There and Back Again: Mantle Xenon Has a Story to Tell
The Earth has been through a lot of changes in its 4.5 billion year history, including a shift to incorporating and retaining volatile compounds such as water, nitrogen and carbon from the atmosphere in the mantle before spewing them out again through volcanic eruptions.
This transport could not have begun much before 2.5 billion years ago, according to researchers at UC Davis and Washington University in St. Louis, published Aug. 9 in the journal Nature.
Researchers at UC Davis and WUSTL modeled transport of xenon between the Earth’s mantle and the atmosphere as a marker for other compounds such as water, carbon and nitrogen. The model shows that this transport went through a shift not more than 2.5 billion years ago.
Over very long periods of time, the traffic of compounds between the deep Earth and the atmosphere controls the habitability of the planet’s surface.
“Life on Earth cares about changes in the volatile budget of the surface,” said Rita Parai, assistant professor of geochemistry at WUSTL and first author of the study. “And there’s an interplay between what the deep Earth was doing and how the surface environment changed over billion-year timescales.”
Volatiles such as water, carbon dioxide and the noble gases come out of the mantle through volcanism and may be injected into the Earth’s interior from the atmosphere, a pair of processes called mantle degassing and regassing. The exchange determines the surface availability of compounds that are critical to life — such as carbon, nitrogen and water.
Modeling xenon transport
Parai and Sujoy Mukhopadhyay, professor of earth and planetary sciences in the UC Davis College of Letters and Science, developed a model for how the noble gas xenon cycles between the mantle and the atmosphere. Xenon isotopes can act as tracers for compounds such as water, because minerals that carry xenon also carry water.
The model shows that not more than about 2.5 billion years ago, the Earth shifted from net degassing, with compounds being released from the mantle into the atmosphere, towards regassing with carbon, nitrogen and water moving into the mantle. This shift was potentially enabled by subduction, the conveyor-belt action of tectonic plates moving under each other.
The mechanical properties of the mantle change as water is added or removed, so the onset of regassing had an important effect on the internal churning of the mantle, known as convection, which controls plate motions at the surface, Parai said.
Mukhopadhyay’s laboratory studies how subduction zones – places where one tectonic plate slides beneath another and dives into the mantle – move chemical compounds between the Earth’s surface and the mantle. Parai and Mukhopadhyay have previously published work on transport of water at subduction zones and the group has ongoing projects using isotopes of boron and strontium to measure this traffic.
The project was supported by a grant from the National Science Foundation.
— Talia Ogliore, Senior News Director for Science at the Office of Public Affairs, WUSTL