New research on ancient Alpine rocks may unveil clues to Earth’s evolution
High up in the Western Alps is a swath of rocks that could provide new insight into what happens deep in the Earth's subsurface. A new $4.2 million initiative, known as the ExTerra Field Institute and Research Endeavor (E-FIRE), will allow researchers from nine U.S. institutions to conduct in-depth analyses of these rocks, which will improve our understanding of the forces governing activity beneath the crust and help us better understand the Earth's evolution.
Penn State received $449,000 of this grant to fund graduate research in this area, to analyze how thorium and uranium decay to lead in the Earth's subsurface and to archive samples collected during the field institute in the Earth and Mineral Sciences Museum collections facility.
The rocks formed approximately 45 million years ago when an ancient oceanic plate slipped beneath the African continental plate in a process known as subduction. When the oceanic plate and sediment from the ocean floor were pushed into the subsurface, they were heated and pressurized quickly, resulting in the formation of various types of metamorphic rocks. The rocks formed in the ancient subduction zone were quickly returned to the Earth's surface roughly 30 million years ago during the collision of the African and Eurasian tectonic plates that resulted in the formation of the Alps.
"This project will be like lifting the hood on the insides of a subduction zone," said Andrew Smye, assistant professor of geosciences, Penn State. "The rocks we will be investigating are called eclogites and they formed about 80 to 100 kilometers (50 to 60 miles) beneath the Earth's surface. There's no other place on Earth where you can find samples that are as fresh and unaltered as these."
In 2017 and 2019, The E-FIRE research team will trave to multiple sites in the Italian and Swiss Alps to collect samples. Researchers from each institute will look at a different process that takes place within a subduction zone.
The process of subduction is important yet not well understood because the activity takes place hundreds of miles beneath the Earth's surface. Most tsunamis are generated as a result of earthquakes in subduction fault zones, and subduction allows for many life-important elements, such as water and carbon dioxide, to be transported deep into the Earth. Subduction is a key process that shaped the evolution and formation of Earth's continents and oceans.
"At the depth at which subduction takes place, the cold, wet ocean floor moves into a high-pressure, high-temperature zone when it meets mantle," said Maureen Feineman, assistant professor of geosciences, Penn State, and co-principal investigator for E-FIRE. "New bonds form between elements, new minerals are created and many volatiles are given off — there's a whole series of events happening as a result of this change in pressure, but we unfortunately don't know much about what processes are taking place."
The Penn State researchers — Feineman, Smye and Kirsty McKenzie, doctoral student in geosciences — will investigate the process by which both thorium and uranium decay to lead. This research will complement other research being conducted by partner institutions.
"Uranium and thorium are important for radiogenic dating because they both decay to different isotopes of lead naturally," said McKenzie, whose research at Penn State is being funded by the E-FIRE grant. "We will be seeking a better understanding of how each of these elements are processed and cycled during the subduction process, and which types of minerals they attached to when the eclogites were exhumed."
The Penn State researchers will use an arsenal of analytical tools, such as mass spectrometry and laser ablation, to investigate whether the ratios of uranium, thorium and lead that were formed in the mantle match the ratios found in other parts of the world. This will help them understand how the uranium-thorium decay process differs in a high-pressure subduction zone, which will improve our understanding of how Earth systems have evolved through time.
Partner institutions on E-FIRE include Boise State University, Lehigh University, Virginia Tech, the University of Maryland, the University of Texas at Austin, Woods Hole Oceanographic Institution, the University of California Santa Barbara and Boston College.
A'ndrea Elyse Messer