the proposed project, to perform in collaboration with Henry Dick from the Woods Hole Oceanographic Institution (WHOI, Massachusetts, US) a systematic review of the extensive collections of dredges and dive suites of the WHOI containing rocks from the sheeted dike rooting zone from slow- and ultraslow-spreading ridges. These will be characterized in terms of igneous and metamorphic assemblage, deformation textures, veining, and contact relationships first in hand specimen. For the subsequent work samples will be selected representative for different tectonic/magmatic environments (tectonic emplacement mode vs. high magmatic productivity). This suite will then be investigated petrographically and with the electron microprobe, in order to unravel the petrographic record of primary magmatic, late magmatic, secondary high-temperature (i.e. granoblastic recrystallization), low-temperature hydrothermal, and tectonic processes. Special focus will be to discover the presence of the characteristic granulite-facies granoblastic overprint in the corresponding dike basalts - the key for the development of a CBL. The database to be obtained will then be compared with that reported from fast-spreading systems to make a generalized comparison of the gabbro/dike interface, and its implications for crustal accretion in these very different spreading environments. The working hypothesis is that for those segments of slow-spreading ridges with stronger magmatic activity it is expected that the gabbro/dike dynamics are similar to fast-spread, EPR-like crust, with a well-defined CBL, while for those segments where crustal accretion is controlled by tectonic processes, it is expected that relative simple cutting relations are developed (basalt cuts gabbro).

洋壳的辉长岩/岩脉过渡是地球上最重要的界面之一，岩浆、变质和热液过程在此发生干扰。在快速扩张的山脊处，通常认为地壳是分层且均匀的，辉长岩/岩脉在此发生。岩脉过渡的特征是由变质条件下形成的角岩构成的十米厚的层位，直至麻粒岩相，这对应于导电边界层（CBL）。理论模型预测的薄（<100 m）、热（>650 °C）、不可渗透区域，夹在轴向岩浆房和片状岩脉之间，轴向岩浆房的热量通过该区域进行交换。在慢速和超慢速扩张的海脊处形成的地壳变化很大，从地幔直接就位到大片区域的海底，到局部强磁力扩张形成与地壳大致相似的层状地壳。由于缺乏系统的研究，慢速和超慢速扩张的山脊系统的辉长岩/岩脉过渡的性质受到了很少的限制，因此该项目的目的是与合作进行。伍兹霍尔海洋研究所（美国马萨诸塞州 WHOI）的亨利·迪克（Henry Dick）对 WHOI 的大量挖泥船和潜水套件进行了系统回顾，其中包含来自缓慢和缓慢的片状堤坝生根区的岩石。这些将首先在手标本中根据火成岩和变质岩组合、变形纹理、脉纹和接触关系来表征，随后将选择代表不同构造/岩浆环境的样本（构造就位模式与成位模式）。然后将对这套套件进行岩相学研究并使用电子显微探针，以揭示原始岩浆、晚期岩浆的岩相记录。岩浆、次生高温（即花岗质碎屑重结晶）、低温热液和构造过程将特别关注于发现相应岩脉玄武岩中特征性麻粒岩相花岗质碎屑叠印的存在——这是发育的关键。然后将获得的数据库与快速扩散系统报告的数据库进行比较，以对辉长岩/岩脉界面进行概括比较，其对这些截然不同的扩张环境中地壳增生的影响的工作假设是，对于那些具有较强岩浆活动的缓慢扩张的山脊部分，预计辉长岩/岩脉动力学与快速扩张的类 EPR 地壳相似。 ，具有明确的 CBL，而对于那些地壳增生受构造过程控制的部分，预计会形成相对简单的切割关系（玄武岩切割辉长岩）。