Regardless of magma "burning" fire or not, it was made perfectly clear that Akainu > Ace when they first clashed, seanos also makes an excellent point, if he jumped in to save Luffy disolving into fire before getting hit would acomplish nothing.

Quote Originally Posted by seanos
Though it pisses me off, to say the least, that the whole death of Ace was in part (pretty damn large part too) caused by Luffy getting distracted by a bit of paper
There was a part that i believe they skipped where Luffy tried to stand up and evade Akainu but was too tired to move, that`s when the paper dropped.