[1] We demonstrate that deformation of partially molten ductile rocks can produce melt segregation by two-phase flow. In simple shear experiments on several melt-rock systems at high temperature and pressure, melt segregates into distinct melt-rich layers oriented 20 to the shear plane. Melt segregates in samples in which pressure gradients can develop at length scales less than the sample thickness. A simple scaling argument combined with a comparison of length scale data suggests that such pressure gradients can develop in the samples with compaction lengths less than or on the order of the sample thickness. In nature, stress-driven melt segregation may produce both high-permeability pathways that contribute to rapid extraction of melt and localization of deformation that increases the anisotropy in viscosity of partially molten regions of the upper mantle and lower crust.

The evolution of melt segregation in deforming partially molten olivine-rich rocks has been studied in a series of laboratory experi-ments. During deformation, melt segregates into networks of anasta-mosing channels (or ‘bands’) surrounding lenses of melt-depleted material. We quantify the nature of the melt distribution in the samples, including thickness, angle, spacing, volume fraction, and melt fraction of melt-rich bands, to understand the dynamics of melt-network organization.Two series of experiments were designed to isolate the effects of (1) increasing shear strain (at similar stress levels), and (2) varying stress levels (deformed to similar shear strains). Melt-rich bands develop by a shear strain of unity. In sam-ples deformed at varying stress levels, higher stress produces smaller characteristic band spacings.We relate these variations to the com-paction length, dc, which varies only as a result of the reduction of matrix viscosity with increasing stress. Simple approaches to scaling from experimental to mantle conditions suggest that stress-driven melt segregation can occur in the asthenosphere; if so, it will signifi-cantly affect rheological, transport and seismic properties, with enti-cing consequences for our understanding of plate^mantle interactions. KEY WORDS: melt segregation; rock rheology; magma transport; self-organization; mid-ocean ridges

This paper is primarily concerned with seismically imaging details in the mantle at an intermediate scale length between the large scales of regional and global tomography and the small scales of reflection profiles and outcrops. This range is roughly 0.1– 1 kmbab10–102 km, where a is the scale. We consider the implications of several models for mantle evolution in a convecting mantle, and possible scales present in the non-convecting tectosphere. Reflection seismic evidence shows that the structures preserved from continental accretion within and at the margins of the Archean cratons are subduction related, and we use subduction as an analog for scales left by past events. In modern orogenic belts we expect to find subduction structures, small scale upper mantle convection structures, and basalt extraction structures. We examine some of the scales that are likely formed by orogenic processes. We also examine the seismic velocity and density contrasts expected between various upper mantle constituents, including fertile upper mantle, depleted upper mantle, normal and eclogitized oceanic crust, and fertile mantle with and without partial melt. This leads directly to predicting the size of seismic signals that can be produced by specular conversion, and scattering from layers and objects with these contrasts. We introduce an imaging scheme that makes use of scattered waves in teleseismic receiver functions to make a depth migrated image of a pseudo-scattering coefficient. Image resolution is theoretically at least an order of magnitude better than traveltime

Dunites and the Rheology of the Shallow Mantle

Copyright by

Contents lists available at SciVerse ScienceDirect Lithospyroxenite melt to the oceanic basalts. Finally, we highlight that the fact the very silica depleted compositions (SiO2b42 wt.%) and high TiO2 contents of some ocean island basalts seem to require the contribution of fluids (CO2 or H2O) through melting of either carbonated lithologies (peridotite or pyroxenite) or amphibole-rich veins.

discontinuities beneath the Kaapvaal craton is remarkable for clarity. The migrated receiver function image of the upper mantle

The lithospheric mantle beneath West Antarctica has been characterized using petrology, whole-rock and mineral major element geochemistry, whole-rock trace element chemistry and Mössbauer spectroscopy data obtained on a suite of peridotite (lherzolite and harzburgite) and pyr-oxenite xenoliths from the Mount Morning eruptive centre, Southern Victoria Land. The timing of pyroxenite formation in Victoria Land overlaps with subduction of the Palaeo-Pacific plate beneath the Gondwana margin and pyroxenite is likely to have formed when fluids derived from, or modi-fied by, melting of the subducting, eclogitic, oceanic crustal plate percolated through peridotite of the lithospheric mantle. Subsequent melting of lithospheric pyroxenite veins similar to those repre-sented in the Mount Morning xenolith suite has contributed to the enriched trace element (and iso-tope) signatures seen in Cenozoic volcanic rocks from Mount Morning, elsewhere in Victoria Land and Zealandia. In general, the harzburgite xenoliths reflect between 20 and 30 % melt depletion. Their depleted element budgets are consistent with Archaean cratonization ages and they have mantle-normalized trace element patterns comparable with typical subcontinental lithospheric mantle. The spinel lherzolite mineral data suggest a similar amount of depletion to that recorded in

The effects of buoyancy on shear-induced melt bands in a compacting porous medium