December 7, 2013
Abandoned Mine May Offer Clues About Permanent CO2 Sequestration
April Flowers for redOrbit.com - Your Universe Online
Can an abandoned mine help with the fight against global warming? Researchers from Stanford University think so. They have been using an abandoned mine to gain new insights on how to permanently entomb greenhouse gas emissions inside the Earth.
The team has spent two years trying to unravel a geological mystery at the Red Mountain mine, located about 70 miles east of the campus. Some of the world's largest veins of pure magnesium carbonate, or magnesite — a chalky mineral made of carbon dioxide (CO2) and magnesium — are found in this mine. Scientists have always been puzzled over how magnesite veins formed millions of years ago, however.
The research team has proposed a solution, which could lead to a novel technique for converting CO2, a potent greenhouse gas, into solid magnesite.
"Conventional geological storage involves capturing CO2 from industrial smokestacks and injecting it as a fluid into the subsurface," said Kate Maher, an assistant professor of geological and environmental sciences at Stanford. "But there is concern that the carbon dioxide would eventually leak back into the atmosphere. Our idea is to permanently lock up the CO2 by converting it into a stable mineral."
More than 60 percent of global CO2 emissions are caused by power plants and other industries, according to the International Energy Agency. Maher explained that sequestering the CO2 in magnesite deposits would prevent the gas from entering the atmosphere and warming the planet.
In the early 20th century, magnesite was used for iron smelting and manufacturing cement. The Red Mountain mine closed in the late 1940s, after operating for about 50 years.
The research team has identified more than 20 large veins of pure magnesite at Red Mountain, embedded in magnesium-rich ultramafic rock. The biggest of these veins is approximately 118 feet wide and 886 feet long. They found that more than half of the magnesite in each vein is composed of CO2, with the rest being magnesium.
Approximately one percent of the Earth's surface is made up of ultramafic rocks, which occur near regions undergoing rapid population and industrial growth. In the California Coast Ranges alone, more than 50 other deposits of exceptionally high-grade magnesite can be found. Maher said that sequestering CO2 emissions at these sites could play a significant role in curbing global warming.
"We've been looking at the geologic structure and veining at Red Mountain to try and understand how hard ultramafic rock could be transformed into magnesite," Maher said.
Red Mountain is estimated to have originally held nearly 1 million metric tons of magnesite. Approximately 83 percent has already been mined.
"One million metric tons of magnesite is the equivalent of sequestering 140,000 metric tons of carbon in mineral form," said graduate student Pablo Garcia del Real.
"Our goal is to use the vast reservoirs of magnesium stored in ultramafic rocks to chemically bind with CO2 and form magnesite. But as we discovered at Red Mountain, breaking those rocks is one of the main engineering challenges that we face."
Maher and her team made several field trips to Red Mountain and performed a series of laboratory tests, concluding that tectonic forces played a crucial role in creating the magnesite deposits.
"To unlock the secrets of these deposits, we needed to find clues about both the mineralization process and the geologic history of the area," del Real said.
The San Andreas fault line lies west of Red Mountain by just less than 40 miles. The infamous fault formed approximately 29 million years ago, creating a large gap between the Earth's crust and the hot mantle below. Heat rose to the surface through this gap, raising the temperature of the water and liquid CO2 trapped in the ultramafic rocks.
"When the temperature of a liquid increases, the volume increases," del Real said. "We think that the CO2 enhanced the ability of the water to expand, adding enough pressure to break the ultramafic rock and cause the chemical reaction that formed the magnesite veins."
This process was fast and furious, according to del Real.
"The magnesite veins are very white, homogenous and composed of very tiny crystals, so they probably formed quickly, perhaps instantaneously," del Real explained. "The ultramafic rocks appear shattered and broken, which means that this was a violent event."
The research team performed an isotopic analysis of the magnesite samples back in the lab at Stanford. The findings suggested that when the fault line initially opened, magnesite formed a little over half a mile below the surface as temperatures rose from about 53 degrees Fahrenheit to 86 degrees Fahrenheit. It should be relatively easy for scientists to convert atmospheric CO2 into pure magnesite at such low temperatures. So far, however, del Real and the Stanford team have been unable to replicate the process experimentally.
"If we inject CO2 from a power plant or other point source into ultramafic rock, we would expect it to form magnesite," he said. "But when we try to make magnesite in the laboratory at low temperatures, it fails to form."
To successfully sequester carbon, the researchers will have to figure out a way to make ultramfic rock permeable. "There is no way that CO2 or anything else will flow through these rocks," del Real said. "In our research, we combine a big tectonics approach with the minute thermodynamic behavior of fluids. So we go from the very large scale to the very small scale."
Findings of this research will be presented on December 10 at the 2013 fall meeting of the American Geophysical Union (AGU).