While this may or may not be a depiction of Gabriel, he gave her the news that she would bear a son, this angel is giving her crown or diadem as a symbol of her virtue and faith by allow God’s wil; to be done.
There are many kinds of crowns, some liter and some spiritual. God covered Mary and he will cover us, I like to think that he exchanged His crown of thorn for a royal one, so that I could have a crown of peace.
Rebecca Jones
On Angel's Wings 