In most cases, I'd use ... element in becoming ..., but any of these might be good. Can you finish the sentence? Sometimes the full context influences the choice of preposition.
503 for "key element in becoming" think element necessary in the mixture/mix 10 for "key element for becoming" think element necessary for the success of
0 Yahoo is one of the big search machines on the Web:02br 02br 05002br 02br 00 You type the phrase (use the quotation marks, as I mentioned), in the Web Search form, say: 02br 01b00"key element in becoming"02b02br 00 click on the button, and you will have the hits, the pages on which the phrase is found. 02br