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

火山喷发是由储存在火山下面几公里深处的含水硅质岩浆推动的。要了解这些岩浆如何从下地壳的源区上升到浅层储集层，取决于了解岩浆的物理性质及其在上升过程中的演化。这项事业的核心是了解温度如何随着上升岩浆中压力的下降而变化。沿着垂直裂缝或岩墙上升的浮力、低粘度的硅质岩浆可能是绝热的，也就是说，不会向周围释放热量。绝热冷却涉及每垂直行驶一公里的温度下降约1.5摄氏度。理论分析表明，在相同的深度范围内，这一温降小于液相线温度的降幅。换句话说，在上升过程中，含水的硅质熔体将达到高于其液相线的温度。这就是所谓的“过热”，它为岩浆提供了一种方法，使其从源头地区溶解任何夹带的晶体，并在上升过程中对堤坝的墙壁进行热腐蚀。对于给定的H2O含量，产生的过热量取决于液线表面相对于绝热气体的压力-温度斜率。不幸的是，关于地壳内含水硅质岩浆液相线表面压力-温度关系的详细实验测定太少了。其主要原因是很难用传统的实验技术获得必要的数据，这涉及到在不同的压力、温度和H2O含量下进行大量的实验。由于传统技术使用的是猝灭法，实验结果只能在实验完成后才能识别，使得整个过程非常耗时。我们建议开发一种新的、可供选择的原位实验方法，该方法利用了外部加热水热金刚石压腔(HDAC)的最新技术发展。该设备使用两个尖端扁平的大钻石来压缩少量含H2O的硅质岩粉。加热是通过电阻炉实现的，而压力则通过波纹管式装置非常精确地控制。钻石作为窗口，通过它来观察样品粉末随温度和压力的变化。在加热过程中所有晶体消失的点或在冷却过程中第一次出现晶体的点，在原位很容易识别。通过这种方法，可以在一次实验中绘制出固定H2O含量的起始材料的液相线，这是使用传统淬火方法根本不可能做到的。最新设计的HDAC能够达到900摄氏度的温度，足以熔化重量超过几个百分比的H2O的硅质岩石。这是一项概念验证提案，旨在测试HDAC是否适合在含水硅质岩浆的液相线表面产生所需的精确实验数据。我们已经设计了一系列实验评估，使用已知的压力-温度依赖性质的材料作为校准剂。如果成功，我们将继续撰写一份标准赠款提案，以确定几个具有重要地质意义的硅质岩浆的液面，并探索其对地球内岩浆上升的影响。

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.