The Feedback Between Volatiles and Mantle Dynamics

2011年，NERC开始了一项范围划分工作，以制定一项基于地球深处对这颗宜居星球的控制的研究计划。这项工作的结果是，国家环境研究中心承诺提供大量资金，支持一项题为“可居住星球上的挥发物、地球动力学和固体地球控制”的方案。这项提议是对这一呼吁的直接回应。人们普遍认为，挥发物--尤其是水--强烈影响控制岩石和矿物流动的性质(它们的流变性)。事实上，对石英和橄榄石等低压矿物的实验表明，即使是少量的水也会削弱矿物--使其流动更快--幅度高达几个数量级。这种效应被称为水解性减弱，已经被用来解释一系列地球基本问题--包括板块构造的起源，以及为什么地球和金星不同。水和挥发分对地幔岩石和矿物性质的影响是这项NERC研究计划的中心组成部分。事实上，它构成了英国学术界提出的三个主要问题之一的基础，并在范围界定过程中得到了一些国际专家的支持。问题是，随着时间的推移，挥发通量和地幔对流之间的反馈是什么？直观地说，挥发分和地幔对流之间存在反馈。例如，人们可能会设想这样一种情景，即潜入下地幔的水越多，地幔就应该越弱，从而允许更快的对流，这反过来又会导致更多的水进入下地幔，以此类推。当然，这是一种简化，因为更快的对流冷却了地幔，减缓了对流，也增加了在大洋中脊从地幔中移除的挥发物的数量。然而，人们可以想象许多重要的反馈，其中一些已经通过简单的模型进行了检验。特别是，这些模型表明了挥发分和对流之间的反馈，这种反馈控制着海洋和地幔之间的水分配，以及地幔垂直运动所产生的大量地形(称为动态地形)。参与探测工作的科学家认识到这是一个重大的科学问题，可能会对地球表面和宜居性产生深远的影响。然而，正如提案中详细讨论的那样，我们对地幔岩石如何随着水分的变化而变形的理解非常有限，而且实际上水对大多数地幔矿物的影响从未被测量过。水对大多数地幔矿物流动性质的影响是通过对低压矿物(橄榄石、辉石和石英)的实验简单推断出来的。正如提案中所主张的，人们不能简单地在不同的矿物和岩石之间进行推断，因为不同的矿物对水的反应可能非常不同。此外，目前的研究甚至对橄榄石的实验结果也提出了质疑，使这个问题变得更加紧迫。因此，我们提出了一项综合行动，以量化水对所有主要地幔矿物和岩石的流变性的影响，使用新的实验和多物理模拟的组合。结合3D地幔对流模型，这些信息将使我们能够了解挥发物和地幔对流之间的反馈如何影响地球宜居问题，例如海洋体积和大规模动态地形如何随时间变化。因此，这项研究迎头解决了研究方案的目标和雄心，而且确实是整个方案成功所必需的。

In 2011, NERC began a scoping exercise to develop a research programme based around deep Earth controls on the habitable planet. The result of this exercise was for NERC to commit substantial funding to support a programme entitled "Volatiles, Geodynamics and Solid Earth Controls on the Habitable Planet". This proposal is a direct response to that call. It is widely and generally accepted that volatiles - in particular water - strongly affect the properties that control the flow of rocks and minerals (their rheological properties). Indeed, experiments on low-pressure minerals such as quartz and olivine show that even small amounts of water can weaken a mineral - allowing it to flow faster - by as much as several orders of magnitude. This effect is known as hydrolytic weakening, and has been used to explain a wide range of fundamental Earth questions - including the origin of plate tectonics and why Earth and Venus are different. The effect of water and volatiles on the properties of mantle rocks and minerals is a central component of this NERC research programme. Indeed it forms the basis for one of the three main questions posed by the UK academic community, and supported by a number of international experts during the scoping process. The question is "What are the feedbacks between volatile fluxes and mantle convection through time?" Intuitively, one expects feedbacks between volatiles and mantle convection. For instance, one might envisage a scenario whereby the more water is subducted into the lower mantle, the more the mantle should weaken, allowing faster convection, which in turn results in even more water passing into the lower mantle, and so on. Of course this is a simplification since faster convection cools the mantle, slowing convection, and also increases the amount of volatiles removed from the mantle at mid-ocean ridges. Nevertheless, one can imagine many important feedbacks, some of which have been examined via simple models. In particular these models indicate a feedback between volatiles and convection that controls the distribution of water between the oceans and the mantle, and the amount topography created by the vertical movement of the mantle (known as dynamic topography). The scientists involved in the scoping exercise recognized this as a major scientific question, and one having potentially far reaching consequences for the Earth's surface and habitability.However, as is discussed in detail in the proposal, our understanding of how mantle rocks deform as a function of water content is remarkably limited, and in fact the effect of water on the majority of mantle minerals has never been measured. The effect of water on the flow properties of most mantle minerals is simply inferred from experiments on low-pressure minerals (olivine, pyroxenes and quartz). As argued in the proposal, one cannot simply extrapolate between different minerals and rocks because different minerals may react quite differently to water. Moreover, current research is now calling into question even the experimental results on olivine, making the issue even more pressing. We propose, therefore, a comprehensive campaign to quantify the effect of water on the rheological properties of all the major mantle minerals and rocks using a combination of new experiments and multi-physics simulation. In conjunction with 3D mantle convection models, this information will allow us to understand how the feedback between volatiles and mantle convection impacts on problems of Earth habitability, such as how ocean volumes and large-scale dynamic topography vary over time. This research thus addresses the aims and ambitions of the research programme head on, and indeed, is required for the success of the entire programme.