Zarathushtra the Amesha Spenta

Where in the Khorda Avesta is Zarathushtra called an Amesha Spenta?  In Yasna 3 and 7 he's called a Yazata "angel."  This is interesting to me because some scholars assert that the idea that man is the offspring of a god is a Greek idea.  Many Greek heroes are the offspring of the gods. But Sassanian kings considered themselves to be offspring of the gods, and Zarathushtra is apparently called a Amesha Spenta "archangel" or hypostasis of Ahura Mazda.