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The oceanic crust, covering two-thirds of the Earth's surface, is formed along mid-oceanic ridges by crystallization at shallow levels of melts formed at depth by partial melting of mantle peridotite. Yet, the process of melt transport to the ridge axis remains poorly understood. Ophiolites, which provide a window into the uppermost mantle, contain dunite bodies often interpreted as relics of melt flow conduits, formed by pyroxene dissolution during melt-peridotite interaction. Here, we present structural and geochemical data on peridotites from the southeastern Oman ophiolite where three types of dunite, corresponding to the Moho transition zone (MTZ), the main and basal mantle sections, are identified. We focus on osmium isotopes, which are particularly well-adapted to tracing melt flow through peridotites. Osmium isotope signatures from host harzburgites accord with abyssal peridotite values and do not vary systematically with setting. In contrast, dunite Os compositions depend on structural context. Basal dunites display compositions similar to harzburgite values, while MTZ dunites have highly radiogenic compositions similar to those of the overlying crust, requiring extensive interaction with melts more radiogenic than MORB. Modeling shows that melts percolating through and equilibrating with dunite channels would acquire unradiogenic compositions, inconsistent with the observed Os signatures of MTZ dunites and lower crust. Thus, our findings require melt transport without equilibration with dunite or harzburgite, arguing for rapid or at least chemically isolated melt migration from the mantle source to the Moho.

We present rheological data on samples of partially molten mantle rocks deformed in torsion at 1200 °C and 300 MPa in a gas-medium deformation apparatus. Samples in which stress-driven melt segregation occurs are significantly weaker than those with a homogeneous melt distribution, with strain rate increased by up to a factor of 6. The degree of melt segregation, which is influenced by the permeability of the sample material (and hence the compaction length), is controlled by changing chromite fraction as a minor solid second phase. In general, a higher degree of melt segregation causes a greater strain rate enhancement. Although we focus here on the steady-state rheological properties, a simple model of the evolution of melt segregation and rheological properties is presented. This model fits well the latter part of the stress–strain evolution but fails to fit the early, transient stages of deformation. One implication is that weakening occurs very quickly due to the emergence of smaller-scale melt bands. The model (constitutive and evolution equations) is designed to be incorporated into geodynamic-scale continuum models to explore the effects of segregation without resolving the length scales of the process directly.

Our ability to monitor volcanoes (using seismic signals, ground deformation, gas fluxes, or other ground and satellite based observations) as well as our understanding of melt reservoirs that feed eruptions have evolved tremendously in recent years. The complex plumbing systems that are thought to feed eruptions are, however, difficult to relate to the monitoring signals. Here we show that the record preserved in compositional zoning of erupted minerals may be used to reconstruct sections of the plumbing system. Kinetic modeling of such zoning can yield information on the residence time of magma in different segments of the plumbing systems. This allows a more nuanced evaluation of the link between observed monitoring signals or eruption styles and the magmatic processes and movement of batches of melts at depth. The approach is illustrated through a study of the compositional zoning recorded in olivine crystals from the 1991–1993 SE-flank eruption products of Mt. Etna (Sicily). The zoning patterns in crystals reveal that the plumbing system of the volcano consisted of at least three different magmatic environments between which magma was transported and mixed in the year or two preceding the start of eruption. Quantification of this history indicates that two main pathways of melt migration and three timescales dominated the dynamics of the system. Combination of this information with the timing of observation of various monitoring signals allows a reconstruction of the dynamic evolution of this section of the plumbing system during the early stages of the 1991–1993 eruption. It is seen, for example, how the migration of melt through the same sections of the plumbing system can cause pre-eruptive triggering, enhance Strombolian activity, and through the ensuing eruption cleanse and flush the plumbing system. Different kinds of mixing occur simultaneously at different sections of the plumbing system on different timescales (a few days up to two years).

In the upper part of the Earth there exists a region where the rock is partially molten. The magmatic melt rises through the partially molten region as a form of permeable flow. The timescale and efficiency of the melt migration are controlled by the permeability, which is a function of the melt fraction and the distribution of the melt at the grain scale. Using gel as an analog of the solid phase of the partially molten system, the permeability and the electrical conductivity were measured. With deformation the gel deforms into a polyhedron due to its high deformability, and the fluid phase exists at edge and corner regions. The morphology of the mixture system is similar to that of the partially molten system controlled by the interfacial energy. Both of the permeability and the electrical conductivity obey power-law functions of the liquid fraction. The permeability is close to Kozeny–Carman type permeability and the electrical conductivity obeys Archie's law at higher liquid fraction (about 10% to 30%). They reduce drastically with reduction of the liquid fraction at lower liquid fraction (about 2% to 10%). These behaviors are considered to result from dispersion of pseudo-dihedral angle. The permeability in the upper mantle would behave like that of the gel–fluid system because the dispersion of dihedral angle exists due to poly-mineralic solid phase of the upper mantle and anisotropic interfacial energy. Also, the permeability is found to be proportional to the square of the electrical conductivity. This relation is useful to estimate the permeability at depth from the electrical conductivity measurement.