Lattice Preferred Orientation Research Articles

Lattice-Preferred Orientation and Seismic Anisotropy of Minerals in Retrograded Eclogites from Xitieshan, Northwestern China, and Implications for Seismic Reflectance of Rocks in the Subduction Zone

Various rock phases, including those in subducting slabs, impact seismic anisotropy in subduction zones. The seismic velocity and anisotropy of rocks are strongly affected by the lattice-preferred orientation (LPO) of minerals; this was measured in retrograded eclogites from Xitieshan, northwest China, to understand the seismic velocity, anisotropy, and seismic reflectance of the upper part of the subducting slab. For omphacite, an S-type LPO was observed in three samples. For amphibole, the <001> axes were aligned subparallel to the lineation, and the (010) poles were aligned subnormal to foliation. The LPOs of amphibole and omphacite were similar in most samples. The misorientation angle between amphibole and neighboring omphacite was small, and a lack of intracrystalline deformation features was observed in the amphibole. This indicates that the LPO of amphibole was formed by the topotactic growth of amphibole during retrogression of eclogites. The P-wave anisotropy of amphibole in retrograded eclogites was large (approximately 3.7–7.3%). The seismic properties of retrograded eclogites and amphibole were similar, indicating that the seismic properties of retrograded eclogites are strongly affected by the amphibole LPO. The contact boundary between serpentinized peridotites and retrograded eclogites showed a high reflection coefficient, indicating that a reflected seismic wave can be easily detected at this boundary.

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.

Deformation Microstructures of Phyllite in Gunsan, Korea, and Implications for Seismic Anisotropy in Continental Crust

Muscovite is a major constituent mineral in the continental crust that exhibits very strong seismic anisotropy. Muscovite alignment in rocks can significantly affect the magnitude and symmetry of seismic anisotropy. In this study, deformation microstructures of muscovite-quartz phyllites from the Geumseongri Formation in Gunsan, Korea, were studied to investigate the relationship between muscovite and chlorite fabrics in strongly deformed rocks and the seismic anisotropy observed in the continental crust. The [001] axes of muscovite and chlorite were strongly aligned subnormal to the foliation, while the [100] and [010] axes were aligned subparallel to the foliation. The distribution of quartz c-axes indicates activation of the basal<a>, rhomb<a> and prism<a> slip systems. For albite, most samples showed (001) or (010) poles aligned subnormal to the foliation. The calculated seismic anisotropies based on the lattice preferred orientation and modal compositions were in the range of 9.0–21.7% for the P-wave anisotropy and 9.6–24.2% for the maximum S-wave anisotropy. Our results indicate that the modal composition and alignment of muscovite and chlorite significantly affect the magnitude and symmetry of seismic anisotropy. It was found that the coexistence of muscovite and chlorite contributes to seismic anisotropy constructively when their [001] axes are aligned in the same direction.

Seismic imaging of Northwest Pacific and East Asia: New insight into volcanism, seismogenesis and geodynamics

Recent studies on seismic imaging of the Northwest Pacific and East Asian region are reviewed. High-resolution tomographic images reveal significant lateral heterogeneities in the crust and upper mantle, which shed new light on interplate and intraplate volcanism, earthquake mechanism, and mantle dynamics. Significant recent advances in seismic imaging are tomographic inversions for 3-D distribution of seismic anisotropy and attenuation in the crust and upper mantle, which provide important new information on lithospheric deformation, mantle convection, melts and fluids associated with plate subductions and mantle dynamics. The results show that intraplate volcanism in NE Asia is caused by hot and wet upwelling flows in the big mantle wedge above the stagnant Pacific slab in the mantle transition zone. The subducting Pacific and Philippine Sea slabs exhibit mainly trench-parallel fast-velocity directions (FVDs) of azimuthal anisotropy, which may reflect frozen-in lattice-preferred orientation of aligned anisotropic minerals and/or shape-preferred orientation, such as transform faults formed at the mid-ocean ridge and normal faults produced at the outer-rise area near the trench axis. Trench-normal FVDs are generally revealed in the mantle wedge under the volcanic front and back-arc region, which may reflect corner flows in the mantle wedge due to the plate subduction and dehydration. Trench-normal FVDs also appear in the subslab mantle, which may reflect asthenospheric shear deformation associated with the overlying slab subduction. The nucleation of large earthquakes, including both crustal and megathrust events, is controlled by structural heterogeneities in and around seismogenic faults. The cause of deep-focus earthquakes is a subject of ongoing debate. A recent example is the 2015 Bonin deep earthquake (M 7.9, ~670 km depth), which was probably caused by joint effects of several factors, including the Pacific slab's fast deep subduction, slab tearing and thermal variation, stress changes and phase transformations in the slab, and complex interactions between the slab and ambient mantle. The key to future advances in seismic imaging is instrumentation. Gradual deployment of seismometers on seafloors and in those less instrumented land areas will be the most important task for seismologists from now. For this purpose, international cooperation of seismologists in different countries will be necessary

Characterizing syn-convergent extension along the Neybaz-Chatak detachment shear zone, Central Iran: Insights from microstructures, quartz petrofabrics and flow vorticity analysis

Integrated microstructural, quartz petrofabric and flow vorticity analyses on mylonitic rocks in the lower plate of the Neybaz Chatak Detachment Shear Zone provide significant information regarding the kinematic characteristics coeval with the Chapedony metamorphic core complex development within the Central East Iranian Microcontinent. Several kinematic shear sense indicators including quartz c-axis fabrics, S–C fabrics, rotated feldspar and mica fish reveal that the detachment shear zone has experienced a consistent top-to-the NE-directed progressive shear deformation. Detailed investigations on recrystallization types of quartz and feldspar in the mylonites demonstrate that the deformation occurred under the amphibolite facies condition corresponding to mid-crustal levels. A combination of investigations on mineral assemblages, microstructures of quartz and feldspar, and quartz lattice preferred orientations (LPOs) show that deformation temperatures ranged from 400 to 645 °C that were varied across a distance of ~21 km in the extension direction (NE-SW). The western and eastern parts of the study area experienced lower temperatures compared to the central parts. The gneissic mylonite record sub simple shear deformation (Wm = 0.56–0.85, 64–36%) associated with an increasing of pure shear component. These data highlight that the metamorphic rocks of the study underwent a non-coaxial shear with ductile low-angle normal faulting, resulting in subvertical thinning in the extensional deformation regime responsible for formation of the core complex.

Deposition of cobalt oxide films by reactive pulsed magnetron sputtering

Cobalt oxide films were deposited with the help of reactive high power impulse magnetron sputtering (HiPIMS) and mid-frequency pulsed magnetron sputtering (PMS) in argon gas at an argon gas pressure of 1 Pa and with different oxygen admixtures. The HiPIMS discharge was operated at a repetition frequency f=100 Hz with a duty cycle of 1%. Deposition rate and ion fraction were measured. The intensity of plasma ions, in particular Co+ and O+, is enhanced during HiPIMS compared to PMS and extends to larger ion energies. Films deposited on glass substrates were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and X-ray diffraction. Crystal structure and electrical resistivity of as-deposited films were found to depend on the deposition conditions. Deposited films are compatible with a non-stoichiometric Co3O4 spinel type crystal structure with unoccupied cobalt sites. PMS produces films with a preferred lattice orientation. HiPIMS show a large internal stress which relaxes during annealing. Electrical resistivity is several orders of magnitude smaller for films deposited by HiPIMS compared to PMS.

Stagnant forearc mantle wedge inferred from mapping of shear-wave anisotropy using S-net seafloor seismometers

Shear-wave anisotropy in Earth’s mantle helps constrain the lattice-preferred orientation of anisotropic minerals due to viscous flow. Previous studies at the Japan Trench subduction zone using land-based seismic networks identified strong anisotropy in the mantle wedge, reflecting viscous flow induced by the subducting slab. Here we map anisotropy in the previously uninvestigated offshore region by analyzing shear waves from interplate earthquakes that are recorded by a new seafloor network (the S-net). The newly detected anisotropy is not in the mantle wedge but only in the overlying crust (∼0.1 s time delay and trench-parallel fast direction). The distinct lack of anisotropy indicates that the forearc mantle wedge offshore is decoupled from the slab and does not participate in the viscous flow, in sharp contrast with the rest of the mantle wedge. A stagnant forearc mantle wedge provides a stable and cold tectonic environment that is important for the petrological evolution and earthquake processes of subduction zones.

Evolution of Deformation Fabrics Related to Petrogenesis of Upper Mantle Xenoliths Beneath the Baekdusan Volcano

Knowledge of the formation and evolution of cratonic subcontinental lithospheric mantle is critical to our understanding of the processes responsible for continental development. Here, we report the deformation microstructures and lattice preferred orientations (LPOs) of olivine and pyroxenes alongside petrological data from spinel peridotite xenoliths beneath the Baekdusan volcano. We have used these datasets to constrain the evolution of deformation fabrics related to petrogenesis from the Baekdusan peridotites. Based on petrographic features and deformation microstructures, we have identified two textural categories for these peridotites: coarse- and fine-granular harzburgites (CG and FG Hzb). We found that mineral composition, equilibrium temperature, olivine LPO, stress, and extraction depth vary considerably with the texture. We suggest that the A-type olivine LPO in the CG Hzb may be related to the preexisting Archean cratonic mantle fabric (i.e., old frozen LPO) formed under high-temperature, low-stress, and dry conditions. Conversely, we suggest that the D-type olivine LPOs in the FG Hzb samples likely originated from later localized deformation events under low-temperature, high-stress, and dry conditions after a high degree of partial melting. Moreover, we consider the Baekdusan peridotite xenoliths to have been derived from a compositionally and texturally heterogeneous vertical mantle section beneath the Baekdusan volcano.

Lattice Preferred Orientation and Deformation Microstructures of Glaucophane and Epidote in Experimentally Deformed Epidote Blueschist at High Pressure

To understand the lattice preferred orientation (LPO) and deformation microstructures at the top of a subducting slab in a warm subduction zone, deformation experiments of epidote blueschist were conducted in simple shear under high pressure (0.9–1.5 GPa) and temperature (400–500 °C). At low shear strain (γ ≤ 1), the [001] axes of glaucophane were in subparallel alignment with the shear direction, and the (010) poles were subnormally aligned with the shear plane. At high shear strain (γ > 2), the [001] axes of glaucophane were in subparallel alignment with the shear direction, and the [100] axes were subnormally aligned with the shear plane. At a shear strain between 2< γ <4, the (010) poles of epidote were in subparallel alignment with the shear direction, and the [100] axes were subnormally aligned with the shear plane. At a shear strain where γ > 4, the alignment of the (010) epidote poles had altered from subparallel to subnormal to the shear plane, while the [001] axes were in subparallel alignment with the shear direction. The experimental results indicate that the magnitude of shear strain and rheological contrast between component minerals plays an important role in the formation of LPOs for glaucophane and epidote.

Deformation fabrics and strain localization mechanisms in graphitic carbon-bearing rocks from the Ailaoshan-Red River strike-slip fault zone

Graphitic carbon-bearing rocks generally occur in low-to high-grade metamorphic units. In many brittle faults, graphitic carbon is often associated with gouge or low-grade metamorphic rocks whereas in ductile faults, graphitic carbon commonly occurs in marble, schist or gneiss. Carbonaceous material gradually transforms from an amorphous into an ordered crystalline structure by increasing thermal metamorphism. The degree of graphitization is believed to be a reliable indicator of peak temperature conditions in the metamorphic rock. In this contribution, based on detailed field observations, the variably deformed and metamorphosed graphitic gneisses to phyllites, located within the footwall and hangingwall unit of the Cenozoic Ailaoshan-Red River strike-slip shear zone are studied. According to lithological features and temperatures determined by Raman spectra of carbonaceous material, these graphitic rocks and deformation fabrics are divided into three types. Type I is represented by medium-grade metamorphism and strongly deformed rocks with an average temperature of 509 °C and a maximum temperature of 604 °C. Type II is affected by low-grade metamorphism and deformed rocks with an average temperature of 420 °C. Type III is affected by lower-grade metamorphism and occurs in weakly deformed/undeformed rocks with an average temperature of 350 °C. Slip-localized micro-shear zones and laterally continuous or discontinuous slip-planes constituted by graphitic carbon aggregates are developed in Types I and II. The electron back-scattered diffraction (EBSD) lattice preferred orientation (LPO) patterns of graphitic carbon grains were firstly observed in comparison with LPO patterns of quartz and switch from basal <a>, rhomb <a> to prism<a> slip systems, which indicate increasing deformation temperatures. Comparison of quartz and graphite LPOs indicates that graphite LPOs show higher temperature conditions. According to the graphitic slip-planes, micro-shear zones with grain-size reduction and mylonitic foliation constituted by graphitic carbon minerals, we also propose that the development of fine-grained amorphous carbon plays an important role in rheological weakening of the whole rock during progressive ductile shearing.

Intracrystalline vorticity record of flow kinematics during shear zone reactivation

The vorticity axis of deformation of a shear zone is a rotational axis controlled by the relative movement of its boundaries. When characterized, it is a reliable tool to determine the ductile flow geometry (monoclinic general, monoclinic transpressive, or triclinic) and an asset to identify the kinematics of complex oblique collisions. Advancements in analysis of electron backscattered diffraction (EBSD) maps now permit inference of vorticity axes – known as crystallographic vorticity axes (CVAs) – from the intragranular lattice orientation dispersion of strain-bearing minerals. However, natural shear zones may preserve a record of multiple shear events with contrasting kinematics and it is not understood to which extent the quartz lattice may preserve a kinematic record through multiple deformation events. Herein, we characterize the finite flow kinematic record produced during reactivation of the Eastern Highlands shear zone (EHSZ), in an oblique collision within the Canadian Appalachians. We integrate field and thin section observations, quartz lattice preferred orientations, EBSD maps, and grain classification, leading to a new method to determine and compare CVA within relict and recrystallized quartz grains. Using this method, we were able to conclude that the EHSZ was reactivated in an heterogenous monoclinic flow environment.

Two-dimensional MoSe2/graphene heterostructure thin film with wafer-scale continuity via van der Waals epitaxy

Two-dimensional MoSe2/graphene heterostructure in a wafer-scale was realized by a new method involves physical vapor deposition (PVD), in which the MoSe2 film was vertically stacked on graphene by evaporating the MoSe2 compounds using an electron gun evaporator. A preferred lattice orientation with the underlying graphene was identified by transmission electron microscopy, revealing the van der Waals epitaxy (VdWs) of MoSe2. The interface interaction between the graphene substrate and the MoSe2 epilayer was investigated via Raman spectroscopy. The growth technique provides a facile route to create large area VdWs heterostructures, which can be developed for future mass production of nano-devices.

Lattice preferred orientation of stishovite deformed at high pressure and high temperature

In the lower part of the mantle transition zone, seismic polarization anisotropies, which could be caused by the lattice preferred orientation (LPO) of elastically anisotropic minerals, were observed related with subducted materials. Stishovite, which is one of the dominant minerals in mid-ocean ridge basalt, continental crust and sediment layers, possesses significant anisotropic elasticity and has high potential to contribute to the observed seismic anisotropy. In this study, three types of deformation experiments (uniaxial compression, uniaxial tension and shear) on polycrystalline stishovite were conducted using either a D-DIA apparatus or a D-111 type apparatus with a Kawai-type cell assembly to investigate the LPO of stishovite developed during deformation at pressure of 12 GPa and temperatures of 1600 and 1800 °C. The obtained LPO patterns in all experiments are consistent each other and they suggested the dominant slip is in the [001] direction dispersed on the {110} and {100} planes with [001]{110} slightly more dominant, thus we concluded that the dominant slip system is [001]{hk0}. Based on the present results, the fast vertically and horizontally polarized shear wave anisotropy observed in the lower part of the mantle transition zone could be explained by the LPO of stishovite with vertical and horizontal flow, respectively, related with subducted materials.

The relationship between ductile shear zone and mineralization in the Jiufeng Sn deposit, northern Guangxi, South China: Evidence from structural analysis and cassiterite U-Pb dating

The Jiufeng Sn deposit is located in the outer contact zone southwest of the Neoproterozoic Motianling granitic pluton, in northern Guangxi, South China. The deposit is characterized by a NNE-striking, 30–200-m-wide ductile shear zone, named the Jiufeng ductile shear zone. Majority of the Sn orebodies, including four large ones, are hosted by the shear zone. Field and microstructural observations, electron backscatter diffraction measurements, and magnetic fabric analyses show that the shear zone is characterized by sinistral thrust shearing. The shear zone has also slightly modified the Sn orebodies. Quartz in the shear zone exhibits core-mantle structure and undulose extinction. Quartz has experienced significant sub-grain rotation and grain-boundary migration recrystallization. Lattice preferred orientation analysis reveals primarily basal <a>, rhomb <a>, and prism <a> slips. Cassiterite grains in the shear zone, however, demonstrate brittle deformation. Therefore, the deformation temperature during the ductile shearing is estimated lower than 650 °C and below U-Pb closure temperature of cassiterite. As such, the U-Pb isotopic composition of the cassiterite was not disturbed by the shearing and the U-Pb age of the cassiterite could be used for the age of the Sn mineralization. LA-ICP-MS U-Pb dating of cassiterite grains selected from a cassiterite-bearing tourmalinite sample performed in this study yielded a Tera–Wasserburg U-Pb lower intercept age of 832.3 ± 5.3 Ma. This age is consistent within error with that of the neighboring Neoproterozoic Motianling granitic pluton, which suggests a close genetic relationship between the Sn mineralization and the granitic magmatism. Studies show that the Jiufeng ductile shear zone was produced by a widespread Caledonian (or Kwangsian) tectonic event in South China, indicating that the shearing was much later than the Sn mineralization and has no generic relationship to the Sn deposit.

Strain-Induced Fabric Transition of Chlorite and Implications for Seismic Anisotropy in Subduction Zones

Seismic anisotropy of S-wave, trench-parallel or trench-normal polarization direction of fast S-wave, has been observed in the fore-arc and back-arc regions of subduction zones. Lattice preferred orientation (LPO) of elastically anisotropic chlorite has been suggested as one of the major causes of seismic anisotropy in subduction zones. However, there are two different LPOs of chlorite reported based on the previous studies of natural chlorite peridotites, which can produce different expression of seismic anisotropy. The mechanism for causing the two different LPOs of chlorite is not known. Therefore, we conducted deformation experiments of chlorite peridotite under high pressure–temperature conditions (P = 0.5–2.5 GPa, T = 540–720 °C). We found that two different chlorite LPOs were developed depending on the magnitude of shear strain. The type-1 chlorite LPO is characterized by the [001] axes aligned subnormal to the shear plane, and the type-2 chlorite LPO is characterized by a girdle distribution of the [001] axes subnormal to the shear direction. The type-1 chlorite LPO developed under low shear strain (γ ≤ 3.1 ± 0.3), producing trench-parallel seismic anisotropy. The type-2 chlorite LPO developed under high shear strain (γ ≥ 5.1 ± 1.5), producing trench-normal seismic anisotropy. The anisotropy of S-wave velocity (AVs) of chlorite was very strong up to AVs = 48.7% so that anomalous seismic anisotropy in subduction zones can be influenced by the chlorite LPOs.

Seismic evidence for subduction-induced mantle flows underneath Middle America

Laboratory experiments and geodynamic simulations demonstrate that poloidal- and toroidal-mode mantle flows develop around subduction zones. Here, we use a new 3-D azimuthal anisotropy model constructed by full waveform inversion, to infer deep subduction-induced mantle flows underneath Middle America. At depths shallower than 150 km, poloidal-mode flow is perpendicular to the trajectory of the Middle American Trench. From 300 to 450 km depth, return flows surround the edges of the Rivera and Atlantic slabs, while escape flows are inferred through slab windows beneath Panama and central Mexico. Furthermore, at 700 km depth, the study region is dominated by the Farallon anomaly, with fast axes perpendicular to its strike, suggesting the development of lattice-preferred orientations by substantial stress. These observations provide depth-dependent seismic anisotropy for future mantle flow simulations, and call for further investigations about the deformation mechanisms and elasticity of minerals in the transition zone and uppermost lower mantle.

Analysis of electron backscattered diffraction (EBSD) mapping of geological materials: precautions for reliably collecting and interpreting data on petro-fabric and seismic anisotropy

Automated electron backscattered diffraction (EBSD) data yield abundant information on the lattice-preferred orientations (LPOs) and deformation microstructures and mechanisms of minerals in a rock, which aid in our understanding of the tectonic conditions and history of a region. Additionally, this information allows us to interpret seismic anisotropies in the crust and mantle. However, great care must be taken in the collection and production of crystallographic orientation data via automated EBSD to more precisely interpret LPOs, deformation microstructures, and seismic anisotropies because petro-fabrics can be different depending on sampling methods such as all-points-per-grain (whole) data and one-point-per-grain (one orientation per grain) data. Here, we report a detailed comparison of the crystallographic orientation data produced using both all-points-per-grain and one-point-per-grain techniques to analyze eclogites from the Yuka terrane in the North Qaidam ultrahigh-pressure metamorphic belt of northwestern China. By comparing eclogite crystallographic orientations between different sampling methods, we found that there was no marked difference in the LPOs, fabric strengths (J-index and M-index), and seismic anisotropies for relatively small mineral grains (e.g., garnet and omphacite). However, there were large differences in the LPOs, fabric strengths, and seismic anisotropies of relatively large minerals grains (e.g., amphibole). This result can be attributed to the acquisition method of crystallographic orientation data, which can influence the LPO, fabric strength, and seismic anisotropy of minerals and rocks. Therefore, the pole figures of minerals should be constructed from one point per grain data in order to avoid the oversampling of large grains for samples with highly heterogenous grain size distributions, as well as to permit the comparison of crystallographic orientations obtained using different tools and in other studies.

Microstructural Evolution of Amphibole Peridotites in Åheim, Norway, and the Implications for Seismic Anisotropy in the Mantle Wedge

The microstructure of amphibole peridotites from Åheim, Norway were analyzed to understand the evolution of the lattice-preferred orientation (LPO) of olivine throughout the Scandian Orogeny and its implication for the seismic anisotropy of the subduction zone. The Åheim peridotites had a porphyroclastic texture and some samples contained an abundant amount of hydrous minerals such as tremolite. Detailed microstructural analysis on the Åheim peridotites revealed multiple stages of deformation. The coarse grains showed an A-type LPO of olivine, which can be interpreted as the initial stage of deformation. The spinel-bearing samples showed a mixture of B-type and C-type LPOs of olivine, which is considered to represent the deformation under water-rich conditions. The recrystallized fine-grained olivine displays a B-type LPO, which can be interpreted as the final stage of deformation. Microstructures and water content of olivine indicate that the dominant deformation mechanism of olivine showing a B-type LPO is a dislocation creep under water-rich condition. The observation of the B-type LPO of olivine is important for an interpretation of trench-parallel seismic anisotropy in the mantle wedge. The calculated seismic anisotropy of the tremolite showed that tremolite can contribute to the trench-parallel seismic anisotropy in the mantle wedge.

A potential post-perovskite province in D″ beneath the Eastern Pacific: evidence from new analysis of discrepant SKS–SKKS shear-wave splitting

SUMMARY Observations of seismic anisotropy in the lowermost mantle—D″—are abundant. As seismic anisotropy is known to develop as a response to plastic flow in the mantle, constraining lowermost mantle anisotropy allows us to better understand mantle dynamics. Measuring shear-wave splitting in body wave phases which traverse the lowermost mantle is a powerful tool to constrain this anisotropy. Isolating a signal from lowermost mantle anisotropy requires the use of multiple shear-wave phases, such as SKS and SKKS. These phases can also be used to constrain azimuthal anisotropy in D″: the ray paths of SKS and SKKS are nearly coincident in the upper mantle but diverge significantly at the core–mantle boundary. Any significant discrepancy in the shear-wave splitting measured for each phase can be ascribed to anisotropy in D″. We search for statistically significant discrepancies in shear-wave splitting measured for a data set of 420 SKS–SKKS event–station pairs that sample D″ beneath the Eastern Pacific. To ensure robust results, we develop a new multiparameter approach which combines a measure derived from the eigenvalue minimization approach for measuring shear-wave splitting with an existing splitting intensity method. This combined approach allows for easier automation of discrepant shear-wave splitting analysis. Using this approach we identify 30 SKS–SKKS event–station pairs as discrepant. These predominantly sit along a backazimuth range of 260°–290°. From our results we interpret a region of azimuthal anisotropy in D″ beneath the Eastern Pacific, characterized by null SKS splitting, and mean delay time of $1.15 \, \mathrm{ s}$ in SKKS. These measurements corroborate and expand upon previous observations made using SKS–SKKS and S–ScS phases in this region. Our preferred explanation for this anisotropy is the lattice-preferred orientation of post-perovskite. A plausible mechanism for the deformation causing this anisotropy is the impingement of subducted material from the Farallon slab at the core–mantle boundary.

Phase transition of sanidine (KAlSi3O8) and its effect on electrical conductivity at pressures up to 11 GPa

Sanidine is an important crustal mineral whose high-pressure phase, liebermannite, can be carried down to the mantle transition zone by the subducting slab. Owing to the characteristic channel structure of liebermannite, which can provide a pathway for K+ along the [001] direction, it can serve as a highly conductive phase. In this study, we measured the electrical conductivity of natural sanidine at pressures of up to 11 GPa. The results show that electrical conductivity decreased during the phase transition of sanidine to a mixture of K2Si4O9 wadeite, kyanite, and coesite at 8 GPa, and further to liebermannite at 11 GPa. Accordingly, the activation enthalpy increased from 1.24 to 1.39 eV, and further to 1.47 eV during the phase transitions. Polycrystalline liebermannite with a high concentration of K+ vacancies (35%) did not exhibit highly conductive behavior when the lattice-preferred orientation was not developed. Compared with the theoretically calculated conductivity of liebermannite with a K+ vacancy of 25% along the [001] direction, the single-crystal liebermannite was highly anisotropic (given simply by the difference between logσmax and logσmin), at least higher than 4. Furthermore, the electrical conductivity of polycrystalline liebermannite was approximately one order of magnitude lower than that measured by geophysical observations (0.1 S/m) in the transition zone beneath Northeastern (NE) China and the Philippine Sea, and could not be used to interpret high-conductivity anomalies without a preferred orientation along the [001] direction.