Pre-eruptive conditions of basaltic andesite at Arenal Volcano, Costa Rica

阿雷纳尔火山是目前仅有的经历数十年活动的弧形火山之一。在近40年的持续活动中，岩浆喷发的主体成分几乎保持不变。然而，矿物和熔融包裹体记录了一个复杂的开放系统演化，其中玄武岩安山岩浆的稳定产生与玄武岩向Arenal岩浆储集层系统的幕式注入有关。芳烃岩浆喷发的单调性意味着分馏、补给、上升和喷发很好地平衡，以便岩浆在喷发时基本均匀，同时包含不同生长历史的斑晶。在本项目中，将进行高温高压实验，以确定阿雷纳尔火山喷发的玄武岩安山岩岩浆喷发前的条件。我们计划研究温度(900-1100℃)、压力(200-400兆帕)、水活度(XH2O=0.3-1)和氧逸度对相平衡、矿物和熔体比例和组成的影响。这些结果将制约镁铁质岩浆在深部的储存，流体相的存在和出溶，在分异到更多长英质岩浆的过程中演化的过程，以及玄武岩和英安岩浆中镁铁质矿物的结晶(例如富An的斜长石)。将讨论在十年时间尺度上接近稳定状态的全岩成分的产生，以及更早的英安岩特弗拉斯的产生，以及过去40年来成分相似的镁铁质岩浆从爆炸或喷发到喷发的变化。这一结果对火山中高铝玄武岩和玄武岩安山岩的产生以及岩浆储集层动力学、成岩史和火山喷发方式等问题也具有普遍意义。

Arenal volcano is currently one of the only arc volcanoes undergoing multi-decade activity. The bulk composition of magma erupted has remained nearly constant during almost 40 years of continuous activity. However, minerals and melt inclusions record a complex open-system evolution in which steady state production of the erupted basaltic andesitic magma is linked to episodic injections of basalt into Arenal magma reservoir system. The monotony of erupting Arenal magmas implies that fractionation, recharge, ascent, and eruption are well balanced in order for magmas to be essentially uniform while containing phenocrysts with different growth histories at the time of eruption. In this project, high-temperature and high-pressure experiments will be conducted to determine the pre-eruptive conditions of basaltic andesitic magmas erupted at Arenal volcano. We planed to investigate the influence of temperature (900-1100°C), pressure (200- 400 MPa), water activity (XH2O=0.3-1), and oxygen fugacity on phase equilibria, mineral and melt proportion and composition. The results will constrain the storage of mafic magma at depth, the presence and exsolution of the fluid phase, the processes evolved during differentiation to more felsic magma and the crystallization of mafic mineral in both basaltic and dacitic magmas (e.g. An-rich plagioclase). The production of a near steady-state whole rock composition over decadal time scales will be discussed as well as the production of earlier dacitic tephras and the changes from explosive or effusive eruption with compositionally similar mafic magmas during the last 40 years. The results are also of general interest for the production of high-alumina basalts and basaltic andesites in arcrelated volcano and for the questions about magmatic reservoir dynamics, petrogenetic history, and eruptive style of volcanic eruption.