Subduction zones, where the edge of one tectonic plate slips beneath another, have earned the reputation as “factories.” Carbon from bits of ocean crust, sediments, and pieces of mantle enter the subduction zone, which recycles some carbon back into the atmosphere through volcanoes, and sends some into the mantle for further processing. Accounting for what’s coming in and out of the subduction zone factory is vital to understanding the finer points of the deep carbon cycle.
In a new paper in Geochimica et Cosmochimica Acta , DCO members present evidence that when carbonates and water within an ocean plate enter the subduction zone factory, they transform into graphite and light hydrocarbons, such as methane, ethane, and propane. Renbiao Tao (Université Claude Bernard Lyon, France, formerly of the Carnegie Institution for Science, USA), Lifei Zhang, Meng Tian, Jianjiang Zhu (all at Peking University, China), Vincenzo Stagno (University of Rome, Italy), and Yingwei Fei (Carnegie Institution for Science, USA) examined rock samples from the Southwest Tianshan subduction zone in China. The researchers discovered bubbles of hydrocarbons trapped in eclogite, a metamorphic rock that forms at high pressure from the material in subducted ocean plates. The researchers also simulated the subduction environment in the lab, using a high-pressure multi-anvil press, to show that carbonate and water can react under the conditions found in nature to form a mix of light hydrocarbons and graphite.
Most hydrocarbons on Earth come from the remains of living cells, but researchers think that smaller amounts of hydrocarbons can form in the subsurface abiotically, in the absence of life. Previous studies, however, have found few natural examples of hydrocarbons forming abiotically within subduction zones.
At the Tianshan subduction zone, an ocean plate sank beneath a continental plate, and carried large amounts of carbonate and water into the subduction zone factory. The materials experienced increasing pressures as they sank, but lower temperatures than other subduction zones, resulting from the high water content stored in the subducting slab. “These rocks are already subducted to deep Earth, and then exhumed to the surface,” said Tao. When he and his colleagues examined the bubbles of fluids inside the rocks, they “found that hydrocarbons are trapped under high-pressure conditions in minerals, like omphacite.”
Additional analysis of the eclogite samples from Tianshan suggests that the rocks have low oxygen fugacity, which is a way of expressing how much oxygen is in an environment. Under low levels of oxygen, the carbonates could transform into hydrocarbons and graphite, which are less oxidized forms of carbon.
To further explore the possibility that the hydrocarbons came from carbonates and water in the subducting slab, the researchers recreated subduction zone and upper mantle conditions in the lab using a large-volume multi-anvil press. They mixed carbonates, in the form of the mineral iron-bearing dolomite, with water inside a gold capsule and exerted pressures up to 6 GPa (almost 60,000 times the atmospheric pressure at sea level) and temperatures of 600 to 1200 degrees Celsius.
After the samples cooled down, the researchers extracted the gas phase and analyzed the contents. They discovered methane, ethane, and propane, suggesting that hydrocarbons can form from carbonate and water under these conditions. In the solid phase, they detected the same assemblage of high-pressure minerals that they found in the natural samples from Tianshan, suggesting that the same reaction they observed in the lab may also occur within the subduction zone factory.
Tao will continue investigating how abiotic hydrocarbons form within subduction zones as a new postdoctoral researcher at the Université Claude Bernard Lyon. Tao attended several early career workshops through the DCO and his current work is DCO-funded. He plans to complicate the water-carbonate system through the addition of mantle minerals such as olivine, which will more closely simulate the subduction zone environment. “In the future we want to expand our system to see which kinds of conditions can yield more abiotic hydrocarbons,” he said.