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Physical properties of oceanic lower crustal and uppermost mantle rocks from the Atlantis Massif, Mid-Atlantic Ridge

大西洋中脊亚特兰蒂斯地块的海洋下地壳和上地幔岩石的物理性质

基本信息

批准号：
NE/N012402/1
负责人：
Timothy Minshull
金额：
$ 7.53万
依托单位：
University of Southampton
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%2FN012402%2F1
关键词：
Physical properties oceanic lower crustal

项目摘要

Gabbro is an intrusive rock that forms when molten rock cools slowly. It is the main constituent of the Earth's crust beneath the oceans. In a wide range of geological settings beneath the oceans, gabbro co-exists with a rock called peridotite that makes up most of the Earth's interior beneath the crust. At high temperatures, peridotite reacts with water in a process called serpentinisation. This reaction happens under certain pressure and temperature conditions and is often incomplete. Geophysical methods involving sound or electromagnetic waves provide a way to determine the nature of rocks beneath the Earth's surface without directly drilling into them, which is very expensive. These methods can be used to estimate the physical properties of rocks, such as density, seismic velocity (compressional, Vp, or shear, Vs) or electrical resistivity. Laboratory measurements on rock samples from the ocean floor have established a linear relationship between seismic velocity of the partially serpentinised peridotite and the degree of serpentinisation, and thus the amount of water chemically bound within the rock. However, it is difficult to distinguish remotely gabbroic rocks from partially serpentinised peridotite because their seismic velocities are similar. The International Ocean Discovery Program Expedition 357 will sample the southern wall of the Atlantis Massif, a geological structure close to the Mid-Atlantic Ridge where gabbro and serpentinised peridotite have been exposed at or close to the seafloor by faulting. Many previous studies have measured in laboratories the seismic properties of these rock types. In this study, we will use a similar approach on the new samples, but to expand our knowledge we will also measure the variation with direction of the electrical resistivity. The electrical resistivity has one of the widest ranges of any common physical property of solids. Therefore its variation with direction is more easily detectable than that of seismic velocities. Any significant difference of electrical resistivity or its variation with direction between partially serpentinised peridotite and gabbro might enable scientists in future to distinguish between these rock types by recording electromagnetic waves that pass through them. Expedition 357 expects also to collect strongly deformed samples along a near-horizontal fault. These samples will allow us to investigate the relationship between the directional variation of electrical resistivity and the direction of motion along the fault. The chemical reaction that leads to the formation of serpentinite can also generate methane gas, and this gas can ultimately provide a food supply for microbes that live on the seafloor. Therefore the flow rates water and methane in these conditions are of significant interest. Flow rates are controlled by a property of the rock called permeability, which measures how easily fluids can pass through it. Therefore the permeability is a valuable additional parameter to measure, but the measurement is very difficult. If Expedition 357 recovers suitable samples, we will measure directly their permeability and explore the relationship between directional variations in permeability and directional variations in electrical resistivity. Establishment of such a relationship would be very valuable because it would allow us to use geophysical data to tell us something about permeability deep in the Earth, in regions that cannot be sampled directly.

辉长岩是一种侵入岩，形成时熔融岩石冷却缓慢。它是海洋下面地壳的主要组成部分。在海洋下面的各种地质环境中，辉长岩与一种称为橄榄岩的岩石共存，橄榄岩构成了地壳下面的大部分地球内部。在高温下，橄榄岩与水发生反应，称为蛇纹石化。该反应在一定的压力和温度条件下发生，并且通常是不完全的。涉及声波或电磁波的地球物理方法提供了一种方法来确定地球表面下岩石的性质，而无需直接钻入它们，这非常昂贵。这些方法可用于估计岩石的物理性质，例如密度、地震速度（压缩速度Vp或剪切速度Vs）或电阻率。对海底岩石样品的实验室测量已经建立了部分蛇纹化橄榄岩的地震速度与蛇纹化程度之间的线性关系，从而建立了岩石中化学结合的水量。然而，由于辉长岩和部分蛇纹化橄榄岩的地震速度相似，很难远距离区分它们。国际海洋发现计划第357次考察将对亚特兰蒂斯地块的南壁进行取样，这是一个靠近大西洋中脊的地质结构，辉长岩和蛇纹橄榄岩因断层作用而暴露在或接近海底。之前的许多研究都在实验室中测量了这些岩石类型的地震特性。在本研究中，我们将对新样品使用类似的方法，但为了扩展我们的知识，我们还将测量电阻率随方向的变化。电阻率是固体的任何常见物理性质中范围最广的一种。因此，它随方向的变化比地震速度更容易检测。部分蛇纹化橄榄岩和辉长岩之间电阻率的任何显著差异或其随方向的变化可能使科学家在未来能够通过记录穿过它们的电磁波来区分这些岩石类型。357号考察队还希望沿着一个近水平的断层沿着收集强烈变形的样本。这些样品将使我们能够研究电阻率的方向性变化与断层沿着运动方向之间的关系。导致蛇纹岩形成的化学反应也可以产生甲烷气体，这种气体最终可以为生活在海底的微生物提供食物。因此，在这些条件下的水和甲烷的流速是重要的。流量由岩石的一种称为渗透率的性质控制，渗透率测量流体通过岩石的难易程度。因此，渗透率是一个有价值的附加参数，但测量非常困难。如果远征357恢复合适的样品，我们将直接测量它们的渗透率，并探索渗透率的方向变化和电阻率的方向变化之间的关系。建立这种关系将是非常有价值的，因为它将使我们能够使用地球物理数据来告诉我们一些关于地球深处的渗透性，在无法直接采样的区域。