Looking into the smaller to see the bigger: nanometer observations of minerals as clues to thermal histories and reservoir sizes of silicic magmas

Abstract:

One of the least well constrained but most important aspects of silicic magmas are the sizes and configuration of their host reservoirs. For medium to large systems (e.g. > 10 km³) geophysical and geodetical data have provided in some cases first order insights into the amount and density of melt or melt + supercritical fluids. These “images” are still however poorly resolved and often open to various interpretations. Another side of the coin could be to extract reservoir information from studying the geochemical and petrological characteristics of past eruption deposits. However, this remains a complex and elusive approach also fraught with various interpretations.

Here we take another look at exploring the size and thermal evolution of reservoirs with the use of time-temperature kinetic phase transformations that various groups of minerals ought to experience. We will present a variety of textural and chemical observations at the nanometer scale from pyroxene crystals of a variety of eruptions. Focused ion beam sample preparation applied to transmission electron microscopy, including in situ heating experiments, allows for site and orientation specific observation. The data are able to record temperature-time evolution spanning thousands of years which can be further translated into sizes of magmatic systems using a variety of geomtertical configurations and thermal models.