I am an Italian son of Noah.
The Torah (written and especially oral) establishes a detailed regulation of the divorce institute. According to Jewish tradition, before the Matan Torah were there prescriptions on the matter given by the Creator? I do not know if the episode of Abraham who sends Hagar away can provide precise information in this sense, as it is true that in Genesis 16:3 it is written:
"So Sarai, Abram's wife, took Hagar the Egyptian, her handmaid, at the end of ten years of Abram's dwelling in the land of Canaan, and she gave her to Abram her husband for a wife."
But in Genesis 21:12-13, HaShem addresses Abraham by calling Hagar simply "your handmaid", and not "your wife". I do not understand therefore if in this case there has been a real divorce.