Describe the mineralogy, micro-structure and the genetic processes involved in the formation of manganese nodules in the present-day ocean.

Mineralogy of Manganese Nodules

Manganese nodules are typically spherical or ellipsoidal in shape, ranging in size from millimeters to tens of centimeters in diameter. Their mineralogical composition is complex and varies depending on the location and geological setting. The primary mineral phases include:

• Manganese Oxides: These constitute the bulk of the nodule, primarily birnessite (δ-MnO₂), todorokite (α-MnO₂), and hollandite ((Ba,Pb)Mn₈O₁₆(OH)₂).
• Iron Hydroxides: Goethite (α-FeOOH) and lepidocrocite (γ-FeOOH) are common iron-bearing phases.
• Metal Hydroxides/Oxides: Nodules often contain significant amounts of nickel, copper, and cobalt, present as hydroxides or oxides. These include asbolane (NiO₂), birnessite-like phases with incorporated Ni, Cu, and Co, and vernadite (δ-MnO₂) with metal substitutions.
• Clay Minerals: Small amounts of clay minerals, such as smectite and illite, are often present as detrital components or formed during diagenesis.
• Authigenic Carbonates: Calcite and aragonite can occur, particularly in nodules from shallower depths.

Microstructure of Manganese Nodules

The microstructure of manganese nodules is characterized by a layered growth pattern, reflecting episodic precipitation of minerals. This layering can be observed in polished sections using optical microscopy and scanning electron microscopy (SEM). Key microstructural features include:

• Concentric Layers: Alternating layers of manganese oxides, iron hydroxides, and metal-rich phases are arranged concentrically around a nucleus.
• Nuclei: Nodule growth typically initiates around a nucleus, which can be a small rock fragment, a biogenic particle (e.g., fish tooth, radiolarian skeleton), or even a pre-existing nodule fragment.
• Dendritic Growth: Within the layers, dendritic or fibrous textures are common, indicating rapid precipitation from solution.
• Microscopic Pores: Nodules contain microscopic pores and voids, which can influence their density and permeability.
• Colloform Textures: Some nodules exhibit colloform textures, suggesting precipitation from colloidal suspensions.

Genetic Processes Involved in Formation

The formation of manganese nodules is a complex process that is not fully understood. Several genetic models have been proposed, and it is likely that multiple processes contribute to their formation. The main theories are:

1. Hydrogenous Precipitation (Inorganic Precipitation)

This theory suggests that manganese and other metals are leached from hydrothermal vents, volcanic activity, and the dissolution of seafloor basalts. These metals are transported in seawater and precipitate as oxides and hydroxides due to changes in redox conditions, pH, and temperature. The deep-sea environment provides ideal conditions for this process, with high concentrations of dissolved metals and low sulfate concentrations. The oxidation of Mn²⁺ to Mn⁴⁺ is a key step, often catalyzed by manganese oxides themselves.

2. Biogenic Precipitation

This theory proposes that microorganisms play a crucial role in the formation of nodules. Microbes can catalyze the oxidation of manganese and other metals, promoting their precipitation. They can also create microenvironments with favorable conditions for mineral formation. Evidence supporting this theory includes the presence of microbial fossils within nodules and the correlation between nodule abundance and organic matter flux to the seafloor.

3. Authigenic Growth & Deep-Sea Currents

Authigenic growth refers to the in-situ precipitation of minerals from seawater. Deep-sea currents play a vital role in transporting dissolved metals to the abyssal plains and influencing the precipitation rates. Bottom currents can also resuspend sediment, providing nucleation sites for nodule growth. The Pacific Ocean, particularly the Clarion-Clipperton Zone (CCZ), is a prime location for nodule formation due to its slow sedimentation rate and strong bottom currents.

4. Role of Hydrothermal Activity

While not directly forming nodules, hydrothermal vents contribute significantly to the metal budget in the ocean. The plumes from these vents release large quantities of dissolved metals, which are then dispersed by ocean currents and contribute to nodule formation in distant locations.

Recent research suggests a combination of these processes is most likely responsible for nodule formation. Hydrothermal activity provides the metal source, while microbial activity and authigenic precipitation control the precipitation and growth of the nodules.