Strong Lattice Preferred Orientation Research Articles

Characterization of the lattice preferred orientation of hcp iron transformed from the single-crystal bcc phase in situ at high pressures up to 80 GPa

Studying the anisotropic physical properties of hexagonal closed-packed (hcp) iron is essential for understanding the properties of the Earth’s inner core related to the preferred orientation of the inner core materials suggested by seismic observations. Investigating the anisotropic physical properties of hcp iron requires (1) the synthesis of hcp iron samples that exhibit several distinctive types of strong lattice preferred orientation (LPO) and (2) the quantitative LPO analysis of the samples. Here, we report the distinctive LPO of hcp iron produced from single-crystal body-centered cubic (bcc) iron compressed along three different crystallographic orientations ([100], [110], and [111]) in a diamond anvil cell based on synchrotron multiangle X-ray diffraction measurements up to 80 GPa and 300 K. The orientation relationships between hcp iron and bcc iron are consistent with the Burgers orientation relationship with variant selection. We show that the present method is a way to synthesize hcp iron with strong and characteristic LPO, which is beneficial for experimentally evaluating the anisotropic physical properties of hcp iron.

Textures Induced by the Coesite‐Stishovite Transition and Implications for the Visibility of the X‐Discontinuity

AbstractThe coesite‐stishovite phase transition is considered the most plausible candidate to explain the X‐discontinuity observed at around 300 km depth in a variety of tectonic settings. Here, we investigate the microstructure in SiO2 across the coesite‐stishovite transition in uniaxial compression experiments. We apply the multigrain crystallography technique (MGC) in a laser‐heated diamond‐anvil cell (LH‐DAC) to identify the seismic signature of the transition and the amount of SiO2 in the mantle. While coesite displays weak lattice‐preferred orientations (LPO) before the transition, stishovite develops strong LPO characterized by the alignment of [112] axes parallel to the compression direction. However, LPO has little effect on the impedance contrast across the transition, which is up to 8.8% for S‐waves in a mid‐ocean ridge basalt (MORB) composition at 300 km depth along a normal mantle geotherm, 10 GPa‐1700 K. Therefore, 10–50 vol.% of a MORB component, corresponding to 0.6–3.2 vol.% SiO2, mechanically mixed with the pyrolytic mantle would be required to explain the range of impedance (and velocity) contrasts observed for the X‐discontinuity. Based on the reflection coefficients computed for the coesite‐stishovite transition, we show that the incidence angle or epicentral distance is critical for the detection of silica‐containing lithologies in the upper mantle, with highest detection probabilities for small incidence angles. The intermittent visibility of the X‐discontinuity may thus be explained by the seismic detectability of the coesite‐stishovite transition rather than by absence of the transition or chemical heterogeneities in some specific tectonic settings.

Seismic Anisotropy in the Lower Mantle Transition Zone Induced by Lattice Preferred Orientation of Akimotoite

AbstractSeismic anisotropy has been widely observed near the subducting slabs in the lower mantle transition zone (MTZ) and is often interpreted by the lattice preferred orientation (LPO) of constituent minerals. Akimotoite is one of the dominant minerals near the cold subducting slabs. Therefore, we conducted the well‐controlled uniaxial and shear deformation experiments on the MgSiO3 akimotoite aggregates at 21–23 GPa and 900–1300°C by using the D111‐type Kawai‐type multianvil apparatus. We observed strong LPOs and the most dominant slip system of akimotoite is suggested to be (0001). The elastic wave velocities of deformed samples were calculated to be strong azimuthal and polarization anisotropy with the velocities of horizontally polarized shear waves greater than that of vertically polarized shear waves for the horizontal mantle shearing. Our results provide important implications for the origin of observed seismic anisotropies and the mantle flow directions in the lower MTZ.

Porosity induced by dislocation dynamics in quartz-rich shear bands of granitic rocks

The production of micro-pores is a driving mechanism for fluids to interact with deep environment and influence rock properties. Yet, such a porosity still remains misunderstood to occur in viscous rocks and may be attributed to either grain boundary sliding (GBS), dissolution effects or sub-grain rotation. Here we focus on quartz-rich shear bands across the Naxos western granite (Aegean Sea, Greece), where we document sub-micron pores at quartz boundaries. While most of these pores are observed along grain boundaries, some of them occur at intra-grain boundaries, which excludes dissolution or GBS to produce them, but instead involves the dynamic of dislocations. We then confirm that quartz is dominated by dislocation creep with evidence of a moderate to strong lattice-preferred orientation (LPO) and numerous tilt/twist boundaries, including at the pluton margin where rocks embrittled. These features coincide with (1) randomly oriented ‘inclusion’ quartz grains along tilt/twist boundaries and (2) a partial dependency of the LPO strength on grain size. Our findings suggest that pores arise from coalescing dislocations at boundaries of rotating sub-grains, providing nucleation sites for new grains to be precipitated during plastic flow. Fluid infiltration, rock embrittlement and related implications are also expected through pores accumulation with increasing strain.

Seismic Anisotropy in Subduction Zones: Evaluating the Role of Chloritoid

Subduction zones are often characterized by the presence of strong trench-parallel seismic anisotropy and large delay times. Hydrous minerals, owing to their large elastic anisotropy and strong lattice preferred orientations (LPOs), are often invoked to explain these observations. However, the elasticity and the LPO of chloritoid, which is one of such hydrous phases relevant in subduction zone settings, are poorly understood. In this study, we measured the LPO of polycrystalline chloritoid in natural rock samples, obtained the LPO-induced seismic anisotropy, and evaluated the thermodynamic stability field of chloritoid in subduction zones. The LPO of chloritoid aggregates displayed a strong alignment of the [001] axes subnormal to the rock foliation, with a girdle distribution of the [100] axes and the (010) poles subparallel to the foliation. New elasticity data of single-crystal chloritoid showed a strong elastic anisotropy of chloritoid with 47% for S-waves (VS) and 22% for P-waves (VP), respectively. The combination of the LPO and the elastic anisotropy of the chloritoid aggregates produced a strong S-wave anisotropy with a maximum AVS of 18% and a P-wave anisotropy with an AVP of 10%. The role of chloritoid LPO in seismic anisotropy was evaluated in natural rock samples and a hypothetical blueschist. Our results indicate that the strong LPO of chloritoid along the subduction interface and in subducting slabs can influence the trench-parallel seismic anisotropy in subduction zones with “cold” geotherms.

Lattice-preferred orientation of amphibole, chlorite, and olivine found in hydrated mantle peridotites from Bjørkedalen, southwestern Norway, and implications for seismic anisotropy

Understanding lattice-preferred orientations (LPOs) of olivine is important in the study of seismic anisotropy and mantle flow of the upper mantle in the earth. Although olivine is the major mineral of the upper mantle, both amphibole and chlorite in a deformed peridotite may develop LPO and thus affect the seismic anisotropy in water-rich environments. However, it is not known whether the coexistence of LPO of amphibole and chlorite contributes to seismic anisotropy constructively or destructively. Therefore, the LPOs of amphibole, chlorite, and olivine in hydrated mantle peridotites from Bjørkedalen, southwestern Norway, were examined. In this study, the hydrated mantle peridotites showed three types of the LPOs of olivine, classified as A type LPO, B type-like LPO, and mixed LPO between the two types. Most of the olivines showed weak LPOs but amphibole and chlorite showed strong LPOs. High water content in olivine (440–690 ppm H/Si) and numerous hydrous inclusions in olivine and orthopyroxene indicated that water induced the fabric change of olivine from A type to B type-like LPO. The seismic velocity and seismic anisotropy were calculated on the basis of the LPOs of the studied minerals. It is found that the LPOs of both amphibole and chlorite can contribute to a strong trench-parallel seismic anisotropy in the subduction zone constructively, depending on the dip angle of flow. The results suggest that the development of the LPO of amphibole and chlorite can contribute significantly to seismic anisotropy of the subduction zone.

Fabric heterogeneity in the Mojave lower crust and lithospheric mantle in Southern California

AbstractWe use xenoliths from young cinder cones in the eastern Mojave region of Southern California to investigate deformation fabrics and their implications for strain localization, lithospheric viscosity, and controls on mineral lattice‐preferred orientations (LPOs) and seismic anisotropy at Moho depths. Lower crustal gabbros and upper mantle peridotites were collected from two areas separated by ∼80 km—the Cima and Deadman Lake Volcanic Fields. At both localities, mantle peridotites exhibit solid‐state deformation fabrics that show strong LPOs and other evidence for dislocation creep as the dominant deformation mechanism. The preservation of microstructures ranging from annealed, granular, to mylonitic indicates that there is substantial strain localization in the Mojave mantle near the Moho. Paleopiezometry conducted on subgrains in olivine indicates that stress magnitudes during deformation were 12–26 MPa. Calculations using olivine flow laws yield strain rates on the order of ∼10−12/s and an associated viscosity of ∼1019 Pa s, consistent with estimates of mantle lid viscosity from postseismic relaxation studies. Two types of olivine LPOs were observed in the peridotites: A‐type and E‐type fabrics, both of which predict seismic fast axes that align parallel to the lineation within the foliation plane. The occurrences of the two fabric types appear to correlate with strain magnitude but not with water contents measured using Fourier transform infrared spectroscopy. Lower crustal fabrics are dominated by magmatic foliations and LPOs in plagioclase, which produce seismic fast axes oriented perpendicular to the foliation plane. This suggests that even where mantle and lower crustal fabrics are kinematically linked, their seismic fast axes may appear anticorrelated across the Moho.

Ductile deformation and its geological implications for retrograded eclogites from the Hongqiyingzi Complex in Chicheng, northern Hebei, China

The rheology of rocks at depth remains a key point in earth sciences. Deformation of high-temperature–high-pressure rocks from the subduction zone has not been fully studied; in particular, the deformation behavior of eclogites remains poorly understood. This research is focused on the microstructure and fabric of retrograded eclogites from the Hongqiyingzi Complex in Chicheng, north Hebei, China, based on photomicrography, scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) analysis. The analytical results show that plastic deformation occurred in the garnets during exhumation, but they do not show an obvious lattice preferred orientation (LPO). This can be interpreted as being caused by the simultaneous activation of multiple slip systems in the garnets during deformation by dislocation creep. The plagioclases have a special fabric; the (001) LPO presents a maximum in Z direction which can be correlated with a new [100](001) slip system. The misorientation angle distribution (MAD) of the plagioclases in the deformed retrograded garnet augens shows a special bimodal distribution with peaks in both a low-angle range ( 140°). In the retrograded garnet augens (i.e. “white eye socket” garnets) the maximum peak moves from a high-angle range to a low-angle range when we analyze only those plagioclases surrounding the residual garnet porphyroclast. Deformation behavior is controlled by the crystallographic orientation of the host grain and the grain boundary sliding process. Hornblendes in different layers and the retrograded garnet augens show almost the same strong LPO patterns, which are correlated with the [001](010) slip system; and MAD diagrams show a peak in a low-angle range (<40°). Integrating recent studies on metamorphism and geochronology, we argue that ductile deformation occurred during the exhumation periods together with retrograded metamorphism.

Early Tertiary deformation of the Zhongba–Gyangze Thrust in central southern Tibet

As the boundary thrust between India and Asia in southern Tibet, the Zhongba–Gyangze Thrust (ZGT) emplaced the Yarlung Zangbo Suture Zone (YZSZ) units in the hanging wall southward onto Tethyan Himalaya sequences (THS) of the northern Indian continental margin in the footwall. Detailed field investigation, electron backscatter diffraction (EBSD) analysis, detrital zircon U–Pb geochronology and 40Ar–39Ar thermochronology were conducted to understand the evolution of the ZGT in Sangsang area, central southern Tibet. The shear zone of ZGT is located within the sedimentary-matrix mélange of YZSZ that is mainly composed of matrix of blueschist with meta pelagic–hemipelagic siliceous and siliciclastic rocks and blocks of basalt, limestone and sandstone. Penetrative F1 foliation and kink band structure were recorded within the matrix both on outcrop and under microscope. Strong lattice preferred orientation (LPO) fabric initiated by the low-temperature (350−450°C) (010)[001] slip system was detected by EBSD in the sodic amphiboles of the blueschist. The 40Ar–39Ar ages of the phengites from blueschist and sericites from the phyllite in the shear zone indicate that the activity of ZGT occurred between 71 and 60Ma. In the THS, a newly documented younger unit preserving detrital zircons from the southern Asian margin lies above the Triassic–Cretaceous sequences that carries only detrital zircons from the Indian continent. This unit is dated to be ~61Ma by the detrital zircon ages, similar to the Sangdanlin Formation and representing the Yarlung Zangbo Foreland Basin (YZFB) system. The ZGT had probably been active due to the initial India–Asia collision and acted as the frontal thrust controlling the development of YZFB.

An experimental study of effect of pre‐existing fabric on deformation of foliated mylonite at high temperature and pressure

We performed deformation experiments on a foliated mylonite under high temperature and pressure conditions in this study. To investigate the effect of pre‐existing fabric on the rheology of rocks, our samples were drilled from natural mylonite with the cylinder axis parallel to the foliation (PAR) and perpendicular to the foliation (PER). We performed 25 tests on seven PAR samples and 21 tests on seven PER samples at temperatures ranging from 600 to 890 °C, confining pressures ranging from 800 to 1400 MPa, and steady‐state strain rates of 1 × 10−4, 1 × 10−5 and 2.5 × 10−6 s−1. In the temperatures of 600–700 °C, the deformation is accommodated by semi‐brittle flow, with the average stress exponent being 6–7 assuming power law flow; in the temperature range of 800–890 °C, deformation is mainly by plastic flow, with an average stress exponent of n = 3 and activation energies of Q = 354 ± 52 kJ/mol (PER and PAR samples). The experimental results show that the strengths of PER samples are higher than those of PAR samples. Deformation microstructures have been studied by optical and electron microscopy. The original foliation of PER samples is destroyed by deformation and replaced by a new foliation, but the deformation of PAR samples followed the original foliation. Electron backscatter diffraction (EBSD) measurements show a strong lattice preferred orientation (LPO) of the quartz c axis fabrics of the starting samples and deformed PER and PAR samples. However, the c axis fabric of quartz in experimentally deformed PER and PAR samples varied with temperature and strain rate is different from that seen in the starting mylonite sample. The initial quartz c axis fabric of the starting mylonite sample has been transformed into a new fabric during experimental deformation. Dehydration melting of biotite and hornblende occurred in both PER and PAR samples at temperatures of 800–890 °C. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Pegmatite mylonites: origin and significance

Abstract Pegmatites generally behave as resistant objects compared to their host rocks during solid-state deformation. They usually show folding or boudinage within their host rocks, features observed from low-grade to high-grade deformations, up to upper-amphibolite metamorphic conditions. Here we describe mylonitic pegmatite veins emplaced within S-C fabric-bearing two-mica granites. Pegmatites consist mainly of feldspar and quartz ribbons, with remnants of coarse-grained Kfs phenocrysts. Quartz ribbons are made of large grains elongated in the shearing direction, with evidence of dynamic recrystallization involving grain boundary migration. Ribbons show very strong lattice-preferred orientations of c-axes at low angles to the stretching lineation. From these, we propose that quartz ribbons are formed by oriented growth during syn-shearing crystallization of the pegmatite. Subsequent syn-shearing solid-state deformation involved grain size reduction accompanied by prism &lt;a&gt; slip. The occurrence of pegmatite mylonites within granitoid intrusions should be considered as a univocal argument for synkinematic emplacement.

Deformation microstructures of olivine and chlorite in chlorite peridotites from Almklovdalen in the Western Gneiss Region, southwest Norway, and implications for seismic anisotropy

Chlorite peridotites from Almklovdalen in southwest Norway were studied to understand the deformation processes and seismic anisotropy in the upper mantle. The lattice preferred orientation (LPO) of olivine and chlorite was determined using electron backscattered diffraction (EBSD)/scanning electron microscopy. A sample with abundant garnet showed [100] axes of olivine aligned sub-parallel to lineation, and [010] axes aligned subnormal to foliation: A-type LPO. Samples rich in chlorite showed different olivine LPOs. Two samples showed [001] axes aligned sub-parallel to lineation, and [010] axes aligned subnormal to foliation: B-type LPO. Two other samples showed [100] axes aligned sub-parallel to lineation, and [001] axes aligned subnormal to foliation: E-type LPO. Chlorite showed a strong LPO characterized by [001] axes aligned subnormal to foliation with a weak girdle subnormal to lineation. Fourier transform infrared (FTIR) spectroscopy of the specimens revealed that the olivines with A-type LPO contain a small amount (170 ppm H/Si) of water. In contrast, the olivines with B-type LPOs contain a large amount (340 ppm H/Si) of water.The seismic anisotropy of the olivine and chlorite was calculated. Olivine showed Vp anisotropy of up to 3.8% and a maximum Vs anisotropy of up to 2.7%. However, the chlorite showed a much stronger Vp anisotropy, up to 21.1%, and a maximum Vs anisotropy of up to 31.7%. A sample with a mixture of 25% of olivine and 75% of chlorite can produce a Vp anisotropy of 14.2% and a maximum Vs anisotropy of 22.9%. Because chlorite has a wide stability field at high pressure and high temperature in the subduction zone, the strong LPO of chlorite can be a source of the observed trench-normal or trench-parallel seismic anisotropy in the mantle wedge as well as in subducting slabs depending on the dipping angle of slab in a subduction zone where chlorite is stable.

Deformation microstructures of olivine and pyroxene in mantle xenoliths in Shanwang, eastern China, near the convergent plate margin, and implications for seismic anisotropy

Deformation microstructures, including lattice-preferred orientations (LPOs) of olivine, enstatite, and diopside, in mantle xenoliths at Shanwang, eastern China, were studied to understand the deformation mechanism and seismic anisotropy of the upper mantle. The Shanwang is located across the Tan-Lu fault zone, which was formed due to the collision between the Sino-Korean and South China cratons. All samples are spinel lherzolites and wehrlites, and LPOs of minerals were determined using scanning electron microscope/electron backscattered diffraction. We found two types of olivine LPO: type-B in spinel lherzolites and type-E in wehrlites. Enstatite showed two types of LPO (types BC and AC), and diopside showed four different types of LPO. Observations of strong LPOs and numerous dislocations in olivine suggest that samples showing both type-B and -E LPOs were deformed in dislocation creep. The seismic anisotropy of the P-wave was in the range of 2.2–11.6% for olivine, 1.2–2.3% for enstatite, and 2.1–6.4% for diopside. The maximum seismic anisotropy of the shear wave was in the range 1.93–7.53% for olivine, 1.53–2.46% for enstatite, and 1.81–6.57% for diopside. Furthermore, the thickness of the anisotropic layer was calculated for four geodynamic models to understand the origin of seismic anisotropy under the study area by using delay time from shear wave splitting, and S-wave velocity and anisotropy from mineral LPOs. We suggest that the seismic anisotropy under the study area can be most likely explained by two deformation modes that might have occurred at different times: one of deformed lherzolites with a type-B olivine LPO by lateral shear during/after the period of the Mesozoic continental collision between the Sino-Korean and South China cratons; and the other deformed the wehrlites with a type-E olivine LPO by horizontal extension during the period of change in absolute plate motion in relation to the westward-subducting Pacific plate.

Amphibole fabric formation during diffusion creep and the rheology of shear zones

Research Article| June 01, 2014 Amphibole fabric formation during diffusion creep and the rheology of shear zones A.J. Getsinger; A.J. Getsinger * Department of Geological Sciences, Brown University, Box 1846, 324 Brook Street, Providence, Rhode Island 02912, USA *E-mails: Amanda_Getsinger@brown.edu; Greg_Hirth@brown.edu. Search for other works by this author on: GSW Google Scholar G. Hirth G. Hirth * Department of Geological Sciences, Brown University, Box 1846, 324 Brook Street, Providence, Rhode Island 02912, USA *E-mails: Amanda_Getsinger@brown.edu; Greg_Hirth@brown.edu. Search for other works by this author on: GSW Google Scholar Geology (2014) 42 (6): 535–538. https://doi.org/10.1130/G35327.1 Article history received: 24 Nov 2013 rev-recd: 19 Mar 2014 accepted: 21 Mar 2014 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A.J. Getsinger, G. Hirth; Amphibole fabric formation during diffusion creep and the rheology of shear zones. Geology 2014;; 42 (6): 535–538. doi: https://doi.org/10.1130/G35327.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We conducted experiments to examine the effect of amphibole-forming hydration reactions on mineral fabric development and the strength of mafic rocks. Both hydrostatic and general shear deformation experiments were conducted on powdered basalt with added water at lower continental crust conditions (800 °C, 1 GPa). Amphibole that formed under hydrostatic conditions exhibits a random lattice-preferred orientation (LPO). In contrast, amphibole formed during deformation exhibits both a strong shape-preferred orientation (SPO) and LPO with a [001] maximum aligned subparallel to the shear direction. Plagioclase in both hydrostatic and deformed samples shows a very weak to random LPO. At low effective strain rates (10−5 s−1 to 10−6 s−1), the stress exponent is ≈1–1.5, consistent with deformation accommodated by diffusion creep. The correlation of the SPO and LPO coupled with the rheological evidence for diffusion creep indicates that the amphibole fabric results from oriented grain growth and rigid grain rotation during deformation. The experimentally produced fabrics are strikingly similar to those observed in amphibolite-grade natural shear zones, supporting interpretations that such rocks deform by diffusion creep. In addition, the rheology of the fine-grained experimental amphibolite is comparable to that predicted using flow laws for wet anorthite. Thus, both our experiments and field analyses indicate that wet plagioclase rheology provides a good constraint on the strength of hydrated lower crust. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Internal structure of basalt flows: insights from magnetic and crystallographic fabrics of the La Palisse volcanics, French Massif Central

We present a new interpretation of anisotropy of magnetic susceptibility (AMS) fabrics in basaltic lava flows based on the detailed study of magnetic mineralogy and silicate crystallographic fabric of a Quaternary lava flow from the French Massif Central (La Palisse). We consider the model of AMS fabric imbrication between magnetic foliation and flow surface, as initially proposed for dykes. At the two sampling sites, the concordance between the flow direction deduced from the AMS foliation and that deduced from field observations indicates that the imbrication model could apply to the lava flows. However, the flow senses inferred fromAMSare systematically opposed between the two sampling sites suggesting permutations between K1 and K3 AMS axes, a configuration referred to as inverse fabric. Electron backscatter diffraction (EBSD) measurements show strong lattice-preferred orientations (LPO) for plagioclase, especially the (010) plagioclase plane, which tends to be parallel to the flow. Clinopyroxene LPO remains less marked than plagioclase LPO, whereas titanomagnetite does not display a significant LPO. Comparison between magnetic and crystallographic fabrics suggests that the AMS fabric of the lava flow results from the distribution of titanomagnetite grains, which is in turn controlled by the fabric of the silicate framework. Magnetic hysteresis parameters and anisotropy of remanent magnetization (ARM) measurements exclude a significant contribution from single-domain grains, often called upon to explain inverse magnetic fabrics. The origin of the observed inverse magnetic fabric may relate to the dip of the palaeosurface, which is the only remarkable difference between the two sampling sites. AMS appears as a good tool to determine the direction of basaltic lava flows and coupling with local crystallographic fabric data provides a valuable control of relationships between magnetic fabrics and flow and thus contributes to better constrain the AMS signature of lava flows.

Quantitative texture analysis from powder-like electron diffraction data

The textures of an Al thin film and of α-MnS nanocrystals deposited on a carbon film grid have been analysed using powder electron diffraction. For each sample a series of powder electron diffraction patterns tilted with respect to two orthogonal axes were collected, to adapt to this type of data the texture analysis procedures commonly used in synchrotron X-ray transmission geometry. Both pattern sets have been analysed with the Rietveld procedure embedded in the softwareMAUD. The fit is satisfactory with agreement factors of 7.03% for the Al film and 3.42% for α-MnS and reveals in both cases a (111) preferred orientation with a pronounced cylindrical symmetry. The (111) and (100) pole figures, plotted in terms of multiples of random distribution (m.r.d.), show a fairly strong lattice preferred orientation in the Al thin film and a stronger one in the deposited α-MnS nanocrystals, with maxima, for the (111) pole figures, of 8.8 and 19.7 m.r.d., respectively.

Deformation mechanisms of plagioclase and seismic anisotropy of the Acebuches metabasites (SW Iberian massif)

Abstract Samples of the Acebuches metabasites (SW Spain), deformed under low-pressure/medium-to-high temperature metamorphic conditions, have been analysed via electron backscattered diffraction (EBSD) to obtain their plagioclase crystal lattice preferred orientations (LPO). Plagioclases from the highest temperature amphibolites show moderate LPO and a good correlation between 180° misorientation angles and both the crystal and the kinematic coordinate systems, which is attributed to dislocation glide accommodated by mechanical albite+pericline twinning. Plagioclases from medium-temperature amphibolites exhibit well-developed LPO, suggesting that dislocation creep was active during plagioclase deformation. Plagioclases from the more intensively deformed mafic schists exhibit weak LPO, indicating the activity of LPO-destroying deformation mechanisms. Evidence points to grain-boundary sliding accompanied by limited fracturing. The observed LPO are characterized by the alignment of [100] parallel to the kinematic X -direction. This association suggests that [100] was the preferential slip direction during dislocation creep of plagioclase, with (010) and/or (001) appearing to have acted as the dominant slip planes. The observed plagioclase LPO is combined with hornblende LPO to define the seismic fabric of the Acebuches metabasites. In samples with strong plagioclase LPO, the resulting seismic fabrics are highly influenced by this phase.

Origin of felsic granulite microstructure by heterogeneous decomposition of alkali feldspar and extreme weakening of orogenic lower crust during the Variscan orogeny

AbstractThis study answers the question of origin and evolution of a granulitic microstructure typically developed in felsic granulites of the European Variscan belt. It shows that the precursor of the Variscan felsic granulites was a high‐pressure alkali feldspar‐rich coarse‐grained layered orthogneiss. Its S1 subhorizontal layering is defined by the alignment of alkali feldspar porphyroclasts alternating with monomineralic bands of quartz and bands rich in plagioclase and garnet. The alkali feldspar porphyroclasts contain inclusions of quartz, garnet, kyanite, biotite and rutile, reflecting peak P–T conditions of 1.6–1.8 GPa and 850 °C during S1 formation. Superimposed steep folds and steep cleavage, S2, are associated with recrystallization of alkali feldspar, plagioclase and quartz, and garnet chemistry modifications that correspond to 0.9–1.0 GPa and 800 °C. During exhumation, involving 0.8 GPa decompression and cooling, the probably perthitic alkali feldspar underwent an unusual process of heterogeneous decomposition along irregular reaction fronts forming a fine‐grained matrix composed of plagioclase and K‐feldspar grains. Regular grain distributions in the matrix, nucleation‐dominated crystal size distribution and preservation of lattice orientation of the parental perthite crystals are all explained by a discontinuous precipitation process. This heterogeneous decomposition of alkali feldspar solid solution is controlled by chemically and strain induced grain‐boundary migration. During exhumation and decompression, the fine‐grained matrix underwent viscous deformation, forming the typical microstructure of the Variscan granulites. Random phase distributions, minor coarsening and feldspar textures are interpreted as a result of strain softening due to diffusion creep‐accommodated grain‐boundary sliding. Subordinate large quartz ribbons were rheologically stronger than the feldspar‐dominated matrix due to the activity of different deformational mechanisms. Finally, in mid‐crustal levels, the subvertical structure was overprinted by a perpendicular steep fabric associated with the growth of sillimanite, heterogeneous hydration and local partial melting, development of aggregate phase distributions and significant coarsening. This evolution is accompanied with the development of a strong lattice preferred orientation of quartz, K‐feldspar and plagioclase, reflecting a switch to dislocation creep mechanism and a general hardening of the granulites under amphibolite facies conditions.

Microfabric characteristics and rheological significance of ultra‐high‐pressure metamorphosed jadeite‐quartzite and eclogite from Shuanghe, Dabie Mountains, China

AbstractQuantitative analysis of the structural evolution of jadeite‐quartzite, a rare ultra‐high pressure (UHP) rock type from the Dabie Mountains of eastern China, sheds light on the formation and evolution of UHP orogenic belts worldwide. Geological mapping of the Shuanghe area, where jadeite‐quartzites crop out, was carried out to determine the spatial relationships between different UHP rocks within this orogen. The deformation mechanisms of jadeite‐quartzite, geodynamical parameters (stress, strain, strain rate), and microstructure including lattice preferred orientation (LPO) were determined from six jadeite‐quartzite samples from the Shuanghe area. LPOs of clinopyroxene (jadeite and omphacite), garnet, rutile and quartz from these jadeite‐quartzite samples are compared with those of three eclogites preserving different degrees of deformation from the Shuanghe area. Microstructural LPOs of jadeite, omphacite, garnet, rutile and quartz were determined using electron backscattered diffraction (EBSD) analysis. Quartz fabrics were largely recrystallized during late, low‐grade stages of deformation, whereas garnet shows no strong LPO patterns. Rutile fabrics show a weak LS fabric along [001]. Jadeite and omphacite show the strongest eclogite facies LPO patterns, suggesting that they may provide important information about mantle deformation patterns and control the rheology of deeply subducted continental crust. Microstructural data show that the jadeite LPO patterns are similar to those of omphacite and vary between L‐ and S‐types, which correlate with prolate and oblate grain shape fabrics (SPO); quartz LPOs are monoclinic. Microstructural analysis using TEM shows that the dominant slip systems of jadeite in one sample are (100)[001], (110)[001] and (1 1 0)1/2[110], while in another sample, no dislocations are observed. Abundant dislocations in quartz were accommodated by the dominant slip system (0001)[110], indicating basal glide and represents regional shearing during the exhumation process. This suggests that dislocation creep is the dominant fundamental deformation mechanism in jadeite under UHP conditions. The protoliths of jadeite‐quartzite, metasedimentary rocks from the northern passive continental margin of the Yangtze craton, experienced the same deep subduction and were deformed under similar rheological conditions as other UHP eclogite, marble and paragneiss. Experimental UHP deformation of quartzo‐feldspathic gneiss with a chemical composition similar to the bulk continental crust has shown that the formation of a jadeite–stishovite rock is associated with a density increase of the host rock similar to the eclogite conversion from basaltic protoliths. The resulting rock can be denser than the surrounding mantle pyrolite up to depths of 660 km (24 GPa). Thus, processes of deep continental subduction may be better‐understood through understanding the rheology and mechanical behaviour of jadeite. Jadeite‐quartzites such as those from the Shuanghe may be exhumed remnants of deeply‐subducted slabs of continental crust, other parts of which subducted past the ‘depth of no return’, and remain in the deep mantle.

Deformation microstructures of olivine in peridotite from Spitsbergen, Svalbard and implications for seismic anisotropy

AbstractTo understand the deformation mechanism and seismic anisotropy in the uppermost mantle beneath Spitsbergen, Svalbard, in the Arctic, the deformation microstructures of olivine in the peridotite of Spitsbergen were studied. Seismic anisotropy in the upper mantle can be explained mainly by the lattice‐preferred orientation (LPO) of olivine. The LPOs of the olivine in the peridotites were determined using electron backscattered diffraction patterns. Eight specimens out of 10 showed that the [100] axis of the olivine was aligned subparallel to the lineation and that the (010) plane was subparallel to the foliation, showing a type A LPO. In the other two specimens the [100] axis of olivine was aligned subparallel to the lineation and both the [010] and [001] axes were distributed in a girdle nearly perpendicular to the lineation, showing a type D LPO. The dislocation density of the olivine in the samples showing a type D LPO was higher than that in the samples showing a type A LPO. The result of an Fourier transformation infrared study showed that both the types A and D samples were dry. These observations were in good agreement with a previous experimental study (Tectonophysics, 421, 2006, 1): samples showing a type D LPO for olivine were observed at a high stress condition and samples showing both types A and D LPO were deformed under dry condition. Observations of both strong LPOs and dislocations of olivine indicate that the peridotites studied were deformed by dislocation creep. The seismic anisotropy calculated from the LPOs of the olivine could be used to explain the seismic anisotropy of P‐ and S‐waves in the lithospheric mantle beneath Spitsbergen, Svalbard.