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

洋中脊海底扩张形成的洋壳形成了地球表面的三分之二。下地壳是理解这一过程的关键。它由辉长岩组成，辉长岩是一种缓慢冷却的粗粒火成岩，由洋中脊地幔熔融产生的岩浆结晶而成。因此，下地壳起着关键层的作用，从下面接收岩浆，使其结晶形成辉长岩，并使剩余的熔体向上流动形成上地壳。然而，令人惊讶的是，我们只有很少的下地壳辉长岩样本来测试海底扩张的不同假设。这些样本来自世界海洋中数量有限的深钻孔，所有样本的体积加起来可以装进一个2.5米的盒子里！除取样外，确定海洋地壳年龄的一个重要工具是解释海底海洋地壳岩石磁化产生的地球磁场强度的微妙变化。这些“海洋磁异常”提供了全球海底扩张的磁带记录，其形式是对称分布在大洋中脊两侧的磁场强度的正负变化，这是过去扩张期间磁场方向逆转的结果。然而，迄今为止，我们从未在下地壳的这些重要反转边界中的任何一个进行过采样，而获取这样的样本将为理解海洋中的磁信号向前迈出重要一步。下地壳的重要新样本只能通过科学的海洋钻探获得。国际海洋发现计划（IODP）第360次考察“西南印度洋脊下地壳和莫霍面”将使用钻井船JOIDES Resolution从亚特兰蒂斯银行的一个新的1.3公里深的下洋壳孔中回收岩心样本，这些岩石因切割地壳的主要断层系统的运动而被带到海底附近。通过描述和分析这一特殊构造背景下的辉长岩岩心样本，考察队将解决以下重要问题：火成岩下地壳如何形成（或增生）;岩浆在增生过程中如何迁移、结晶和演变（改变成分）;地壳的构造变形（断层）如何通过吸收海洋板块的拉伸而促成海底扩张。重要的是，Atlantis Bank还提供了一个了解海洋磁性异常来源的独特机会，因为据信在该位置海底以下几百米处存在一个反转边界。辉长岩在结晶后不久就获得了地球磁场方向的磁化。岩石中被捕获的磁化方向可以作为一个标记，通过与该地点已知的现今地球磁场方向进行比较，来确定岩石在断层作用期间是否倾斜。然而，这种方法的一个主要问题是，IODP样品在钻井期间在岩心筒中自由旋转，并且不相对于现今的北方定向。为了克服这一点，我们将检查钻孔壁内部的定向地球物理“图片”，并将这些图片中已知方向的地质结构与岩心样本上的相应结构相匹配，以重新定位岩心块。这将使我们能够将从岩心中获得的磁性数据恢复到真正的地理参考框架，从而使用它们的磁化方向来确定构造倾斜在多大程度上促进了北东裂谷的结构发展。我们还将研究辉长岩的磁化强度如何随着孔中反转边界的接近和穿过而变化，从而对海洋磁异常源的性质提供新的认识。

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.