Category: Extreme Earth Phenomena
Arc volcanism along convergent plate margins—responsible for the explosive stratovolcanoes of the Pacific “Ring of Fire”—is driven by an apparent thermodynamic contradiction: how do descending, cold oceanic plates trigger widespread partial melting in the overlying mantle? The answer lies in metamorphic dehydration. Oceanic lithosphere is profoundly hydrated during seafloor hydrothermal circulation. As the cold slab descends into high-pressure metamorphic regimes, hydrous minerals break down, releasing supercritical aqueous fluids that flux the hot mantle wedge, lowering its solidus and sparking arc magmatism.
1. Hydration Architecture of the Subducting Plate
Prior to subduction, oceanic plates are extensively hydrated along mid-ocean ridges and outer-rise fault zones. Seawater infiltration converts dry mantle and crustal silicates into hydrous metamorphic assemblages across three vertical zones:
- Sedimentary Layer: Smectites and hydrous clays containing 7–15 wt% structural H₂O.
- Upper Oceanic Crust (Basalts & Dikes): Chlorite, actinolite, epidote, and zeolites containing 2–4 wt% structural H₂O.
- Serpentinized Mantle Lithosphere: Seawater penetrating deep bending faults at the oceanic trench transforms mantle peridotite into serpentine minerals (antigorite, lizardite) containing up to 13 wt% structural H₂O.
2. Metamorphic Facies Reactions and the Blueschist-to-Eclogite Transition
As the slab sinks, progressive compaction expels pore water in the shallow forearc (< 20 km depth). At greater depths (40 to 100 km), structural water bound inside mineral crystal lattices is released through discontinuous devolatilization reactions:
The Blueschist-to-Eclogite Metamorphic Transformation
In subducting oceanic basalts, the transition from blueschist facies (characterized by the sodic amphibole glaucophane and lawsonite) to anhydrous eclogite facies (garnet + omphacite) marks the primary crustal dehydration horizon:
Na₂Mg₃Al₂Si₈O₂₂(OH)₂ (glaucophane) → 2NaAlSi₂O₆ (omphacite) + Mg₃Al₂Si₃O₁₂ (pyrope) + SiO₂ + H₂O
This reaction density increases the crust from ~3,000 kg/m³ to ~3,500 kg/m³, driving slab pull while liberating immense fluxes of water directly beneath volcanic arc fronts (~80–120 km depth).
Serpentine Antigorite Breakdown
In the slab’s peridotitic mantle, antigorite serpentine breaks down at temperatures of 600–650°C and pressures of 2 to 4 GPa:
Antigorite → Olivine + Orthopyroxene + H₂O
This massive devolatilization event releases up to 9% water by weight from the slab mantle, channeling fluid pulses into the base of the mantle wedge.
3. Mantle Wedge Flux Melting: Depression of the Peridotite Solidus
The released aqueous fluids are supercritical, enriched in dissolved silica, alkalis (K, Rb, Ba, Sr), and fluid-mobile trace elements. These ascending fluids infiltrate the hot, convective mantle wedge (T ≈ 1,200–1,300°C).
Lowering the Melting Temperature
Dry peridotite does not melt at 100 km depth because the ambient geotherm lies well below the anhydrous solidus (~1,450°C). However, the introduction of dissolved water depresses the peridotite solidus temperature by 300 to 450°C:
T_solidus(wet) = T_solidus(dry) − ΔT_H₂O
The mantle wedge immediately undergoes partial melting (F = 10–25%), generating hydrous primary basaltic magmas that ascend through the crust to fuel subduction zone volcanoes.
The Planetary Scale Metric
Global Subduction Water Flux: ~ 1.0–2.0 × 10¹² kg H₂O / year | Solidus Depression: ΔT ≈ 300–450°C | Sub-Arc Dehydration Depth: 80–120 km
| Metamorphic Hydrous Phase | Structural H₂O Content | Dehydration Temperature | Breakdown Reaction Product |
|---|---|---|---|
| Antigorite (Serpentine) | ~ 12.0–13.0 wt% | 600–650°C | Forsterite + Enstatite + H₂O |
| Glaucophane (Sodic Amphibole) | ~ 2.0–2.5 wt% | 500–550°C | Omphacite + Garnet + Quartz + H₂O |
| Lawsonite | ~ 11.5 wt% | 700–850°C (Cold slabs) | Grossular + Kyanite + Coesite + H₂O |
| Phengitic Muscovite | ~ 4.0–4.5 wt% | 800–900°C | K-feldspar + Kyanite + Melt |
4. Key Takeaways and Geophysical Summary
- Slab Water Carrier: Altered oceanic basalts, clays, and serpentinized mantle lithosphere carry over 10¹² kg of water into subduction zones each year.
- Eclogitization Release: The blueschist-to-eclogite transition densifies the slab, driving slab pull while discharging supercritical hydrous fluids at 80–120 km depth.
- Flux Melting Physics: Water lowers the melting temperature of peridotite by over 300°C, generating the hydrous magmas that power explosive arc volcanoes.
- Deep Water Recycling: Cold slabs with stable lawsonite transport water past the arc into the deep transition zone, replenishing the mantle’s vast internal water reservoir.
