The physiks of continental magmatic systems

H. Schmeling, G. Marquart (Aachen), H. Wallner, R. Weinberg, S. Cruden (both at Monash, Melbourne)

The physics of formation and evolution of magmatic systems within the continental crust is not well understood. We approach this problem by a thermo-mechanical-compositional two-phase flow formulation based on the conservation equations of mass, momentum, and energy for melt and solid. The approach includes compaction of the solid matrix and melting, melt segregation, melt ascent and freezing. We use a simplified melting law of a binary solid solution system to track chemical composition, i.e. the enrichment or depletion in SiO2 of the advected silicic melt and solid. We discovered a new melt ascent mechanism, CATMA , which stands for Compaction/decompaction Assisted Two-phase flow Melt Ascent. This is a combination of compaction and decompaction of the contact zones of the accumulated magma with the solid rock that dislodges solid material from the roof which sinks through and partly dissolves in the magma. Melt - solid separation together with chemical separation result in a dual melt porosity distribution with melt rich magma bodies (> 60% melt) collected in a cap on top of low melt fraction mushes (< 20 %). Such magma systems are typical for natural examples such as the Altiplano-Puna magma body.

Figure. Snapshots of the volumetric melt percentage for a model of the evolution of a magmatic system. Phase 1: Early melt segregation-compaction, Phase 2: Diapir rise, Phase 3: Melt accumulation at the top of the partially molten zone, Phase 4: Lateral spreading of diapir and melt accumulation in diapir head, Phase 5: Melt penetrating into overburden ("CATMA", or "micro-stoping"), formation of melt sills and pillow, and final freezing, Phase 6: Slow sub-diapir melt segregation. Black arrows indicate the solid matrix velocity, grey arrows the melt velocity. (From Schmeling et al., 2018, under revision)

On the possibility of thermal convection in terrestrial ice sheets

(H. Schmeling, P. Bons (Tübingen), NN.)

The rheology of ice sheets is strongly anisotropic and non-linear. Thus flowing ice sheets may exhibit effective rheologies, which, if combined with appropriate temperature boundary conditions, may result in overcritical Rayleigh numbers. We investigate the parameter space of such scenarios including heat advection due to ice compaction to predict scenarios in which thermal convection may becom important.

Figure. Model setup

Transport of volatiles and isotopes with two-phase flow

J. Dohmen, H. Schmeling

In my PhD we want to model the redistribution of radioactive, heat generating isotopes or volatiles such as water and carbon dioxides in partially molten asthenosphere-lithosphere systems and magmatic continental crust systems due to melting and two-phase flow assisted magma ascent. One of the aims is to investigate the self-consistent formation of depth-dependence radiogenic heat sources, which empirically follow an exponential depth distribution. This redistribution of crustal heat sources would have a feedback on crustal convection and melting processes.

Melt migration evolution from two-phase flow to dikes

L. Chevalier, H. Schmeling

Melt (liquid rock) forms at depth, due to the partial fusion of rocks. In such regions, melt porous migration through the compacting residual rock can be modelled as a two-phase flow. However, in shallower, colder rocks, melt flows through dikes that propagate by fracturing the hosting solid rock. The aim of this project is to better understand the transition between these two transport processes, as well as dike initiation, at the transition from partially molten to solid rocks. While the melt migrates towards the surface, a thermal disequilibrium between melt and residual rock may progressively build up due to melt pathway merging and widening. We focus on the development and consequences of such a disequilibrium on melt migration, using a two-temperature model for implementing Local Thermal Non-Equilibrium.

Figure : We solve mass, momentum and heat equations for a two-phase flow. The two heat equations, for the liquid and the solid phases, are coupled through a heat exchange term Qb, which depends on S (interfacial area density), dm (boundary layer thickness), temperature difference and mean thermal conductivity. The two phases are assumed to be continuous and homogeneous at the meso-scale.

Mantle deformation beneath the Alps and the physics of the subduction polarity switch: Constraints from thermomechanical modelling, seismic anisotropy and waveform modelling

Jan Philipp Kruse & Harro Schmeling in cooperation with Frederik Link & Georg Rümpker (all Uni Frankfurt)

The deep dynamics of continental collision is one of the least understood plate tectonic processes. One interesting process that is believed to be a feature of continental collision is a flip in subduction polarity. A prominent location where such a flip is proposed by different seismic tomography studies is the eastern alps region.

We use the thermo-mechanical 2D finite-difference code FDCON to model continental collision. In our study we vary several input parameters (initial thermal structure and model geometry, lower crustal rheology, convergence velocity) to get detailed knowledge under which conditions a flip is possible and to get an idea of the physical mechanisms that can produce a flip in subduction polarity.

In the second stage of the project we want to estimate the seismic anisotropy produced by the flow of mantle related to a polarity switch scenario (2D/3D) to compare this with results from the seismology (Frederik Link & Georg Rümpker).