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Deep Water: Hydrous Silicate Melts and the Transition Zone Water Filter

深水：水合硅酸盐熔体和过渡区水过滤器

基本信息

批准号：
NE/P002951/1
负责人：
Oliver Lord
金额：
$ 74.68万
依托单位：
University of Bristol
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2016
资助国家：
英国
起止时间：
2016 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FP002951%2F1
关键词：
Deep Water Hydrous Silicate Melts

项目摘要

Earth is a wet planet, and its habitability is intrinsically tied to water at the surface. Water also plays a key role inside the Earth because it has the affect of drastically lowering the melting point of mantle rocks, and indeed, the water at the surface ultimately comes from the interior through magmatism. Water is returned to the interior at subduction zones, and over geological time the surface and mantle water inventories are regulated by a deep water cycle. Nearly all of the volcanism we see at the Earth's surface is caused by melting in the shallow mantle, especially above subduction zones, and this constitutes an important part of the deep water cycle. However, water is also transported deeper down into the mantle, and what happens to it, and the controls it has on interior processes, is a mystery. Deep mantle rocks are generally too cool to melt, but there are regions in the mantle, most notably at about 400 and 700 km, where seismic signals are interpreted to represent partial melting. Water reduces the solidus of the mantle by hundreds of degrees in the upper and lower mantle, but much less so in the mantle transition zone (410-660 km), and this is because water is soluble in the main minerals that make up this region, wadsleyite and ringwoodite, but it is not soluble in the minerals that constitute the regions above and below the transition zone. This means that if there is water in the transition zone, and it potentially can store a couple of oceans worth, when mantle is transported from the transition zone into either the upper or lower mantle it is expected to melt when water is released. This concept was originally applied to the region above the transition zone by Bercovici and Karato in a landmark paper in 2003, and was called the 'transition zone water filter' because of the important predicted affects on mantle geochemistry. Interestingly, since then seismic evidence has been mounting indicating melted regions above and below the transition zone. If these signals do indeed represent the presence of hydrous melt in these regions, then the transition zone may act as a double-sided mid-mantle water filter, and the melting that occurs at its boundaries could have modulated the chemistry and geodynamics of the mantle throughout its history. Currently we cannot adequately test this model or understand its implications because we do not know accurately the composition of hydrous silicate melts of the mantle at these depths, nor do we know their physical properties, such as the density and viscosity. Because of this, we are currently unable to model accurately the seismic response expected for hydrous silicate melting at these depths, and we cannot model the dynamic behavior of such melts should they exist in these locations. Here we propose to collect this data.We propose to make high P-T experimental measurements to determine the compositions of hydrous silicate melts in the mantle at depths corresponding to the deep upper mantle, transition zone and upper part of the lower mantle. We will also use novel experiments where we combine diamond anvil cell techniques with synchrotron X-ray scattering methods to determine melt densities. Simultaneously we will use first principles molecular dynamics methods to calculate the physical and seismic properties of these melts, supplemented with experiments to measure their wetting properties. With these data, we will be able for the first time to develop dynamic and seismic models to explicitly test the transition zone water filter model, and make predictions about its chemical and dynamical affects on the mantle, and on the deep water cycle, throughout geological time.

地球是一个潮湿的星球，它的宜居性本质上与地表的水有关。水在地球内部也发挥着关键作用，因为它具有大幅降低地幔岩石熔点的作用，事实上，地表的水最终通过岩浆作用来自内部。水在俯冲带返回到内陆，随着地质时间的推移，地表和地幔的水储量受到深水循环的调节。我们在地球表面看到的几乎所有火山活动都是由浅地幔融化引起的，特别是在俯冲带上方，这构成了深水循环的重要组成部分。然而，水也被输送到地幔更深的地方，它发生了什么，以及它对内部过程的控制，是一个谜。深部的地幔岩石通常太冷而不能融化，但地幔中有一些区域，最明显的是大约400和700公里处，那里的地震信号被解释为部分熔融。水使上地幔和下地幔的固相线降低了数百度，但在地幔过渡带(410-660公里)中的影响要小得多，这是因为水可溶于构成该区域的主要矿物--瓦兹利石和环木石，但不溶于构成过渡带上下区域的矿物。这意味着，如果过渡带中有水，并且它可能储存了几个海洋的价值，当地幔从过渡带输送到上地幔或下地幔时，当水释放时，它预计会融化。这一概念最初是由Bercovici和Karato于2003年在一篇里程碑式的论文中应用于过渡带上方的区域，并因其对地幔地球化学的重要预测影响而被称为“过渡带水过滤器”。有趣的是，从那时起，越来越多的地震证据表明，过渡带上方和下方的区域正在融化。如果这些信号确实表明这些地区存在含水熔融，那么过渡带可能起到双侧地幔中水过滤器的作用，在其边界发生的熔融可能在其整个历史上调制了地幔的化学和地球动力学。目前，我们无法充分测试这一模型或了解其含义，因为我们不能准确地知道这些深度的地幔水合硅酸盐熔体的组成，也不知道它们的物理性质，如密度和粘度。正因为如此，我们目前无法准确地模拟这些深度的水合硅酸盐熔融预期的地震响应，而且我们也无法模拟此类熔体的动态行为，如果它们存在于这些位置的话。在这里，我们建议收集这一数据。我们建议进行高P-T实验测量，以确定对应于上地幔深部、过渡带和下地幔上部的地幔中水合硅酸盐熔体的成分。我们还将使用新颖的实验，将钻石顶顶盒技术与同步辐射X射线散射法相结合来确定熔体密度。同时，我们将使用第一性原理分子动力学方法计算这些熔体的物理和地震性质，并辅之以实验来测量它们的润湿性质。有了这些数据，我们将首次能够开发动力学和地震模型，以明确测试过渡带水过滤模型，并预测其在整个地质时期对地幔和深水循环的化学和动力学影响。

项目成果

期刊论文数量（10）

专著数量（0）

科研奖励数量（0）

会议论文数量（0）

专利数量（0）

Boron Incorporation in Silicate Melt: Pressure-induced Coordination Changes and Implications for B Isotope Fractionation

硅酸盐熔体中硼的掺入：压力引起的配位变化及其对 B 同位素分馏的影响

DOI：
10.3389/feart.2022.870892
发表时间：
2022
期刊：
Frontiers in Earth Science
影响因子：
2.9
作者：
Drewitt J
通讯作者：
Drewitt J

Configurational constraints on glass formation in the liquid calcium aluminate system

DOI：
10.1088/1742-5468/ab47fc
发表时间：
2019-10-01
期刊：
JOURNAL OF STATISTICAL MECHANICS-THEORY AND EXPERIMENT
影响因子：
2.4
作者：
Drewitt, James W. E.;Jahn, Sandro;Hennet, Louis
通讯作者：
Hennet, Louis

In situ observation of nanolite growth in volcanic melt: A driving force for explosive eruptions.

DOI：
10.1126/sciadv.abb0413
发表时间：
2020-09
期刊：
Science advances
影响因子：
13.6
作者：
Di Genova D;Brooker RA;Mader HM;Drewitt JWE;Longo A;Deubener J;Neuville DR;Fanara S;Shebanova O;Anzellini S;Arzilli F;Bamber EC;Hennet L;La Spina G;Miyajima N
通讯作者：
Miyajima N

From Short to Medium Range Order in Glasses and Melts by Diffraction and Raman Spectroscopy