> Longer-term - it's mid-ocean ridges, not subduction zones, where you find all the volatiles and volcanoes.

That's simply not true. Yes, there are volcanoes at mid-ocean ridges, but subduction zones also have plenty of activity.

https://en.wikipedia.org/wiki/Mud_volcano

"There are 10 active mud volcanoes in the Izu–Bonin–Mariana Arc which can be found along a north to south trend, parallel to the Mariana trench.[43] The material erupted at these mud volcanoes consists primarily of blue and green serpentinite mud which contains fresh and serpentinized peridotite material from the subduction channel. Fluid from the descending Pacific Plate is released by dehydration and alteration of rocks and sediment."

https://en.wikipedia.org/wiki/Volcanic_arc

(for description of how subduction zones create magmatic volcanoes)

Subducted material is very wet, and as it descends the volatiles ascend to melt the rock above, creating magma (water reduces the melting point of rocks just like it reduces the melting point of sugar.)