Arc Magmatism Source Explained

Arc magmatism occurs above subduction zones, where one tectonic plate slides beneath another. Understanding the origin of this magma is key.

Principal Magma Source: Mantle Wedge

The principal source of magma in arc settings is the mantle wedge. This is the wedge-shaped piece of the upper mantle lying above the subducting slab and below the overriding plate.

Why the Mantle Wedge?

• The subducting oceanic plate releases water (released from hydrous minerals) into the overlying mantle wedge.
• This water lowers the melting point of the mantle rock (peridotite).
• This process, known as flux melting, generates magma within the mantle wedge.
• This newly formed magma is less dense and rises towards the surface, leading to volcanic activity in the overlying arc.

Other Contributions

While the mantle wedge is the primary source, the subducting plate itself also contributes. Materials from the subducted oceanic crust and sediments can be incorporated into the rising magma, modifying its composition. However, the initial melting and magma generation predominantly occur in the mantle wedge.

Evaluating Options

• Subducted oceanic crust & Subducted sediments: These provide fluids and some solid material but are not the primary site of melting.
• Mantle wedge: This region undergoes flux melting due to the introduction of water, making it the principal source.
• D'' layer: This deep mantle layer is typically associated with different processes and is not the direct source for arc magmatism.

Therefore, the mantle wedge is the correct answer.