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

阿纳尔火山是目前仅有的几个经历几十年活动的弧火山之一。在近40年的持续活动中，喷发岩浆的整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的演化，其中喷发的玄武安山质岩浆的稳态生产与玄武岩到阿纳尔岩浆储集系统的幕式注入有关。喷发的阿纳尔岩浆的单调性意味着分馏，补给，上升和喷发是很好的平衡，以使岩浆基本上是均匀的，同时包含斑晶在喷发时具有不同的生长历史。在该项目中，将进行高温高压实验，以确定阿纳尔火山喷发的玄武安山质岩浆喷发前的条件。我们计划研究温度（900-1100°C）、压力（200- 400 MPa）、水活度（XH 2 O =0.3-1）和氧逸度对相平衡、矿物和熔体比例和组成的影响。这一结果将限制镁铁质岩浆在深部的储存、流体相的存在和出溶、向长英质岩浆分异过程的演化以及玄武质和英安质岩浆中镁铁质矿物的结晶（如富An斜长石）。近稳定状态的整个岩石成分在十年的时间尺度上的生产将被讨论，以及生产较早的英安质火山岩和变化，从爆炸性或喷发成分相似的镁铁质岩浆在过去的40年。这一结果对弧相关火山中高铝玄武岩和玄武安山岩的产出以及岩浆储集动力学、成岩历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.