1. Early Stage:
At the onset of subduction, the mantle wedge above the subducting plate is relatively dry. As the oceanic plate descends, it carries hydrated oceanic crust and sediments into the mantle. The release of water from these materials leads to an increase in mantle hydration in the forearc region.
2. Intermediate Stage:
As subduction continues, the mantle wedge becomes progressively more hydrated due to the continuous influx of water from the subducting slab. This hydration causes melting in the mantle, leading to the formation of magmas that can erupt to form arc volcanoes. The presence of water also lowers the melting temperature of the mantle, contributing to increased magma production.
3. Advanced Stage:
With prolonged subduction, the mantle wedge becomes extensively hydrated, leading to the formation of hydrous minerals such as serpentine and amphibole. This high degree of hydration can cause the mantle to weaken and experience a phenomenon known as "serpentinization". The serpentinized mantle is less dense and can rise buoyantly, leading to the formation of topographic features such as seamounts or island arcs.
4. Stagnant Stage:
Eventually, the subduction zone may enter a stagnant phase where the subduction process slows down or ceases. During this stage, the mantle hydration beneath the subduction zone remains high due to the accumulated water from the previous stages. However, the absence of ongoing subduction reduces the supply of new water, and the mantle hydration gradually decreases over time.
5. Late Stage:
In the final stages of a subduction zone, the mantle wedge may experience dehydration as the subducting slab reaches greater depths and higher temperatures. This dehydration process occurs due to the breakdown of hydrous minerals and the release of water back into the overlying mantle. The mantle hydration decreases, and the subduction zone eventually becomes inactive.
In summary, mantle hydration changes significantly over the lifetime of a subduction zone. It increases during the early and intermediate stages due to water input from the subducting slab, reaches a peak during the advanced stage, and then gradually decreases during the stagnant and late stages as dehydration processes occur. Understanding these changes in mantle hydration is essential for unraveling the complex processes that shape subduction zones and influence their volcanic and tectonic activities.
