Mapping out the liquidus surface of hydrous silicic magmas in a hydrothermal diamond anvil cell

绘制热液金刚石砧室中含水硅质岩浆的液相线表面

• 批准号: NE/E007953/1
• 负责人: Jonathan Blundy
• 金额: $ 5.12万
• 依托单位: University of Bristol
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2007
• 资助国家: 英国
• 起止时间: 2007 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FE007953%2F1
• 关键词: Mapping; out; liquidus; surface; hydrous

Explosive volcanic eruptions are fuelled byhydrous silicic magmas stored at depths of a few km below a volcano. Understanding how such magmas ascend from their source regions in the lower crust to the shallow storage reservoir relies on knowing the physical properties of the magma and their evolution during ascent. Central to this enterprise is a knowledge of how temperature changes with decreasing pressure in ascending magma. Buoyant, low-viscosity silicic magmas that ascend along vertical factures, or dykes, may do so adiabatically, that is without losing heat to their surroundings. Adiabatic colling involves a temperature drop of about 1.5 degrees Celsius for each vertical km travelled. Theoretical considerations suggest that this temperature drop is less than the decrease in liquidus temperature over the same depth range. In other words, hydrous silicic melts will attain temperatures above their liquidus during ascent. This is known as 'superheat' and provides a means for the magma to dissolve any entrained crystals from its source region and to thermally corrode the walls of the dyke during ascent. The amount of superheat generated depends on the pressure-temperature slope of the liquidus surface for a given H2O content compared to that of the adiabat. Unfortunately there are too few detailed experimental determinations of the precise pressure-temperature dependence of the liquidus surface of hydrous silicic magmas within the crust. The principal reason for this is the difficulty of obtaining the requisite data using conventional experimental techniques, which invole performing a large number of experiments at different pressure, temperatures and H2O contents. Because conventional techniques use the quench method, the outcome of an experiment can only be discerned after it has finished, making the whole process extremely time-consuming. We propose to develop a novel, alternative, in situ experimental method which takes advantage of the latest technical developments in externally-heated hydrothermal diamond anvil cells (HDAC). This apparatus uses two large diamonds with flattened tips to compress a small volume of H2O-bearing silicic rock powder. Heating is achieved via a reistance furnace, while pressure is controlled very precisely with a bellows-type device. The diamonds serve as windows through which to observe changes in the sample powder with changes in temperature and pressure. The liquidus can be readily identified in situ as the point at which all crystals disappear during heating or the first appearance of crystals during cooling. In this way it is possible to map out the liquidus for a starting material of fixed H2O content in a single experiemntal run, something that is simply not possible using conventional quench methods. The latest design of HDAC is capable of temperatures up to 900 degrees Celsius, plenty high enough to melt silicic rocks with more than a few weight per cent H2O. This is a proof of concept proposal to test the suitability of HDAC to generate the required precise experimental data on liquidus surfaces of hydrous silicic magmas. We have devised a series of experimental evaluations using materials with known pressure-temperature dependent properties as calibrants. If successful, we will go on to write a Standard Grant proposal to determine the liquidus surfaces of several geologically important silicic magmas and explore the implications for magma ascent within the Earth.

火山爆发是由储存在火山下几公里深处的含水岩浆推动的。要了解这些岩浆是如何从下地壳的源区上升到浅储层的，就必须了解岩浆的物理性质及其在上升过程中的演变。这项工作的核心是了解温度如何随着岩浆上升压力的降低而变化。具有浮力的、低粘度的岩浆沿着沿着垂直裂缝或岩脉上升，可能是以热的方式上升的，也就是说，不会将热量损失到周围环境中。绝热冷却意味着每垂直行驶一公里，温度下降约1.5摄氏度。理论上的考虑表明，在相同的深度范围内，该温度下降小于液相线温度的下降。换句话说，含水的氢氧镁熔体在上升过程中将达到高于其液相线的温度。这被称为“岩浆热”，为岩浆提供了一种溶解来自其源区的任何夹带晶体的方法，并在上升过程中对岩墙进行热腐蚀。对于给定的H2O含量，生成的H2O的量取决于液相线表面的压力-温度斜率。遗憾的是，对于地壳内含水岩浆液相线表面的精确压力-温度依赖性的详细实验测定太少。其主要原因是使用常规实验技术难以获得所需数据，常规实验技术涉及在不同压力、温度和H2O含量下进行大量实验。由于传统技术使用淬火方法，实验结果只能在完成后才能看出，使得整个过程非常耗时。我们建议开发一种新的，替代的，原位实验方法，利用外部加热的水热金刚石对顶砧细胞（HDAC）的最新技术发展。该设备使用两个具有扁平尖端的大金刚石来压缩小体积的含H2O的滑石粉。加热是通过电阻炉实现的，而压力是通过波纹管型装置非常精确地控制的。金刚石作为窗口，通过它观察样品粉末随温度和压力变化的变化。液相线可以很容易地在原位确定为在加热过程中所有晶体消失或在冷却过程中晶体首次出现的点。以这种方式，可以在单次实验运行中绘制出固定H2O含量的起始材料的液相线，这是使用常规淬火方法根本不可能的。HDAC的最新设计能够达到900摄氏度的温度，足够高到足以融化含有超过几个重量百分比的H2O的岩石。这是一个概念验证的建议，以测试HDAC的适用性，以产生所需的精确的实验数据的液相线表面的含水岩浆。我们已经设计了一系列的实验评估使用已知的压力-温度依赖性的材料作为校准。如果成功的话，我们将继续写一个标准赠款计划，以确定几个地质上重要的岩浆的液相线表面，并探索地球内岩浆上升的影响。