Compositional Buffering of Magmas as an Indicator of Magmatic Processes
Whereas 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.