Compositional Buffering of Magmas as an Indicator of Magmatic Processes

许多火山在很大的温度范围内喷发成分不同的岩浆，而另一些火山在很小的温度范围内喷发成分几乎相同的岩浆。当岩浆上升穿过岩石圈时，必须有一个或多个岩浆作用缓冲后一类系统中的成分，而确定和量化这些过程是本提议的目标。由于缓冲作用所涉及的过程对整个岩石成分的变化很小，因此研究的主要手段将是再均一化熔体包裹体的电子和离子探针分析。这项研究的很大一部分，以及其他许多研究，都依赖于熔融包裹体可靠地反映岩浆成分演化的假设。为了验证这一假设，设计了一系列实验来评估包裹体成分的可靠性程度。生长的晶体改变了边界层中液体的组成，边界层的厚度取决于化学成分的扩散系数和晶体的生长速度。成分边界层对熔体包裹体成分的影响将通过在与其处于平衡状态的熔体存在的情况下对预裂晶体进行热处理来创建人造熔体包裹体来模拟。不同厚度的边界层应由不同的冷却速率和不同的总冷却量产生。

GeistEAR-0003367Whereas many volcanoes erupt magmas of diverse composition over a wide range of temperatures, others erupt magmas of nearly identical composition over a narrow range of temperature. One or more magmatic processes must buffer the compositions in the latter type of system as the magmas ascend through the lithosphere, and determining and quantifying those processes are the objectives of this proposal. Because the processes involved in buffering cause little variability in whole rock compositions, the main means of the study will be electron and ion microprobe analysis of rehomogenized melt inclusions. Much of this study, and many others, depends on the presumption that melt inclusions reliably reflect the compositional evolution of the magma. To test this presumption, a series of experiments has been designed to assess the degree that inclusions' compositions are reliable. Growing crystals change the compositions of the liquid in a boundary layer whose thickness depends on the diffusivity of the chemical constituents and the growth rate of the crystal. The effect of a compositional boundary layer on the composition of melt inclusions will be simulated by creating artificial melt inclusions by annealing a pre-fractured crystal in the presence of a melt with which it is in equilibrium. Boundary layers of different thicknesses should be generated by a range of cooling rates and different amounts of total cooling.