Oxygen isotope variation in Icelandic gabbros: Deep hydrothermal flow or mantle heterogeneity?

岩石的化学成分可以用来研究发生在地球深处的过程。其中许多过程都对人们的生活方式产生了影响，尤其是在冰岛这样的国家，那里有著名的火山和温泉。这些活火山构成了巨大的危险，在过去的250年里，许多火山爆发造成了生命损失，摧毁了城镇或基础设施。1783年的一次拉基火山爆发导致约20%的人口死亡。冰岛也受益于火山活动，因为它导致了地球上热水的循环。这些地热系统现在已经被利用，以一种廉价、清洁和可再生的方式提供了冰岛50%以上的能源需求。我的项目目的是利用冰岛火山喷发产生的岩石成分来提高我们对火山和地热活动的理解。特别是，我想试着确定地热活动期间水渗透的最大深度。这些知识将有助于地热发电站的深层钻探的进一步规划。此外，我将计算在火山深处融化的岩石的成分。我对这种成分的估计将进一步加深我们对推动火山活动的力量的理解。在25公里深处形成的固体岩石碎片，在极少数情况下，会在火山爆发时被带到地表。研究这些晶体碎片的科学家在化学成分上发现了意想不到的变化。他们发现晶体中的氧原子出奇地轻，比其他地方类似熔岩流中分析的氧轻得多。这些观察结果引发了两组科学家之间激烈的争论，他们对这种轻氧的起源提出了不同的假设。这场辩论尚未得到解决。两组人都认为，大部分熔岩是由地幔最上层30至150公里深的区域融化产生的。他们还同意，在这种融化形成后，它向上移动到地壳中，地壳是在地表和20-30公里深度之间发现的岩层。当熔体到达地壳时，它被储存在岩浆房中，并开始冷却和凝固。如果发生喷发，岩浆会从岩浆室迅速向上移动到地表的喷发点。一组科学家认为轻氧起源于地壳，而另一组则认为它来自地幔。如果轻氧来自地壳，那么它很可能与冰岛最近的地热活动有关。冰岛的水含有比地壳更轻的氧，当水在地热活动期间穿过地壳时，一些轻氧被转移到地壳中。然后，如果一个岩浆房在这个改变的地壳中形成，轻氧可能会从地壳传递到岩浆房中的岩浆中。如果这个假设是正确的，那么地热循环必须延伸到超过20公里的深度，比先前假设的深度要深得多。或者，如果轻氧来自地幔，那么这个信号很可能最终来自于古代海底的碎片，可能有3亿年的历史，它们在板块构造过程中被送回了地幔。如果这一假设是正确的，那么这些观测结果对这些过程和地球内部大规模运动的本质就有了启示。目前还不可能区分这两种假设，因为还没有人对单个晶体的成分变化进行过正确的测量。通过利用最近发展的一些微分析技术，如探针和微钻，我将是第一个进行观察的人，可以用来确定轻氧是来自地幔还是地壳。

The chemical composition of rocks can be used to investigate processes which occur deep in the Earth. Many of these processes have an impact on the way that people live, particularly in countries like Iceland, with its well-known volcanoes and hot-springs. These active volcanoes present a significant hazard and over the last 250 years many eruptions have caused loss of life and destroyed towns or infrastructure. In one case the Laki eruption in 1783 lead to the death of about 20% of the population. Iceland also benefits from volcanic activity because it leads to the circulation of hot water in the Earth. These geothermal systems have now been tapped to provide more than 50% of Iceland's energy needs from a cheap, clean and renewable source. The aim of my project is to use the composition of rocks produced by Icelandic eruptions to improve our understanding of the volcanic and geothermal activity. In particular, I want to try to determine the maximum depth of water penetration during geothermal activity. This knowledge will be useful in the further planning of deep drilling for geothermal power stations. In addition, I will calculate the composition of the rocks which melt in the deep roots of the volcanoes. My estimation of this composition will further our understanding of the forces that drive volcanic activity. Solid fragments of rocks which formed at depths of up to 25 km can, in exceptional cases, be transported to the surface during eruptions. Scientists studying these crystalline fragments have found unexpected variation in their chemical composition. They found that oxygen atoms in the crystals were surprisingly light, much lighter than oxygen analysed in similar lava flows from elsewhere. These observations sparked a lively debate between two groups of scientists with competing hypotheses for the origin of this light oxygen. This debate has yet to be resolved. Both groups agree that most of the molten rock is generated by melting of a region between 30 and 150 km depth sitting in the uppermost layer of the Earth's mantle. They also agree that after this melt forms, it moves upwards into the crust, the layer of rock found between the surface and 20-30 km depth. When the melt reaches the crust it is stored within magma chambers and starts to cool and solidify. If an eruption occurs, magma moves swiftly upwards from the chamber to the eruption site at the surface. One group of scientists argues that the light oxygen originates in the crust, and the other group believes that it comes from the mantle. If the light oxygen is derived from the crust, then it is likely to be related to recent geothermal activity on Iceland. Icelandic water contains lighter oxygen than the crust, and as the water passes through the crust during geothermal activity, some of the light oxygen is transferred to the crust. Then, if a magma chamber forms within this altered crust, light oxygen may then be passed from the crust into the magma in the chamber. If this hypothesis is correct, then geothermal circulation must extend to depths of over 20 km, much deeper than had been previously assumed. Alternatively, if the light oxygen comes from the mantle, then it is likely that this signal is ultimately derived from slivers of ancient seafloor, perhaps 300 million years old, which have been returned to the mantle by plate tectonic processes. If this hypothesis is correct, then the observations have implications for the nature of these processes and large-scale motions within the interior of the Earth. It has not yet been possible to distinguish between these two hypotheses because nobody has yet made the right set of measurements of compositional variation within individual crystals. By taking advantage of a number of recently developed micro-analytical techniques, such as probes and micro-drilling, I will be the first to make observations that can be used to determine whether the light oxygen originates in the mantle or crust.