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

我们生活的地球外壳是由易碎的“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，两个板块碰撞，迫使一个板块在另一个板块的下面，这一过程被称为“俯冲”。俯冲带导致了地球上大部分的爆炸性火山活动，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化学元素的交换和循环也是至关重要的。当大洋板块俯冲时，它受到高压和高温的影响。在这一过程中，板通过流体、脱气、反应和有时熔化而失去化学成分。控制元素损失多少的关键参数之一是可用氧。地球化学家将可用的氧气量称为系统的“氧逸度”，可以简单地认为是氧气的分压。氧逸度对碳(C)、氢(H)和其他“挥发性”元素在俯冲系统中的表现有很大影响。挥发物（如H2O和CO2）是指在低温下蒸发的物质。与其他俯冲的物理和化学条件相结合，氧逸度控制着爆炸性火山活动期间通过脱气损失的氢和碳的数量，以及被拖入地球深处的氢和碳的数量。与地球内部的其他部分相比，含氧俯冲带要多多少，这是一个长期存在的争论。俯冲带火山样品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的其他部分，那么它们可能提供一种将氧气再循环到地球内部的有效手段。因此，限制俯冲带的氧逸度对于评估挥发性元素的整个地球循环及其随时间的变化是至关重要的。必须找到一种可靠的方法来测定氧逸度。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”，因此它们不能给出原始来源的真实指示。因此，对于俯冲系统中可用氧的“真实”量有多少存在相当大的不确定性。我以前的研究和专业知识集中在化学元素同位素的新应用，以解决地球问题。在这条广阔的研究道路上，我继续研究一种精确的分析方法来测量钒稳定同位素的变化。测量结果并非微不足道，但它们非常有价值。钒稳定同位素的特别之处在于，它们的分馏应该与氧逸度直接而有力地联系在一起。本研究小组分析了来自马里亚纳（西南太平洋）、阿留申（阿拉斯加）和墨西哥俯冲带的熔岩、沉积物和深部地球样本中的钒稳定同位素变化。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的挥发性元素（如碳和氢）的通量方面的表现。这将有助于我们解决与碳循环如何运作有关的影响深远的当今问题，也有可能提供一种方法来研究地球上的氧气含量是如何随着时间的推移而变化的，它与大气进化的联系，以及最终与我们的星球如何能够维持生命有关。

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.