Serpentinized Peridotite: Formation, Tectonics, and Carbonation

Serpentinized peridotite plays a crucial role in geological processes, particularly in cycling elements like carbon and hydrogen within Earth’s lithosphere. Its transformation through hydration and carbonation influences geochemical reactions, tectonic activity, and potential carbon sequestration strategies.

Understanding its behavior requires examining its mineral composition, formation environments, and interactions with fluids under varying pressure and temperature conditions.

Mineralogical Framework

Serpentinized peridotite forms through the hydration of olivine and pyroxene, producing serpentine-group minerals such as lizardite, chrysotile, and antigorite. Lizardite and chrysotile typically form at lower temperatures, while antigorite dominates in higher-temperature regimes. This transformation also generates magnetite, brucite, and hydrogen gas, altering the rock’s physical and chemical characteristics. Magnetite contributes to magnetic anomalies, often used as geophysical indicators of serpentinization.

The degree of serpentinization depends on fluid availability and temperature. Complete hydration reduces rock density and increases volume, causing fracturing that facilitates further fluid infiltration. The reaction between olivine and water follows:
\[ \text{Mg}_2\text{SiO}_4 + H_2O \rightarrow \text{Mg}_3\text{Si}_2\text{O}_5(OH)_4 + \text{Mg(OH)}_2 \]
Here, forsterite, the magnesium-rich end-member of olivine, converts into serpentine and brucite, releasing hydrogen gas, which affects redox conditions and microbial ecosystems.

Trace elements and accessory minerals provide insights into geochemical evolution. Nickel and cobalt, concentrated in olivine, redistribute during serpentinization, affecting the rock’s economic potential for metal extraction. Boron and lithium incorporation into serpentine minerals reflects fluid-rock interactions, with isotopic signatures tracing past hydrothermal activity. Carbonate minerals like magnesite and dolomite indicate secondary alteration, especially where CO₂-bearing fluids have interacted with the rock.

Tectonic Formation Settings

Serpentinized peridotite forms in tectonic environments where mantle rocks undergo hydration due to fluid infiltration, including mid-ocean ridges, subduction zones, and ophiolites. At mid-ocean ridges, peridotite is exposed through faulting and interacts with seawater, leading to extensive hydration. This process is prominent along slow- and ultraslow-spreading ridges, where tectonic extension emplaces mantle material onto the seafloor. Detachment faults in these environments create permeability pathways, enhancing serpentinization.

In subduction zones, serpentinized peridotite helps transport volatiles into the mantle. In the forearc region, peridotite interacts with fluids from the subducting slab, forming serpentinite mélange complexes. These hydrated rocks exhibit reduced density and strength, influencing subduction dynamics by modifying the mechanical behavior of the plate interface. Serpentinization also contributes to cycling elements like water, sulfur, and halogens, which are later released during dehydration reactions, affecting arc magmatism and mantle wedge chemistry.

Ophiolites, representing ancient oceanic lithosphere thrust onto continental margins, preserve records of seafloor alteration and serve as analogs for active serpentinization. In many ophiolitic sequences, serpentinized peridotite occurs within shear zones and faulted contacts, indicating fluid infiltration along structural weaknesses. The degree of alteration varies, with highly serpentinized sections containing significant secondary minerals that reflect prolonged fluid interaction.

Carbonation Reactions

The reaction between serpentinized peridotite and CO₂-rich fluids forms carbonate minerals, impacting natural geochemical cycles and engineered carbon sequestration. CO₂-bearing fluids interact with brucite, serpentine, and magnesium-rich phases, precipitating magnesite, dolomite, and hydrous carbonates. The efficiency of this transformation depends on temperature, fluid composition, and permeability, with higher CO₂ concentrations driving more complete mineralization.

Fluid chemistry dictates reaction pathways, as pH and dissolved ions influence carbonate stability. In low-silica environments, brucite reacts with CO₂ to form magnesite:
\[ \text{Mg(OH)}_2 + CO_2 \rightarrow \text{MgCO}_3 + H_2O \]
Magnesite is stable over geological timescales, making it effective for carbon storage. Higher silica activity promotes serpentine breakdown, leading to more complex dissolution-precipitation reactions that generate mixed carbonate phases. Hydrous carbonates in low-temperature systems suggest carbonation occurs under various conditions, including near-surface weathering.

Reaction kinetics depend on reactive surface availability and rock fracturing. Increased porosity from serpentinization enhances fluid access and accelerates carbonate precipitation. However, carbonation reactions can clog pores, reducing permeability and limiting further CO₂ uptake. This self-limiting behavior is relevant for engineered carbon storage, where maintaining fluid pathways is critical. Studies of natural ultramafic systems, such as those in Oman and British Columbia, provide insights into long-term carbonation rates, showing mineral trapping of CO₂ can occur over decades to millennia.

Rheological Characteristics

The mechanical behavior of serpentinized peridotite is influenced by hydration and mineral composition, affecting strength, ductility, and deformation mechanisms. Hydration weakens the rock, as serpentine-group minerals lower friction and facilitate strain localization. This effect is evident in shear zones, where serpentinization promotes fault slip and enhances aseismic deformation. Laboratory tests show fully serpentinized peridotite has a friction coefficient as low as 0.2-0.3, compared to over 0.6 for dry olivine, highlighting hydration’s role in modifying mechanical properties.

Deformation depends on temperature and strain rate, with brittle failure at shallow depths and ductile flow at higher temperatures and pressures. Antigorite, the high-temperature polymorph of serpentine, is stronger than lizardite and chrysotile, leading to variations in rheological behavior. Experimental studies show antigorite-bearing rocks can sustain differential stresses exceeding 1 GPa before plastic deformation, while low-temperature serpentine phases accommodate strain through cataclastic flow and microfracturing. These differences affect subduction zone stability, where serpentine mineralogy influences seismic activity depth and nature.

Hydrothermal Interactions

Hydrothermal fluids drive complex geochemical transformations in serpentinized peridotite, affecting deep-sea and subduction-zone systems. Circulating fluids, derived from seawater or metamorphic dehydration, infiltrate the rock and trigger further mineral alterations. Elevated temperatures accelerate dissolution-precipitation reactions, forming secondary minerals such as talc, carbonate phases, and additional serpentine-group minerals. These changes modify permeability and influence fluid compositions, creating localized variations in pH, redox conditions, and element mobility. In ultramafic-hosted hydrothermal systems, such as those along mid-ocean ridges, these reactions generate highly alkaline fluids with pH values above 10, supporting unique microbial ecosystems.

Hydrogen gas production through the oxidation of ferrous iron in olivine and pyroxene plays a key role in hydrothermal processes. This reaction, a continuation of serpentinization, provides energy for chemosynthetic microbial communities in hydrothermal vent systems. At sites like the Lost City Hydrothermal Field in the Atlantic Ocean, serpentinization-driven hydrogen sustains microbial metabolisms based on methanogenesis and sulfate reduction. The presence of methane and other reduced carbon species in these systems has implications for deep biosphere studies and the search for extraterrestrial life, as similar reactions could occur on planetary bodies with ultramafic rock compositions. Ongoing research into hydrothermal alteration continues to shed light on Earth’s deep carbon cycle and the evolution of life in extreme environments.