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

辉长岩是一种侵入岩，当熔岩慢慢冷却时形成。它是海洋下地壳的主要组成部分。在海洋下的各种地质环境中，辉长岩与一种名为橄榄岩的岩石共存，这种岩石构成了地壳下的大部分地球内部。在高温下，橄榄岩与水发生反应，这一过程称为蛇纹岩化。这种反应在一定的压力和温度条件下发生，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无需直接钻探就能确定地球表面下岩石性质的方法，这是一种非常昂贵的方法。这些方法可以用来估计岩石的物理性质，如密度、地震速度(纵波、横波或横波)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岩橄榄岩的地震速度与蛇纹岩程度之间的线性关系，从而建立了岩石中化学结合水的数量。然而，很难区分遥远的辉长岩和部分蛇纹石化的橄榄岩，因为它们的地震速度相似。国际海洋发现计划探险357号将对亚特兰蒂斯地块的南壁进行采样，亚特兰蒂斯地块是靠近大西洋中脊的地质结构，辉长岩和蛇纹状橄榄岩因断层而暴露在或接近海底。许多以前的研究已经在实验室中测量了这些岩石类型的地震性质。在这项研究中，我们将对新的样品使用类似的方法，但为了扩大我们的知识，我们还将测量电阻率随方向的变化。在固体的任何常见物理性质中，电阻率具有最宽的范围之一。因此，它随方向的变化比地震速度的变化更容易检测到。部分蛇纹状橄榄岩和辉长岩之间的任何显著的电阻率差异或其随方向的变化，都可能使科学家在未来通过记录穿过这些岩石的电磁波来区分这些岩石类型。探险357预计还将沿着近水平的断层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变化与沿断层的运动方向之间的关系。导致蛇纹岩形成的化学反应也会产生甲烷气体，这种气体最终可以为生活在海底的微生物提供食物供应。因此，在这些条件下的流速、水和甲烷是很有意义的。流量由岩石的一种名为渗透率的属性控制，该属性衡量流体通过岩石的难易程度。因此，磁导率是一个很有价值的附加参数，但测量难度很大。如果勘探357找到合适的样品，我们将直接测量它们的渗透率，并探索渗透率的方向性变化与电阻率的方向性变化之间的关系。建立这样的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无法直接采样的地区的渗透率的信息。

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.