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

我们赖以生存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喷发等重大地质事件直接相关。在一些地区，两个板块发生碰撞，迫使其中一个板块位于另一个板块之下，这一过程被称为“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的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，它会受到高压和高温的影响。在这个过程中，板材通过流体、脱气、反应，有时还会熔化，使化学成分流失。控制元素损失量的关键参数之一是可用氧。地球化学家将可利用的氧气量称为系统的“氧逸度”，它可以简单地理解为氧气的分压。氧逸度对碳(C)、氢(H)和其他挥发性元素在俯冲系统中的行为方式有很大影响。挥发性物质(如H2O和CO2)是那些在低温下蒸发的物质。结合俯冲的其他物理和化学条件，氧逸度控制着在火山爆发期间通过脱气损失了多少H和C，以及有多少可以被拖入地球更深的地方。长期以来，人们一直在争论，与地球内部的其他地方相比，含氧俯冲带多出了多少。俯冲带火山样品的逸度告诉我们当今的过程和挥发循环。此外，如果俯冲带的含氧量明显高于地球内部的其他部分，那么它们可能会提供一种将氧气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整个地球挥发性元素的循环以及这种循环随时间的变化是至关重要的。找到一种可靠的方法来测定氧逸度是至关重要的。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”，因此它们没有给出原始来源的真实指示。因此，潜水系统中可用氧的“真实”量是多少，存在着相当大的不确定性。我之前的研究和专业知识集中在化学元素同位素的新应用上，以解决地球问题。我继续这一广泛的研究途径，致力于一种精确的分析方法来测量钒的稳定同位素变化。这些测量并不是微不足道的，但它们是非常有价值的。尤其是钒稳定同位素的威力在于，它们的分馏作用应该直接和有力地与氧逸度联系在一起。这项研究分析了马里亚纳(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品中的钒稳定同位素变化。通过这项工作，可以更好地约束氧逸度以及地球系统在地球深层和地表水库之间的挥发性元素(如碳和氢)的通量方面的行为。这将帮助我们解决与碳循环如何运作有关的影响深远的当今问题，还可能提供一种手段，研究地球中的氧气量如何随着时间的推移而变化，它与大气演变的联系，以及最终我们的星球如何能够维持生命。

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.