Reconstructing Magma Storage Depths for the 2018 Kīlauean Eruption from Melt Inclusion CO2 Contents: The Importance of Vapor Bubbles

Wieser, Lamadrid, Maclennan, Edmonds, Matthews, Iacovino, Jenner, Gansecki, Trusdell, Lee, Ilyinskaya, G3 22(2), e2020GC009364 (2020). doi: 10.1029/2020GC009364

The CO2 content of magmas is controlled primarily by pressure. Since pressure increases with depth in the crust, the CO2 contents of magmas can be useful tool for estimating how deep magma chambers sit beneath volcanoes. However, by the time magma reaches the surface it has lost most of its CO2. A widely used to tool to get around this is to measure the CO2 content in tiny droplets of magma trapped within crystals (melt inclusions) as they grew within the magma chamber.

However, crystals never behave as perfect pressure vessels. In many cases they have been shown to leak, removing the signal of magma chamber depth they once preserved. In other cases, they don’t leak, but the CO2 from the magma exsolves to form a bubble within the melt inclusion. This study analyses this behaviour in detail, and derives estimates of magma chamber depths beneath Kīlauea.

This study provided an ideal testing ground for the new VESIcal software I am involved in developing. In particular it allowed comparison of different CO2 solubility models and how they take into account the secondary control of magma composition on CO2 solubility.

The image is Figure 10 from the manuscript. It demonstrates the disparity in pressure estimates derived from different CO2 solubility models. Whilst none of the models are incorrect, they all work best in different situations. Something important to consider when using melt inclusions as barometers.

Decoupling of zircon U-Pb and trace-element systematics driven by U diffusion in eclogite-facies zircon (Monviso meta-ophiolite, W. Alps)

Garber, Smye, Feineman, Kylander-Clark, Matthews, CMP 175, 1-25 (2020). doi: 10.1007/s00410-020-01692-2

The mineral zircon is used extensively for dating metamorphic processes. The U-Pb system is particularly useful; however, understanding U-Pb dates requires a knowledge of their mobility (or lack thereof) during crustal residence and metamorphism.

In this manuscript, data are presented that suggest U diffusion into zircon can be significantly faster than suggested by experiments. This has important implications for the types of process that can be recorded by zircon dating. My contribution to this study was providing an estimate of the redox state of U in the fluids likely to be reacting with the zircons studied here.

The image is Figure 12 from the manuscript. It shows my predictions for the dominant speciation of U in aqueous fluids at elevated temperature and pressure.