Pervasive upper mantle melting beneath the western US

We report from converted seismic waves, a pervasive seismically anomalous layer above the transition zone beneath the western US. The layer, characterized by an average shear wave speed reduction of 1.6%, spans over an area of similar to 1.8 x10(6) km(2) with thicknesses varying between 25 and 70 km. The location of the layer correlates with the present location of a segment of the Farallon plate. This spatial correlation and the sharp seismic signal atop of the layer indicate that the layer is caused by compositional heterogeneity. Analysis of the seismic signature reveals that the compositional heterogeneity can be ascribed to a small volume of partial melt (0.5 +/- 0.2 vol% on average). This article presents the first high resolution map of the melt present within the layer. Despite spatial variations in temperature, the calculated melt volume fraction correlates strongly with the amplitude of P-S conversion throughout the region. Comparing the values of temperature calculated from the seismic signal with available petrological constraints, we infer that melting in the layer is caused by release of volatiles from the subducted Farallon slab. This partially molten zone beneath the western US can sequester at least 1.2 x 10(17) kg of volatiles, and can act as a large regional reservoir of volatile species such as H or C. (C) 2017 Elsevier B.V. All rights reserved

An inversion approach for analysing the physical properties of a seismic low-velocity layer in the upper mantle

International audienceIn this article, we propose a new inversion scheme to calculate the melt volume 17 fractions from observed seismic anomalies in a low-velocity layer (LVL) located atop 18 the mantle transition zone. Our method identifies the trade-offs in the seismic 19 signature caused by temperature, solid composition, melt volume fraction, and 20 dihedral angle at the solid-melt interface. Using the information derived from the 2

Stability and migration of slab-derived carbonate-rich melts above the transition zone

We present a theoretical model of the stability and migration of carbonate-rich melts to test whether they can explain seismic low-velocity layers (LVLs) observed above stalled slabs in several convergent tectonic settings. The LVLs, located atop the mantle transition zone, contain small (similar to 1 vol%) amounts of partial melt, possibly derived from melting of subducted carbonate-bearing oceanic crust. Petrological and geochemical evidence from inclusions in superdeep diamonds supports the existence of slab-derived carbonate melt, which may potentially explain the origin of the observed melt in the LVL. However, the presumptive reducing nature of the ambient mantle can be an impediment to the stability of carbonated melt. To reconcile this apparent contradiction, we test the stability and migration rates of carbonate-rich melts atop a stalled slab as a function of melt percolation, redox freezing, amount of carbon supplied by subduction, and the metallic Fe concentration in the mantle. Our results demonstrate that carbonaterich melts in the LVL can potentially survive redox freezing over long geological time scales. We also show that the amount of subducted carbon exerts a stronger influence on the stability of carbonate melt than does the mantle redox condition. Concentration dependent melt density leads to rapid melt propagation through channels while a constant melt density causes melt to migrate as a planar front. Our calculations suggest that the LVLs can sequester significant fractions of carbon transported to the mantle by subduction. (C) 2019 Elsevier B.V. All rights reserved

Grain boundary wetting during magma migration by two-phase flow

We employ the theory of two-phase flow to investigate the influence of grain boundary wetting during segregation of magma in a partially molten aggregate. In partially molten aggregates the `disaggregation melt fraction', the volume fraction of partial melt at which grain boundaries are completely wetted, is crucial in determining the total interfacial force per unit volume. Surface tension on grain boundaries in melt fractions less than the disaggregation melt fraction tend to homogenize melt distribution when the solid-liquid interfacial tension is smaller than the interfacial tension arising from grain boundaries. We also demonstrate that in the presence of large grain-grain interfacial tension, the solitary wave solution to the mass, energy, and momentum equations for two-phase aggregates becomes dissipative. Under such conditions, the rate of buoyancy driven melt segregation is reduced due to melt retention by grain boundaries

Evidence for Melt Leakage from the Hawaiian Plume above the Mantle Transition Zone

Dehydration reactions at the top of the mantle transition zone (MTZ) can stabilize partial melt in a seismic low-velocity layer (LVL), but the seismic effects of temperature, melt and volatile content are difficult to distinguish. We invert P-to-S receiver function phases converted at the top and bottom of a LVL above the MTZ beneath Hawaii. To separate the thermal and melting related seismic anomalies, we carry out over 10 million rock physics inversions. These inversions account for variations arising from the Clapeyron slope of phase transition, bulk solid composition, dihedral angle, and mantle potential temperature. We use two independent seismic constraints to evaluate the temperature and shear wave speed within the LVL. The thermal anomalies reveal the presence of a hot and seismically slow plume stem surrounded by a “halo” of cold and fast mantle material. In contrast to this temperature distribution, the plume stem contains less than 0.5 vol% melt, while the surrounding LVL—within the coverage area—contains up to 1.7 vol% melt, indicating possible lateral transport of the melt. When compared to the melting temperatures of mantle rocks, the temperature within the LVL, calculated from seismic observations of MTZ thickness, suggests that the observed small degrees of melting are sustained by the presence of volatiles such as CO2 and H2O. We estimate the Hawaiian plume loses up to 1.9 Mt/yr H2O and 10.7 Mt/yr CO2 to the LVL, providing a crucial missing flux for global volatile cycles