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Volcanoes and hydrogen in nominally anhydrous minerals (NAMs)

Small amounts of hydrogen (sometimes called water or hydroxyl) in deep-earth minerals such as olivine and clinopyroxene can significantly affect the melting temperature of rocks and therefore the movement of lithospheric plates and occurrence of volcanic eruptions. Understanding how hydrogen behaves in these minerals will help to explain these phenomena and perhaps even provide a clock to measure the magma ascent rate, which is a key factor determining the explosiveness of a volcanic eruption.

The whole-block method

Paper in American Mineralogist

Python code on github

I developed a way to measure hydrogen distributions in a NAM cut into the shape of a prism by FTIR without cutting the sample. This "whole-block" method has several advantages over previously established approaches. For instance, I can perform multiple experiments (e.g., a time series or reversal) and estimate diffusivities in three orthogonal directions using the same sample.

Movie 1, Movie 2, Movie 3
(mp4; may require plugin)

Site-specific hydrogen incorporation

The FTIR spectra of most nominally anhydrous minerals reveal multiple peaks that correspond to different sites in which hydrogen is incorporated. These peaks often move at different rates, potentially providing information about geologic processes operating over multiple timescales.

FTIR spectra show changes in hydrogen site distribution

Site-specific hydrogen diffusion in clinopyroxene

Paper in Contributions to Mineralogy and Petrology

Python code on github

How fast hydrogen diffuses can be described by the diffusivity (higher diffusivities, so higher up on the diagram, indicate faster movement). H incorporated in different ways, as indicated by different FTIR peak positions, may differ by an order of magnitude. This difference, though large, is smaller than the range in diffusivities resulting from differences correlated with iron (Fe) content. The large majority of natural clinopyroxenes have so much iron that they are expected to have diffusivities on the high end of this range, which is so fast that the hydrogen in mantle xenoliths potentially equilibrates with the host magma.

Bulk H (large symbols) and peak-specific H diffusivities in clinopyroxene change with sample chemistry

Hydrogen diffusion in olivine

NSF award abstract

I have ongoing experimental work meausuring the diffusivity of hydrogen in olivine, with particular emphasis on site-specific behavior during dehydration.

Oriented whole block of San Carlos olivine

Actinide-bearing zircon-structured solid solutions

Paper in American Mineralogist

The mineral zircon is very common in the natural world and has been considered for long-term disposal of actinides such as plutonium. I used computational techniques to calculate thermodynamic properties of seven related systems, including phase diagrams and ordering schemes. ZrSiO4 and HfSiO4 form a nearly ideal solid solution, but the miscibility of (Zr,Th)SiO4, (Zr,U)SiO4 (Zr,Pu)SiO4,(Zr,Ce)SiO4, (Hf,Pu)SiO4, and (U,Th)SiO4 was limited to no more than 12 mole % of the substituting cation.

UO2 oxidation in the presence of corroding steel

Paper in Journal of Nuclear Materials

Solid uranium dioxide (UO2) is chemically similar to nuclear fuel. When UO2 corrodes in air and water, the U is oxidized to form the highly mobile ion UO22+. My experiments indicated that surrounding the UO2 with corroding steel may significantly slow the corrosion process.

Ti-in-zircon geothermometry

Paper in Journal of European Mineralogy

The concentration of titanium in zircon can give a rough estimate of the temperature at which a rock last reached thermodynamic equilibrium. I used computational techniques to show that the pressure correction for this geothermometer is likely on the order of 100°C/GPa, and it may not be useful at all for systems that have experienced pressures above ~3.5 GPa (a depth of ~110 km).