just got the shock of my life today......... fitting new tyres to the 748 and lo and behold the rear marvic is starting to split along all 5 spokes..........DOH! lucky i was having a close look as i could of easly missed it...(the black marker pen is either side of the cracks .. realy am shocked, i did 8 trackdays last year with these wheels i supose the old wifes take of mag wheels going off after a few years is true then.....
I'd contact Marvic through their dealer and arrange to return it. It looks like a manufacturing defect to me. Probably associated with the heat treatment cooling rate given the symmetry of the cracking. If there's no evidence of an curb impact etc. to indicate that it's your fault, I'd bet that a good customer relations policy would prompt them to replace it without charge.
I couldnt help wondering about product liability for racing though...fit for purpose is one thing but if they had been used constantly say, every weekend during summer - then thats potentially a lot of tracktime.
Having said all that - from the symetrical fractures it does possibly look like a manufacturing fault.
For those of you that can remember that far back, there was a load of trouble with Marvic Mag wheels in the mid nineties, 125 bikes and up had the same problems, culminating in Robert Dunlops big get of an RC45 at about 130mph, ripped the middle out of the spokes exactly the way this wheel was on its way doing.