[Brian Duff]

I'll bet freezing water caused that crack, as I dont see any drain holes and the bottom of your mast. A thought anyway
If you want to keep your rig height the same you could, say, cut the mast 1/2" or whatever is needed to pass the crack, then raise the step that much with a piece of alum plate or something else suitable. Also, are there no holes in your mast or step (per photos) to fasten the spar to the step ?