Should probably be Death according to the ability. Otherwise it's basically this but more limited: http://elementscommunity.org/forum/index.php/topic,27139.0.html
How about "gain X quanta of the element of the target creature when it dies"? Otherwise, another passive ability similar to airborne would have to be added (and it would have a very limited use).
I don't think there are many "dead" creatures in other elements...So am I correct in assuming that Cats, Ashes, Fire/Leaf Spirits (?), Ghosts of the Past, Pharaohs, and Vampires (?) are considered "dead"?