Life and Earth Sciences - First Year - Partial melting of peridotites in subduction zones

Magmatism in Subduction Zones
📌 Subduction zones exhibit magmatic rocks such as andesites, rhyolites (microcrystalline texture, rapid cooling), and plutonic/acidic rocks (fully crystallized, slow cooling).
🌋 These rocks indicate the presence of magma formation originating from partial melting of peridotite in the mantle.
❓ The central question addressed is how magma forms within subduction zones.

Geothermal Gradient vs. Peridotite Melting
🌡️ The red curve shows the geothermal gradient in subduction zones: temperature is slightly below $1000^\circ\text{C}$ at $80\text{ km}$ depth and slightly below $1500^\circ\text{C}$ at $240\text{ km}$ depth.
🧪 Laboratory experiments on dry peridotite define the dry solidus (limit between solid and partially liquid) and the liquidus (limit between partially liquid and fully liquid).
❌ Theoretically, the subduction zone geothermal gradient never crosses the dry peridotite solidus, meaning dry peridotite should never partially melt under these conditions.

The Role of Hydration in Melting
💧 Experiments using hydrated peridotite (e.g., serpentinites) show a significantly lower partial melting temperature compared to dry peridotite.
🔥 At $80\text{ km}$ depth, the partial melting temperature for hydrated peridotite is about $700^\circ\text{C}$, whereas for dry peridotite it is around $1400^\circ\text{C}$.
✅ The geothermal gradient in subduction zones intersects the hydrated peridotite solidus between approximately $80\text{ km}$ and $170-180\text{ km}$ depth, creating a window for partial melting.

Key Points & Insights
➡️ Magmatic rocks in subduction zones are definitively sourced from partial melting of peridotite in the mantle.
➡️ The reason melting occurs is that the subducting slab carries water (hydration) into the mantle, which lowers the solidus temperature of the surrounding peridotite.
➡️ Melting is confined to a specific depth range ($80\text{ km}$ to $170-180\text{ km}$), indicating that hydrated peridotite must exist below the subduction zone where temperatures are sufficient.

📸 Video summarized with SummaryTube.com on Jan 06, 2026, 19:05 UTC