Saswata Hier-Majumder

Magma transport and storage

We are interested in studying transport and storage of magmatic melts in the deep interior of the Earth. Efficiency of magma transport determines the rate at which mass and energy are circulated in the interior of the Earth. Beneath mid-oceanic ridges and within plume conduits, buoyant magma rises in discrete pulses through a matrix of viscous rocks. Capillary action on matrix grain boundaries tend to retain melts from these pulses in a way similar to a damp sponge retaining a small amount of water. This movie shows melt retention by matrix beneath a rising pulse of magma.
Seismic evidence indicates the likely presence of a dense melt-rich layer at the core-mantle boundary. Thickness of this layer is a crucial parameter controlling the thermal evolution of the mantle. Our recent results show that a balance between capillary forces and gravitational pull controls the thickness of such a dense melt-rich layer.

Acoustic Properties and melt geometry

Bulk physical properties of partially molten rocks depend on the volume fraction of melt and the melt geometry. One area of our research involves modeling the microstructure of partially molten rocks using analytical and numerical techniques. Our recent results indicate that for a constant melt fraction, ratio of viscosities between the melt and the matrix as well as interfacial tension significantly influences the microstructure, thus controlling the seismic velocities of the rock. This result can be used an extremely powerful tool to determine the determination of melt and the viscosity structure of the deep interior of the Earth.

Permeability

Melt travels through a network of interconnected melt tubules along edges of matrix grains. The presence of such a network, however, is precluded by high dihedral angle melts such as water-rich fluids in shallow subduction environment and Fe-FeS melts in a protoplanet. One important aspect of our research involves modeling the establishment of permeability in partially molten rocks containing high dihedral angle melts. Pramod Doguparty's research indicates that deformation of the matrix opens up grain boundaries, thus establishing an interconnected network of melt sheets along grain boundaries.

Chemical signature

Mass exchange between melt and the matrix determines the observed chemical signature of rocks. In collaboration with Roberta Rudnick, Xiaoming Liu is developing a model of distribution of lithium isotopes in metamorphic aureoles, combined with direct measurement of lithium concentration.

Mass exchange upon melting also controls the rate of growth or decay of melt-rich layers in a cooling planet. In a collaborative project with Yasuko Takei at the Earthquake Research Institute in Tokyo, we are calculating the `half-life' of such melt-rich layers. Our preliminary results indicate that the rate of decay of such melt-rich layer depends strongly on the initial layer thickness.

Vertical profile of a dense melt-rich layer. Strong capillary tension of the overlying rock in the blue curve prevents the melt from pooling into a thin layer.

Colormap of normalized decay rate of a reacting melt-rich layer as a function of logarithmic, normalized matrix grain size (x-axis) and initial layer thickness (y-axis).