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

Abstract

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, Fo90.7; 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. Copyright 2007 by the American Geophysical Union.