Differentiation by Deformation Revisited

In a paper published in 1920, N.L Bowen conceived of a situation where forces acting on a crystalline mesh could extract the liquid phase from the solid and in doing so cause variations in chemistry distinct from the purely gravitational effects of fractional crystallisation. His paper was a call-to-arms to explore the deformation potential as a cause of variation in igneous rocks, but was never followed up in a rigorous way. Inspired by this, we have developed a quantitative model showing how shear deformation of a dense magma with poroelastic properties can be related quantitatively to its phase petrology during crystallisation. The critical link between the mechanics and associated compositional changes is the degree to which the magma undergoes dilation (volume increase) during shear. It is important to note the effect can only take place after the initially loose magma has undergone compaction. Key parameters governing the dilatancy effect include the permeability, mush strength, the shear modulus and the contact mechanics and geometry of the granular assemblage. Calculations show that dilation reduces the interstitial fluid (melt) pressure causing, in Bowens words, “the separation of crystals and mother liquor” via a suction effect. At strain rates in excess of the tectonic background, shear-induced melt flow can redistribute chemical components and heat between regions of crystallising magma with contrasting rheological properties, at velocities far in excess of diffusion or buoyancy forces, the latter of course the driving force behind fractional crystallisation and compaction. The local influx of hotter, less evolved melt into regions where crystallisation is more advanced may result in reverse zoning and/or resorption of crystals. Post-failure instabilities include hydraulic rupture of the mush along shear zones governed by the angles of dilation and internal friction. Evolved, compositionally distinct melt fractions removed at this late stage may for example lead to certain miarolitic alkaline rocks and intrusive granophyres in basaltic systems, as proposed by Bowen, and late stage aplites and pegmatites in granites - differentiation by deformation.