Final answer:
Volcanoes formed as a result of oceanic crust subduction are enriched in iron and magnesium compared to the upper mantle.
Step-by-step explanation:
Volcanoes formed as a result of oceanic crust subduction are enriched in iron and magnesium, compared to the upper mantle. Oceanic crust, which is formed by the cooling of volcanic lava, is mainly composed of elements such as silicon, oxygen, iron, aluminum, and magnesium. When oceanic crust subducts beneath another plate, the subducting plate melts and enriches the resulting volcano with iron and magnesium.