Three Reasons Why Petrologists Should Study Compaction J. Connolly, ETH Zurich

What is compaction driven fluid flow?

Objectives Provide a conceptual understanding of porosity waves in a viscous rock matrix Insights from compaction on melt extraction at mid-ocean ridges

A simple model for regional metamorphism

What happens with time?

Numerically computed porosity and pressure profiles above a metamorphic dehydration front

Birth of the Blob Model or Fluid Flow through a 2D Rock Matrix with Constant Viscosity · Length scale for fluid flow ~ 

Tod des Blobs oder Fluidfluss durch eine sich aufwärts verstärkende Matrix

Has anyone ever seen a porosity wave? Sedimentary Basin Compaction

Pannonian basin

Inverse analysis of sedimentary compaction profiles for pressure solution creep parameters

Lateral flow during regional metamorphism? A World Where Fluids Flow Upward => Mid-Ocean Ridges How does melt produced during mantle upwelling get focused at mid-ocean ridges? How can highly incompatible short-lived isotopes be fractionated and preserved in MORB?

A World Where Fluids Flow Upward => Mid-Ocean Ridges How does melt produced during mantle upwelling get focused at mid-ocean ridges? How can highly incompatible short-lived isotopes be fractionated and preserved in MORB?

How does melt get to the ridge?

Steady State

What was wrong?

Fast Fluid Transport in Ductile Rocks y

Initial State

Final State

Return of the Blob

Final State

Conclusion The combination of models suggested here can reconcile the geochemical signature of MOR basalts, with the possible exception of near surface matrix-melt disequilibrium. Reports of the death of the porosity wave model are premature and premised on a rheological model that is almost certainly false.

Viscoelastic porosity wave model for Pannonian Basin sediments

Viscoplasticity Viscous porosity waves are propagated by high fluid pressures. Under such conditions rocks even ductile rocks will deform plastically.

Morb the Movie

What happens beneath a mid-ocean ridge?

What next? Composition and depth of devolatilization => global volatile budget, deep seismicity Amount of pore fluid => subduction zone seismic structure

Model Formulation

What next? Experimental and microscopic models to characterize differential compaction rheology The mantle wedge

The models assumed constant porosity and lithostatic melt pressure. Lithostatic melt pressure is fundamentally inconsistent with expulsion. Variations in porosity, and therefore permeability, may cause significant focusing. To assess these effects it is essential to account both for the process that creates porosity (melting) and destroys it (compaction). What was wrong with previous models of the corner flow effect?

What next? Dynamic modelling of the matrix deformation, thermal controls of melting rates, and melt advection => details of the focusing

Ergo The corner flow pressure effect is not dependent on the mantle viscosity and is capable of explaining extraction of asthenospheric melts at mid-ocean ridges What next? Evaluate the influence of the mantle compressibility on the strength of the pressure effect => future work? Consider details necessary to explain geochemical peculiarites of MORB => next slide.

What is wrong with “conventional” porosity waves? Require high initial porosity to nucleate, but there is no Th/U fractionation at high porosity Unlikely to propagate at velocities > 3 v 0 Based on an inappropriate rheological model

So what point am I trying to make? The first order control on the time and length scales of fluid flow in many petrologic systems is mechanical. To attempt to understand such processes solely through the study of petrological and geochemical tracers is like wagging a dog by its tail. Lateral flow during regional metamorphism?

What causes the pressure difference?