Lecture 5A- Deformation by transfer of dissolved material

VIEPS/Mainz Microstructure Course 

| TOC | Lecture 1 2 3 4 a b 5 a b | Lab 1 a b c 2 a b c 3 a b 4 a b 5 a b | Glossary Table 1 2 3 4 5 Index |

Introduction

Deformation can be achieved by removing material from some sites and bringing it to other sites. At length scales below the transport scale, there are sites of volume increase and volume decrease, whereas other sites remain undeformed.

Several (micro-) structures resulting from material transport due to applied stresses:

(a) Compaction of a porous rock with material dissolving at grain contacts and precipitating as cement in pores.

(b) Preferential dissolution at grain contacts that are normal to compression and re-precipitation at grain contacts that are normal to extension leads to deformation. Original grain shapes can often be discerned by differences in inclusion content of original grains and overgrowths and/or by dust rims.

(c) Preferred dissolution of quartz at quartz-mica contacts. A dust rim reveals the original grain shape.

(d) Localised dissolution (net material loss) at strain cap and localised precipitation (net material gain) at strain shadow or pressure shadow around a relatively rigid object or grain (e.g. pyrite, feldspar or quartz augen).

(e) Precipitation in veins and dissolution at stylolites. Material transport can be from stylolites to veins, or into / out off the system.

(f) Segregation of quartz and mica's forming domainal cleavage.

- Fluid reservoirs

A fluid is needed to transport dissolved material. Only at very high temperature can diffusional transport without a fluid be significant. Fluid can be present in a variety of different sites (reservoirs) in a rock.

1. In open cracks (> cm scale). Open fluid-filled cracks can exist at deeper levels despite the high pressure if the fluid pressure is high enough. However, open cracks greatly enhance the permeability of a rock, allowing fluids to flow and decrease the fluid pressure. There must be a dynamic equilibrium between opening of cracks (increase in permeability, decrease in fluid pressure) and closure of cracks (decrease in permeability, allowing build-up of fluid pressure).


2. Open pore space between grains. Sediments at shallow depths typically have a high porosity, which decreases with burial. At deeper levels the geometry of the pore space depends on the balance between grain-grain boundary surface energy and grain-fluid boundary surface energy: the wetting angle. The wetting angle (a) determines the shape of the pore space: (b) at a high wetting angle fluids reside in pockets where 4 grains meet; (c) at a medium wetting angle in tubes where 3 grain meet and (d) at a low (0°) wetting angle all grain boundaries are wetted. It seems that in general fluids tend to reside in pockets or tubes.

3. micro-cracks. Typical length <0.1 mm, width/length<0.01. Short-lived and dynamic structures due to rapid propagation (stress corrosion cracking), due to stress concentration at crack tip (a), and rapid healing, due to surface energy effects (b), possibly resulting in fluid inclusions.

4. Grain boundaries. The nature of grain boundaries is very important, since fluids here actually come in contact with all grains. Two general models for grain boundaries:

(a) Thin film model:

Thin fluid films have been observed at low normal stress, but seem to get squeezed out between 0.1 and 20 MPa. Wetting angles >0o also suggest that the thin film is probably not the correct model for most mineral aggregates.

(b) Island-and-channel model:

In the island-and-channel model there is a stable, but dynamic, structure of grain-grain contacts that support the stress and a fluid in the channels. As islands and channels constantly migrate, the fluid can access the whole grain surface over time.

5. Inside the crystal, incorporated in the lattice (only a small amount) and in fluid inclusions.

Dissolution precipitation creep

Dissolution-precipitation creep is a deformation mechanism that involves three serial steps. The slowest of these three steps is the rate controlling step.

(a) Dissolution reaction at (relatively) high normal stress grain boundaries

(b) Diffusional transport along chemical potential (m) gradient in grain boundary fluid

(c) Precipitation reaction at (relatively) low normal stress grain boundaries.

DP creep is the dominant ductile deformation mechanism at low temperatures in wet rocks (<= greenschist facies), where other mechanisms, such as dislocation creep are slow. Provided there is a suitable fluid, it may also be important at higher temperatures.

- Driving force

Diffusional transport and the interfacial reactions are driven by a stress induced chemical potential differences along the grain boundaries (->diffusion) and across the interfaces (->reaction). The equilibrium chemical potential (m) of a solid dissolved in a fluid adjacent to the surface of the solid can be described as:

(with f = Helmholtz free energy of solid, P = pressure in solid, V = molar volume of solid, c = a material constant, g = surface energy of solid-fluid interface, R = local curvature of that interface). Grain boundaries have to transmit stresses from grain to grain. Based on this one can argue that effectively P=sn (Fig. 4.5). Chemical potential is therefore higher on compressive grain boundaries (sn=s1) than on extensional grain boundaries (sn=s3). Neglecting the surface energy term, the drop in m that drives the material transport is:

- Diffusions is rate controlling

The whole of Dm is used to drive the diffusion, if precipitation and dissolution are relatively fast. The flux (J) through the GB-fluid is proportional to the concentration gradient along the grain boundary, which is proportional to Dm and inversely proportional to the grain size (g):

The flux (F) has to go through an area proportional to the cross-sectional area of the grain boundaries, which is proportional to the grain size: The dissolved solid arriving on the extensional grain boundary adds a layer of solid to it of width w: Extension rate (E) is: When diffusion is rate controlling, the strain rate is proportional to the differential stress (linear or Newtonian viscous) and inversely proportional to the cube of the grain size.

-Reaction is rate controlling

The whole of Dm is used to drive the interfacial reactions, if precipitation and dissolution are relatively slow compared to diffusional transport. The rate (w) of precipitation and dissolution are (normally) proportional to the chemical potential difference across the interface:

The extension rate is given by the growth rate divided by the grain size: When interfacial reactions are rate controlling, the strain rate is again proportional to the differential stress (linear or Newtonian viscous) and inversely proportional to the grain size. Pressure solution creep is favoured by a small grain size, and the grain size must be especially small for reaction controlled pressure solution creep.

- Material transfer on larger length scales (veins, stylolites, around objects)

Deformation induced material transfer can also occur on length scales larger than one grain. Since the 'effective grain size' would then be much larger, it is clear that the rate of such transfer to produce deformation is low (1/g or 1/g3) compared to grain scale pressure solution creep. As a deformation mechanism, long distance transport is not very important, but it is as a significant process that produces structures in rocks, such as veins, stylolites and cleavages.

Material transfer on large length scales (>>g) occurs when some areas within a rock volume experience a net volume loss and/or other areas a net volume gain. In other words a heterogeneous distribution of precipitation and dissolution.

If dissolution consistently outweighs precipitation at a plane, this plane is a site of net volume loss: a stylolite. Stylolites are often oriented normal to the maximum compression direction. The material that is removed can be carried away by diffusion and/or flow of a fluid through the rock.

If precipitation consistently outweighs dissolution at a plane, this plane is a site of net volume gain: a vein. Veins are typically oriented normal to the minimum compression direction, and are then usually called tension gashes. The material that is added can be carried in by diffusion and/or flow of a fluid through the rock.

- Why & where localised precipitation?

a) not controlled by dissolution precipitation creep.

Primary mechanical heterogeneities provide heterogeneities in stress state and pressure and preferred sites for precipitation

b) localisation of dissolution and/or precipitation coupled to dissolution precipitation creep: development of regular alterations of dissolution & precipitation.

Continue to lecture 5.b: (Micro) structures in veins