Studies of Exhumed Seafloor Show Fate of Subducting Carbon

Two new studies show that carbonate minerals in subducting ocean plates can dissolve and be funneled toward the surface once they encounter the heat and pressure of the mantle, creating carbon-rich minerals along the paths of these fluids. Some of the carbon, however, remains trapped in the sinking plate, where it likely stays in the mantle.

Ocean floor is born when lava wells out of underwater volcanoes along mid-ocean ridges. It slowly crawls across the globe due to the movement of other plates, picking up water and carbon-containing minerals through weathering along the way. Then it ends its life by plunging into the mantle beneath another tectonic plate through a process called subduction. We know that the subduction of ocean plates is a major sink of surface carbon because it transports carbon to the mantle where it stays over long timescales. Scientists, however, are still working on the finer details of how much of that accumulated carbon stays in the mantle, and how much dissolves or melts and washes up to the surface, where it gets stuck in the crust or escapes through nearby volcanoes.

These are exactly the details that DCO scientists Manuel Menzel and Carlos Garrido (both from the Spanish Research Council and the University of Granada, Spain) are trying to figure out. In two new papers, they and their colleagues investigate what happens to the carbon in the subducting edge of an ocean plate, called the slab. Specifically, they are looking at serpentinites, which are weathered mantle rocks of an oceanic plate that take up large amounts of water and carbon, in one ophiolite and two metamorphic complexes – pieces of the ocean crust and mantle, respectively, that tectonic forces pushed onto a continent. In the two metamorphic complexes, the serpentinites experienced high pressures on the way to the mantle to deliver their carbon payload before being shoved back to the surface. Through modeling and fine-scale analyses of the mineral composition of these rocks, the researchers show that after losing the fluids stored in serpentinites during subduction, these rocks still retain a large portion of their carbon, which can head to the mantle. Furthermore, carbon-rich fluids that escape from the slab as it sinks and heats up become trapped again higher up as a carbon-rich rock called listvenite. This rock may represent a reservoir of carbon that does not escape through volcanoes. The findings can help researchers to improve their estimates of the fluxes of carbon moving in and out of the mantle in similar locations worldwide.

In a new paper [1] in the Journal of Metamorphic Geology, the researchers examined the structure and chemistry of the minerals in the Milagrosa and Almirez Massifs in southern Spain. The high-pressure metamorphic complexes comprising these massifs enabled the researchers to describe for the first time the fate of carbon compounds during subduction of ophicalcites – serpentinite rocks that also contain calcite minerals. The ophicalcites reached depths of about 60 kilometers before being exhumed. The researchers estimate that the Milagrosa Massif reached up to 600 degrees Celsius, while the Almirez was about 680 degrees Celsius. 

graphic of the Milagrosa and Almirez Massifs
This graphic of the part of southern Spain that contains the Milagrosa and Almirez Massifs shows how the ocean plate was subducted and exhumed. Image courtesy of JMG

The analyses show that during subduction, much of the seawater that became incorporated into the minerals during serpentinization is released again when heated to 660 degrees Celsius. “The Almirez is one of the few places where we can study directly what happens to carbonate minerals when serpentinites dehydrate,” said Menzel. He estimates that about nine percent of the rock turns to fluid during this process, which is responsible for a large part of the fluids produced in most subduction zones worldwide, and for washing away some of the carbon contained in ophicalcites and other carbon-bearing rocks in slabs.

But not all of the carbon in the ophicalcite washes away – the rocks in the Almirez Massif are still composed of up to 40 percent carbonate minerals. Metamorphic reactions make the ophicalcites denser than the surrounding serpentinites, which prevents more fluids from percolating through the rock and removing the remaining carbon. “These rocks show unequivocally that the ophicalcites survived immense amounts of fluid flushing,” said Menzel, which shows that some carbon compounds can be retained in the slab for delivery deeper into the mantle.

The researchers also published related work in a recent paper [2] in the journal Lithos. They describe the minerals and processes that affected mantle rocks of the Advocate Ophiolite in Newfoundland, Canada. The ophiolite contains listvenite, which forms when deeper sources of dissolved carbon dioxide, in the form of carbon-rich fluids, interact with serpentinite. Listvenite often occurs along faults because the fractures concentrate and channel these carbon-rich fluids. “Up to 35 percent of listvenite is carbon dioxide, bound in the mineral structure,” said Menzel. “This setting can act as storage for quite a large amount of carbon.” 

Menzel inspects a wall of a listvenite ridge
Menzel inspects a wall of a listvenite ridge. The orange rind on its surface comes from weathering of the iron-bearing magnesite. Credit: Manuel Menzel

The researchers show that the listvenite formed when the subducting oceanic plate below the Advocate mantle encountered elevated heat and pressure, which released water and dissolved carbon from the plate. These fluids rise and react with olivine and serpentine minerals higher up in the mantle above the subducting slab and convert them into another mineral, magnesite. This reaction releases silicon, which solidifies as talc and quartz. The researchers can see the complete sequence of minerals related to this reaction in the rocks of the Advocate Ophiolite, because microstructures recorded the different “frozen” partial reaction steps in a remarkable way.

Further analysis confirmed that these series of reactions occurred within the mantle, before the ophiolite had returned to the surface. “The isotopic signatures show that the carbon is not oceanic,” said Menzel. “These fluids likely derived from graphite and carbonate-bearing rocks,” which points to ocean sediments transformed by high temperatures and pressure as the source. Additionally, thermodynamic modeling suggests that the serpentinite became carbonated at a temperature of 300 degrees Celsius and at pressures that are 2000 to 5000 times higher than standard atmospheric pressure.

This reaction sequence is also related to turning iron in the minerals from its oxidized state (the form iron takes in rust) to a more reduced form because oxidized iron cannot incorporate into the mineral structure of magnesite. Menzel expects that further studies of the iron in the Advocate Ophiolite may reveal additional details of the role of reduced hydrogen and organic carbon compounds in deep fluids at that time, and how the formation of listvenites may contribute to oxidizing the mantle.
 
Now that Menzel has examined these three locations in great detail, he is working on a more general model of what happens to serpentinites during subduction and how the dissolution of carbonate rocks into carbon-rich fluids impacts the deep carbon cycle. He points out that in both these situations, the field record shows that fluids follow focused pathways in the rocks, which means that the carbon content and the dissolution of carbonate in these rocks will vary greatly, depending on whether they lay in the path of the fluids. This variability must be considered when making global estimates of carbon fluxes from the slab to the overlying mantle.

Main image: A listvenite ridge with abundant quartz veins extends into Flatwater Pond in Newfoundland, Canada. Credit: Manuel Menzel

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