Magnetization and tectonic evolution of ultraslow-spreading rate lower oceanic crust, Atlantis Bank, SW Indian Ridge (IODP Expedition 360)

南印度洋中脊亚特兰蒂斯浅滩超慢速扩张速度下洋地壳的磁化作用和构造演化（IODP Expedition 360）

• 批准号: NE/N019210/1
• 负责人: Antony Morris
• 金额: $ 14.73万
• 依托单位: Plymouth University
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2015
• 资助国家: 英国
• 起止时间: 2015 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FN019210%2F1
• 关键词: Magnetization; tectonic; evolution; ultraslow; spreading

Generation of ocean crust by seafloor spreading at mid-ocean ridges is responsible for forming two-thirds of the Earth's surface. The lower crust is the key to understanding this process. It consists of the rock gabbro, a slowly-cooled, coarse-grained igneous rock formed by crystallization of magma derived from melting of the mantle at mid-ocean ridges. The lower crust therefore acts as a critical layer, receiving magma from below, crystallizing it to form gabbros and passing remaining melt upwards to form the upper crust. However, we have surprisingly few samples of lower crustal gabbros on which to test different hypotheses for seafloor spreading. These have come from a limited number of deep drill holes in the world's oceans, and the volume of all samples combined would fit inside a 2.5 m box! In addition to sampling, an important tool in determining the age of oceanic crust is the interpretation of subtle variations in the strength of the Earth's magnetic field produced by the magnetization of oceanic crustal rocks on the seafloor. These "marine magnetic anomalies" provide a global tape recording of seafloor spreading in the form of positive and negative variations in field strength that are distributed symmetrically on either side of mid-ocean ridges as a result of past reversals of the direction of the field during spreading. However, to date we have never sampled across one of these important reversal boundaries in the lower crust, and retrieving such samples would provide a major step forward in understanding of the magnetic signal in the oceans.Significant new samples of the lower crust can only be obtained by scientific oceanic drilling. International Ocean Discovery Program (IODP) Expedition 360 "SW Indian Ridge Lower Crust and Moho" will use the drillship JOIDES Resolution to recover core samples from a new 1.3 km deep hole into lower oceanic crust at Atlantis Bank, where these rocks have been brought near to the seafloor by movement on a major fault system that cuts the crust. By describing and analyzing core samples of gabbros from this special tectonic setting, the expedition will address important questions such as: how the igneous lower crust forms (or accretes); how magma migrates, crystallizes and evolves (changes composition) during accretion; and how tectonic deformation (faulting) of the crust contributes to seafloor spreading by taking up stretching of the oceanic plates. Importantly, Atlantis Bank also provides a unique opportunity to understand the source of marine magnetic anomalies as a reversal boundary is believed to lie a few hundred metres beneath the seafloor at this location.This project will analyse the magnetic properties of the lower crustal rocks recovered at Atlantis Bank. The gabbros acquired a magnetization in the direction of the Earth's field shortly after they crystallized. The direction of the magnetization trapped in the rocks can be used as a marker to determine whether the rocks have been tilted during faulting, by comparing with the known present day direction of the Earth's field at the site. However, a major problem with this approach is that IODP samples are free to spin in the core barrel during drilling, and are not oriented relative to present day north. To overcome this, we will examine oriented geophysical "pictures" of the inside of the borehole wall and match up geological structures of known orientation in these images with corresponding structures on the core samples to reorient core pieces. This will allow us to restore the magnetic data obtained from the cores to a true geographic reference framework and hence to use their magnetization directions to determine the extent to which tectonic tilting has contributed to the structural development of the massif. We will also look at how the magnetization of the gabbros changes as the reversal boundary in the hole is approached and crossed, shedding new light on the nature of source of marine magnetic anomalies.

在大洋中脊扩张的海底形成了洋壳，形成了地球表面的三分之二。下层地壳是理解这一过程的关键。它由岩石辉长岩组成，这是一种缓慢冷却的粗粒火成岩，由大洋中脊地幔熔融产生的岩浆结晶形成。因此，下层地壳起到了临界层的作用，接受来自下层的岩浆，使其结晶形成辉长岩，并将剩余的熔体向上传递，形成上层地壳。然而，令人惊讶的是，我们几乎没有下部地壳辉长岩的样本，可以用来测试海底扩张的不同假设。这些样品来自世界海洋中有限数量的深钻孔，所有样品的体积加起来可以放在一个2.5米长的盒子里！除取样外，确定洋壳年龄的一个重要工具是解释海底洋壳岩石磁化所产生的地球磁场强度的细微变化。这些“海洋磁异常”提供了海底扩散的全球磁带记录，其形式是磁场强度的正负变化，由于过去扩散期间磁场方向的反转，这些变化对称地分布在大洋中脊的两侧。然而，到目前为止，我们还没有在下地壳的这些重要的反转边界上采集过样品，取回这些样品将为理解海洋中的磁性信号向前迈出一大步。只有通过科学的海洋钻探才能获得有意义的新的下地壳样品。国际海洋发现计划(IODP)360“西南印度洋海脊下地壳和莫霍面”将使用JOIDES分辨率钻井船，从亚特兰蒂斯浅滩一个新的1.3公里深的海底地壳中提取岩心样本，这些岩石是通过切割地壳的主要断层系统的运动而接近海底的。通过描述和分析来自这一特殊构造环境的辉长岩岩芯样品，考察队将解决以下重要问题：火成岩下地壳是如何形成(或吸积)的；岩浆在吸积过程中如何迁移、结晶和演化(改变成分)；地壳的构造变形(断层)如何通过拉伸大洋板块来促进海底扩张。重要的是，亚特兰蒂斯浅滩还提供了一个独特的机会来了解海洋磁性异常的来源，因为据信在该地点海底下几百米处有一个反转边界。辉长岩在结晶后不久就获得了沿地球磁场方向的磁化。通过与现今已知的地磁场方向进行比较，岩石中磁化的方向可以作为判断岩石在断裂过程中是否倾斜的标志。然而，这种方法的一个主要问题是，IODP样品在钻井过程中可以在岩心桶中自由旋转，并且相对于今天的北方没有定向。为了克服这一问题，我们将检查井壁内部的定向地球物理“图片”，并将这些图像中已知方位的地质结构与岩心样品上的相应结构进行匹配，以重新定位岩心碎片。这将使我们能够将从岩心获得的磁性数据恢复到一个真正的地理参考框架，从而利用它们的磁化方向来确定构造倾斜在多大程度上促进了地块的结构发展。我们还将观察辉长岩的磁化强度是如何随着洞中反转边界的接近和交叉而变化的，这将为海洋磁异常的来源提供新的线索。

项目成果

Dynamic Accretion Beneath a Slow‐Spreading Ridge Segment: IODP Hole 1473A and the Atlantis Bank Oceanic Core Complex

• DOI: 10.1029/2018jb016858
• 发表时间: 2019-12
• 期刊: Journal of Geophysical Research: Solid Earth
• 作者: H. J. Dick;C. J. MacLeod;P. Blum;N. Abe;D. K. Blackman;J. Bowles;M. Cheadle;K. Cho

Magnetic Mineral Populations in Lower Oceanic Crustal Gabbros (Atlantis Bank, SW Indian Ridge): Implications for Marine Magnetic Anomalies

• DOI: 10.1029/2019gc008847
• 发表时间: 2020-03
• 期刊: Geochemistry
• 影响因子: 3.7
• 作者: J. Bowles;A. Morris;M. Tivey;I. Lascu