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

冰岛辉长岩中的氧同位素变化：深部热液流还是地幔异质性？

• 批准号: NE/E001254/1
• 负责人: John Maclennan
• 金额: $ 10.08万
• 依托单位: University of Cambridge
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2006
• 资助国家: 英国
• 起止时间: 2006 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FE001254%2F1
• 关键词: Oxygen; isotope; variation; Icelandic; gabbros

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.

岩石的化学成分可用于研究地球深处发生的过程。这些过程中的许多过程都会影响人们的生活方式，尤其是在冰岛等国家，其著名的火山和热弹簧。这些活的火山构成了重大危害，在过去的250年中，许多爆发导致生命丧失并破坏了城镇或基础设施。在一种情况下，1783年的Laki爆发导致约20％的人口死亡。冰岛也受益于火山活动，因为它导致地球热水的循环。这些地热系统现在已经被挖掘出来，从便宜，清洁和可再生的来源提供了超过50％的冰岛能源需求。我项目的目的是利用冰岛喷发产生的岩石的组成，以提高我们对火山和地热活动的理解。特别是，我想尝试确定地热活动期间水渗透的最大深度。这些知识将有助于对地热电站进行深入钻井的进一步计划。此外，我将计算在火山深根部融化的岩石的成分。我对这一组成的估计将进一步理解驱动火山活动的力量。在爆发期间，在特殊情况下，在高达25公里的深度形成的岩石的实心碎片可以运输到表面。研究这些结晶片段的科学家发现其化学成分意外变化。他们发现，晶体中的氧原子令人惊讶地轻，比在其他地方的类似熔岩流中分析的氧气轻得多。这些观察结果引发了两组科学家之间对这种光氧的起源的竞争假设之间的辩论。这场辩论尚未解决。两组都同意，大多数熔融岩石都是通过坐在地球地幔上层的30至150 km深度之间的区域熔化而产生的。他们还同意，在这种熔体形成后，它向上移动到地壳中，在表面和20-30 km深度之间发现的岩石层。当熔体到达地壳时，它将被存储在岩浆室内，并开始冷却和凝固。如果发生喷发，岩浆会从腔室迅速向上移动到表面的喷发部位。一群科学家认为，光氧源于地壳，另一组认为它来自地幔。如果光氧源自地壳，则可能与冰岛最近的地热活动有关。冰岛水含有比地壳较轻的氧气，当水在地热活动过程中穿过地壳时，一些光氧被转移到地壳上。然后，如果岩浆腔室内形成这种改变的地壳，则可以将光氧从地壳中传递到腔室中的岩浆中。如果该假设是正确的，那么地热循环必须延伸至20公里以上的深度，比以前假定的要深得多。另外，如果光氧来自地幔，那么该信号可能最终来自古代海底的片，可能是3亿年的历史，这些历史已被板块构造工艺返回到地幔。如果该假设是正确的，那么观察结果对这些过程的性质和地球内部内部的大规模运动具有影响。尚未有可能区分这两个假设，因为还没有人对单个晶体内的组成变化进行正确的测量。通过利用许多最近开发的微分析技术（例如探针和微钻），我将成为第一个进行观察的人，可以使用该观察来确定光氧是否起源于地幔还是地壳。

项目成果

Short Length Scale Oxygen Isotope Heterogeneity in the Icelandic Mantle: Evidence from Plagioclase Compositional Zones

• DOI: 10.1093/petrology/egu066
• 发表时间: 2014-12
• 期刊: Journal of Petrology
• 影响因子: 3.9
• 作者: B. Winpenny;J. Maclennan