Michel Rabinowicz

A rolling mill effect in asthenosphere beneath oceanic spreading centers

Abstract From structural studies in seventeen ophiolite massifs, information has been drawn about the activity of the asthenosphere beneath oceanic spreading centers. This information, together with geophysical data pertaining to oceanic ridges, has been integrated into a numerical model. It is inferred that for a fast-spreading ridge (

Compaction in a mantle mush with high melt concentrations and the generation of magma chambers

Abstract Most compaction models in the partially molten mantle have addressed the case of low intergranular melt concentrations (up to a few percent). Here we develop a mathematical and numerical formalism adapted to the two-dimensional modeling of compaction in mushes with higher melt concentrations (up to a few tens of percents). Experimental data on mantle-like mushes (olivine crystals+basaltic melts) suggest that the mush viscosity depends in a complex way on the melt concentration: three rheological thresholds occur at melt concentrations of about 5%, 20%, and 40%, respectively. The first threshold corresponds to the establishment of full interconnectivity of the intergranular melt, the second to the formation of a very dense suspension of crystals and the last to the development of crystal clusters in the suspension. The present models take into account the stiff and drastic viscosity drops associated with these rheological thresholds. Intergranular melt migration associated with an initial melt pulse generates a horizon of high melt/crystal ratio. If the melt concentration in the initial pulse presents a local excess, the horizon becomes slightly tilted. As a consequence, melt percolates upslope inside the tilted horizon, pools at its summit and generates a ‘pocket-like’ zone. Due to the higher melt concentration, the upward Darcy velocity in the pocket markedly exceeds that in the horizon. The result is that the pocket-like impregnation is rapidly disconnected from the horizon and a new pocket develops at the summit of the partially fragmented horizon. Eventually, the intergranular melt contained in the horizon is completely redistributed into pockets. Increasing the background melt concentration in the mush from 5%, 20%, and 40% leads to an increase of the maximum melt concentration of 10%, 40%, and 100%, in the transient horizons and of 25%, 60%, and 100% in the pockets. These models suggest that magma chambers with a kilometer extent naturally result from the compaction of a mantle mush with an initial melt concentration exceeding 5%.

Melt percolation in a partially molten mantle mush: Effect of a variable viscosity

Abstract Data collected in ophiolite massifs indicate that some horizons with very high melt fractions have been frozen during the accretion of the harzburgites just below the crust. We show that the existence, distribution and amplitude of these impregnations are better understood if rheological bifurcations due to the increase in melt fraction, in addition to a power-law dependence of permeability on porosity, are taken into account in the compaction models. As shown by deformation experiments on partially molten rock, at melt fractions of around 5–10% a first rheological bifurcation occurs when the melt wets the grain boundaries of mantle minerals, enhancing diffusion processes. A second one occurs when the melt fraction exceeds the second percolation threshold (around 25–30%). Our 1D numerical experiments based on the compaction equations integrating the effect of these bifurcations show striking features reminicent of observations in ophiolites.

Thermal and petrological consequences of melt migration within mantle plumes

The high temperatures and high degrees of melting expected in the core of mantle plumes have virtually no expression in the eruption temperatures of hotspot lavas, nor in the composition of their glasses, which is restricted in the basaltic field. A solution to this paradox is looked for in the melt migration processes within the melting region of mantle plumes. Three dimensional convective calculations at Rayleigh number of 106 allow estimates of the possible temperature, melt fraction and stress fields within a plume. Two regions with different melt migration patterns can be distinguished. A lower zone ranging in depth from the base of the melting region (150 km) to around 80—100 km where the first melt fraction is redistributed in a sub-horizontal vein network and convects in response to the steep horizontal temperature gradient. This process is able to homogenize the temperature within the melting region very efficiently. The high (300 °C) temperature contrast between the centre of the plume and the surrounding mantle can be reduced to a few tens of degrees at the top of this zone. Fractional crystallization of high pressure phases will strongly modify the composition of the melt as it circulates toward the periphery of the melting region. A second upper zone, where the sub-vertical vein orientation will make possible rapid melt migration toward the surface, extends to the base of the lithosphere. Due to the buffering of the plume temperature around a value close to the mean upper mantle temperature, the degree of adiabatic melting within this upper zone will not greatly exceed that beneath normal spreading centres, even in the case of on-ridge hotspots. The lavas erupted at hotspots are likely to result from the mixing in various proportions of these low pressure melts (basalts) with the highly evolved liquids (possibly with kimberlitic to alkalic affinities) resulting from fractional crystallization of the high-pressure melt fractions produced at the base of the melting region. This scenario could account for the low eruption temperatures and Mg contents of hotspot lavas, in spite of a complex high pressure, and thus high temperature, history evidenced by some geochemical trends.

Is the 670 km phase transition able to layer the Earth's convection in a mantle with depth‐dependent viscosity?

The effect of a viscosity stratification on phase change dynamics have been investigated with axi-spherical convection models. As in previous studies with a constant viscosity mantle an intermittent layering appears for a Clapeyron slope from −2 MPa/K to −3 MPa/K. A viscosity increase in lower mantle requires a more negative Clapeyron slope to produce the layering. This shift is sensitive to the mechanical boundary condition. With a viscosity contrast of 30, a no-slip top condition does not lead to layering in the range of the possible values for the Clapeyron slope. With a free-slip condition, the threshold is at −4 MPa/K. Just below this threshold, a whole mantle circulation driven by a cylindrical hot plume coexists with layered mantle domains over several billion years.

2D modeling of melt percolation in the mantle: The role of a melt dependent mush viscosity

2D-melt percolation models in the partially molten mantle are presented. Previous work has shown that melt solitary waves develop in response to the non-linear dependence of mush permeability to melt concentration. The present study focuses on the wave evolution when the mush viscosity abruptly drops, around a critical concentration fc. 2D-waves develop with a diameter, that shortens as the melt concentration increment for viscosity drop narrows. They are initiated inside 1D-maturing horizontal waves at melt concentration of about 4×fc. Both 1D- and 2D-waves collect all the melt whose concentration exceeds fc. These features may help to explain the distribution and organization of some melt impregnations observed in mantle outcrops.

Subsurface Hydrology of the Lake Chad Basin from Convection Modelling and Observations

In the Lake Chad basin, the quaternary phreatic aquifer (named hereafter QPA) presents large piezometric anomalies referred to as domes and depressions whose depths are ~15 and ~60xa0m, respectively. A previous study (Leblanc et al. in Geophys Res Lett, 2003, doi:10.1029/2003GL018094) noticed that brightness temperatures from METEOSAT infrared images of the Lake Chad basin are correlated with the QPA piezometry. Indeed, at the same latitude, domes are ~4–5xa0K warmer than the depressions. Leblanc et al. (Geophys Res Lett, 2003, doi:10.1029/2003GL018094) suggested that such a thermal behaviour results from an evapotranspiration excess above the piezometric depressions, an interpretation implicitly assuming that the QPA is separated from the other aquifers by the clay-rich Pliocene formation. Based on satellite visible images, here we find evidence of giant polygons, an observation that suggests instead a local vertical connectivity between the different aquifers. We developed a numerical water convective model giving an alternative explanation for the development of QPA depressions and domes. Beneath the depressions, a cold descending water convective current sucks down the overlying QPA, while, beneath the dome, a warm ascending current produces overpressure. Such a basin-wide circulation is consistent with the water geochemistry. We further propose that the thermal diurnal and evaporation/condensation cycles specific to the water ascending current explain why domes are warmer. We finally discuss the possible influence of the inferred convective circulation on the transient variations of the QPA reported from observations of piezometric levels and GRACE-based water mass change over the region.

Development and Evolution of the Size of Polygonal Fracture Systems during Fluid-Solid Separation in Clay-Rich Deposits

In continental and oceanic conditions, clay-rich deposits are characterised by the development of polygonal fracture systems (PFS). PFS can increase the vertical permeability of clay-rich deposits (mean permeability ≤10-16 m2) and are pathways for fluids. On continents, the width of PFS ranges from centimeters to hundreds of meters, while in oceanic contexts they are up to a few kilometres large. These structures are linked to water-solid separation during deposition, consolidation and complete fluid squeeze of the clay horizon. During the last few decades, modeling of melt migration in partially molten plastic rocks led to rigorous quantifications of two-phase flows with a particular emphasis on 2D and 3D induced flow structures. The numerical modeling shows that the melt migrates on distances at most equal to a few times the compaction length L that depends on permeability and viscosity. Consequently, polygonal structures in partially molten plastic rocks result from the melt-rock separation and their sizes are proportional to L. Applying these results to fluid-solid separation in clay-rich horizons, we show that (1) centimetric to kilometric PFS result from the dramatic increase of L during compaction and (2), this process involve agglomerates with 100 μm to 1 mm size.

Can high-temperature, high-heat flux hydrothermal vent fields be explained by thermal convection in the lower crust along fast-spreading Mid-Ocean Ridges?

We present numerical models to explore possible couplings along the axis of fast-spreading ridges, between hydrothermal convection in the upper crust and magmatic flow in the lower crust. In an end-member category of models corresponding to effective viscosities μM lower than 1013 Pa.s in a melt-rich lower crustal along-axis corridor and permeability k not exceeding ∼10−16 m2 in the upper crust, the hot, melt-rich, gabbroic lower crust convects as a viscous fluid, with convection rolls parallel to the ridge axis. In these models, we show that the magmatic-hydrothermal interface settles at realistic depths for fast ridges, i.e., 1–2 km below seafloor. Convection cells in both horizons are strongly coupled and kilometer-wide hydrothermal upflows/plumes, spaced by 8–10 km, arise on top of the magmatic upflows. Such magmatic-hydrothermal convective couplings may explain the distribution of vent fields along the East (EPR) and South-East Pacific Rise (SEPR). The lower crustal plumes deliver melt locally at the top of the magmatic horizon possibly explaining the observed distribution of melt-rich regions/pockets in the axial melt lenses of EPR and SEPR. Crystallization of this melt provides the necessary latent heat to sustain permanent ∼100 MW vents fields. Our models also contribute to current discussions on how the lower crust forms at fast ridges: they provide a possible mechanism for focused transport of melt-rich crystal mushes from moho level to the axial melt lens where they further crystallize, feed eruptions, and are transported both along and off-axis to produce the lower crust.