why plate boundary type divergent ocean-ocean is quite common?

Divergent plate boundaries result in thin crust, and felsic crust ultimately gets replaced by mafic crust as new volcanic rocks are generated at the constructive boundary. Therefore the rifts are topographic lows and are ocean-ocean boundaries, or will become ocean-ocean boundaries with continued rifting.

I'm beautiful sure that they are fashioned in subduction zones when you consider that they are the result of fashionable metamorphic techniques, which you aren't getting at divergent zones. You most effective get contact metamorphosis at divergent zones.