A Step Toward Modeling Carbonatites in the Subsurface

Using high-temperature and high-pressure experiments, researchers have determined the conditions causing calcium carbonate to take the form of calcite, aragonite, or a liquid. This represents a major step toward modeling the behavior of calcium carbon-rich rocks, called carbonatites, in the subsurface.

When generations of phytoplankton with calcium carbonate shells die, their shells accumulate on the ocean floor in the form of calcite. This calcium-rich ooze is a major carbon sink. After it descends into the mantle through subduction, it transforms at elevated pressures and temperatures into an alternate form of calcium carbonate called aragonite. At even greater temperatures, it becomes a liquid. Scientists have performed multiple experiments to predict the exact pressure and temperature conditions that will produce calcite, aragonite or liquid calcium carbonate in the mantle, but these studies have produced disparate estimates.

One of the forms that calcium carbonate can take is aragonite, which tends to form from calcite under high-pressure and low-temperature conditions in the shallow mantle. Scientists have found aragonite in mantle xenoliths from the Granatifera Tuff in the Northern Andes, Colombia. Large aragonite crystals form from interactions between mantle peridotites and percolating carbonatitic liquids. Credit: [2] Image courtesy of Fabio Ferri, 

In an attempt to resolve those discrepancies, members of the DCO Reservoirs and Fluxes and the Extreme Physics and Chemistry Communities Stefano Poli, Erwin Schettino, and Marco Merlini (all at the Università degli Studi di Milano, Italy) and former post-doc Sutao Zhao (now at China University of Geosciences, China), performed high-temperature and pressure experiments in the lab and thermodynamic modeling to develop a definitive phase diagram that shows how calcium carbonate behaves under different conditions. These determinations are the first step in modeling a type of calcium carbonate-rich rock called carbonatite in the mantle. They published their findings in a new paper in Lithos.   

“I think it’s an important step forward in modeling because for the first time we provide thermodynamic parameters for liquid calcium carbonate,” said Poli. 

Carbonatite is an unusual rock because it contains a lot of carbon and very little silica compared to other rocks in the mantle. When it melts, the magma tends to form narrow dikes that shoot up through existing rocks, often erupting near continental rift areas, where tectonic plates are splitting apart. Calcium carbonate (CaCO3) is a major component of carbonatite, and to a lesser extent, magnesium in the form of magnesite (MgCO3) and iron in the form of siderite (FeCO3). 

Despite having the same chemical formula, calcite and aragonite have different crystal structures and they interact differently with traces elements. Compared to calcite, aragonite tends to associate more strongly with strontium and rare earth elements, like neodymium and samarium, which have various technological applications. Thus, aragonite is more likely to transport these trace elements through the subsurface as it moves than calcite.

To determine whether calcium carbonate exists as aragonite, calcite, or a liquid under a range of temperature and pressure conditions, the researchers used a multi-anvil apparatus that exerts intense pressure on a sample. They exposed powdered calcium carbonate and synthetic aragonite to pressures of 3 to 6 gigapascals (about 30000 to 60000 times atmospheric pressure at sea level) and temperatures of 1300 to 1750 degrees Celsius. Once the samples returend to room temperature and pressure, the researchers examined them under a microscope and used a technique called X-ray diffraction, which uses the X-rays scattered off of a compound’s crystal structure to determine its form, to see whether the conditions produced aragonite, calcite, or liquid calcium carbonate. 

Phase Diagram for CaCO3
This phase diagram shows the form that calcium carbonate will take at different temperatures and pressures based on the experimental results from this study, shown as solid lines and squares. The chart includes results from previous studies as dotted lines and other shapes. Credit: Image courtesy of Zhao, et al. and Lithos

Next, the researchers used this data to perform thermodynamic modeling and to develop a phase diagram showing calcium carbonate’s form across a range of temperatures and pressures. They discovered that aragonite exists down to pressures of 4.7 gigapascals, at temperatures of 1650 degrees Celsius, suggesting that it is stable in parts of the mantle, where it may affect the distribution of rare Earth elements. 

“This is just the first brick for modeling carbonatite,” said Poli. The researchers are already working on modeling the behavior of magnesite and siderite, in collaboration with Max Schmidt (ETH Zurich, Switzerland), which will enable them to build a full model of carbonatitic liquids in the subsurface. 

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