Andrew Smye, assistant professor of geosciences at Penn State, will use a $640,000 Faculty Early Career Development Program grant from the National Science Foundation to shed light on a geological mystery while advancing educational opportunities for underrepresented students.
Smye will use the five-year grant to investigate why the Earth retains water at the surface instead of the necessary-for-life substance being sequestered by the Earth’s thirsty mantle. Geoscientists understand that subduction — the process by which hydrated oceanic plates are buried into the mantle — transports surface water to mantle depths, but they are not sure how that water is released from the subducting plate and returned to the surface through volcanic activity. This geophysical anomaly is key to understanding how Earth and similar planets can sustain life over billions of years.
“I’ve always been fascinated that Earth has been able to retain its surface water,” Smye said. “Without the oceans and other surface waters, obviously, life wouldn’t persist. What’s interesting is that if we compare Earth to Mars there’s good evidence to suggest that Mars once had surface water. We don’t yet know what makes our planet so different.”
To investigate this, Smye will develop a novel technique that links the concentrations of noble gases — neon, argon, krypton and xenon — with concentrations of water in rock samples that were previously subducted and returned to the surface during episodes of mountain building.
Noble gases are strongly concentrated in the Earth’s surface reservoirs — the atmosphere, seawater and sediments — making them powerful tracers of seawater-derived materials through the subduction process, Smye said, adding that his research will investigate the path water takes as the Earth’s oceanic crust is subducted.
The rock samples Smye and students will investigate are mostly from the European Alps. After samples are prepped and characterized at Penn State, Smye will work with collaborators to measure noble gas concentrations and isotopic compositions at the Woods Hole Oceanographic Institution.
“What I find very exciting about this research is that it’s novel ground,” Smye said. “There are very few measurements of the noble gases on metamorphic rocks because they’re so difficult to measure in these small concentrations.”
These metamorphic rock samples had quite a journey. Before finding their way to geoscientists’ microscopes and mass spectrometers they were below the ocean floor some 160 to180 million years ago before being subducted roughly 40 to 60 miles below the Earth’s surface. Later, they rose to the surface through a combination of buoyancy and erosion.
Smye said these samples are key because they are the only direct record of the physical and chemical conditions of subducting plates beneath volcanic arcs — the location at which juvenile continental crust is extracted from the mantle. Previous attempts to quantify the passage of water through subduction have been limited by theoretical assumptions or the use of elements with small numbers of isotopes, such as oxygen. Noble gases, Smye said, are well-suited to the problem of tracing fluid flow through metamorphic rocks because they are chemically inert, prefer to reside in fluid over minerals and have abundant isotopes for chemical fingerprinting of the process.
By measuring the noble gas composition of minerals in these exhumed rocks, Smye aims to link the concentrations and compositions of the gases to the pressures and temperatures at which the minerals formed, and the concentrations of water stored in the rock. From there, he hopes to construct a map of water concentration across pressures and temperatures relevant to subduction, constraining the depths at which noble-gases and, by inference, water are removed from the subducting plate.
These historic tracers can then be used to study the processes as they play out in real time. If the link is established, scientists can travel to active subduction zones, measure the noble gases coming out of the volcanoes, and measure the origin and fluxes of the water being returned to the surface versus how much is conveyed to the mantle.
If Smye and his students can figure out how and how much water is transported from the subducting plate to the surface, instead of being swallowed by the mantle, he will contribute to solving a geological mystery that is critical for explaining what promoted habitable conditions on Earth.
“In my mind, this is one of the most fundamental questions that remains unsolved in the Earth sciences,” Smye said. “I think understanding what goes on deep in the planet’s subduction zones is key to answering the question.”
Increasing diversity in STEM fields
A portion of the grant will be used to develop and implement a series of summer learning opportunities, designed to enhance accessibility to petrological research for undergraduate students from minority-serving institutions. Students from these institutions will get a taste of geosciences research during three-week projects at Penn State that use an array of analytical instrumentation, including the newly established geochronology lab, of which Smye is a co-director.
Participants will shadow faculty and graduate students, which, Smye said, will give students an exciting introduction to petrological research while offering the experiences and connections needed to pursue a career in the geosciences.