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Apatite as a quantitative tool for tephrochronology and magmatic evolution

磷灰石作为岩石年代学和岩浆演化的定量工具

基本信息

批准号：
NE/K003852/1
负责人：
Madeleine Humphreys
金额：
$ 25.29万
依托单位：
University of Oxford
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2013
资助国家：
英国
起止时间：
2013 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FK003852%2F1
关键词：
Apatite quantitative tool tephrochronology magmatic

项目摘要

Apatite is a calcium phosphate mineral that crystallises in small quantities from a wide variety of different magmas. It is particularly useful because it incorporates 'volatile' species such as water (H2O), carbon dioxide (CO2), chlorine, fluorine and sulphur, into its mineral structure. Volatiles are dissolved gases, which are an important component of many magmas. During magma ascent underneath a volcano the volatiles form bubbles, which expand and accelerate the magma upwards, fuelling explosive volcanic eruptions. Apatite has the potential to be a particularly reliable recorder of volatile concentrations in the magma before eruption, but in order to exploit the information it contains, we need independent measurements of the 'partition coefficients' for H2O and CO2 - the amount found in the mineral for a given concentration in the melt. In the first part of the project, we will measure these partition coefficients in the laboratory, so that we can infer magmatic volatile contents from their concentrations in apatite. We will also measure partition coefficients for fluorine and chlorine. We will test the partition coefficients by using apatite to infer volatile concentrations in a magmatic system, known as the Laacher See Tuff, that has already been well characterised using alternative methods. This volcanic deposit was produced in a major explosive eruption nearly 13,000 year ago, with ash layers dispersed across central continental Europe up to 1100 km away from the volcanic vent. In the second part of the project, we propose to develop apatite as a tool for correlating ash deposits from past explosive eruptions. Explosive volcanic eruptions inject large quantities of ash into the atmosphere, dispersing it over large areas, sometimes thousands of kilometres from the vent. These deposits are geologically instantaneous, and therefore widely used as time marker horizons for constraining age in studies ranging from past climate reconstructions to archaeology. It is therefore crucial to be able to identify a given eruption very precisely in sites that may be far apart. Usually volcanic glass compositions are used to identify different eruptions, but sometimes the deposits of different eruptions from the same volcano are similar: ash layers from these eruptions cannot be reliably used as marker horizons. We propose to use apatite crystals in place of glass chemistry to correlate between different ash layers. Apatite is an ideal candidate for this because it: (i) is common in a wide range of eruption deposits; (ii) is stable and insensitive to weathering, and (iii) has high concentrations of the elements of interest, which allows very precise measurements of chemical composition. This development would facilitate the identification and correlation of ash layers and hence improve the robustness of correlations that are used in climate reconstructions and other fields. Better identification of ash layers would also help ash dispersal to be mapped and modelled accurately, which has clear implications for understanding the practical and economic outcomes of future explosive eruptions.

磷灰石是一种磷酸钙矿物，从各种不同的岩浆中少量结晶。它特别有用，因为它将“挥发性”物质，如水（H2O），二氧化碳（CO2），氯，氟和硫，纳入其矿物结构。挥发物是溶解的气体，是许多岩浆的重要组成部分。在火山下面的岩浆上升过程中，挥发物形成气泡，这些气泡膨胀并加速岩浆向上，从而引发火山爆发。磷灰石有可能成为喷发前岩浆中挥发性浓度的特别可靠的记录器，但为了利用它所包含的信息，我们需要独立测量H2O和CO2的“分配系数”-在熔体中给定浓度的矿物中发现的量。在项目的第一部分中，我们将在实验室中测量这些分配系数，以便我们可以从磷灰石中的浓度推断岩浆挥发分含量。我们还将测量氟和氯的分配系数。我们将测试的分配系数，用磷灰石来推断挥发性的岩浆系统，被称为Laacher看到凝灰岩，这已经很好地表征了使用替代方法的浓度。这座火山存款是在近13,000年前的一次大爆发中产生的，火山灰层散布在欧洲大陆中部，距离火山口长达1100公里。在该项目的第二部分，我们建议开发磷灰石作为一种工具，从过去的爆发火山灰沉积相关联。爆炸性火山爆发将大量火山灰注入大气层，将其分散在大片地区，有时距离喷口数千公里。这些沉积物在地质学上是瞬时的，因此被广泛用作时间标记层，用于从过去的气候重建到考古学的研究中限制年龄。因此，至关重要的是，能够在可能相距甚远的地点非常精确地识别特定的喷发。通常，火山玻璃成分被用来识别不同的喷发，但有时来自同一火山的不同喷发的沉积物是相似的：这些喷发的灰层不能可靠地用作标志层。我们建议用磷灰石晶体代替玻璃化学来关联不同的灰层。磷灰石是一个理想的候选人，因为它：（i）是常见的火山喷发沉积物的范围很广;（ii）是稳定的，对风化不敏感，（iii）具有高浓度的感兴趣的元素，这使得非常精确的测量化学成分。这一发展将促进灰层的识别和相关性，从而提高气候重建和其他领域中使用的相关性的鲁棒性。更好地确定火山灰层也有助于准确地绘制火山灰散布图和建立模型，这对了解未来爆炸性喷发的实际和经济后果具有明确的意义。