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).

洋壳辉长岩/岩墙过渡区是地球上最重要的界面之一，岩浆、变质和热液作用都在此发生。在快速扩张的海脊，地壳通常被认为是分层的和均匀的，辉长岩/岩墙过渡的特点是十米厚的层位，由角岩组成，角岩是在变质条件下形成的，直到麻粒岩相。这对应于传导边界层（CBL），理论模型预测的薄（<100 m），热（>650 °C），不可渗透的区域，夹在轴向岩浆房和席状岩墙之间，轴向岩浆房的热量通过该区域交换。 相比之下，在慢速和超慢速扩张海脊形成的地壳变化很大，从地幔在广阔区域直接侵位到海底，到局部高度岩浆扩张形成与快速扩张海脊大致相似的层状地壳。由于缺乏系统的调查，辉长岩/岩墙过渡的性质缓慢和超低扩展的山脊系统，只有很少的约束。 拟议项目的目的是与伍兹霍尔海洋研究所（WHOI，马萨诸塞州，美国）的亨利迪克合作，对WHOI的大量挖泥船和潜水套件进行系统审查，这些挖泥船和潜水套件含有来自缓慢和超缓慢扩张脊的席状岩脉生根区的岩石。这些特征将在火成岩和变质组合，变形结构，脉，接触关系的第一个在手标本。对于后续工作，将选择代表不同构造/岩浆环境（构造侵位模式与高岩浆生产力）的样品。然后，该套件将进行岩相学和电子探针研究，以解开原生岩浆，晚期岩浆，次生高温（即花岗变晶重结晶），低温热液和构造过程的岩相学记录。特别关注的是发现相应岩脉玄武岩中特征麻粒岩相花岗变晶叠印的存在-这是CBL发展的关键。将获得的数据库，然后将与快速扩展系统的报告进行比较，使辉长岩/岩墙界面的广义比较，并在这些非常不同的扩展环境中的地壳增生的影响。 工作的假设是，对于那些部分的缓慢扩张的脊与较强的岩浆活动，预计辉长岩/岩墙动力学是类似的快速蔓延，EPR样的地壳，具有明确的CBL，而对于那些部分的地壳增生是由构造过程控制，预计相对简单的切割关系（玄武岩切割辉长岩）。