Abyssal peridotites have been long interpreted to be residues of mantle melting beneath midocean ridges,but the recent studies of their modal proportions and reconstructed whole-rock compositions show that they are mixtures of mantle melting residues and excess olivine beneath an ocean ridge.The origin of abyssal peridotites results from a combination of decompression melting of upwelling mantle and olivine crystallization as a result of cooling of acending melts passing through the cold thermal boundary layer atop the mantle.The greater the extent of mantle melting,the more depleted the peridotites,and the higher the excess olivine contents.Hence peridotite modes can be used to reflect efficiently mantle melting and melt extraction processes beneath ocean ridges only if the excess olivine is removed appropriately.