Igneous differentiation by deformation

N Petford,J D Clemens,M A Koenders

doi:10.1007/s00410-020-1674-3

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.

Highlights

“It is not difficult to conceive of a distribution of forces in the heterogeneous surroundings of an igneous mass, such that locally the liquid might be sucked out of the crystalline mesh... such physical separation of two magmatic phases is the complement of chemical differentiation” (Bowen 1920)
Order-of-magnitude estimates for the time Γ to dilation from Eq (2) are shown in Table 2 for each deformation régime and skeletal stresses caused by buoyancy (Σb), for a mineral assemblage corresponding to a generic mafic and silicic mush
We present expressions for (1) the time to dilation, (2) the pressure change in the interstitial melt phase, and (3) associated melt flow rates as a function of shear strain rate

Summary

Introduction

“It is not difficult to conceive of a distribution of forces in the heterogeneous surroundings of an igneous mass, such that locally the liquid might be sucked out of the crystalline mesh... such physical separation of two magmatic phases is the complement of chemical differentiation” (Bowen 1920). Multiphase compaction models assume that buoyancy effects, coupled with viscous deformation of the granular matrix, drive the flow instability (segregation). Connolly and Podladchikov 1998), still rely on buoyancy-forced melt flow This situation is quite different from Bowen’s original idea of melt being ‘sucked’ out of a rigid, sparingly deformable matrix in an externally imposed stress field. Marsh 2004) of crustal magma reservoirs as vertically extensive mush columns (Cashman et al 2017; Jackson et al 2018; Sparks et al 2019) In these models, fractionation and compositional diversity is achieved mainly as a consequence of melt migration through a porous, viscously compacting matrix. Our analytical model captures a dilation effect inherent in densely packed poro-elastic materials undergoing shear, that—importantly with respect to the current debate on melt segregation— occurs after compaction

Results

Discussion

Conclusion