SESolid EarthSESolid Earth1869-9529Copernicus PublicationsGöttingen, Germany10.5194/se-8-1193-2017Hierarchical creep cavity formation in an ultramylonite and implications for phase mixingGilgannonJamesjames.gilgannon@geo.unibe.chhttps://orcid.org/0000-0003-2597-1762FusseisFlorianhttps://orcid.org/0000-0002-3104-8109MenegonLucahttps://orcid.org/0000-0003-0625-2762Regenauer-LiebKlaushttps://orcid.org/0000-0002-2198-5895BuckmanJimInstitute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, 3012 Bern, SwitzerlandSchool of Geosciences, The University of Edinburgh, Grant Institute, Edinburgh EH9 3JW, UKSchool of Geography, Earth and Environmental Sciences, Plymouth University, Plymouth PL4 8AA, UKSchool of Petroleum Engineering, The University of New South Wales, Kensington NSW 2033, AustraliaInstitute of Petroleum Engineering, Heriot-Watt University, Edinburgh EH14 4AS, UKJames Gilgannon (james.gilgannon@geo.unibe.ch)19December2017861193120922August201728August201730October201714November2017This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://se.copernicus.org/articles/8/1193/2017/se-8-1193-2017.htmlThe full text article is available as a PDF file from https://se.copernicus.org/articles/8/1193/2017/se-8-1193-2017.pdf
Establishing models for the formation of well-mixed polyphase domains in
ultramylonites is difficult because the effects of large strains and
thermo-hydro-chemo-mechanical feedbacks can obscure the transient phenomena
that may be responsible for domain production. We use scanning electron
microscopy and nanotomography to offer critical insights into how the
microstructure of a highly deformed quartzo-feldspathic ultramylonite
evolved. The dispersal of monomineralic quartz domains in the ultramylonite
is interpreted to be the result of the emergence of synkinematic pores,
called creep cavities. The cavities can be considered the product of two
distinct mechanisms that formed hierarchically: Zener–Stroh cracking and
viscous grain-boundary sliding. In initially thick and coherent quartz
ribbons deforming by grain-size-insensitive creep, cavities were generated by
the Zener–Stroh mechanism on grain boundaries aligned with the YZ plane of
finite strain. The opening of creep cavities promoted the ingress of fluids
to sites of low stress. The local addition of a fluid lowered the adhesion
and cohesion of grain boundaries and promoted viscous grain-boundary sliding.
With the increased contribution of viscous grain-boundary sliding, a second
population of cavities formed to accommodate strain incompatibilities.
Ultimately, the emergence of creep cavities is interpreted to be responsible
for the transition of quartz domains from a grain-size-insensitive to a
grain-size-sensitive rheology.
Introduction
Microstructural observations of shear zones in nature and experimental
investigations of monomineralic systems in the laboratory have demonstrated
that the evolution of a ductile fault rock through the mylonite series can
entail a switch from a dislocation creep-controlled (grain-size-insensitive,
GSI) to a diffusion creep-controlled (grain-size-sensitive, GSS) bulk
rheology e.g.. In
quartzo-feldspathic rocks at mid-crustal conditions, this progressive
evolution often leads to the development of distinct microstructural elements
in close spatiotemporal proximity: feldspathic porphyroclasts, monomineralic
quartz bands and well-mixed, fine-grained polyphase domains (Fig. 1). Each
of these elements have been shown to accommodate deformation differently,
e.g. feldspars fracture and react, quartz experiences GSI creep and the
polyphase domains deform by GSS processes . Generally speaking, with
ongoing deformation the proportion of fine-grained material deforming by GSS
creep increases synkinematically so that the polyphase domains form an
interconnected weak layering. It is the establishment of these well-mixed,
anti-clustered polyphase domains that is recognised to ultimately promote a
switch in the bulk rheology of the rock . However, the exact modes by which these domains are established
are still poorly understood and the subject of intense research.
BSE images from the quartzo-feldspathic ultramylonite. Panel (a) shows
the overall strain gradient in the sample, with the highest strain domain
found at the top of the image. Panel (b) is a high-resolution BSE SEM mosaic of
a representative area of the sample (41 448 × 40 282 pixels, scale of 1 px : 35.5 nm).
All results presented for pore shape and orientation analysis are
from the area of Fig. 1b. The greyscale values identify minerals as follows:
black: porosity, dark grey: Qtz, grey: Plg, light grey: Kfs,
bright: accessory phases. Panel (c) presents the edge of a disaggregating quartz domain
and highlights the minerals present in the poorly mixed polyphase domains.
One of the details under investigation is the role of fluids and their
pathways. Models have been proposed for brittle-fracture-based pumping and
the advection of fluids , diffusion-dominated granular flow
and more recently a dynamic granular fluid pump
that is dominated by fluid advection .
postulated that in ultramylonites, fine-grained polyphase domains deforming
by viscous grain-boundary sliding (VGBS) develop the dynamic granular
fluid pump. This pump operates during deformation and utilises a
synkinematic porosity known as creep cavitation. Here, creep cavitation is
defined as a porosity that results from entropy production during
deformation. A stringent consequence of this definition is that pore
nucleation must arise directly from the active deformation mechanism's
response to the shortening and stretching of the rock mass. It is clear that
a model generating fluid pathways in the middle crust that does not require
brittle fracturing has significant implications for the controls of the
exchange of fluids between the hydrostatic and lithostatic pore fluid
pressure regimes , phase nucleation in mylonites
and, by extension, rheology.
The phenomenon of creep cavitation has been well described in material
science, with several distinct types of creep cavitation being distinguished
. To date, geological research has identified evidence for
creep cavitation in natural ultramylonites from the middle crust
, the lower crust and
in mantle rocks . Experimental work has
shown that octachloropropane, quartzite, diabase, feldspar aggregates,
anorthite–diopside aggregates, olivine–clinopyroxene aggregates and
calcite–muscovite aggregates can develop creep cavities .
This dataset is small but diverse and suggests that creep cavities can occur
in many types of deforming rocks across varying pressure, temperature and
rate conditions.
The wide variety of metamorphic conditions at which cavities form suggests
that a range of micro-scale processes contribute to creep cavitation. Many of
the geological works cited interpret creep cavities as the product of
VGBS that form to accommodate strain incompatibilities . In the VGBS-based model of
, cavitation at one grain triple junction is balanced by cavity
closure at other grain triple junctions. This dynamic model of cavity formation is limited
to domains deforming by some form of diffusion creep and does not account
for all reports of creep cavities in geology. In other studies, creep
cavitation was linked to the production of crystal defects . It is unclear how these mechanisms relate to each other across
rock types or if it is possible for multiple cavitation mechanisms to be
active simultaneously.
Despite a growing body of observations on creep cavitation in rocks, some
important open questions remain, including the following.
How do creep cavities effect an evolving rock rheology and how does this ultimately influence rock deformation?
How ubiquitous are creep cavities in deformed rocks and what combinations of deformational processes facilitate their
formation?
This contribution addresses the first question by examining in detail the nature
and occurrence of creep cavities in a mid-crustal ultramylonite from the
Redbank Shear Zone (Australia) and furthers our understanding of the second
question. We use a sophisticated workflow combining electron microscopy,
image analysis, electron backscatter diffraction (EBSD) and
synchrotron-based x-ray nanotomography (nCT) to show that creep cavities can
form by multiple mechanisms in one sample. We present a high-resolution map
of porosity distribution on the millimetre scale in an ultramylonite and demonstrate
how this porosity evolved during mylonitic deformation.
Geological setting and sample description
The Redbank Shear Zone (RBSZ) is part of a crustal-scale thrust duplex that
formed during the Alice Springs orogeny in Central Australia
. Due to its geometry, where higher-grade shear zones
piggy-backed on lower-grade shear zones, the RBSZ has experienced no
significant retrograde metamorphic overprint during its exhumation, and the
synkinematic mineral fabrics and parageneses are preserved
. Micro-fabrics in the RBSZ are therefore ideal for the
investigation of transient chemo-physical processes that characterise
mid-crustal shear zones.
The RBSZ is a network of shear zones that cascades across scales, with shear
zone thicknesses that range from 10-3 to 101 m displaying a
characteristic protomylonite – mylonite – ultramylonite succession
. This contribution focusses on a quartzo-feldspathic
ultramylonite sampled from the amphibolite facies shear zones in the Black
Hill area of the RBSZ (sample BH02; 23∘32′46.81′′ S,
133∘25′14.42′′ E; temperature 350–550 ∘C; lithostatic
pressure 500 MPa; ). The sample is a banded
ultramylonite that displays a striking and extensive grain-boundary porosity
that is hosted in fine-grained (∼< 20 µm), monomineralic
quartz bands (Fig. 1). In addition, the sample shows thick domains of
well-mixed polyphase material and a network of fine-grained
(∼ 1–2 µm) polyphase layers that envelop large, fractured
augen porphyroclasts (∼ 1 mm). In general, the sample's foliation is
defined by the monomineralic quartz bands and the thicker polyphase domains,
whereby the quartz bands display no signs of boudinage. This work focusses on
the microstructure of the quartz bands and the nature of the porosity they
host. We interpret the disaggregating quartz domains at different stages of
dispersal cf. to offer an insight into the locally
evolving quartz micro-fabric and an associated porosity.
MethodsSample preparation
We analysed a small sample block, which was cut parallel to the stretching
lineation and perpendicular to the foliation (long- and short-axis dimensions
of sample: 22.9×19.4 mm) and then polished and carbon coated (thickness
∼20 nm) for electron microscopy and EBSD. To split the sample along
the mylonitic foliation (after electron microscopy), it was pre-cut parallel
to the stretching lineation and cleaved in a vice. The split surface was
gold coated (thickness ∼3 nm).
Data acquisition and processingMicrostructural analysis
A large (41 448 × 40 282 pixels on a scale of
1 : 35.5; px : nm) backscatter electron (BSE) map was acquired on an FEI
Quanta FEG 650 SEM operated at an accelerating voltage of 20 kV. This map was stitched from individual images using the Maps software by FEI.
Image analysis
The BSE map formed the basis of a detailed analysis of porosity. In BSE
images, pores appear black. They were segmented using binary thresholding and
labelled in Fiji after a pre-processing workflow was
applied to reduce noise (see the Supplement). Data were visualised with
Matplotlib Python libraries . Hexbin plots were used to
help visualise large data scatter plots. A hexbin plot works by laying a
hexagon grid over the data and then preforming a chosen operation on the data
within the bounds of each hexagon. An example of operations that can be
performed on data within a hexagon include count, sum and calculation of the
mean. The kernel density for point features in the ESRI ArcGIS v10.1
software was used in pore cluster analysis for Fig. 2. The kernel smoothing
factor was automatically calculated with reference to the population size and
the extent of analysis and contoured based on a 1/4σ kernel. For Fig. 5
kernel density calculations were made using the SciPy and
NumPy Python libraries (see the Supplement for parameters used).
The spatial distribution of porosity in Fig. 1b. In the BSE mosaic
of Fig. 1b, 8129 individual pores were identified. For reference, panel (a) displays
the micro-fabric of the area analysed. Panel (b) is the mask used for
distinguishing microstructural domains for analysis: quartz is coloured
black. Panels (c, d) present masked kernel density analysis highlighting
regions of pore clustering. Panel (c) is masked to remove all quartz and panel (d)
is masked to only show the quartz. The clustering of pores is most
prevalent in the thickest, most coherent quartz domains and in the largest
feldspar porphyroclasts. Panels (e, f) are hexbin plots that are coloured to
show the absolute porosity per hexbin, masked in the same fashion as
panels (c, d). See Table 1 for porosity values per domain.
From the segmented data, the following parameters were evaluated.
Pore size.
Defined as the cross-sectional area (in µm2) of a pore.
Pore orientation.
Pore orientations were determined by using the long axis of the best-fit
ellipse and calculating its deviation from the vertical axis of images shown.
For ease of viewing, the orientation measures were folded along their
symmetry axis, 90∘, and presented as the value β. For
example, β=0∘ describes a long axis aligned parallel to the
vertical axis of the image analysed (and orthogonal to the mylonitic
foliation in Fig. 1) and β=90∘ would be orthogonal to this
(and parallel to the mylonitic foliation).
Pore shape descriptions.
Circularity=4π(Porearea)(Poreperimeter)2
Circularity is a shape descriptor that quantifies the complexity of
a shape by linking the area and perimeter. It is important to note that
circularity values are not unique but simply describe a deviation in shape
from the area and perimeter relationship of a circle. A circularity value of
1 describes such a circle and decreasing values can represent either an
increase in shape complexity (e.g. a star) or shape elongation or a
combination of both .
Roundness=4(Porearea)πMajoraxis2
Roundness is a shape descriptor that only quantifies elongation.
Used together, circularity and roundness can characterise pore shape
complexity and elongation more correctly .
Electron backscatter diffraction, EBSD
EBSD data were collected at the University of Bern on a Zeiss Evo 50 SEM equipped
with a Digiview II camera. The sample was tilted to 70∘, and a 20 kV
accelerating voltage was applied (beam current was ≈ 2.5 nA, working
distance ≈ 11.5 mm). Crystallographic orientation data were
obtained with a 0.65 µm step size from automatically indexed EBSD
patterns in TSL OIM. EBSD data were processed and plotted with MTEX
. Raw data points with a < 0.1 confidence index (CI)
and calculated grains with < 10 indexed points in total were excluded from
analysis. Orientation distribution functions (ODFs) calculated for pole
figures use a kernel half-width of 7∘, while ODFs calculated for
misorientation angle histograms use a kernel half-width of 5∘.
Split sample
Broken surfaces explored in the SEM provide insight into pore morphologies
and evidence for the redistribution of material cf.. Images from the split sample were acquired on a Carl Zeiss
SIGMA HD VP FEG-SEM using a 20 kV accelerating voltage and a ∼ 6.5 mm
working distance. Oxford AZtecEnergy energy dispersive x-ray (EDX) analysis
was conducted at an aperture setting of 60 µm and used to
qualitatively evaluate phase compositions on the split surface. This form of
mineral identification was necessary to interpret individual pores in their
microstructural context.
X-ray nanotomography
We used an Xradia synchrotron-based nanotomograph at the Advanced Photon
Source (beam line APS/32-ID) to acquire 3-D nanotomographic
datasets of porous ultramylonitic quartz ribbon bands and assess the
potential interconnectivity of pores. A cylindrical sample
(20 µm diameter × 100 µm length) was extracted
using focussed ion beam techniques from a polished thin section that was cut
from the sample. In the tomograph, radiographic projections were acquired
over a rotation of 180∘ at an 8 keV beam energy and reconstructed to
yield a 3-D nanotomographic dataset with a spatial resolution on
the order of 70 nm. Porosity was segmented with binary thresholding and
labelled in Avizo Fire. Due to the small sample size and the noisiness of the
data, no quantitative analysis was attempted. Volumetric data were visualised
in Avizo.
ResultMicro-fabric domains
The investigated high-strain micro-fabric is composed of four microstructural
components (Fig. 1).
Monomineralic quartz domains are elongated parallel to the foliation,
exhibiting a varying degree of coherency as domains. Here coherency is used
in the context of the spatial distribution of quartz grains to qualitatively describe
the degree to which quartz grains are aggregated into
monomineralic bands vs. dispersed into segregated grains. The most coherent
quartz domains wrap around porphyroclasts and in some cases mantled
porphyroclasts. The fringes of the thickest quartz domains show evidence for
the removal of individual quartz grains and their progressive assimilation
into neighbouring, poorly mixed polyphase domains. We assume that thinner
quartz domains have advanced further on the path of disaggregating and that
the progression from thicker to thinner quartz domains reflects a progressive
microstructural evolution. This assumption does not presuppose that all thin
quartz domains were once very thick, but we consider it highly unlikely that
any thin quartz ribbon was initially just one or two grains wide (of a
10–20 µm diameter; Fig. 1).
Porosity that can be considered tri-modal: pores on quartz grain
boundaries, pores hosted within porphyroclasts and cracks that cross-cut and
run parallel to the foliation. Feldspar porphyroclasts host an
intra-crystalline, angular porosity which is distinctly different from the
inter-crystalline porosity observed in association with quartz (see detailed
discussion below).
Well-mixed and poorly mixed polyphase domains: the finest-grained (< 1–2 µm)
parts of the ultramylonite make up the well-mixed polyphase domains (see
left-hand side of Fig. 1a). There are also less well-mixed polyphase
domains which contain disaggregated quartz (< 10 µm; Fig. 1b
and c). The polyphase domains is comprised of plagioclase, K-feldspar, mica,
epidote, ilmenite and quartz (Fig. 1c).
Porphyroclasts, which are generally either sodic plagioclase or K-feldspar.
The K-feldspar porphyroclasts occasionally display flame perthites. Sodic oligoclase
plagioclase porphyroclasts exhibit what appears to be a reaction to the fine-grained mantles
of K-feldspar and mica (Fig. 1). No quartz porphyroclasts are observed.
Plots of relationships for pore shape, size and orientation for all
pores observed in association with the quartz domains of Fig. 1b. Due to the
large sample size, pertinent clustering in data is more clearly observed when
presented in hexbin plots (actual data points are overlain). In Fig. 3a and
b, the hexbin plot colouring displays the number of data points contained
within each hexbin. Panel (a) presents area as a function of pore long-axis
orientation (β=0∘ is parallel to the Z plane of finite
strain and β=90∘ is parallel to the X plane of finite strain).
Circular pores (circ = 1) are excluded from panel (a) because the long of a
circle will not have a unique or meaningful orientation. Panel (b) compares
each pore's circularity with its roundness. One large cluster (circ = 0.3,
round = 0.8) and two minor clusters (circ = 1, round = 1; circ = 1,
round = 0.8) are observed. Panel (c) shows the relationship of pore perimeter with
increasing cross-sectional area. Each data point is additionally coloured for
its circularity. Two power-law trends are identified in panel (c). See text for
further discussion.
Image analysis in the XZ plane of finite strain
Pores were analysed in a representative area (2.1 mm2) of the sample.
Figure 1b shows the full extent of the area used for both spatial and pore
shape analysis. Areas for subset analysis are also marked in Fig. 1b. Pores
in all domains were extracted and considered in bulk for density analysis.
Subsequently, masks were applied to quantify total porosity and analyse
pore shapes in quartz domains separate from porphyroclasts and the polyphase
domains. Pores in porphyroclasts and the fine-grained polyphase domains were
analysed together.
Spatial distributions of pores
Kernel density analysis demonstrates that the porosity is anisotropically distributed,
with a bimodal clustering in respect to domains (Fig. 2c and d). The majority of
observable pores (86 %) exist in direct spatial association with quartz ribbon bands.
In quartz domains, the highest density is recorded in the thickest, most coherent ribbons.
In the porphyroclast and polyphase domains, the larger feldspar porphyroclasts that have
seen the least fracture or reaction to smaller components show the highest density of
pores. The total porosity measured in the area shown in Fig. 2e and f is presented in Table .
Porosity data from Fig. 2.
DomainNumber of pores% of totalAbsolute porosityPorosity presentedin domainporosity (%)(µm2)as % of total areaof Fig. 1b (%)Quartz69918615150.07Porphyroclast + polyphase1138142470.01Pores in monomineralic quartz
Segmented pores were analysed to identify any systematic changes in pore
size, shape and orientation. Figures 3 and 4 show the analysis for the area
shown in Fig. 1b, while Fig. 5 shows the subset analyses.
The link between pore orientations, pore sizes and their shapes in
detail. Figure 4 presents only data for pores with circularity values < 1. A
delineation (circ = 0.8) is presented to show the change in power-law
relations observed in Fig. 3c. Panel (a) shows that with decreasing circularity
there is an increase in the variability of β values. Panel (b) presents
the same data as Fig. 4a but with the data points coloured for
cross-sectional area. The largest pores are observed to mostly have high
β values.
Pore analyses for subsets 1 and 2 shown in Fig. 1b corresponding to
decreasing quartz domain width and increasing quartz dispersion. Lines
marking the change in pore shape relations described by Eqs. () and ()
are presented in each plot. Panels (a, d) present pore long-axis orientation against pore area data in hexbin plots,
while panels (b) and (e) show the same data but with true point density analysis.
Both sets of figures show that with decreasing quartz band thickness, there
is an increase in the range of pore orientations observed.
Panels (c) and (f) demonstrate that this change is also concordant with a change in the pore
shapes from roughly elliptical to more complex. More specifically, it can be
seen in panels (b) and (d) that with decreasing domain width there is a loss of
circular and elliptical pores.
Pore sizes
It can be seen that pores cover a limited range of values in
cross-sectional area (focussed strongly around a median value of
0.18 µm2) but vary greatly in long-axis orientation (Fig. 3a).
The lower limit of pore area may be controlled by the resolution of the
imaging technique. At first inspection there are two maxima in Fig. 3a:
pores with a low β and pores with a high β. The maximum for low
β values appears to be more significant.
Pore shapes
As stated above, a pore's shape complexity and elongation can be
characterised by combining circularity and roundness. When circularity is
plotted against roundness, three salient clusters are observed (Fig. 3b):
pores with a circular character (circularity ≈ 1,
roundness ≈ 1);
pores with an elliptic character (circularity ≈ 1,
roundness ≈ 0.8);
and pores with a complex shape but only moderate elongation (circularity ≈ 0.3, roundness ≈ 0.8).
Figure 3c shows that when area is plotted against perimeter and coloured for
circularity, there are some systematics that can be described by two power-law relationships.
Area=0.062⋅Perimeter1.498Area=0.072⋅Perimeter1.081
We assign pores described by these equations to two distinct populations.
From Fig. 3, it is evident that the pores characterised by
Eq. () have very high circularity. Furthermore, Fig. 3b and c
suggest that the very circular pores are the smallest (in cross-sectional
area). These small, circular pores are then linked by Eq. () to
the elliptical pores (elliptical pores having a circularity ≈ 0.8
and
roundness ≈ 0.8). This relation can be most clearly seen in
Fig. 3c, in which Eq. () describes pores of a circularity ranging
from 1 to ≈ 0.75. Similarly, Eq. () suggests that
all pores with circularity values below ∼0.8 are systematically linked
and scale in shape with a power-law relationship.
Changes in pore orientations
We assume that the long axis of a pore's best-fit ellipse is roughly parallel
to the orientation of the pore's boundary with the host minerals and
therefore representative of pore orientation. For pores with a circularity
less than 1, the Feret diameter is seen to have the same orientation as the
long axis of the best-fit ellipse (see the Supplement Fig. S1). Figure 3a shows a
variation in pore orientations but does not readily highlight any
systematics. However, if Fig. 3a is considered with Fig. 3c, it can be seen
that pores whose shape is governed by Eq. (3) generally have a lower value of
β (see pores with areas ∼≤0.1µm2). Figure 4
decomposes this observation to clearly show that the orientations of the more
circular pores (>0.8) rarely exceed 45∘ and predominately
assume a low angle to the Z direction of finite strain (Fig. 4a). The
change in pore orientation at a circularity of 0.8 corresponds to the
change in the equation governing pore shape (see Fig. 3c). There is also a
clear propensity for the largest, least circular pores
(>2µm2) to be oriented more parallel to the shear plane
(Fig. 4b). It is unclear if these largest pores are foliation parallel cracks
that post-date deformation.
Porosity with a changing quartz microstructure
Spatial analysis of pore occurrences has already shown that pore density
decreases with quartz domain thickness (Fig. 2d). In combination with
microstructural evidence for the disaggregation of quartz domains, it is
possible to consider the evolution of porosity congruent with that of quartz
domains (Fig. 5). It can be observed that the pore shape descriptors change
with the quartz microstructure. Firstly, in the thicker quartz domain, both
pore populations (described in Eqs. and ) are
observed (Fig. 5c). In this domain, pores generally have their long axes
aligned with the Z direction of finite strain (Fig. 5a and b). However,
both the pore population and orientation change as quartz domains become
thinner. It can be seen that in the thinner quartz domain there is an absence
of pores from the trend described by Eq. () (Fig. 5f) and that
pore orientations become far more variable (Fig. 5d and e).
Observations of pores in the XY plane of finite strainPore shapes and orientations
On the broken surface, the porosity present in the thickest quartz domains
shows a clear preference to occur along grain boundaries roughly parallel to
the YZ plane (Fig. 6a). When the pore morphology is considered with respect
to the grain-boundary arrangement at the pore location, two end-member shapes
can be identified. Firstly, there are roughly elliptical pores in
the YZ plane. These pores can either show an asymmetry, truncating on a flat
grain boundary (e.g. uppermost white arrow in Fig. 6a), or be symmetrical
about the grain boundary (e.g. smaller pore in Fig. 6b). Secondly,
there is an occurrence of angular pores at quartz grain triple junctions (see
all yellow arrows in Fig. 6).
Secondary electron images from broken surfaces of quartz domains.
White and yellow arrows point respectively to pores on grain boundaries
with a low angle to the YZ plane of finite strain and to pores at grain-boundary trip junctions. Blue arrows highlight “precipitation” features, with
the empty blue arrow in panel (d) identifying crystallites interpreted to be
incipient precipitates found on a less dilatant grain boundary. Panel (a) shows
a thick, coherent, monomineralic quartz domain that has an abundance of
pores. Panel (b) highlights the difference between a pore found at a triple
junction and a pore found in the middle of a grain boundary (see Fig. 1c for
comparison in XZ plane). Panels (c), (d) and (e) show the change in porosity and
grain-boundary features with increasing distance from a plagioclase
porphyroclast (see text for more detail).
Precipitates on grain boundaries
The broken surface also reveals information about material redistribution in
spatial association with the porosity in the quartz directly surrounding a
plagioclase porphyroclast. The porphyroclast itself shows reaction to more
K-rich material, which appears to have a flakey morphology (see the area
highlighted as Kfs in Fig. 6c). In contrast, nearby quartz grain boundaries
are covered in a dendritic material (see lower left-hand side of Fig. 6c).
The EDX conducted on the broken surface showed the chemistry of the dendrites
to be Si rich, with no other obvious chemical signal. Sharply truncated
dendrites (see blue arrow in Fig. 6c) seem to preferentially occur on
dilatant quartz grain boundaries. Figure 6d highlights textural evidence
linking crystallite precipitation (empty blue arrow in Fig. 6d) and a
dendrite on a dilatant grain boundary (filled blue arrow in Fig. 6d). It is
noteworthy that very little evidence for the dissolution of quartz can be found.
Etch pits were observed only on one site (Fig. 6d).
Interestingly, many pores appear empty, but some also seem filled with
crystallites. With increasing distance from the porphyroclast there is a
transition from the dendritic features on the dilatant grain boundaries to
clusters of crystallites in pores and along grain boundaries (see all blue
arrows in Fig. 6e). At the furthest distances from the porphyroclast in the
quartz domain, only small amounts of very isolated crystallites are observed
(Fig. 6b).
Observations of pores in 3-D
Due to a range of technical difficulties, nCT yielded only one dataset that
provided insights into a porous quartz layer of about 50 µm width.
Figure 7 shows a visualisation of labelled pores and highlights
interconnected creep cavities in three dimensions. The pores in this layer
are mostly oblate and seem to occupy a range of orientations, mostly at high
angles to the foliation (parallel to the top and bottom surfaces of the
bounding frame). Most importantly, it is clear from Fig. 7 that pores are
indeed interconnected and not constrained to the polished surface
investigated in this study. We consider this proof that at most a small
minority of pores observed in Fig. 1 are formed by plucking during sample
preparation.
3-D rendering of cavities segmented from a nanotomographic
dataset. Pores are coloured individually to highlight connectivity. This
means that a single colour allows for the tracing out of pore connections.
For example, note the large interconnected pore cluster in orange. The dimension
of the cube is 700 voxel3, with a voxel size of ∼ 35 nm. The top and
base of the cube are parallel to the mylonitic foliation. The figure
indicates the oblate shape of most cavities and proves that they are indeed
3-D features.
EBSD analysis
To better understand any potential link between the porosity and the
mechanisms accommodating mylonitic deformation in quartz domains, EBSD
analysis was undertaken. The results show clear evidence for crystal plastic
processes with the presence of a crystallographic preferred orientation
(CPO), subgrains and the occurrence of lattice distortions (Figs. 8 and 9).
These crystal plastic processes are not uniform across the area analysed
(Fig. 8b). It can be seen that the thicker quartz band hosts more features
that are considered diagnostic of dislocation creep (Figs. 8b and 9 subset 1). These processes then become less well-articulated in the thinner quartz
band (Figs. 8b and 9, subset 2). The abatement of lattice distortions and
subgrains coincides with a reduction in grain size and texture strength. The
loss of texture strength in the pole figures is further expressed in the
misorientation angle distributions, with the thinner quartz band showing a
nearly uniform distribution of misorientations (Fig. 9, subset 3).
EBSD maps for the region of interest in Fig. 1b. Panel (a) shows an
inverse pole figure map coloured for the X direction of finite strain.
Colours in panel (b) relate each pixel's orientation to the mean orientation of
the grain that hosts the pixel.
EBSD subset analyses of quartz domains. The EBSD analyses were
preformed in the same quartz bands used for subset image analysis (see Figs. 1
and 5). EBSD data are presented in equivalent subsets corresponding to
decreasing quartz domain width and increasing quartz dispersion. A clear CPO
is observed in the pole figure analysis of subset 1 with two c[0001] maxima
and two corresponding a< 11–20> maxima. As quartz domain thickness
decreases there is a randomisation of the CPO highlighted by the decrease in
the J-Index and supported by the shift to a nearly random distribution of the
misorientation angle histogram of subset 3. Note that only grain-boundary
data are presented in the misorientation angle histograms. Subgrains and
Dauphine twin boundaries are excluded from the visualisation.
A model for synkinematic creep cavity formation by two different
mechanisms. Panel (a) graphically represents the inferred trajectory of a creep
cavity's life cycle. Initially creep cavities grow from circular (1) to
elliptical cavities (2). These cavities generally have long axes aligned in
the Z direction (when viewed in the XZ plane). As cavities become larger they
develop more complex shapes and rotate (3), ultimately elongating and
becoming more aligned with the shear plane (4). Panel (b) schematically
integrates our observations of cavity evolution with the evolution of the
microstructure and the associated micro-mechanisms. First cavities form in
quartz domains deforming by GSI creep through the Zener–Stroh mechanism (b,
time 1). As the grain-boundary strength is weakened by the ingress of
fluids into cavities, VGBS is promoted. This increase in the contribution of
VGBS drives the production of new creep cavities of a more complex shape
(b, time 2). It is the production of creep cavities that initiates the
increased contribution of VGBS and ultimately prompts a switch to a GSS
creep. See text for details.
DiscussionA model for synkinematic creep cavitation by different mechanisms
Our study not only advances our understanding of porosity distribution in
mid-crustal ultramylonites but also provides critical insight into the
mechanisms behind synkinematic cavity formation and the associated effects
on rock rheology.
To develop this in detail, we interpret different quartz microstructures in
our sample as representing different stages in a progressive evolution
(“space for time”; see also). If the
well-mixed polymineralic domains are the most mature parts of the studied
ultramylonite, monomineralic quartz domains must be considered as relics of
an original mylonitic fabric that has been captured in the process of
disaggregation. The mechanisms of disaggregation can be observed at the
jagged edges of monomineralic quartz domains and by comparison of
thinner, less coherent quartz domains with thicker, more coherent ones. Our
EBSD data from these domains suggest that during progressive disaggregation,
quartz micro-fabrics with a clear CPO get randomised (Fig. 9). This is
typically interpreted as indicating a transition from GSI creep to GSS creep
accommodated by viscous grain-boundary sliding
.
Our data show how a synkinematic porosity can be associated with this
inferred change in rheology and the formation of a quartzo-feldspathic
ultramylonitic micro-fabric. In our sample, quartz domains are associated
with an overt porosity and we consider the pores in the quartz domains to be
creep cavities . We claim that the cavities evolved
synkinematically with both the microstructure and the dominant deformation
mechanisms, finding that the disintegration of quartz domains is a result of
the emergence of creep cavities. These creep cavities have two distinct
populations and formed hierarchically. We integrate our findings in a model
(Fig. 10) in which Zener–Stroh cracking produced small cavities in domains
deforming by GSI creep (Fig. 10a, blue trend; Fig. 10b, time 1). We infer
that their formation promoted fluid ingress, which in turn lowered the
adhesion and cohesion of grain boundaries cf.. The
addition of a fluid locally increased the contribution of VGBS to strain
energy dissipation, which led to the formation of a second population of
creep cavities (Fig. 10a, green trend; Fig. 10b, time 2) .
Image analysis constrained the two populations of creep cavities in detail
(Fig. 3c).
Cavities with a perimeter smaller than about ∼2µm with
long axes that have a low angle to the YZ plane of finite strain (Figs. 3c
and 4a) and are governed by the power-law relation in Eq. ().
A population of larger cavities that has initially jagged or incised
perimeters and generally elliptic shapes (Fig. 3b and c). The long axes of
these larger cavities exhibit a wider variation in orientation (Fig. 4a).
However, the largest of these cavities tend to be aligned with grain
boundaries that are parallel to the shear plane (Fig. 4b). This population is
governed by the power-law relation in Eq. ().
In our sample, thicker, more coherent quartz domains show more of the former
population, and thinner, less coherent quartz domains contain only cavities
with the latter characteristics (Fig. 5). It can also be observed that
congruent with the change in cavity characteristics and domain width, there
is a randomisation of the CPO present (Fig. 9). We interpret these
observations to indicate that larger cavities are associated with VGBS.
Synkinematic creep cavitation by VGBS is not a new idea in geology: several
microstructural and experimental works invoke the process. Experimental work by both
and show that pores generated during
deformation by VGBS have a complex, angular and elongated shape. This agrees
well with our observations of increasingly disaggregated, thinner quartz
domains (Fig. 5, subset 2) with their weakened CPO (Fig. 9, subset 2). As
this type of cavity formation has been well discussed in previous
contributions we will not examine this further.
In contrast, geological descriptions of creep cavities that formed by
Zener–Stroh cracking are still rare . In the quartz domains
of our sample that deformed dominantly by GSI creep, the population of very
small cavities does not occur at grain triple junctions but rather seems to
nucleate along grain boundaries that are aligned in the YZ plane of finite
strain (Figs. 4a, 5, subset 1; Fig. 6, all white arrows). Boundaries in these
orientations are mechanically unlikely to experience significant sliding,
which suggests that VGBS is not conditional for cavity formation and
alternative mechanisms might be relevant. In material science, it has been
shown that creep cavities in an environment that is characterised by work
hardening can nucleate by the coalescence of dislocations. This will occur
where crystallographic slip bands intersect with grain boundaries or
grain-boundary precipitates, allowing dislocations to pile up and thus forming
Zener–Stroh cracks . On the basis of our
observations we speculate that cavitation by Zener–Stroh cracking could have
provided an initial porosity in monomineralic quartz domains that emerged
directly out of GSI creep and potentially played an important role in the
rheological evolution of the ultramylonite.
Nucleation of creep cavities during GSI creep
Despite evidence provided by cavity shapes and orientations (Figs. 3, 4, 5
and 6), unequivocal proof of Zener–Stroh cracking is difficult. For the
Zener–Stroh mechanism to act, the material must have an abundance of crystal
defects produced by deformational work . In our sample, the
presence of a CPO, subgrains and the occurrence of lattice distortions in
the thick quartz domains are evidence for the activity of dislocation creep
processes and hence the production of crystal defects in the same
microstructural domains that host creep cavities. Our analysis further shows
that the orientation of the dominant slip direction in these domains is
aligned with the X direction of finite strain (Fig. 9, subset 1).
Grain boundaries at high angles to the X direction (i.e. those in the YZ plane) could provide the obstacles required for dislocations to pile up,
which could result in cavitation by Zener–Stroh cracking .
Evidence for quartz grain-boundary porosity in association with high
dislocation density has been reported in previous studies of ultramylonites
. Recently,
explicitly invoked the Zener–Stroh mechanism to explain
small grain-boundary cavities next to dislocation pile-ups. In light of
these studies and despite the fact that we can only provide indirect
evidence, its seems reasonable to speculate that the process of cavity
nucleation could be that of Zener–Stroh cracking.
A dislocation-driven cavitation would require an appropriate density of
dislocations to be present for a creep cavity to nucleate. It follows that
cavity production consumes defects, and in this way it is a process that may
directly compete with other recovery processes such as subgrain wall
formation. Our SEM observations of cavities and subgrains suggests that both
coexist. has demonstrated the importance of subgrain wall
formation in the disaggregation process of quartz domains in
quartzo-feldspathic ultramylonites that have experienced similar geological
conditions as those investigated here. From an irreversible thermodynamic
perspective, integrating cavitation as a dissipative component of dislocation
creep would expand our current understanding of a crystal's internal entropy
production (see Eq. 1 of ). This would alter Eq. (18) of
, which describes the total rate of reduction (ρ-) of
the dislocation density at a steady state (ρ). Integrating Zener–Stroh
crack formation as a ρ- mechanism would yield
dρ-γ=dρDRX-γ+dρDRV-γ+dρCAV-γ,
where dρDRX-γ,
dρDRV-γ and
dρCAV-γ are the dislocation
annihilation rates due to dynamic recrystallisation, dynamic recovery and
creep cavitation, respectively.
have recently demonstrated in a fine-grained calcite
ultramylonite (∼ 3 µm) that recovery can occur without the
formation of subgrain walls. Internal strain is recovered by extensive glide
and dislocation networks characteristic of cross-slip and network-assisted
dislocation movement. In conjunction with this mechanism,
also observed Zener–Stroh crack formation. This may outline a scenario in
which
cavity production dominates over subgrain wall formation.
While we infer Zener–Stroh cracking to be responsible for cavity nucleation,
proposed an alternative model to explain a concentration
of pores on grain boundaries at high angles to the fabric attractor in a
sheared micaceous quartzite (see Fig. 10 in ).
suggested that the initial porosity is loosely connected
to the preferential dissolution of quartz at sites where dislocation tangles
intersect grain boundaries cf.. In both the sample of
this study and of , there is a clear link between the
pore orientation and the bulk finite strain. Considering the inferred
orientations of compression and extension in our sample, preferential
dissolution should be expected to be orthogonal to the observed cavities (see etch
pit formation in Fig. 6d), i.e. creep cavities in our sample open at sites
that would be favourable for precipitation, not dissolution. We therefore
consider the model of as incompatible with our
observations.
The role of creep cavities in the activation of GSS creep
The rheological evolution of mid-crustal ultramylonites to GSS creep is
prompted by the establishment of a very fine grain size. In ultramylonites
that deform by GSS creep, grain growth is usually inhibited by the presence
of secondary phases, a process called Zener pinning .
When synkinematic creep cavities control fluid transport in ultramylonites
, cavities should also control secondary
phase precipitation and hence directly influence Zener pinning
. This invites a discussion on how creep cavitation could
influence Zener pinning and thereby facilitate the transition of a rock's
rheology from GSI to GSS creep. A critical step in the rheological evolution
of the ultramylonite investigated here is the transition from GSI to GSS
creep in disintegrating quartz domains.
Conspicuously, secondary minerals are generally absent in the monomineralic
quartz domains. We speculate that fluid-filled creep cavities will have
affected grain-boundary migration directly by acting as pinning phases,
therefore arresting grain sizes at sub-equilibrium dimensions and promoting
the transition to VGBS. This idea can be further explored by combining our
data with the Zener parameter, which quantifies the influence of second
phases on rheology :
Z=dspfsp,
where Z is the Zener parameter, dsp is the size and
fsp is the volume fraction of the secondary phases.
Equations () and () give an empirical indication
of the value of dsp for creep cavitation via Zener–Stroh cracking and
VGBS, respectively. The dynamic nature of creep cavitation suggests that
dsp is not constant but varies between a maximum and a minimum
that can be taken from Fig. 3a. However, it is unclear how fsp
would evolve with the different cavity formation mechanisms. Any porosity
derived exclusively from dislocation creep would be expected to have a
characteristic spacing between pores on a grain boundary dictated by the
crystal volume and the amount of strain that an individual slip system can
accommodate. Therefore in this scenario fsp would be directly
linked to this characteristic spacing. On the other hand, values of
fsp generated by porosity linked to VGBS would have a different
character. The volume fraction in this case may be linked to a space problem
in which the amount of dilatancy is limited by the surrounding grains. In either
case, cavitation should be considered as a mechanism that is capable of
evolving the Zener parameter and hence the rheology of a domain from GSI to
GSS.
Our model ties in with more recent experimental observations by
, who also identify creep cavitation as a means of
producing domains that deform by GSS creep. In contrast to our results,
discuss the deformation of clinopyroxene embedded in an
olivine matrix in which phase mixing occurs in clinopyroxene tails. This
process is interpreted to be initiated by micro-cracking.
advocate a model in which the nucleation rate of secondary
phases is high. New phases are precipitated simultaneously with
micro-cracking and each new cavitation site becomes filled with new phases,
which suppresses the development of a CPO. On the other end of the spectrum,
our observations highlight a scenario in which the rates of precipitation are so
slow that cavities remain fluid filled. Evidence of quartz precipitation is
possibly observed in the form of Si-rich grain-boundary features (Fig. 6, see
all blue arrows), but in our interpretation any precipitation is
volumetrically not significant enough to fill cavities. Another major
difference between the two models is that our model does not require brittle
fractures to initiate the disaggregation of a monomineralic domain. The
results of our work probably showcase an example in which the nucleation of
phases is not kinetically or energetically favourable.
A lack of boudinage but maintenance of strain compatibility
A striking feature of the ultramylonite is the lack of any evidence for
boudinage in quartz layers. This implies that either the synkinematic
viscosity contrast between the polyphase and the quartz domains was small
, or that the quartz layer was not able to
achieve localisation because the local temperature fluxes were efficiently
dissipated . Boudinage by either of these processes is
considered a ductile instability through which irrecoverable change occurs and grows
over time cf.. In our sample a synkinematic porosity
is observed and can itself be considered a ductile instability. As discussed
above, creep cavitation by Zener–Stroh would be a dissipative feature of
dislocation creep that would act to lower the internal energy of a grain. In
thermodynamic terms, creep cavities could act as an energy sink. From a
micromechanical perspective, a synkinematic porosity offers the possibly of
lowering grain-boundary adhesion and cohesion as fluid is drawn to low-stress
sites , thereby compromising the rheological
integrity of the monomineralic quartz domains and promoting sliding.
Therefore, it may be the case that cavity formation in quartz domains
inhibits strain localisation via boudinage and the increase in the
contribution of VGBS accommodates the extension of quartz layers,
facilitating the quartz bands' ultimate demise.
Conclusions and outlook
In this study we utilise a workflow of SEM-based techniques and synchrotron
x-ray nanotomography to rigorously examine the nature and occurrence of a
grain-boundary porosity found in recrystallised quartz ribbons of a
quartzo-feldspathic ultramylonite. We find that the porosity developed
synkinematically from the deformation mechanisms active in quartz and the
pores can thus be considered as creep cavities. We propose a model of
hierarchical creep cavity formation that has implications for both the
mircostructural and rheological maturation of an ultramylonitic fabric. We
interpret based on the orientation of creep cavities and the crystallographic
texture of quartz domains that Zener–Stroh cracking is responsible for the
initial nucleation of creep cavities. The opening of creep cavities promotes
the ingress of fluids to sites of low stress, and the local addition of a
fluid lowers the adhesion and cohesion of grain boundaries, promoting VGBS.
The increased activity of VGBS is documented in the thinning of quartz
domains. In thinner quartz domains the texture weakens and cavities
become more complex, eventually elongating. We suggest that cavitation at
this stage of the quartz microstructural evolution is governed by VGBS.
Zener–Stroh cracking can be directly linked to crystal plasticity, and our
observations therefore potentially point to a wider significance of creep
cavitation in mylonitic deformation. It remains unclear if the emergence of
Zener–Stroh cracking is contingent on quartz becoming the locally stronger
phase. This would restrict the model presented here to scenarios in which some
fine-grained mixtures have already emerged. Most importantly our findings
document a micromechanical path for clustered quartz grains to be dispersed
into a well-mixed phase mixture.
Both of the invoked creep cavity formation mechanisms are well known from
material sciences and are intimately linked to ductile failure in metals and
ceramics . Our model
points to the coeval activity of both mechanisms in mid-crustal
ultramylonites. This raises questions about how these creep cavities
interact. While it is unclear if natural samples can reveal such transient
aspects, it is clear that such questions are of critical importance in furthering
our understanding of mylonitic processes and crustal deformation in general.
The high-resolution BSE image is available from James Gilgannon
(james.gilgannon@geo.unibe.ch).
The Supplement related to this article is available online at https://doi.org/10.5194/se-8-1193-2017-supplement.
The authors declare that they have no conflict of
interest.
Acknowledgements
We would like to thank Nicola Cayzer (Edinburgh Materials and Micro-Analysis
Centre) and Natasha Stephen (Plymouth Electron Microscopy Centre) for their
help with the acquisition of SEM data. James Gilgannon would like to thank
Marco Herwegh for invaluable discussions about Zener pinning, Cees-Jan De Hoog
for council on fluids and phase precipitation and finally Alfons Berger
for EBSD procurement. We kindly thank the reviewers, Jacques Précigout
and Luiz F. G. Morales, for their helpful reviews. This work was financially
supported by the School of Geosciences, University of Edinburgh. Use of the
Advanced Photon Source at Argonne National Laboratory was supported by the
US Department of Energy, Office of Science, Office of Basic Energy
Sciences under contract no. DE-AC02-06CH11357.
Edited by: Renée Heilbronner
Reviewed by: Luiz Morales and Jacques Précigout
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