Igneous differentiation by deformation

Abstract

In a paper published in 1920, 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 role of deformation 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 crystallised dense magma (ϕ > 70%) with poro-elastic properties is analogous to a granular material. The critical link between the mechanics and associated compositional changes of the melt is the degree to which the crystallising magma undergoes dilation (volume increase) during shear. It is important to note that the effect can only take place after the initial loose solid material has undergone mechanical compaction such that the grains comprising the rigid skeleton are in permanent contact. Under these conditions, the key material parameters governing the dilatancy effect are the physical 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 Bowen’s words, “the separation of crystals and mother liquor” via a suction effect. At shear strain rates in excess of the tectonic background, deformation-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 viscous compaction. Influx of hotter, less evolved melt drawn internally from the same magma body into regions where crystallisation is more advanced (auto-intrusion), may result in reverse zoning and/or resorption of crystals. Because dilatancy is primarily a mechanical effect independent of melt composition, evolved, chemically distinct melt fractions removed at this late stage may explain miarolitic alkaline rocks, intrusive granophyres in basaltic systems and late stage aplites and pegmatites in granites (discontinuous variations), as proposed by Bowen. Post-failure instabilities include hydraulic rupture of the mush along shear zones governed by the angles of dilation and internal friction. On the macro-scale, a combination of dilatancy and fracturing may provide a means to extract large volumes of chemically evolved melt from mush columns on short (< 1000 year) geological timescales.