Based on the white powder between the stem and seatpost, I say corrosion stress crack. Corrosion began on account of two widely different dissimilar materials.
See it all the time in aerospace stuff.
PK
Very good point. This is why Calfee lines their seattube/seatpost interface so that alloy posts do not interact with the carbon frame, though other mfr like Santana does not spec any special sleave with the Tamer Carbon posts used with Santana's stoker stem or the AL frames. Dunno.