Experimental and field studies of the timescale of igneous processes

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Continents drift and mountains grow on timescales much longer than a human lifespan. Geologists study these processes by measuring changes in the amount of radioactive elements, like uranium, in minerals. The long half-life of most of these elements restricts their use to timescales of at least a few hundred thousand years. We often forget that there are geologic processes, such as those at volcanoes, which occur over timescales of seconds to years. To understand volcanic processes we need ways that we that can use minerals in lava to resolve these short timescales. In the last six years, my students and I have used reactions between magma and the mineral olivine as a time capsule. Olivine picked up by magma reacts with it and creates a rind of different composition. As the magma rises, chemical elements are exchanged between the original grain and the rind making a region of intermediate composition between them. The change in composition and width of this region depend on many factors, the most important of which is time. Experiments with olivine, at temperatures equivalent to those in magma chambers, led to mathematical models that describe how composition changes with time. We have used these models and olivine time capsules in lava to show that magma can rise to the earth's surface from a depth of 50 km in less than two hours.***Unfortunately, olivine time capsules are not present in all volcanic rocks, so we need to develop a similar method for a range of different minerals. My proposal is to work with eight students to develop the mineral clinopyroxene, a common mineral in volcanic rocks, as a time capsule. Clinopyroxene is attractive for this because it often has zones of different composition arranged in a “tree-ring” like pattern around the core of the crystal. The first step will be to do experiments to find out how composition and temperature affect the speed of exchange of the chemical components in clinopyroxene. We will do this by sticking together two crystals of different composition and heating them to the temperature of a magma chamber; up to 1200 Celsius for, at least 100 days. The composition where the two crystals meet will change over a distance similar to the diameter of a human hair. We will use an electron microscope to measure the changes in composition and then make mathematical models that will account for changes in composition with temperature and time.***Once we develop our models, we will apply them to clinopyroxene in lava from ancient and modern volcanoes and build up a picture of how they have worked in the past. When we use this technique at volcanoes that have erupted recently and have been monitored before, during and after an eruption, we will be able to relate changes in mineral and magma composition, monitoring information, and the duration of events in the magma chamber from our time capsules. This will allow us to make better models of how volcanoes work and make better predictions of future eruptions.