Organic Carbon Shapes Minerals and Metals at Low Temperature in Serpentinite Rocks

The organic carbon leftover from microbes colonizing cracks in the seafloor along mid-ocean ridges or deriving from abiotic reactions, independent of life, affects the minerals and the metals that occur in the cracks, following water-rock reactions.

When exhumed mantle material creates new seafloor along mid-ocean ridges, seawater seeps into cracks and interacts with the rock to make serpentine minerals. These serpentinization reactions generate hydrogen and small organic compounds, supporting microbes colonizing the pores and cracks. New findings suggest that the remnants of these organic compounds, whether from biological or abiotic origins, take an active role in how serpentinization reactions proceed.

DCO Deep Energy Community members Bénédicte Ménez (Institut de Physique du Globe de Paris, France) and Daniele Brunelli (University of Modena e Reggio Emilia, Italy), and Deep Life Community member François Guyot (Muséum National d’Histoire Naturelle, France) used high-resolution microscopy techniques to visualize the fractures and mineral boundaries in serpentinite rocks collected from the Mid-Atlantic Ridge. The researchers discovered that organic carbon causes unusual bead-like spheres of the mineral serpentine to form, and also traps metals like cobalt, manganese, and nickel. The findings, published in a new paper in Lithos [1], suggest that organic carbon may play an active role in low-temperature serpentinization and in the formation of some supergene ore deposits, which occur close to the surface.

Serpentine balls
A scanning electron microscopy image of polyhedral serpentine in a serpentinite rock from the Mid-Atlantic Ridge. Credit: Images courtesy of IPGP/Univ. Modena e Reggio Emilia and Lithos.

“Until now, no one had shown that organic compounds might be involved in low-temperature serpentinization reactions in the rocks,” said Ménez. “Normally, all these reactions are seen from an inorganic perspective, but here we showed that the organic fraction is directly involved in the reactions.” 

In previous work, Ménez and her colleagues showed that microbes had once colonized serpentinized rock from the oceanic crust [2]. These rocks contain a mineral called hydrogarnet, and a type of serpentine that looks much like tiny balls of Styrofoam, called polyhedral serpentine.

In the new study, the researchers took a closer look at the polyhedral serpentine using electron microscopy. They saw carbon gel, likely leftover from old microbial biofilms, dissolving away the edges of the hydrogarnet and seeding the formation of polyhedral serpentine. “It was really when we used a series of techniques with high spatial resolution down to the nanometric scale that we were able to see this organic interface between the minerals,” said Ménez.

Ultrathin section of polyhedral serpentine
Ultrathin section of polyhedral serpentine bathed in a carbon gel in the cavity of a hydrogarnet mineral. Credit: Images courtesy of IPGP/Univ. Modena e Reggio Emilia and Lithos.

The researchers also used scanning transmission X-ray microscopy to characterize the carbon gel and confirm its organic nature. Using energy dispersive X-ray spectrometry, they identified elements within the organic phase. The organic gel accumulated large quantities of metals, primarily cobalt and manganese, but also iron and nickel. 

These rocks initially came from an area of seafloor near the Mid-Atlantic Ridge that underwent low-temperature (less than 200 degrees Celsius) serpentinization. This region is representative of large portions of the seafloor, suggesting that the interaction between organic carbon, serpentine minerals, and metals may be a widespread occurrence.

The discovery that organics drive metals accumulations during low-temperature serpentinization may have implications for supergene ore deposits. One example is New Caledonia, a French territory in the South Pacific made of an ophiolite, which is a piece of the ocean floor, pushed up onto the continental crust. The island has ores rich in nickel, manganese, and cobalt, from low-temperature serpentinization of the ophiolite. “If organics play a role in cobalt and manganese accumulations during low-temperature alteration, it may represent one of the drivers for metal accumulation in ophiolites,” said Ménez. 

This composite of elemental maps, shows how the presence of organic carbon impacts metal distribution. In the image, carbon is green, manganese is red, and cobalt is blue. Credit: Images courtesy of IPGP/Univ. Modena e Reggio Emilia and Lithos.


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