Image based modelling of lateral magma flow: the Basement Sill, Antarctica

摘要

The McMurdo Dry Valleys magmatic system, Antarctica, provides a world-class example of pervasive lateral magma flow on a continental scale. The lowermost intrusion (Basement Sill) offers detailed sections through the now frozen particle microstructure of congested magma slurry. We simulated the flow regime in 2 and 3D using numerical models built on a finite element mesh derived from field data. The model captures the flow behaviour of the Basement Sill magma over a viscosity range of 1-104 Pa s where the higher end (≥102 Pa s) corresponds to a magmatic slurry with crystal fractions varying between c. 30 and 70%. A novel feature of the model is the discovery of transient, low viscosity (≤ 50 Pa s) high Reynolds number eddies formed along undulating contacts at the floor and roof of the intrusion. Numerical tracing of particle orbits implies crystals trapped in eddies segregate according to their mass density. Recovered shear strain rates (10 3 to 10-5 s-1) at viscosities equating to high particle concentrations (> c. 40%) in the Sill interior point to shear-thinning as an explanation for some types of magmatic layering there. Model transport rates for the Sill magmas imply a maximum emplacement time of c. 105 years, consistent with geochemical evidence for long range lateral flow. It is a theoretically possibility that fast-flowing magma on a continental scale will be susceptible to planetary-scale rotational forces.