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.

岩石的化学成分可以用来研究地球深处发生的过程。这些过程中的许多都对人们的生活方式产生了影响，特别是在冰岛这样拥有著名火山和温泉的国家。这些活火山构成了重大危险，在过去250年中，许多火山喷发造成了生命损失，摧毁了城镇或基础设施。在一个案例中，1783年的拉基火山喷发导致了大约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开发，以廉价、清洁和可再生的来源提供冰岛50%以上的能源需求。我的项目的目的是利用冰岛喷发产生的岩石的成分来提高我们对火山和地热活动的了解。特别是，我想要尝试确定地热活动期间水的最大渗透深度。这一认识将有助于地热发电站深部钻探的进一步规划。此外，我还将计算熔化在火山根部深处的岩石的成分。我对这种成分的估计将加深我们对驱动火山活动的力量的理解。在喷发期间，在25公里深处形成的固体岩石碎片在特殊情况下可以被带到地表。研究这些晶体碎片的科学家们发现，它们的化学成分发生了意想不到的变化。他们发现，晶体中的氧原子轻得惊人，比从其他地方分析的类似熔岩流中的氧要轻得多。这些观察结果在两组科学家之间引发了激烈的辩论，他们对这种轻氧的起源提出了相互竞争的假说。这场争论尚未得到解决。两个小组都同意，大部分熔岩是由地幔最上层30至150公里深的区域熔化产生的。他们还同意，在熔体形成后，它向上移动到地壳中，地壳是在地表和20-30公里深之间发现的一层岩石。当熔体到达地壳时，它被储存在岩浆室中，并开始冷却和凝固。如果发生喷发，岩浆会迅速从洞穴向上移动到地表的喷发地点。一组科学家认为，轻氧来自地壳，另一组认为它来自地幔。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气，在地热活动期间水穿过地壳时，一些轻氧被转移到地壳。然后，如果岩浆室在这个蚀变的地壳中形成，那么轻氧可能会从地壳进入岩浆室。如果这一假设是正确的，那么地热循环一定会延伸到超过20公里的深度，比之前假设的要深得多。或者，如果轻氧来自地幔，那么这个信号很可能最终来自古老海底的碎片，可能是3亿年前的，这些碎片已经通过板块构造过程返回到地幔。如果这一假设是正确的，那么这些观测结果对这些过程的性质和地球内部的大规模运动都有意义。目前还不可能区分这两个假设，因为还没有人对单个晶体内的成分变化进行正确的测量。通过利用最近开发的一些微观分析技术，如探头和微钻孔，我将成为第一个进行观测的人，这些观测可以用来确定轻氧来自地幔还是地壳。

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