Mantle wehrlite from Hess Deep as a crystal cumulate from an ultra‐depleted primary melt in East Pacific Rise

A piece of wehrlite, containing about 20 volume % of clinopyroxene, was found in harzburgite in a drill core of ODP Leg 147 from Hess Deep near the East Pacific Rise. Minerals are as refractory (olivine, Fo 90.7 ; spinel, Cr# = 0.52) as those in the harzburgite. The clinopyroxene is depleted in REE (rare earth elements), being similar in chondrite‐normalized pattern for middle to heavy REE to that of the harzburgite. The calculated melt in equilibrium with the clinopyroxene is as depleted as ultra‐depleted MORB melts ever documented. The wehrlite is a cumulate from an in‐situ segregated ultra‐depleted MORB melt. Dimension of individual masses of the ultra‐depleted MORB involved may be at least as large as a few tens of centimeters. The Hess Deep wehrlite is different from the lower crustal wehrlite from the ocean floor or some ophiolites, which is either a cumulate or a peridotite/melt reaction product.