Plagioclase-wehrlites and peridotites on the East Pacific Rise (Hess Deep) and the Mid-Atlantic Ridge (DSDP Site 334): evidence for magma percolation in the oceanic upper mantle

Abstract

Textural and petrological data are presented that record the formation of plagioclase-bearing wehrlites and peridotites at slow-spreading and fast-spreading ridges that are comparable to the wehrlites and peridotites formed in ophiolitic complexes. Evidence is provided for locally pervasive magma percolation in the oceanic residual upper mantle. The samples studied come from the East Pacific Rise (EPR) (Hess Deep) and the Mid-Atlantic Ridge (MAR) (DSDP Site 334). At both sites, parts of the peridotites display poikilitic textures with oikocrysts of clinopyroxene and interstitial plagioclase that include xenocrysts of strained and partly annealed olivine and subidiomorphic spinel crystals. Petrofabric data for olivine suggest a magmatic origin for the Hess Deep wehrlite. Its phase chemistry is comparable to intrusive plagioclase-wehrlites and lherzolite from ophiolites that are thought to have crystallized from a crystal mush. It differs drastically from the associated Hess Deep diopside-bearing harzburgites and dunites, which are quite similar to oceanic and ophiolitic residual peridotites. The EPR wehrlite is therefore considered as an intrusive rock. The Leg 37 plagioclase-peridotites display a weak lattice fabric of olivive which may have resulted from high-temperature plastic flow; these rocks have a phase chemistry that is typical of ophiolitic and oceanic residual rocks. These rocks are hence interpreted as residual dunite or harzburgite which has been pervasively impregnated by a melt from which the clinopyroxene and plagioclase crystallized. These peridotites could have an intrusive origin or may have formed in situ at the crust-mantle transition in association with intrusions of large gabbro bodies.