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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中。挥发物是溶解的气体，是许多岩浆的重要组成部分。岩浆在火山下上升时，挥发物形成气泡，膨胀并加速岩浆上升，为火山爆发提供燃料。磷灰石有可能成为喷发前岩浆挥发性浓度的特别可靠的记录者，但为了利用它所包含的信息，我们需要对水和二氧化碳的“分配系数”进行独立测量——在给定浓度的熔体中，矿物中发现的量。在项目的第一部分，我们将在实验室测量这些分配系数，这样我们就可以从它们在磷灰石中的浓度推断岩浆挥发性含量。我们还会测量氟和氯的分配系数。我们将通过使用磷灰石来推断岩浆系统（被称为Laacher See Tuff）中的挥发性浓度来测试分配系数，该系统已经使用其他方法进行了很好的表征。这些火山沉积物是在近13000年前的一次大爆发中形成的，火山灰层散布在离火山口1100公里远的欧洲大陆中部。在该项目的第二部分，我们建议开发磷灰石作为一种工具，从过去的爆发对比灰烬矿床。火山爆发将大量的火山灰注入大气，将其分散到大面积地区，有时距离火山口数千公里。这些沉积物在地质上是瞬时的，因此在从过去的气候重建到考古学的研究中被广泛用作限制年龄的时间标记。因此，能够在相隔很远的地点非常精确地识别给定的喷发是至关重要的。通常，火山玻璃成分被用来识别不同的喷发，但有时同一座火山不同喷发的沉积物是相似的：这些喷发的火山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者，因为它：(i)在广泛的喷发沉积物中很常见；(2)稳定，对风化不敏感，(3)有高浓度的感兴趣的元素，可以非常精确地测量化学成分。这一发展将有助于灰层的识别和关联，从而提高用于气候重建和其他领域的关联的稳健性。更好地识别火山灰层也将有助于准确地绘制和模拟火山灰的扩散，这对理解未来爆炸性喷发的实际和经济后果具有明确的意义。