The Feedback Between Volatiles and Mantle Dynamics

挥发物与地幔动力学之间的反馈

基本信息

批准号：
NE/M000281/1
负责人：
Jeroen Van Hunen
金额：
$ 34.82万
依托单位：
Durham University
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2017
资助国家：
英国
起止时间：
2017 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FM000281%2F1
关键词：
Feedback Between Volatiles Mantle Dynamics

项目摘要

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.

2011年，NERC开始了一项范围界定工作，以开发一个基于地球深层对宜居行星的控制的研究项目。此次活动的结果是 NERC 承诺投入大量资金来支持名为“宜居星球上的挥发物、地球动力学和固体地球控制”的项目。该提案是对这一呼吁的直接回应。人们普遍认为，挥发物（特别是水）会强烈影响控制岩石和矿物流动的特性（它们的流变特性）。事实上，对石英和橄榄石等低压矿物的实验表明，即使少量的水也会削弱矿物的强度，使其流动得更快，强度可达几个数量级。这种效应被称为水解弱化，并已被用来解释各种基本的地球问题 - 包括板块构造的起源以及地球和金星为何不同。水和挥发物对地幔岩石和矿物性质的影响是 NERC 研究计划的核心组成部分。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许多国际专家的支持。问题是“随着时间的推移，挥发性通量和地幔对流之间的反馈是什么？”直观上，人们期望挥发物和地幔对流之间存在反馈。例如，人们可能会设想一种情况，越多的水被俯冲到下地幔中，地幔就越弱化，从而允许更快的对流，这反过来又导致更多的水进入下地幔，等等。当然，这是一种简化，因为更快的对流会冷却地幔，减慢对流，并且还会增加从洋中脊地幔中去除的挥发物的量。然而，我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，这些模型表明挥发物和对流之间的反馈控制着海洋和地幔之间的水分布，以及地幔垂直运动产生的地形量（称为动态地形）。参与范围界定工作的科学家们认识到这是一个重大的科学问题，并且可能对地球表面和可居住性产生深远的影响。然而，正如提案中详细讨论的那样，我们对地幔岩石如何变形的理解是一个函数水的含量非常有限，事实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特性的影响是通过低压矿物（橄榄石、辉石和石英）的实验简单推断出来的。正如该提案中所指出的，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截然不同。此外，当前的研究甚至对橄榄石的实验结果提出了质疑，使这个问题变得更加紧迫。因此，我们建议开展一项综合运动，结合新实验和多物理场模拟来量化水对所有主要地幔矿物和岩石流变特性的影响。与 3D 地幔对流模型相结合，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性问题，例如海洋体积和大规模动态地形如何随时间变化。因此，这项研究直接解决了研究计划的目标和抱负，事实上，这是整个计划成功所必需的。