Using element diffusivities to constrain the timescales of igneous processes
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Assessing mineralogical controls on high-temperature stable isotope fractionation
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Connecting laboratory studies to remotely-sensed geochemical observations
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Overview
My research focuses on the petrogenesis of igneous rocks, i.e., the processes of magma generation, solidification, and eruption. I am particularly interested in planetary differentiation and volcanism, and in understanding the geochemical differences and similarities amongst rocky planetary bodies. To better understand the chemistry and time scales of processes occurring in planetary interiors, I conduct experiments that characterize the high-temperature behavior of elements and their isotopes. While a primary focus of my Ph.D. has been the petrology of the Moon, I have also studied the magmas of Mars and Mercury, and am eager to study additional planets, moons, and asteroids as my career progresses. Additional details regarding specific research projects are provided below. |
"Stacking" a piston cylinder.
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ilmenite diffusion timescales
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At magmatic temperatures, elements rapidly exchange between minerals and silicate melt. As a magma cools, this exchange slows down, eventually coming to a halt when the magma drops below a critical temperature. Volcanic eruptions are one way in which the migrating elements are frozen in place, effectively preserving the ongoing elemental exchange as a temperature- and time-dependent diffusion profile. By quantifying the speed of elemental exchange in minerals, we can decipher important information regarding the timescales of volcanic processes.
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Back scattered electron image of a chemically zoned ilmenite megacryst from Monastery kimberlite.
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Oxide pairs from volcanic rocks, data from Ghiorso and Evans (2008). Red points identify ilmenite-magnetite pairs that are not equilibrated.
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Ilmenite is an iron- and titanium- bearing oxide mineral commonly found in volcanic rocks. Part of my Ph.D. work has been to experimentally determine how fast Fe, Mg, and Mn cations diffuse in ilmenite. We do this by heating a "diffusion couple" in a piston cylinder, allowing elements to exchange between the two parts of the couple, then measuring the resulting compositional gradient. One important finding from our work is that cation diffusion in ilmenite is strongly affected by oxygen fugacity. This result has important implications for the equilibrium between ilmenite and other oxide minerals in volcanic systems, as ilmenite (FeTiO3, all Fe2+) forms a solid solution with hematite (Fe2O3, all Fe3+), and the oxidation state of Fe greatly influences the migration of elements.
Diffusion profile fits to experimental data from geikielite-ilmenite diffusion couples.
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mineral-melt iron isotopic fractionation
Iron isotopic compositions of terrestrial and lunar volcanic rocks plotted as a function of TiO2. The high-Ti lunar basalts have a different composition than that of the low-Ti basalts, illustrating one instance of high-temperature Fe isotopic fractionation.
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The "isotopes" of an element refer to atoms that have the same number of protons but different atomic masses (resulting from a difference in the number of neutrons). The most common way to fractionate stable isotopes of an element is through mass-dependent processes. Mass-dependent fractionations are largest at low temperature and for elements with low atomic number, however, modern analytical precision has allowed geologists to resolve significant high-temperature fractionations of "non-traditional" stable isotopes, such as Fe.
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Evidence for Fe isotopic fractionation in igneous rocks suggests there are igneous processes capable of fractionating Fe isotopes to a measurable extent. As a major part of my Ph.D. work, I have been experimentally investigating whether the crystallization of minerals from silicate melt fractionates Fe isotopes. Additional influences on Fe isotopic fractionation in igneous rocks include Fe oxidation state, melt composition, and kinetic processes. In our recent GCA publication, we conclude that no measurable equilibrium fractionation exists between olivine and melt in reducing (all Fe2+) conditions across a wide range of TiO2 contents. We are continuing this work by determining mineral-melt fractionations for pyroxene and Fe-Ti oxides, with the goal of identifying the igneous processes responsible for generating the observed variation in the Fe isotopic compositions of the lunar mare basalts.
Iron isotopic compositions of the Fe reservoirs in an olivine crystallization experiment. While there is no resolvable difference between the olivine and melt (glass) compositions, we do see evidence for Fe isotopic fractionation between Re metal and silicate melt, as well as Fe isotopic fractionation via evaporation.
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Back scattered electron images of olivine crystallization experiments. Dark gray crystals = olivine; light gray matrix = glass (quenched melt).
Isotopic effect of evaporative Fe loss during one-atmosphere gas-mixing furnace experiments as determined by our work.
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magma oceans & basaltic volcanism
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The formation of rocky planets involves a "magma ocean" stage wherein the planetary body is wholly or partially molten and subsequently crystallizes. Segregation and sinking of dense metal within the melt leads to the formation of a metallic core. As the molten body cools, elements are either sequestered in crystals, retained in the melt, or lost to vapor (potentially forming an atmosphere). Post-differentiation, re-melting of crystals within the planetary body generates magmas that may eventually erupt to the surface. Understanding how elements are distributed during all of these processes is essential in placing constraints on both the extent and timing of planetary differentiation and magmatism.
My research in planetary differentiation and magmatism includes:
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Comparison of liquid compositions from experimental studies of lunar magma ocean crystallization. I created this figure as part of my involvement with the "New Views of the Moon 2" Reviews in Mineralogy and Geochemistry volume.
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planetary samples & Remote sensing
Back scattered electron image of Apollo sample 70275,51, a high-Ti lunar mare basalt.
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Planetary geochemistry is unique in that physical samples are limited to meteorites and samples returned by missions. Remote sensing is a powerful tool for studying the chemistry and petrology of planetary bodies. My research links high-temperature experiments with planetary samples and remote sensing in an effort to characterize the petrology and geochemistry of planetary bodies.
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Research topics I have studied by combining experiments with planetary samples and/or remote sensing:
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Reflectance spectroscopy of synthetic spinel. Increased chromium abundance gives rise to a sharp feature near 550 nm (Williams et al., 2016), while increased iron content gives rise to a broad absorption near 1000 nm (Jackson et al., 2014).
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