Unravelling disequilibrium in basaltic crystal cargoes: an experimental and petrological study of mixing in primitive Icelandic magmas

海洋玄武岩构成了地球表面喷发的大部分物质，它是熔体和晶体的复杂混合体，是在地幔上升和岩浆室加工过程中熔体的混合和结晶过程中形成的。不平衡的记录，即两个或两个以上相互不平衡的相组合的并置，在玄武岩晶体货物的结构和组成中被广泛保存。因此，岩浆组合和演化的现实模型必须将不平衡的影响纳入它们对岩浆过程的描述中。尽管相平衡研究为岩浆过程的本质提供了无数的见解，但使用平衡实验来理解自然样品中的不平衡特征的有效性仍然不清楚。因此，进行约束良好的不平衡实验来了解反应动力学是改进对天然玄武岩解释的关键下一步。该项目的主要目的是定量记录玄武岩岩浆的晶体含量如何响应岩浆混合导致的化学和热不平衡。这将通过进行高压（3 kbar）、高温（1160-1280°C）实验来实现，这些实验旨在模拟在冰岛特征良好的天然玄武岩中发现的混合过程。为了尽可能地分离化学不平衡和热不平衡的影响，一系列实验将研究含晶和成分不均匀的原始玄武岩（开放系统化学不平衡）的等温和等压混合。第二个系列将研究冷却和加热对含单晶原始岩浆组成的影响，分别模拟夹带和补给（封闭系统热不平衡）。不平衡实验将只在已经在以前的项目中确定了平衡相关系的初始组合物上进行。运行后的产品将被成像（BSE）和分析，以量化晶体的结构（如晶体尺寸分布）和成分对化学和热不平衡的反应。晶体生长、溶解和再平衡的速率将通过在一系列与冰岛火山系统爆发前动荡重叠的时间尺度上进行实验来评估。新的实验数据将用于重新评估冰岛火山喷发中天然晶体的不平衡特征，这些火山喷发包括1784-1784年的拉基火山和2014-2015年的霍鲁劳恩火山。例如，混合对岩浆过程结晶记录的影响将被研究。虽然这个项目将重点放在冰岛系统上，但由于其良好的地质环境，其发现将与任何携带晶体货物不平衡记录的玄武岩广泛相关。

Oceanic basalts, which form the bulk of material erupted at the surface Earth, are complex hybrids of melts and crystals assembled during mixing and crystallisation of melts during ascent from the mantle and processing in magma chambers. Records of disequilibrium, i.e., the juxtaposition of two or more phase assemblages that are out of equilibrium with each other, are widely preserved in the textures and compositions of basaltic crystal cargoes. Realistic models of magma assembly and evolution must therefore incorporate the effects of disequilibrium into their descriptions of magmatic processes. Although phase equilibria studies have provided innumerable insights into the nature of magmatic processes, the validity of using equilibrium experiments to understand disequilibrium features in natural samples remains unclear. Carrying out well-constrained disequilibrium experiments to understand reaction kinetics is thus a crucial next step in improving the interpretation of natural basalts.The primary aim of this project is to document quantitatively how the crystal contents of basaltic magmas respond to chemical and thermal disequilibrium as a result of magma mixing. This will be achieved by performing high-pressure (3 kbar), high-temperature (1160-1280 °C) experiments designed to the mimic mixing processes that have been identified in well-characterised natural basalts from Iceland. In order to separate the effects of chemical and thermal disequilibrium as much as possible, one series of experiments will investigate the isothermal and isobaric mixing of crystal-bearing and compositionally heterogeneous primitive basalts (open system chemical disequilibrium). A second series will investigate the effect of cooling and heating on a single crystal-bearing primitive magma composition, simulating entrainment and recharge respectively (closed system thermal disequilibrium). Disequilibrium experiments will only be performed on starting compositions for which equilibrium phase relationships have already been determined in a previous project. Run products will be imaged (BSE) and analysed to quantify the textural (e.g. crystal size distribution) and compositional response of crystals to both chemical and thermal disequilibrium. Rates of crystal growth, dissolution and re-equilibration will be evaluated by performing experiments over a range of timescales that overlap with those of pre-eruptive unrest in Icelandic volcanic systems. The new experimental data will then be used to re-assess disequilibrium features in natural crystals from a selection of Icelandic eruptions including the 1784-1784 Laki and 2014-2015 Holuhraun eruptions. For example, the ability of mixing to bias the crystalline record of magmatic processes will be investigated. While this project will focus on Icelandic systems, because of their excellent geological context, its findings will be widely relevant to any basalts that carry records of disequilibrium in their crystal cargoes.