Experimentation at high pressures in peridotite + CO2 Systems has suggested that some CO2-rich, Si-undersaturated magmas originate in mantle source regions consisting of carbonated peridotite. However, a caveat to the petrological application of such experiments has been the virtual absence of carbonate in mantle xenoliths. Decompression experiments were undertaken to resolve this problem and to test the hypothesis that carbonated peridotite exists in the upper mantle, but decomposes during ascent as xenoliths in its host magma. I demonstrate that carbonate coexisting with silicates in mantle-derived xenoliths could not survive entrainment even in the fastest ascending magmas due to rapid decarbonation upon decompression. I conclude that carboante may indeed exist in the upper mantle, and that the paucity of this phase in mantle-derived xenoliths does not require its absence in the mantle source regions of many primary, alkalic magmas.