Summer Project Description:

The Dependence of the Hydrogen Incorporation Mechanism of Mantle Olivine on Oxygen Fugacity

It is hypothesized that nominally anhydrous minerals function as reservoirs for water in the upper mantle. This class of mineral includes important mantle constituents such as olivine, clinopyroxene, garnet, and orthopyroxene. Olivine ((Fe,Mg) 2 SiO 4 ) is by far the most important nominally anhydrous mineral because it is thought to make up an overwhelming percentage of mantle composition, approximately 60%. In the high-pressure/temperature conditions of the upper mantle, olivine, though anhydrous on the earth's surface, has the ability to hold water in its structure through hydrogen occupancy at certain lattice sites. There are important implications for water in the mantle. Though olivine's water storage capacity per mole is small, its shear abundance in the mantle would correlate to a volume of water as great as four times that of the entire hydrosphere. Water affects mantle viscosity, phase transition depth intervals, seismic wave velocity, convection, and mantle rheology. Therefore, it is important to understand the hydrogen incorporation mechanism in mantle olivine to better understand these effects. However, there is great debate as to exactly where and how hydrogen atoms are incorporated in the structure; either at point defect locations, where a cation is missing, or in interstitial spaces.

Oxygen fugacity, or the non-ideal partial pressure due to oxygen, is also thought to play an important role in governing the mechanism of hydrogen incorporation in the mantle. Oxygen fugacity decreases with increasing depth, creating a reducing environment in the mantle. However, as mantle material rises towards the surface, oxygen fugacity increases. The exact dependence of the hydrogen incorporation mechanism on oxygen fugacity remains largely unknown.

I will be synthesizing a minimum of three olivine samples in a multi-cell anvil press (D-DIA) located at Brookhaven National Laboratories. Oxygen fugacity will be systematically varied by using nickel, iron, and rhenium metal capsules to contain the sample. Fourier Transform Infrared Spectroscopy, from the National Synchrotron Light Source, will then be employed to analyze the samples and determine where hydrogen atoms have been incorporated as oxygen fugacity changes. It is the goal of this study to obtain a quantitative mechanism of hydrogen incorporation in mantle olivine.

Thomas Kennell
SUNY Geneseo, NY
Major: Chemistry
Mentor: Liping Wang