We jumped off the cars and stared at the exposed wall of the quarry. Everyone was speechless. Facing us, a six meter-thick silvery band of graphite, cutting the wall and the surrounding rocks like a knife.
It was an inerasable mark left by the migration of ancient deep fluids, carrying enormous amounts of methane and other potential energy sources for deep life, like hydrogen and ammonia. But where and how did these deep energetic fluids originate?
We were a group of eleven researchers and geology students from five nations (Italy, France, China, Canada and the United States), all seeking answers to this question. After a long journey, starting at Montreal airport in Canada and driving south, we had finally reached Vermont in the United States. It was early September, maple leaves were just starting to turn orange, and the sun illuminated the forested hills of the Northern Appalachians. As picturesque as this is, this is not exactly the place where, as a geologist, you would expect to find nice outcrops! Luckily for us, close to the small town of Eden lies an impressive abandoned quarry; the twenty meter-tall steps excavated during the now-terminated mining activity offers a unique perspective on these ancient rocks and their complex geological history.
The rocks that we were looking at belong to the so-called Belvidere Mountain Complex (BMC): strongly deformed and metamorphosed mafic and ultramafic rocks that more than 500 million years ago formed the basement bottom of the Iapetus Ocean. Around 470 million years ago, a major shift in plate motions led to the subduction of the BMC rocks underneath a now long-gone continent, Laurentia.
During this collision, the BMC rocks were hauled to great depths. Fluids circulating at high temperature and pressure interacted with the rocks, resulting in intriguing chemical reactions. Iron-bearing minerals reacted with aqueous fluids, generating hydrogen. Gases, solutes and minerals in the subducting system reacted and combined, like ingredients in a hot cauldron. The final result was an energetic cocktail of gases – hydrogen, methane, ammonia, and hydrogen sulfide – which are potential energy sources for deep life somewhere above the subducting plate.
Surprisingly, not all of these ancient geological fluids have been lost. Extremely small amounts remained trapped inside minerals. These ‘fluid inclusions’, visible under at the optical microscope, are filled with bubbles of gas and liquid water. Fluid inclusions are a window into deep geological fluids produced in subduction zones, therefore allowing us to study the geological processes that generate deep energy sources for life, like methane and hydrogen.
During the campaign, which lasted from the 4th to the 13th of September 2023, we walked through every corner of the quarry, filling our backpacks with heavy rock samples. We sought possible evidence of past methane migration like the graphite band that we observed at the beginning of this story. Back at the Deep Carbon Lab in Bologna, the collected samples are studied at the optical microscope and the fluid inclusions are analyzed through different techniques.
This work remains ongoing, and it will help us better understand the processes that generated hydrogen and methane hundreds of millions of years ago in this area, with possible further implications for deep energy production at convergent margins.
Orlando Olivieri
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