Fluid-assisted Grain Research Articles

Fluid-assisted grain size reduction leads to strain localization in oceanic transform faults

Oceanic Transform Faults are major plate boundaries representing the most seismogenic part of the mid ocean ridge system. Nonetheless, their structure and deformation mechanisms at depth are largely unknown due to rare exposures of deep sections. Here we study the mineral fabric of deformed mantle peridotites - ultramafic mylonites - collected from the transpressive Atobá ridge, along the northern fault of the St. Paul transform system in the Equatorial Atlantic Ocean. We show that, at pressure and temperature conditions of the lower oceanic lithosphere, the dominant deformation mechanism is fluid-assisted dissolution-precipitation creep. Grain size reduction during deformation is enhanced by dissolution of coarser pyroxene grains in presence of fluid and contextual precipitation of small interstitial ones, leading to strain localization at lower stresses than dislocation creep. This mechanism potentially represents the dominant weakening factor in the oceanic lithosphere and a main driver for the onset and maintenance of oceanic transform faults.

Deformation mechanisms accommodating progressive simple shear thrusting of quartzite and metacarbonate in the southwestern Espinhaço Range, Brazil

The accommodation of low-temperature ductile deformation in foreland fold-and-thrust belts is often described in terms of outcrop-based geometric analysis, but microtextural observations are also important, as they relate stress, strain, fluids and temperature and thus the rheology in the peripheral part of the orogen. Such microtextural observations are lacking from the foreland part of the Araçuaí orogen in Brazil, and are here investigated through EBSD-based textural analysis of quartzites and metacarbonate samples from the southwestern Espinhaço Range. The quartzites overthrust the metacarbonates, and deform solely by the activation of dissolution-precipitation creep, whereas the metacarbonates show a much greater variety of deformation mechanisms that closely relate to grain-size, composition and strain, in the long (low-angle) and short (steep) limbs of shear-related folds. The metacarbonates show a centimeter-scale alternation of coarse-grained domains of calcite and quartz, and fine-grained domains of a mix of phases (calcite, dolomite, phyllosilicates, quartz, and apatite). In the strongly sheared long limbs, coarse calcite was deformed by dislocation creep with slip on (c)<a>, whereas fine calcite displays evidence of oriented dissolution-precipitation creep, with both cases displaying <c> axis orientations consistent with the top-to-the-west thrusting. Coarse calcite shows evidence of recovery by subgrain rotation and grain boundary migration. Coarse-grained calcite in the short limbs was deformed by dislocation creep, assisted by twin boundary migration recrystallization with twinning on {e}, indicating simple shear-dominated deformation. In contrast, fine-grained calcite was deformed by fluid-assisted grain boundary sliding. Dissolution-precipitation in both short and long limbs produced random textures, with the exception of coarse quartz in the short limbs, where dissolution-precipitation creep led to <c> axis orientation parallel to X. This study demonstrates that the foreland thrust zone of the Araçuaí orogen underwent consistent W-directed progressive thrusting, and that this deformation produced a wide variety of texture types in the metacarbonates while a simpler structural and textural pattern developed in adjacent quartzitic metasediments.

Myrmekite and strain weakening in granitoid mylonites

Abstract. At mid-crustal conditions, deformation of feldspar is mainly accommodated by a combination of fracturing, dissolution–precipitation, and reaction-weakening mechanisms. In particular, K-feldspar is reaction-weakened by the formation of strain-induced myrmekite – a fine-grained symplectite of plagioclase and quartz. Here we use electron backscattered diffraction to (i) investigate the microstructure of a granodiorite mylonite, developed at ∼ 450 °C during cooling of the Rieserferner pluton (Eastern Alps); and (ii) assess the microstructural processes and the weakening associated with myrmekite development. Our analysis shows that the crystallographic orientation of plagioclase in pristine myrmekite was controlled by that of the replaced K-feldspar. Myrmekite nucleation resulted in both grain-size reduction and anti-clustered phase mixing by heterogeneous nucleation of quartz and plagioclase. The fine grain size of sheared myrmekite promoted grain-size-sensitive creep mechanisms including fluid-assisted grain boundary sliding in plagioclase, coupled with heterogeneous nucleation of quartz within creep cavitation pores. Flow laws, calculated for monomineralic quartz, feldspar, and quartz + plagioclase aggregates (sheared myrmekite) during deformation at 450 °C, show that grain-size-sensitive creep in sheared myrmekite accommodated strain rates several orders of magnitude higher than monomineralic quartz layers deforming by dislocation creep. Therefore, diffusion creep and grain-size-sensitive processes contributed significantly to bulk rock weakening during mylonitization. Our results have implications for modelling the rheology of the felsic middle crust.

Fluid distribution in grain boundaries of natural fine-grained rock salt deformed at low differential stress (Qom Kuh salt fountain, central Iran): Implications for rheology and transport properties

We used a combination of broad ion beam cross-sectioning and cryogenic SEM to image polished surfaces and corresponding pairs of fractured grain boundaries in an investigation of grain boundary microstructures and fluid distribution in naturally deformed halite from the Qom Kuh salt glacier (central Iran). At the scale of observations, four types of fluid-filled grain boundary can be distinguished by morphology (from straight to wavy), thickness (from 5000 to 50 nm) and the presence of fluid inclusions. The mobility of the brine is shown after cutting the inclusions by broad ion beam (BIB) in vacuum and fine-grained halite forms efflorescence and precipitates on internal walls of inclusions. At cryogenic temperature, grain boundary brine is shown either as continuous film or in isolated inclusions. The halite–halite grain boundary between isolated fluid inclusions is interpreted to have formed by fluid-assisted grain boundary healing. Preliminary experiments on the samples at shear stress conditions of natural salt glacier show very slow strain rates (7.4 × 10−10 s−1 and 1 × 10−9 s−1), which are less than expected for pressure solution creep. Both microstructures and deformation experiments suggest interfacial energy-driven grain boundary healing and therefore rendering inactive the pressure solution creep in our samples. This result disagrees with previous microstructural studies of the same sample, which showed microstructural evidence for pressure solution (and dislocation creep). Different explanations are discussed, which imply that both healing and reactivation of grain boundaries are important in salt glaciers, leading to heterogeneous distribution of deformation mechanisms and strain rates in both space and time.

Brittle-plastic deformation in initially dry rocks at fluid-present conditions: transient behaviour of feldspar at mid-crustal levels

We present detailed microstructural and chemical analyses from an initially dry anorthositic rock deformed during wet amphibolite facies conditions. Three different domains representing the microstructural variation of the deformed samples are investigated in detail in terms of fracture morphology and mode, grain characteristics and chemistry of present phases. Results show transient deformational behaviour where a close interaction between brittle, plastic and fluid-assisted deformation mechanisms can be observed. Our analysis allows us to describe the succession, interrelationships and effects of active mechanisms with progressively increasing strain in three so-called stages. In Stage 1, initial fracturing along cleavage planes promoted fluid influx that caused fragmentation and chemical reactions, producing fine-grained mineral assemblages in the fractures. Deformation twins and dislocations developed in clast pieces due to stress relaxation. Passive rotation of conjugate fracture sets and interconnection of intracrystalline fractures formed micro-shear-zones, constituting Stage 2. Microstructures and grain relationships indicate the activity and fluctuation between fracturing, dissolution-precipitation creep, grain boundary sliding and locally dislocation creep, reflecting the transient behaviour of brittle and plastic deformation mechanisms. Further rotation and widening of fractures into overall foliation parallel shear-bands (Stage 3) promoted strain partitioning into these areas through increased fluid influx, influence of fluid-assisted grain boundary sliding, phase mixing and presence of weak phases such as white mica. We suggest that local differences in fluid availability, volume fraction of weak phases produced by fluid present metamorphic reactions coupled with volume increase and local variations in stress concentration induced transient brittle-plastic behaviour. The studied shear-zone represents an example of the transformation of a rigid dry rock to a soft wet rock during deformation through syntectonic fracturing.

The Sierra Ballena Shear Zone in the southernmost Dom Feliciano Belt (Uruguay): evolution, kinematics, and deformation conditions

The Sierra Ballena Shear Zone (SBSZ) is part of a high-strain transcurrent system that divides the Neoproterozoic Dom Feliciano Belt of South America into two different domains. The basement on both sides of the SBSZ shows a deformation stage preceding that of the transcurrent deformation recognized as a high temperature mylonitic foliation associated with migmatization. Grain boundary migration and fluid-assisted grain boundary diffusion enhanced by partial melting were the main deformation mechanisms associated with this foliation. Age estimate of this episode is >658 Ma. The second stage corresponds to the start of transpressional deformation and the nucleation and development of the SBSZ. During this stage, pure shear dominates the deformation, and is characterized by the development of conjugate dextral and sinistral shear zones and the emplacement of syntectonic granites. This event dates to 658–600 Ma based on the age of these intrusions. The third stage was a second transpressional event at about 586 to <560 Ma that was associated with the emplacement of porphyry dikes and granites that show evidence of flattening. Deformation in the SBSZ took place, during the late stages, under regional low-grade conditions, as indicated by the metamorphic paragenesis in the supracrustals of the country rocks. Granitic mylonites show plastic deformation of quartz and brittle behavior of feldspar. A transition from magmatic to solid-state microstructures is also frequently observed in syntectonic granites. Mylonitic porphyries and quartz mylonites resulted from the deformation of alkaline porphyries and quartz veins emplaced in the shear zone. Quartz veins reflect the release of silica associated with the breakdown of feldspar to white mica during the evolution of the granitic mylonites to phyllonites, which resulted in shear zone weakening. Quartz microstructures characteristic of the transition between regime 2 and regime 3, grain boundary migration and incipient recrystallization in feldspar indicate deformation under lower amphibolite to upper greenschist conditions (550–400°C). On the other hand, the mylonitic porphyries display evidence of feldspar recrystallization suggesting magmatic or high-T solid-state deformation during cooling of the dikes.

Deformation mechanisms of deeply buried and surface-piercing Late Pre-Cambrian to Early Cambrian Ara Salt from interior Oman

We compared microstructures of Late Pre-Cambrian to Early Cambrian Ara Salt diapirs from the deep subsurface (3.5–5 km) of the South Oman Salt Basin and from surface-piercing salt domes of the Ghaba Salt Basin. Laterally, these basins are approximately 500 km apart but belong to the same tectono-sedimentary system. The excellent data situation from both wells and outcrops allows a unique quantification of formation and deformation mechanisms, spanning from sedimentation to deep burial, and via re-activated diapir rise to surface piercement. Microstructures of gamma-irradiated and etched thin sections indicate dislocation creep and fluid-assisted grain boundary migration as the main deformation mechanisms operating in the deep subsurface. Microstructures from the surface are characterised by large ‘old’ subgrain-rich crystals. These ‘old’ grains are partly replaced by ‘new’ subgrain-free and subgrain-poor crystals, which show gamma irradiation-decorated growth bands and fibrous microstructures, indicative of pressure solution creep and static recrystallisation, most likely due to surface piercement and exposure. Using subgrain size piezometry, the maximum differential stresses for the subsurface salt is 1.7 MPa and those for the surface-piercing salt is 3.4 MPa, the latter value displaying the high stress conditions in the diapir ‘stem’ as the salt rises on its way to the surface.

Dynamic recrystallization near the brittle-plastic transition in naturally and experimentally deformed quartz aggregates

The electron backscattering diffraction technique (EBSD) was used to analyze bulging recrystallization microstructures from naturally and experimentally deformed quartz aggregates, both of which are characterized by porphyroclasts with finely serrated grain boundaries and grain boundary bulges set in a matrix of very fine recrystallized grains. For the Tonale mylonites we investigated, a temperature range of 300–380 °C, 0.25 GPa confining pressure, a flow stress range of ~ 0.1–0.2 GPa, and a strain rate of ~ 10 − 13 s − 1 were estimated. Experimental samples of Black Hills quartzite were analyzed, which had been deformed in axial compression at 700 °C, 1.2–1.5 GPa confining pressure, a flow stress of ~ 0.3–0.4 GPa, a strain rate of ~ 10 − 6 s − 1 , and to 44% to 73% axial shortening. Using orientation imaging we investigated the dynamic recrystallization microstructures and discuss which processes may contribute to their development. Our results suggest that several deformation processes are important for the dismantling of the porphyroclasts and the formation of recrystallized grains. Grain boundary bulges are not only formed by local grain boundary migration, but they also display a lattice misorientation indicative of subgrain rotation. Dynamic recrystallization affects especially the rims of host porphyroclasts with a hard orientation, i.e. with an orientation unsuitable for easy basal slip. In addition, Dauphiné twins within porphyroclasts are preferred sites for recrystallization. We interpret large misorientation angles in the experimental samples, which increase with increasing strain, as formed by the activity of fluid-assisted grain boundary sliding.

Fluid-assisted grain boundary sliding in bedding-parallel quartz veins deformed under greenschist metamophic grade

Iron formation rocks of Quadrilátero Ferrífero, Brazil, were deformed at greenschist facies. Quartz grains in bedding parallel veins were sheared and deformed by a combination of mechanisms assisted by aqueous fluids. Veins in the outcrop appear to be stretched parallel to the compositional layering. The overall vein shapes resemble those of boundinage and pinch and swell. In thin sections, veins show microstructures similar to those observed in hand samples, where domains of large quartz crystals are pulled apart for several millimeters. The voids between quartz fragments are filled with domains of polycrystalline quartz. The microstructural and orientation data show that the strain imposed on the vein as a rigid and competent layer was not accommodated in the quartz polycrystals exclusively by crystal plastic deformation or dynamic recrystallization. The new grains are strain-free, with straight boundaries and with weak to random crystallographic fabrics. We interpret these features to have resulted from a combination of processes, which included grain boundary sliding accomplished by solution transfer. We propose that the coeval operation of both mechanisms allows the aggregate to deform at higher strain rates without necking of the vein layer in a type of flow similar to those described in superplastic regimes.

Dynamic and static recrystallization-related microstructures in halite samples from the Klodawa salt wall (central Poland) as revealed by gamma-irradiation

Microstructures of 33 rocksalt specimens from gallery walls from the highly strained part of the K odawa Salt Structure (Poland) were analyzed in gamma-irradiated and etched thin sections in reflected and transmitted light. Two types of microstructures could be distinguished i) dynamic recrystallization related ones caused by natural deformation and ii) static recrystallization related ones with uncertain origin, perhaps caused by subsequent mine-wall convergence. Natural deformation-related microstructural features are the abundant subgrains and strain-free regions at grain boundaries together with elongated subgrains which imply dislocation climb controlled creep accompanied with fluid-assisted grain boundary migration as the deformation mechanism. Using subgrain size piezometry, differential stresses between 0.9 to 3.1 MPa were calculated. All the samples contain a few percent (< 5 %) of euhedral statically recrystallized phase. In three samples the euhedral, statically recrystallized grains may comprise up to 85 % of the material. The microstructure suggests that the static recrystallization post-date the natural deformation-related features. The static recrystallization is thought to be due to mine-wall convergence. The pervasive crack system in the samples may also be explained by the stress concentration and subsequent minor mine-wall convergence. Detailed observations of the mutual relationship between the microcracks and the euhedral, statically recrystallized phase imply that as the new grains have grown, and

Deformation and recrystallization mechanisms in mylonitic shear zones in naturally deformed extrusive Eocene–Oligocene rocksalt from Eyvanekey plateau and Garmsar hills (central Iran)

Microstructural processes in mylonitic shear zones from extrusions of Eocene–Oligocene rocksalts from Garmsar hills and Eyvanekey plateau (central Iran) are described and analyzed. The halite samples were studied by transmitted light microscopy of gamma-irradiated thin sections, subgrain size palaeopiezometry of polished and chemically etched samples and texture measurements by electron backscatter diffraction (EBSD). The studied shear zones are thin (<5 cm), subhorizontal and bedding parallel. The protomylonite comprises 2–6 mm sized grains, occasionally rich in primary fluid inclusions indicative of their primary non-recrystallized state. Abundant, well-developed subgrains suggest that the protomylonite was deformed mainly by dislocation processes. Elongated subgrains at grain edges point to recrystallization by fluid-assisted grain boundary migration. Recrystallized, strain-free grains are common. The material in the mylonitic zones is fine grained (∼0.6 mm) with strong shape-preferred orientation. Microstructures such as oriented fibrous overgrowths and growth banding (observed in gamma-irradiated sections) suggest that the principal deformation mechanism was solution-precipitation creep (non-conservative grain boundary migration and grain boundary sliding accommodated by solution-precipitation). Crystal fabrics measured by EBSD show a weak crystallographic preferred orientation that is consistent with solution-precipitation accommodated grain boundary sliding. Using published flow laws, the strain rate in the fine-grained mylonites is about 10 −10 l/s, which is in a good agreement with earlier in situ measurements on glacier salt flow rate.

Rheological behaviour of synthetic rocksalt: the interplay between water, dynamic recrystallization and deformation mechanisms

The ductile deformation of rocks in nature can be greatly enhanced by the presence of water. Part of the water-induced weakening of rocks at depth may come from fluid-assisted deformation or recrystallization mechanisms that are absent in dry rocks. In this study, we investigate the effect of water on the rheological behaviour of rocksalt. We focus on quantification of the contribution of individual deformation and recrystallization mechanisms to deformation. We also aim to calibrate a flow law that incorporates the effect of all the relevant microphysical processes and hence more accurately describes the flow of rocksalt in nature. For this purpose, the mechanical behaviour and microstructural evolution of synthetic rocksalt samples that are similar, except for differences in water content (determined using FTIR analysis), are investigated. The samples are deformed to natural strains of 0.07–0.46 at 50 MPa confining pressure, strain rates of 5×10 −7–1×10 −4 s −1 and temperatures of 75–240 °C, resulting in flow stresses of 7–22 MPa. The flow stress of samples with a water content below ∼5 ppm (‘dry’) is higher than that of samples with a water content of ∼9–46 ppm (‘wet’) at all strains under the investigated conditions. The difference in flow stress can be explained as due to the operation of only work hardening dislocation creep without dynamic recrystallization in the dry material versus combined dislocation and solution-precipitation creep plus fluid-assisted grain boundary migration in the wet material. The results allow us to calibrate a flow law for wet rocksalt that incorporates the effects of solution-precipitation creep and fluid-assisted grain boundary migration. The results also suggest that strain localization in natural rocksalt is more likely to be localized due to fluid infiltration and associated rheological weakening, than due to progressive removal of strain hardening substructure by grain boundary migration.

Sulfur and oxygen isotope studies of the interaction between pelitic xenoliths and basaltic magma at the Babbitt and Serpentine Cu-Ni deposits, Duluth Complex, Minnesota

Sulfur and oxygen isotope compositions of two pelitic Virginia Formation xenoliths and surrounding igneous rocks have been measured in drill core intersects at the Babbitt and Serpentine Cu-Ni deposits, Duluth Complex. The at the Babbitt deposit (xenolith 1) shows compositional and textural evidence for partial melting, whereas the xenolith at the Serpentine deposit (xenolith 1) may still be in part connected to the footwall, and primary sedimentary mineral assemblages are present in its center. The delta 34 S values of the xenoliths range from 6 to 27 per mil. A well-developed vertical profile of upward-increasing delta 34 S values in 2 is suggestive of initial pyrite formation due to bacterial reduction of sulfate in a sedimentary environment where the rate of sulfate reduction was rapid relative to sulfate supply. The extreme variation in delta 34 S values over a vertical interval of less than 20 m illustrates the potential variability of delta 34 S values in the Virginia Formation. The delta 34 S values of magmatic sulfides that formed in response to S assimilation may be equally variable, depending upon local sites of contamination and convective homogenization in the magma. In situ conservation of sulfur is suggested for 2 where the Fe/Mg ratio of biotite decreases with increasing bulk-rock sulfur content. Sulfidation reactions involving iron-bearing silicates or oxide minerals limited the amount of sulfide liberated by the xenolith. Assimilation of country-rock sulfide occurred either at depth within crustal staging chambers or during magma ascent.An oxygen isotope gradient is well-developed at the upper contact between gabbronorite-norite and 2. Oxygen isotope exchange fronts are poorly developed at the lower contact of 2 and at both contacts of 1. At the Babbitt deposit the igneous rock at the lower contact is a compositionally distinct troctolite that shows little evidence for country-rock contamination. Intrusion of distinct, thin, magmatic pulses that cool rapidly, after previous devolatilization and possible partial melting of the xenoliths, can cause the obliteration of well-developed isotopic profiles. The lack of concentration gradients that coincide with the oxygen isotope gradient, together with results of modeling of mass transfer by diffusive processes, suggests that the observed delta 18 O profile was produced during subsolidus cooling. The observed profile may be produced by oxygen isotope exchange via fluid-assisted grain boundary diffusion within 500,000 yr, a duration consistent with the 0.5 to 1 m.y. time span estimated for emplacement of the major intrusions of the Duluth Complex.

Rheology of rocksalt

We review progress in experimental determinations of transient and steady-state flow properties and processes of natural rocksalt aggregates, focusing primarily on results from Avery Island, Louisiana, domal salt. The steady-state flow field-established from constant stress and constant strain-rate tests at temperatures from 50 to 200°C, strain rates from 10 −5 to 10 −9 s −1 and differential stresses, σ, from 20.7 to 2.5 MPa-has been separated into two flow regimes, each fit by a power-law relation. At the higher stresses and strain rates this relation is ε ̇ =1.6 ×10 −4 exp( −68.1 RT.10 −3 )σ 5.3 for which the pre-exponential constant is expressed in MPa −5.3 s −1 and the apparent activation energy is in J mol −1. Creep rates predicted by equation (A) do not differ appreciably from those predicted previously. The relatively low stress, low strain-rate data are very well fit by ε ̇ =8.1 ×10 −5 exp( −51.6 RT.10 −3 σ 2.4 which is comparable conditions, predicts creep rates higher and equivalent viscosities lower than does equation (A) by two orders of magnitude. The change in behavior from (A) to (B) is ascribed to a change in rate-limiting mechanism from cross-slip of screw dislocations to the climb of edge dislocations in this dry material. Subgrain formation dominates microstructural development during steady-state flow of salt and, from determinations of average subgrain diameters in crystals from 20 different rocksalt bodies, flow stress levels between 0.6 and 1.4 MPa have been estimated. These values do not bear directly on arguments concerning the nature of forces initiating salt pillow growth because, apparently, evidence relating to the early deformational history has been overprinted. At that stage, fluid-assisted grain boundary diffusional processes might dominate dislocation creep, leading to a linear stress-strain rate mechanical response. Considering buoyancy forces alone, behavior described by equation (B) rather than (A) reduces the relief necessary to initiate pillow growth but a 1000-fold amplification is required to produce stress differences near 1 MPa. That forces other than buoyancy are important is indicated by the occurrence of these same paleostress levels in bedded salts and in shallow offshore concordant intrusions. Differential loading generally provides the most plausible initial driving force for the growth of diapiric salt structures.