Dissolution-precipitation Creep Research Articles

Lawsonite eclogite records subduction rheology and slab detachment, Faro, Yukon-Tanana Terrane, Yukon Territory, Canada

Lawsonite eclogites represent cold subduction in the rock record, and new localities provide rare natural field laboratories to investigate deep deformation processes. We present a new lawsonite eclogite locality near Faro, Yukon Territory, Canada, the first reported lawsonite eclogite locality of the Yukon-Tanana Terrane. The discovery indicates that mid- to Late Permian subduction in the Yukon-Tanana Terrane subducted cold material in an oceanic subduction zone, in contrast to most previous models. We define a pressure-temperature-deformation (P-T-d) pathway for the lawsonite eclogites using a combination of microstructures, mineral compositional mapping, crystal orientation mapping, and phase equilibrium modeling. Metamorphism in the Faro eclogites followed a clockwise pathway through the lawsonite eclogite facies, reaching peak pressures of 22−24 MPa at 525−550 °C. Rocks then exhumed to 14.5−16 MPa during heating to 630−655 °C. Crystallographic preferred orientations (CPOs) in omphacite developed dominantly by dissolution-precipitation creep and oriented grain growth concurrently with CPOs developed in glaucophane by a combination of dislocation- and dissolution-precipitation creep with oriented grain growth. We link this fabric and CPO development in omphacite and amphibole to lawsonite dehydration, slab decoupling, and initiation of exhumation of the eclogites, providing evidence that lawsonite dehydration plays a significant role in changes in subduction zone rheology.

The Frictional‐Viscous Transition in Experimentally Deformed Granitoid Fault Gouge

AbstractIn crustal faults dominated by granitoid gouges, the frictional‐viscous transition marks a significant change in strength constraining the lower depth limit of the seismogenic zone. Dissolution‐precipitation creep (DPC) may play an important role in initiating this transition, especially within polymineralic materials. Yet, it remains unclear to what extent DPC contributes to the weakening of granitoid gouge materials at the transition. Here we conducted sliding experiments on wet granitoid gouges to large displacement (15 mm), at an effective normal stress and pore fluid pressure of 100 MPa, at temperatures of 20–650°C, and at sliding velocities of 0.1–100 μm/s, which are relevant for earthquake nucleation. Gouge shear strengths were generally ∼75 MPa even at temperatures up to 650°C and at velocities >1 μm/s. At velocities ≤1 μm/s, strengths decreased at temperatures ≥450°C, reaching a minimum of 37 MPa at the highest temperature and lowest velocity condition. Microstructural observations showed that, as the gouges weakened, the strain localized into thin, dense, and ultrafine‐grained (≤1 μm) principal slip zones, where nanopores were located along grain contacts and contained minute biotite‐quartz‐feldspar precipitates. The stress sensitivity exponent n decreased from a large number at 20°C to ∼2.2 at 650°C at the lowest velocities. These findings suggest that high temperature, slow velocity and small grain sizes promote DPC‐accommodated granular flow over cataclastic frictional granular flow, leading to the observed weakening and strain localization. Field observations together with extrapolation suggest that DPC‐induced weakening occurs at depths of 7–20 km depending on geothermal gradient.

Progressive Strain Localization and Fluid Focusing in Mantle Shear Zones during Rifting: Petrostructural Constraints from the Zabargad Peridotites, Red Sea

Abstract This article documents the evolution of pressure and temperature conditions and the successive influence of hydrous melts and aqueous fluids on the operation of extensional shear zones, which exhumed mantle slivers from deep lithospheric or asthenospheric depths, in a rift-to-drift setting. These results are based on a re-analysis of 40 samples from three peridotite massifs of Zabargad island in the northern Red Sea. By integrating high-resolution mapping of the microstructure by electron backscattered diffraction with recent developments in barometry for plagioclase-bearing peridotites and thermodynamic modelling of peridotitic compositions, this study (1) constrains the temporal and spatial evolution of petrological and tectonic processes in the shallow mantle during rifting and (2) documents the presence of melts or aqueous fluids throughout the activity of the shear zones, unravelling substantial feedback between petrological and tectonic processes. Thermobarometry and thermodynamic modelling, constrained by the microstructural observations, document progressive strain localization associated with shearing under decreasing pressure and temperature, from near solidus conditions at >1 GPa (in the north and central peridotite massifs) or ~ 0.7 GPa (in the southern massif) to <600°C and < 0.3 GPa in all three massifs. The data substantiate local aqueous fluid saturation in the shear zones. This together with higher contents of hydrous minerals in ultramylonites indicate fluid focusing in the shear zones, with seawater ingress extending to >10 km depth. The presence of melts or fluids enabled concurrent dislocation and dissolution–precipitation creep, resulting in weakening of the shear zones. However, fluid supply was spatially heterogeneous and likely intermittent, with equilibrium achieved only locally in the ultramylonites. The present study documents therefore how the feedback between progressive strain localization and fluid focusing in extensional shear zones contributes to thinning and exhumation of the mantle during continental rifting and the rift-to-drift transition.

Brittle initiation of dissolution–precipitation creep in plagioclase-rich rocks: insights from the Bergen arcs, Norway

The initiation of ductile shear zones commonly occurs spatially associated with fluid-rock reactions along brittle precursors. In many cases the relative timing of fracturing, fluid infiltration, reaction, and recrystallisation is unclear, making it difficult to disentangle mechanisms of shear zone initiation from subsequent deformation and recrystallisation. Here we present the study of the transition from a dry plagioclase-diopside-garnet-scapolite host granulite-facies lithology to (1) a low strain amphibolite-facies rock, and (2) a transition from low strain to high strain amphibolite-facies lithologies. Hydration of the granulite-facies precursor at amphibolite-facies conditions produces an assemblage comprised dominantly of plagioclase-amphibole-zoisite-clinozoisite-kyanite-scapolite-quartz. Detailed study of plagioclase chemistry and microstructures across these two transitions using Electron Backscatter Diffraction (EBSD) and Wavelength Dispersive Spectrometry (WDS) allows us to assess the degree of coupling between deformation and fluid-rock reaction across the outcrop. Plagioclase behaves dominantly in a brittle manner at the hydration interface and so the initial weakening of the rock is attributed to grain size reduction caused by fracture damage and fluid infiltration at amphibolite-facies conditions. Extensive fracturing-induced grain size reduction locally increases permeability and allows for continuing plagioclase and secondary mineral growth during shear. Based on plagioclase microstructures, such as, an inherited but dispersed crystallographic preferred orientation (CPO), truncation of chemical zoning, and the dominance of fine (5–150 µm), slightly elongate, polygonal grains we conclude that deformation is dominantly facilitated by dissolution–precipitation creep assisted by grain boundary sliding in the shear zone.

Fluid environment controls along-strike variation in slip style: Midcrustal geological signatures from the Red River fault, China

Abstract The slip style of continental midcrustal shear zones plays a crucial role in determining the seismogenic potential of faults, but it remains poorly understood because geological observations that can be directly tied to seismic behavior are scarce. We describe frictional-viscous shear zones in the Red River fault, China, which consists of two segments with distinct seismic behaviors and fluid availabilities. The northern segment hosts moderate to large earthquakes, and midcrustal fault slip is localized into mylonitized pseudotachylyte-bearing layers where dynamically recrystallized quartz records flow stresses exceeding 100 MPa and accelerated viscous creep. The southern segment is dominantly aseismic but active microseismically. Fault slip is accommodated in several mylonitized cataclasite layers, comprising interconnected biotite and intervening fractured clasts, with evidence for pervasive dissolution-precipitation creep. Microstructures, paleopiezometry, and microphysical modeling suggest transient aseismic slip in response to increased strain rates during viscous creep at <50 MPa. We interpret that along-strike variations in fluid environment control fault slip styles and seismic behaviors. The dry and strong northern segment is capable of nucleating large earthquakes, while greater fluid availability in the southern segment activates dissolution-precipitation creep at low driving stresses, which limits interseismic elastic strain accumulation at frictional-viscous transition depths. In this model, compaction-driven fluid pressurization and dilatant hardening are invoked to explain the aseismic slip transients in the southern segment.

Low temperature creep of garnet: Insights from blueschist-facies skarn of the Preveli nappe (Crete, Greece)

We present a comprehensive record of deformation, metamorphism, and the state of stress during interaction between garnet-rich skarn and infiltrating fluids in an Eo-Hellenic subduction zone (Preveli nappe, Crete). The structural data and deformation microfabrics of quartz, coupled with petrological constraints, suggest that subduction of the Preveli rocks occurred under epidote-blueschist-facies conditions (T = 360 ± 40 °C, P > 1.0 GPa) and differential stresses >70 MPa. Localized deformation of fine-grained monomineralic grandite of the skarn led to mode I fractures and discrete shear zones along which synkinematic ferri-winchite developed. Brittle-viscous deformation of grandite in high-strain domains was accommodated by cataclasis, incongruent dissolution precipitation creep (IDPC), and incipient dislocation creep (DC). Cataclasis led to strong grain-size reduction and increased permeability resulting in fluid infiltration and enhanced IDPC as is documented by serrated reaction seams along which actinolite and ferri-winchite grew in sequence while replacing grandite. Incipient DC of grandite is indicated by lattice distortion and possible subgrains. Brittle-viscous deformation of grandite occurred at high differential stress (>250 MPa) and was significantly controlled by externally derived fluids, which contributed to hydraulic fracturing and the redistribution and addition of mobile elements.

Melt-enhanced strain localization and phase mixing in a large-scale mantle shear zone (Ronda peridotite, Spain)

Abstract. Strain localization in upper-mantle shear zones by grain size reduction and the activation of grain-size-sensitive deformation mechanisms is closely linked to phase mixing. With its mylonitic grain size (50–100 µm) and well-mixed phase assemblage, the kilometer-scale shear zone at the northwestern boundary of the Ronda peridotite is, in this respect, no exception. In transects across the high-strain mylonitic into the low-strain tectonitic part of this shear zone, the following four dominant microstructural domains were identified: (1) olivine-rich matrix, (2) mixed matrix, (3) neoblast tails of clinopyroxene porphyroclasts, and (4) neoblast tails of orthopyroxene porphyroclasts. In these domains, phase mixing and its impact on strain localization were investigated by a combination of microstructural (optical microscopy), textural (EBSD), and geochemical (EPMA) analysis. The dominant microstructural domain of all samples is the mixed matrix composed of olivine, orthopyroxene, and clinopyroxene. Its homogenous distribution of interstitial pyroxenes contradicts mechanical mixing. Instead, extensive phase mixing under near-steady-state conditions is documented by the constant grain size and by phase boundary percentages > 60 % for the entire mylonitic unit and all the microstructural domains. Lobate phase boundaries, homogenous phase mixing, and secondary-phase distribution, as well as continuous geochemical trends that are independent of the microstructural domain, point to a reaction-driven, metasomatic formation of the mixed matrix and pyroxene porphyroclast tails in the entire shear zone. An OH-bearing metasomatism by small fractions of evolved melts is indicated by amphibole abundance in pyroxene neoblast tails, olivine B-type-crystallographic-preferred orientations (CPOs), and the microstructural consistency of the garnet–spinel (grt–spl) mylonites from both major peridotite massifs of the Gibraltar arc, Ronda, and Beni Bousera (Morocco). The established syn-deformational temperature of 800–900 ∘C at 1.95–2.00 GPa suggests that the metasomatism did not reset the equilibrium temperatures. Consistent geochemistry and phase assemblage in mylonites and tectonites but a change from equiaxial (tectonites) to wedge-shaped pyroxenes aligned parallel to the foliation (mylonites) point to a pre- to syn-deformational metasomatism, with the potential annealing of the tectonites. For the mylonitic mixed matrix, wedge-shaped pyroxenes, and neoblast tail formation in pyroxene porphyroclast stress shadows point to the activity of incongruent dissolution–precipitation creep. Apart from the dissolution–precipitation creep, strong CPOs of all major phases (ol, opx, and cpx) suggest dislocation creep as being the major deformation mechanism in the entire shear zone.

Microstructures, geochemistry, and geochronology of mica fish: Review and advances

Shear zones are one of the most conspicuous structures in orogenic and rifting settings, accommodating high strain, rock displacement and influencing magma emplacement and fluid flow. Since shear zones may be simultaneously or sequentially affected by multiple processes involving mineral reactions, variations in pressure-temperature conditions, fluid-rock interaction, and diffusion, determining the timing of such structures has been one of the major challenges for modern geochronology. Although low- (up to lower greenschist facies) and high-temperature shear zones (above amphibolite facies) are well-dated through low- and high-closure temperature minerals, medium-temperature shear zones developed within the critical temperature window of ∼450–550 °C, in which conventional chronometers such as 40Ar/39Ar and Rb–Sr applied to mica fish may or may not record the timing of deformation for multiple reasons (e.g., grain size, cooling rate, mineral composition, fluid activity, deformation, neo- and recrystallization). Here, we review the current knowledge on the evolution of mica fish and the effect of deformation on its chemical and isotopic systems. We evaluate the effect on the widely deployed in situ 40Ar/39Ar technique. Furthermore, we demonstrate the potential to assess mica fish evolution applying high-spatial resolution microstructural and chemical mapping techniques such as electron backscatter diffraction (EBSD), time-of-flight secondary ion mass spectrometry (ToF–SIMS) and in situ Rb–Sr via triple quadrupole inductively coupled plasma mass spectrometry (TQ-ICP-MS) to a case study of medium-temperature mylonites from the well-characterized Taxaquara shear zone, SE Brazil. We show that mica fish display complex microstructures with variable strain intensity, commonly with low strain inner cores and high strain edges and along kink planes. Strain shadows in mica fish are commonly characterized by low-strain fine-grained muscovite, suggesting recrystallization coeval with ductile deformation. Despite being intensely deformed, muscovite fish Rb–Sr retain the protolith age (c. 600 Ma), whereas recrystallized fine-grained muscovite yields the timing of deformation (c. 550–540 Ma). Synthetic shear bands cross-cutting coarse-grained muscovite fish induce muscovite recrystallization consistent with their distinct chemistry, with recrystallized muscovite characterized by higher Fe–Mg and lower Na suggesting fluid-assisted recrystallization under lower temperature compared to the muscovite fish host. We propose that these shear bands across mica fish play an important role by accommodating grain size reduction and subsequent deformation, leading to the formation of smaller individual mica fish. Grain size reduction, likely enhanced by dynamic precipitation (i.e., coeval crystal-plastic deformation and dissolution-precipitation creep), appears as the key recrystallization mechanism that allows low strain muscovite in strain shadows and shear bands to record the timing of deformation in medium-temperature shear zones, consistent with qualitative Sr diffusion modelling.

Microfabric evolution during metasomatism and deformation, exemplified by the nodular sillimanite gneisses (Bamble lithotectonic domain, South Norway)

Proterozoic foliated and nodular sillimanite gneisses from the Bamble lithotectonic domain, South Norway, are analysed to unravel their microfabric evolution with mineral reactions during metasomatism and associated deformation. The nodules form cm-scaled spherical to ellipsoidal sillimanite-quartz aggregates that locally grade into foliated sillimanite gneisses. Independent on their fabric, they record incomplete breakdown reactions of biotite and K-feldspar recorded by muscovite lamellae and associated Fe-oxide needles in biotite and by muscovite-quartz aggregates after K-feldspar. Muscovite is partly replaced by sillimanite. Based on immobile Al, the nodular gneiss forming reactions give excess K, Mg and H2O that may leave the nodular gneiss to form a metasomatic agent and caused regional metasomatism (scapolitisation) in the surrounding rocks. Quartz in the foliated gneisses shows a pronounced shape but no marked crystallographic preferred orientation. There is no indication of major strain accumulation by quartz dislocation creep. Muscovite shows lobate phase boundaries to quartz, which is interpreted as reaction fabric, from the breakdown reactions of K-feldspar and biotite. The nodular and sillimanite gneisses formed during metasomatic mineral reactions, where major elements K, Mg and H2O leave the rock and an Al-rich metasomatic restite remains. We suggest that the metasomatism involved a molar volume loss, where reactions forming muscovite, quartz and sillimanite occurred by incongruent dissolution-precipitation creep at low stresses forming the nodular and foliated gneisses. Our study demonstrates that metasomatism with chemical rock changes and mass transfer associated with incongruent dissolution-precipitation contributed to the observed reaction and deformation microfabric.

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.

Shearing-enhanced deep fluid circulation induces seismic anisotropy in the lower crust at slow-spreading oceanic ridges

AbstractAlthough long-lived detachment faulting plays an important role in fluid circulation and in accommodating tectonic extension at slow-spreading oceanic ridges, it is still unclear how the fluid-enriched faults contribute to the observed seismic anisotropy in the lower crust. We investigated sheared and altered gabbros along the detachment fault zones from the Xigaze ophiolite in the southern Tibetan Plateau. Results demonstrate that the positive feedback between fluid circulation and shearing, linked by dissolution-precipitation creep of amphibole, resulted in fluid enrichment during strain localization along the fault zones. Based on this shearing-enhanced fluid circulation model, our calculations of the seismic properties show that amphiboles (de)formed by dissolution-precipitation creep along the fault zones largely contribute to the seismic anisotropy (P and S waves) and S-wave delay time in the lower crust at slow-spreading ridges, with the polarization directions of fast shear waves being subparallel to the ridges. The strength of resulting seismic anisotropy is largely a function of crustal thickness, fault zone attitude, and metasomatism intensity. This study provides a novel explanation for the origin of seismic anisotropy in the lower oceanic crust at slow-spreading ridges. The conclusion may also have implications for the origin of seismic anisotropy at fast-spreading ridges where there are high melt supplies.

The effect of muscovite on the microstructural evolution and rheology of quartzite in general shear

We conducted general shear experiments on synthetic mixtures of quartz and muscovite aggregates at 800 °C and 1.5 GPa with 0.1 wt% H2O added in the Griggs apparatus to investigate the role of muscovite content on the microstructural evolution and rheological properties of quartz aggregates. Muscovite content varied between 0, 5, 10 and 25% muscovite. Mechanically, the sample strengths decrease with an increase in muscovite content. At high strains muscovite grains align sub-parallel to the shear plane with C′- bands commonly observed in the muscovite-bearing samples. The presence of muscovite has significant influence on the amount of dynamic recrystallization in quartz; at high strains the pure quartz sample completely recrystallizes while only ∼5% of the quartz in the 25% muscovite sample is dynamically recrystallized at similar strains. The presence of muscovite also has a significant influence on crystallographic preferred orientations (CPO) and grain shape preferred orientations (SPO) of quartz at high strains. At high strains, muscovite is interpretated to deform primarily by basal glide and dissolution-precipitation creep. Finally, our mechanical results fit well with rheological mixing models, where we estimate aggregates with 25% muscovite may deform 1–2 orders of magnitude faster than pure quartz aggregates at conditions near the brittle-ductile transition.

Interplay between crystal-plasticity, fracturing and dissolution-precipitation creep in lower-crustal ultramylonite from hole U1473A, Atlantis Bank, Southwest Indian Ridge

Microfabrics of a gabbroic ultramylonitic shear zone from the Atlantis Bank oceanic core complex have been studied to investigate strain localization processes during exhumation of the lower oceanic crust through detachment faults. The ∼0.5 cm wide ultramylonite band is hosted in a coarse-grained deformed gabbro. Microfabric analysis reveal undulatory extinction, kink bands, bent exsolution lamellae or twins and healed microfractures decorated by new grains within porphyroclasts of diopside and plagioclase in the ultramylonite, as well as the host gabbro close to the border of the ultramylonite. These microstructures suggest that strain was initially accommodated by dislocation glide associated with microfracturing, i.e. high-stress crystal plasticity. Heterogenous fluid influx along microfractures is suggested to have led to localized phase transformation, as amphibole is abundant in the fine-grained matrix of the ultramylonite but not in the host gabbro. Fluid-rock interaction resulted in the development of the ultramylonite via grain-size sensitive fluid-assisted granular flow. Strain localization is concomitant to increasing fluid-rock interaction and the replacement of the former deformed anhydrous mineral assemblage into a hydrated ultramylonite composed of a plagioclase-amphibole mixture. Our study highlights the importance of fluid-assisted metamorphic reaction in decreasing the viscosity of the metastable lower oceanic crust and creating zones of extreme rheological weakening that deform by diffusion creep on the long term. Fluid infiltration plays a key role in facilitating ductile deformation and tectonic spreading through large-scale detachment faults in the hot, lower oceanic crust.

Quartz rheology constrained from constant-load experiments: Consequences for the strength of the continental crust

The mechanical properties of quartz are fundamental to control the plastic behaviour of the continental crust. Our understanding of quartz rheology is still limited in the following respects: i) the large variability of flow law parameters in the earlier literature (stress exponent n = 4 to ≤ 2 and activation energy Q = 120 to 242 kJ/mol), and ii) the difficulty to identify the rate-limiting deformation mechanism, if several mechanisms are operating simultaneously. These two issues are connected and cannot be resolved separately. The present study has carried out constant-load experiments to constrain the flow law parameters of quartz. A new generation hydraulically-driven Griggs-type apparatus has been employed, resulting in reproducible mechanical data, even at very low strain rates (10−8 to10−9 s−1; so far, closest to the natural ones). Furthermore, the Q-value in constant load experiments can be estimated without prior knowledge of the n value. Our new n (= 2) and Q values (= 110 kJ/mol) are fairly low. We calculated an A-value of 1.56 × 10−9 /MPa/sec. Microstructural analysis suggests that the bulk sample strain in our experiments is achieved by crystal plasticity, i.e., dislocation glide with minor recovery by sub-grain rotation, accompanied by grain boundary migration. Micro-cracking helps to nucleate new grains. It is inferred that strain incompatibilities induced by dislocation glide are accommodated by grain boundary processes, including dissolution-precipitation creep and grain boundary sliding. These grain boundary processes are responsible for the n-value that is lower than expected for dislocation creep (n≥ 3). The new flow law can consistently estimate strain rates (especially at low stresses) in excellent agreement with documented natural case studies and predicts a rapid drop in strength of quartz-bearing rocks in the continental crust below a depth of ∼10 km or at a temperature of ∼300°C and higher.

Rheology of Naturally Deformed Antigorite Serpentinite: Strain and Strain-Rate Dependence at Mantle-Wedge Conditions.

Antigorite serpentinite is expected to occur in parts of subduction plate boundaries, and may suppress earthquake slip, but the dominant deformation mechanisms and resultant rheology of antigorite are unclear. An exhumed plate boundary shear zone exposed near Nagasaki, Japan, contains antigorite deformed at 474°C ± 30°C. Observations indicate that a foliation defined by (001) crystal facets developed during plate‐boundary shear. Microstructures indicating grain‐scale dissolution at high‐stress interfaces and precipitation in low‐stress regions suggest that dissolution‐precipitation creep contributed to foliation development. Analysis of crystal orientations indicate a small contribution from dislocation activity. We suggest a frictional‐viscous rheology for antigorite, where dissolution‐precipitation produces a foliation defined by (001) crystal facets and acts to resolve strain incompatibilities, allowing for efficient face‐to‐face sliding between facets. This rheology can not only explain aseismic behavior at ambient plate boundary conditions, but also some of the contrasting behaviors shown by previous field and laboratory studies.

Monazite as a monitor of shear strain in orogenic crust

The Ramsay-Graham heterogeneous simple shear zone model revolutionized our understanding of finite strain variation in orogenic belts. Many deeply exhumed (>20 km depths) shear zones in orogenic continental crust are a consequence of sub-simple shear strain accumulation during contraction and displacement at high temperature. Here we explore the micro-scale record of this process using syn-kinematic monazite in two shear zones in a >20,000 km2 terrane of exhumed lower continental crust. Field observations combined with high-spatial resolution X-ray mapping, Th–U-Pb electron microprobe petrochronology, and electron backscatter diffraction analysis demonstrate that monazite grains can provide an important textural and temporal record of shear strain in mylonite and ultramylonite gneisses. Monazite grains exhibit aspect ratios of 1.7–4.6 parallel to the stretching lineation and parallel or oblique (<30°) to the penetrative foliation. Monazite EBSD data for grain orientation spread, i.e., average misorientation relative to the mean, are consistent with no significant dislocation-accommodated internal deformation. We conclude that dissolution precipitation creep of monazite produced compositionally-defined geometries compatible with the dextral sense of shear observed in outcrop and thin section. Syn-kinematic rims on monazite grains that develop in low strain domains during accumulation of shear strain can provide absolute ages for μm-to km-scale kinematics in continental orogens.

Biotite supports long-range diffusive transport in dissolution–precipitation creep in halite through small porosity fluctuations

Abstract. Phyllosilicates are generally regarded to have a reinforcing effect on chemical compaction by dissolution–precipitation creep (DPC) and thereby influence the evolution of hydraulic rock properties relevant to groundwater resources and geological repositories as well as fossil fuel reservoirs. We conducted oedometric compaction experiments on layered NaCl–biotite samples to test this assumption. In particular, we aim to analyse slow chemical compaction processes in the presence of biotite on the grain scale and determine the effects of chemical and mechanical feedbacks. We used time-resolved (4-D) microtomographic data to capture the dynamic evolution of the porosity in layered NaCl–NaCl/biotite samples over 1619 and 1932 h of compaction. Percolation analysis in combination with advanced digital volume correlation techniques showed that biotite grains influence the dynamic evolution of porosity in the sample by promoting a reduction of porosity in their vicinity. However, the lack of preferential strain localisation around phyllosilicates and a homogeneous distribution of axial shortening across the sample suggests that the porosity reduction is not achieved by pore collapse but by the precipitation of NaCl sourced from outside the NaCl–biotite layer. Our observations invite a renewed discussion of the effect of phyllosilicates on DPC, with a particular emphasis on the length scales of the processes involved. We propose that, in our experiments, the diffusive transport processes invoked in classical theoretical models of DPC are complemented by chemo-mechanical feedbacks that arise on longer length scales. These feedbacks drive NaCl diffusion from the marginal pure NaCl layers into the central NaCl–biotite mixture over distances of several hundred micrometres and several grain diameters. Such a mechanism was first postulated by Merino et al. (1983).

Dissolution precipitation creep as a process for the strain localisation in mafic rocks

The lower crust is, on average, mafic in composition and composed of minerals that remain mechanically strong up to high temperatures. Here we show that dissolution-precipitation creep (as a type of diffusion creep) plays a major role in deformation of gabbro bodies at upper amphibolite facies conditions. The Kågen gabbro, N. Norway, is comprised of undeformed gabbro lenses enclosed by mylonitised margins that deformed at 690 ± 25 °C and 1.0–1.1 GPa. The evolution of the microstructures and fabric of the low strain gabbro to high strain margins were investigated. Original clinopyroxene and plagioclase dissolved during mineral reactions and precipitated as new minerals phases: new plagioclase and clinopyroxene (different compositions relative to the magmatic parents) and additional amphibole and garnet. Microstructural and crystallographic preferred orientation (CPO) data indicate that dissolution-precipitation creep is the dominant deformation mechanism. Amphibole shows a strong CPO that is primarily controlled by orientated growth in the stretching direction. The progression of mineral reactions and weakening is directly connected to a fluid-assisted transformation process that facilitates diffusion creep deformation of strong minerals at far lower stresses and temperatures than required by dislocation creep. Initially strong lithologies can become weak, provided that reactions proceed during deformation.

Fluid–Mineral Interactions: Controlling Coupled Mechanisms of Reaction, Mass Transfer and Deformation

AbstractFluid–rock interaction has profound effects on the dynamics of the lithosphere. This Perspectives article describes the catalytic effects of water on the kinetics of mineral reactions and on the strength of rocks in terms of coupled dissolution and precipitation mechanisms on a macro- and nano-scale. The length scale of coupling between the dissolution and precipitation steps depends on the fluid composition at the mineral-fluid interface and also on differential stress. Stress-induced mass transport, dependent on the generation of porosity by mineral reactions, results in dissolution-precipitation creep as the principal mechanism of rock deformation in the lithosphere. The heterogeneous distribution of fluid infiltration into dry, strong rocks in the deep crust leads to weak rocks within strong, stressed host rock and the possibility of significant local variations in pressure. Fluid–rock interaction mechanisms are discussed in terms of recrystallisation reactions whereby mass transport and the most favourable nucleation sites determine the distribution and texture of the resultant assemblages. Metamorphic differentiation is a natural consequence of dissolution-precipitation mechanisms under deviatoric stress and similar mechanisms may apply to general pattern formation in rocks.

Evolution of fluid pathways during eclogitization and their impact on formation and deformation of eclogite: A microstructural and petrological investigation at the type locality (Koralpe, Eastern Alps, Austria)

Eclogites representing three strain stages from the type locality (Saualpe-Koralpe Complex, Eastern Alps, Austria) have been analyzed with regard to their microstructural, petrological and mechanical evolution during the gabbro-eclogite transformation and subsequent deformation. Thermodynamic forward models suggest the presence of a H2O dominated fluid phase during eclogitization and following deformation. While all three eclogite types show the same mineral paragenesis composed of garnet, omphacitic clinopyroxene, quartz and a fine grained polyphase aggregate of kyanite, clinozoisite, quartz and retrograde plagioclase, we do observe a microstructural and mechanical evolution with strain.Eclogitization is controlled by dissolution of metastable gabbro and precipitation of the stable finer grained eclogitic mineral paragenesis. This induces a micro-porosity which allows fluids to migrate in an unchannelized way. Distributed eclogitization results in strain weakening, where both transformational and volumetric strain are accommodated by dissolution-precipitation creep. Subsequent deformation of eclogite results in the development of pronounced eclogitic foliation accompanied by grain coarsening and reduction of porosity. The latter results in a reduced efficiency of dissolution-reprecipitation possibly causing apparent strain hardening. Based on our observations we suggest a model for the rheological evolution during the transformation of a dry mafic rock into an eclogite where the combination of volumetric and transformational strain at the onset of eclogitization assisted by fluid induces rapid weakening followed by apparent hardening once eclogitization is completed and steady state deformation of the eclogite mineral paragenesis sets in.