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Volatile cycling and oxygen fugacity of subduction zones using stable vanadium isotopes

使用稳定钒同位素研究俯冲带的挥发性循环和氧逸度

基本信息

批准号：
NE/H01313X/2
负责人：
Julie Prytulak
金额：
$ 25.17万
依托单位：
Imperial College London
依托单位国家：
英国
项目类别：
Fellowship
财政年份：
2011
资助国家：
英国
起止时间：
2011 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FH01313X%2F2
关键词：
Volatile cycling oxygen fugacity subduction

项目摘要

The outer shell of the Earth that we live on is made up of brittle 'plates'. The migration of these plates across the surface of the planet is directly linked to major geologic events such as earthquakes and volcanic eruptions. In some regions, two plates collide, forcing one beneath the other in a process called 'subduction'. Subduction zones are responsible for much of the explosive volcanism on Earth, including the infamous Pacific 'Ring of Fire'. However, these zones are also critical for the exchange and cycling of chemical elements between the surface and interior of the planet. As an oceanic plate subducts, it is subjected to high pressures and temperatures. During this process, the plate looses chemical components through fluids, degassing, reactions, and sometimes melting. One of the key parameters controlling how much of which elements are lost, is the available oxygen. Geochemists refer to the amount of available oxygen as the 'oxygen fugacity' of a system, which can be simply thought of as the partial pressure of oxygen. Oxygen fugacity has a large affect on the way the carbon (C), hydrogen (H), and other 'volatile' elements behave in a subduction system. Volatiles species (e.g., H2O and CO2) are those that vaporize at low temperatures. Combined with other physical and chemical conditions of subduction, oxygen fugacity controls how much H and C is lost through degassing during explosive volcanism, and how much can be dragged deeper into the Earth. There has been a long-standing debate over how much more oxygenated subduction zones are compared with the rest of the interior of the Earth. The fugacity of samples from subduction zone volcanoes tells us about present day processes and volatile cycling. Furthermore, if subduction zones are significantly more oxygenated than the rest of the interior of the Earth, then they may provide an efficient means of recycling oxygen into the interior of the Earth. Therefore, it is critical to constrain the oxygen fugacity of subduction zones to evaluate the whole Earth cycling of volatile elements and how this may change through time. It is essential to find a robust way of determining oxygen fugacity. Unfortunately, previous studies used methods that can be easily 'reset' by later events, so that they do not give a true indication of the original source. Consequently, there is considerable uncertainty in what the 'real' amount of available oxygen is in subduction systems. My previous research and expertise focused on the novel application of isotopes of chemical elements to solving Earth problems. I have continued in this broad avenue of investigation by working on a precise analytical method for the measurement of vanadium stable isotope variations. The measurements are not trivial, however they are very valuable. The power of vanadium stable isotopes in particular, is that their fractionation should be directly and robustly linked to oxygen fugacity. This fellowship analyses vanadium stable isotope variations in lavas, sediments and deep Earth samples from the Mariana (southwest Pacific), Aleutian (Alaska) and Mexican subduction zones. Through this work, better constraint can be placed on oxygen fugacity and how the Earth system behaves in terms of the fluxes of volatile elements such as carbon and hydrogen between deep and surface reservoirs of the Earth. This will help us tackle far-reaching present day issues related to how the carbon cycle works, and also potentially provide a means of investigating how the amount of oxygen in the Earth has changed over time, its links to the evolution of the atmosphere and ultimately to how our planet became able to sustain life.

我们赖以生存的地球的外壳是由易碎的“板块”组成的。这些板块在地球表面的迁移与地震和火山爆发等重大地质事件直接相关。在某些地区，两个板块发生碰撞，迫使一个板块压到另一个板块下方，这一过程称为“俯冲”。俯冲带是地球上大部分火山爆发的原因，包括臭名昭著的太平洋“火环”。然而，这些区域对于地球表面和内部之间化学元素的交换和循环也至关重要。当海洋板块俯冲时，它会承受高压和高温。在此过程中，板材通过流体、脱气、反应，有时甚至熔化而失去化学成分。控制元素损失量的关键参数之一是可用氧。地球化学家将可用氧气量称为系统的“氧逸度”，可以简单地将其视为氧分压。氧逸度对碳 (C)、氢 (H) 和其他“挥发性”元素在俯冲系统中的行为方式有很大影响。挥发性物质（例如 H2O 和 CO2）是在低温下蒸发的物质。与俯冲的其他物理和化学条件相结合，氧逸度控制着火山爆发期间通过脱气损失的 H 和 C 的量，以及可以被拖入地球更深处的量。关于俯冲带的含氧量与地球内部其他部分相比有多少，一直存在争论。俯冲带火山样本的逸度告诉我们当今的过程和挥发性循环。此外，如果俯冲带的含氧量明显高于地球内部其他区域，那么它们可能提供一种将氧气循环到地球内部的有效方法。因此，限制俯冲带的氧逸度对于评估整个地球的挥发性元素循环及其随时间的变化至关重要。找到一种确定氧逸度的可靠方法至关重要。不幸的是，以前的研究使用的方法很容易被后来的事件“重置”，因此它们不能真实地表明原始来源。因此，俯冲系统中“真实”可用氧气量存在相当大的不确定性。我之前的研究和专业知识主要集中在化学元素同位素解决地球问题的新颖应用。我通过研究测量钒稳定同位素变化的精确分析方法，继续在这一广泛的研究领域进行研究。这些测量并不是微不足道的，但它们非常有价值。尤其是钒稳定同位素的力量在于，它们的分馏应与氧逸度直接且牢固地相关。该研究项目分析了马里亚纳（西南太平洋）、阿留申群岛（阿拉斯加）和墨西哥俯冲带的熔岩、沉积物和深层地球样本中钒稳定同位素的变化。通过这项工作，可以更好地限制氧逸度以及地球系统在地球深层和地表储层之间碳和氢等挥发性元素通量方面的行为方式。这将帮助我们解决当今与碳循环如何运作相关的影响深远的问题，并有可能提供一种方法来研究地球中的氧气含量如何随时间变化、它与大气演化的关系以及最终与我们的星球如何能够维持生命的关系。