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Pole figures of orthopyroxene (enstatite) and olivine. (a) Representative pole figures of enstatite. Type-AB: sample SVF-04, n =172; Type-AC: sample SVF-49, n =176; Type-BC: sample SVF-30, n =138; Type-ABC: sample SVF-71, n =168. (b) Pole figures of olivine (from Jung et al., 2009a) for the same sample shown in Fig. 4a. Legends are the same as in Fig. 3.
Source publication
Lattice preferred orientation (LPO) and seismic anisotropy of orthopyroxene (enstatite) in mantle xenoliths from Spitsbergen,
Svalbard, near the Arctic, are studied. LPOs of enstatite were determined using electron backscattered diffraction (EBSD).
We found four types of LPOs of orthopyroxene and defined them as type-AC, -AB, -BC, and -ABC. Type-AC...
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Citations
... Lamellae are not bent by subgrain boundaries which implies that mantle cooling occurred during post-kinematic and static conditions. Unlike (mostly) coherent olivine and orthopyroxene CPOs in harzburgites (e.g., [8,80]), in the same research area, olivine and clinopyroxene CPOs were usually incoherent (Table 1, Figure 4b,c). This difference was probably due to the two pyroxenes experiencing respective evolutionary progress, as mentioned in Section 5.1. ...
The microstructural relationship between olivine and clinopyroxene is significant in recovering the mantle evolution under clinopyroxene-saturated melting conditions. This study focuses on olivine/clinopyroxene-related ultramafic rocks (dunite, wehrlite, olivine clinopyroxenite, and clinopyroxenite) in the Ells Stream Complex of the Red Hills Massif. (Olivine) clinopyroxenites have an A/D-type olivine crystallographic preferred orientation (CPO) whereas peridotites have various olivine CPO types. B-type olivine CPO was newly discovered, which may have been generated under hydrous conditions. The discovery of B-type CPO means that all six olivine CPO types could exist in a single research area. Clinopyroxene CPOs also vary and have weaker deformation characteristics (e.g., lower M index and weaker intracrystalline deformation) than olivine; thus, they probably melted and the clinopyroxene-rich ultramafic bands existed as melt veins. Irregular clinopyroxene shapes in the peridotites and incoherent olivine and clinopyroxene CPOs ([100] OL and [001] CPX are not parallel) also indicate a melted state. The dominant orthorhombic and LS-type CPOs in olivine and clinopyroxene imply that simple shear was the main deformation mechanism. Such complicated microstructural characteristics result from the overprinted simple shear under high temperatures (>1000 • C) and hydrous melting environments until the melt-frozen period. This case study is helpful to better understand the olivine and clinopyroxene relationship.
... Seven CPOs of orthopyroxene have been reported: AB, AC, BC, ABC, CA, CB, and BA-types (Jung et al., 2010;Bernard et al., 2021). The nomenclature by Jung et al. (2010) combines the slip plane and slip direction (e.g., AB-type shows a [010](100) slip system; ABC-type shows a slip system with [001] slip direction and girdles of [100] and [010]). ...
... Seven CPOs of orthopyroxene have been reported: AB, AC, BC, ABC, CA, CB, and BA-types (Jung et al., 2010;Bernard et al., 2021). The nomenclature by Jung et al. (2010) combines the slip plane and slip direction (e.g., AB-type shows a [010](100) slip system; ABC-type shows a slip system with [001] slip direction and girdles of [100] and [010]). In Table 1 Locality (structural unit), setting (the Upper/Lower Zone), mineralogy, lithology, texture, and index ratio (raw and processed data) of analyzed samples in this study. ...
... The numbers inside parentheses mean the modal propotions (%) of minerals calculated from reconstracted grain areas (pixcels) in EBSD map. this study, we determined orthopyroxene CPO types following the nomenclature proposed by Jung et al. (2010). ...
... The CPO pattern of pyroxene in sample DMP-8 is somewhat similar to that in sample DMP-3, featured by the [001] and the [100] axes nearly normal and parallel to the foliation plane, respectively ( Figure 6). Additionally, the orthopyroxene CPOs in sample DMP-19 and DMP-24 are close to type-ABC, while that in sample DMP-25 is close to type-BC in Jung et al. (2010). ...
Extensive melt‐peridotite interactions had been documented in the lithospheric mantle beneath the North China Craton (NCC), a prime example of destroyed cratons in the world. Yet the impacts of melt‐peridotite interactions on the deformation and seismic anisotropy of the NCC upper mantle remain unclear. Here we studied in detail the microstructure, crystallographic preferred orientation (CPO) of minerals, and seismic properties of 26 peridotite xenoliths from the Damaping area of the NCC. The studied samples can be classified into two groups: weakly to nonfoliated and strongly foliated. Petrographic and microstructural observations suggest that multiple melt‐peridotite interactions and at least two stages of deformation had influenced samples from both microstructural groups. Dislocation creep in response to a transpression deformation led to the [010]‐fiber type olivine CPOs in most samples. Variable degrees of annealing followed the last stage of deformation. Due to a higher degree of melt‐peridotite interactions, which had promoted nondislocation creep, and more extensive annealing, olivine and pyroxene in the strongly foliated samples developed weaker CPOs. This in turn leads to weaker maximum P wave propagation anisotropy and S wave polarization anisotropy for this microstructural group. Our data, therefore, cast light on a strong control of intensity of melt‐peridotite interactions on deformation and seismic properties of the upper mantle beneath the NCC. If foliation and lineation are vertical and horizontal, respectively, the measured SKS splitting parameters can be well explained by the “fossil” anisotropy frozen in the lithospheric mantle, with no need to invoke asthenospheric flow as a source of the anisotropy.
... 28,[38][39][40] and are usually attributed to the lower finite strains accommodated by stronger pyroxenes compared with weaker olivine crystals 41,42 . The orthopyroxene CPO shows maxima of the [001] and [100] axes aligning subparallel to the lineation and subnormal to the foliation, respectively (Fig. 2b, also named as Type-AC CPO by Jung, et al. 43 . This CPO type of orthopyroxene www.nature.com/scientificreports/ ...
Constraining the stress related to lithospheric deformation in natural rocks is key to develop and test a geodynamic model. However, the cautions of extrapolating piezometers that are established on experimental samples to natural rocks are less addressed. In this study, we investigated the microstructures of a natural harzburgite sample using the electron backscatter diffraction (EBSD) technique. Subgrain boundary (SGB) geometries suggest large percentages of (010)[100] and {0kl}[100] dislocation slip systems in olivines. More importantly, multiple low-angle misorientation boundaries (LAMBs) variants are recognized for the first time in olivine based on their distinctive characteristics with the change of EBSD mapping step size. The LAMBs that exist at a small step size (≤ 1 μm) are mostly equivalent to real SGBs, while other LAMBs that appear only when the step size is larger (> 1 μm) are artificial SGBs. Besides, the former develop mainly in the high LAMB density grains, whereas the latter are mostly found in the low LAMB density grains. This result reinforces the previous knowledge that the stress calculated using subgrain-related piezometers is meaningful only when real SGBs are captured at sufficiently small step size. Furthermore, we provide a proof of concept that SGB density and kernel average misorientation (KAM) are two viable metrics to estimate stress. These two alternative piezometers, which still need calibrations using the experimentally deformed samples, are anticipated to have wide applications in natural rocks.
... nnot establish from the xenoliths alone whether the olivine, orthopyroxene or another reference frame is correct. The one sample with foliation and lineation does not help, as common CPO types are observed for both olivine (A-type) and orthopyroxene (AC-type, (100) [001]) when samples are oriented using foliation and lineation (Jung & Karato, 2001;H. Jung et al., 2010). Shao et al. (2021) infer that consistent CPOs across the East Otago area, relating to a fossil shear zone, explain the anisotropy data there. We thus speculate that the lower S-wave anisotropy in West Otago is because olivine CPO is not aligned on a large scale. For example, coarse samples oriented using an orthopyroxene reference fram ...
Mantle xenoliths from the Southern Alps, New Zealand, provide insight into the origin of mantle seismic anisotropy related to the Australian‐Pacific plate boundary. Most xenoliths from within 100 km lateral distance of the Alpine Fault are coarse grained, but a small number are finer grained protomylonites. The protomylonites contain connected networks of fine grains with a different crystallographic preferred orientation (CPO) to coarse porphyroclasts in the same xenolith, suggesting that protomylonites and coarse‐grained samples record different deformation kinematics. The CPOs of fine grains in protomylonites have monoclinic symmetry, with the 2‐fold rotation axis normal to a plane that contains olivine [010] and orthopyroxene [100] maxima, suggesting that the protomylonite deformation involved significant simple shear. Some coarse‐grained samples contain unconnected lenses and layers of fine grains with the same CPO as the coarse grains. Microstructures suggest that these fine grains formed by subgrain rotation recrystallization and that protomylonites may represent an up‐strain progression of this microstructure, where the connectivity of fine grains has allowed them to localize shear and develop a new Alpine Fault CPO. The samples tell us about the state of the mantle at 25 Ma, in the early history of the plate boundary. If this suite of samples is representative of the mantle beneath the Alpine Fault in the present day, then we can interpret the complex seismic anisotropy patterns in the lithospheric mantle as representative of blocks containing variably rotated older CPOs juxtaposed by narrow shear zones associated with Alpine Fault deformation.
... A CPO type is chosen as input to the calculation for D-Rex by assigning reference dimensionless resolved shear stresses (RSS) ( Table S2 in Supporting Information S1). Multiple slip systems have been reported for enstatite (Bernard et al., 2021;Jung et al., 2010). If more than 40% of the aggregate is olivine, the effect of enstatite on the orientation of aggregates' anisotropy is relatively small while it may impact the strength of anisotropy (Bernard et al., 2021). ...
Plain Language Summary
As oceanic plates subduct, they drag the overlying mantle material, inducing mantle wedge flow. As the mantle flows, mineral crystals align in certain directions. Earthquake waves that pass through the aligned crystals travel faster in one direction than another. As a result, the waves split into two parts and are characterized by two parameters: fast direction and delay time. We can simplify how we model this material, but it may impact these parameters. In this study, we test the non‐simplified and simplified material for four different directions of mineral alignment and calculate the fast direction and delay time for a 3‐D mantle flow where the oceanic plate is moving toward the edge of another plate at an angle of 45°. We find that simplifying the material has modest effects on the parameters but simplification can cause smaller delay times for one mineral alignment direction, underestimating how aligned the minerals are. Overall, the mineral alignments result in fast directions that are mainly perpendicular to the edge of the overriding plate, which does not reflect the 3‐D mantle wedge flow pattern.
... Qi et al., 2018). Opx also have various CPOs within the same sample, however, with a smaller variety: In eight samples we found girdled distribution of [100] and [010] perpendicular to the lineation, which is defined as ABC-type fabric (Jung et al., 2010). In six samples we also observed maxima in [010] normal to the foliation plane, indicating BC-type fabric (Jung et al., 2010) and in one sample opx [100] is orthogonal to the foliation plane, corresponding to AC-type fabric (Jung et al., 2010). ...
... Opx also have various CPOs within the same sample, however, with a smaller variety: In eight samples we found girdled distribution of [100] and [010] perpendicular to the lineation, which is defined as ABC-type fabric (Jung et al., 2010). In six samples we also observed maxima in [010] normal to the foliation plane, indicating BC-type fabric (Jung et al., 2010) and in one sample opx [100] is orthogonal to the foliation plane, corresponding to AC-type fabric (Jung et al., 2010). Additionally, some opx bands in four samples have no preferred orientation. ...
... Opx also have various CPOs within the same sample, however, with a smaller variety: In eight samples we found girdled distribution of [100] and [010] perpendicular to the lineation, which is defined as ABC-type fabric (Jung et al., 2010). In six samples we also observed maxima in [010] normal to the foliation plane, indicating BC-type fabric (Jung et al., 2010) and in one sample opx [100] is orthogonal to the foliation plane, corresponding to AC-type fabric (Jung et al., 2010). Additionally, some opx bands in four samples have no preferred orientation. ...
Sheared peridotites from the Kaapvaal craton may be broadly divided into two types: (i) high-T and refertilized and (ii) low-T and highly-depleted, which equilibrated at conditions lying either above or along the Kaapvaal craton conductive geotherm, respectively. Here, we have studied 14 low-T sheared peridotites from Kimberley entrained by several Late Cretaceous (90Ma) kimberlites in order to constrain the nature and timing of the deformation. The sample suite comprises: 9 garnet peridotites with various amounts of clinopyroxene ± isolated spinel; 3 garnet-free phlogopite peridotites with minor amounts of spinel; one garnet spinel peridotite and one dunite. The peridotites have intense deformation textures, ranging from porphyroclastic to fluidal mosaic.
Olivine and orthopyroxene compositions (Mg# = 91-94) indicate varying degrees of depletion, similar to coarse-grained peridotites from the same localities. Pre-deformation conditions of the garnet peridotites are preserved in the cores of large (>100 µm-mm diameter) porphyroclasts and give a range in temperature of 930-1000°C at pressures of 4.0 ± 0.4 GPa. The garnet spinel peridotite was equilibrated at 840 °C and 3.1 GPa. Projected onto a 40 mW/m² geothermal gradient, the phlogopite peridotite samples yield temperatures of 850-870°C at 3.3-3.4 GPa.
Trace element measurements by LA-ICP-MS and electron microprobe indicate that the 'cold' sheared peridotites were influenced by several metasomatic events, ranging from 'old' pre-deformation metasomatism to interactions shortly before or during deformation. The old pre-deformation metasomatism is recorded in garnet, clinopyroxene and orthopyroxene porphyroclasts and implies interactions with PIC-(phlogopite-ilmenite-clinopyroxene) or MARID (muscovite-amphibole-rutile-ilmenite-diopside)-related metasomatic agents, which also led to crystallization of phlogopite in the garnet-free peridotites. A 'young' metasomatic event 3 caused an enrichment in Fe, Ti, Ca and Y (+HREE) and is evident in zoned orthopyroxene and clinopyroxene and phlogopite, the crystallization of new clinopyroxene-porphyroclasts and compositional heterogeneities in garnet. This young event marks the beginning of extensive kimberlite-related metasomatism in the late Cretaceous beneath Kimberley. The metasomatism caused the deformation (triggered by a kimberlite pulse?), resulting in the recrystallization of fine-grained, mainly olivine, neoblasts (down to <10 µm). These record the metasomatic conditions at the time of deformation, revealing an increase in temperature up to 1200°C accompanied by an increase in Ti content up to 300 µg/g. Crystal preferred orientations of olivine neoblasts suggest the presence of elevated concentrations of water (B-, C-, E-type) or the presence of a melt during the deformation (AG-type). We suggest that these high water contents led to hydrolytic weakening of the cratonic lithosphere and prepared the pathways for subsequent kimberlite magmas to reach the surface.
We propose that the deformation is a byproduct of extensive metasomatism, resulting in a metasomatism-deformation cycle: In times of extensive magmatism and metasomatism, fluids and melts flow along the pathways established by previous metasomatic agents, leading to further hydrolytic weakening of these mantle segments. Later, deformation was initiated by a new pulse of melt/fluid, with one of the later pulses eventually reaching the surface, and transporting fragments of sheared and undeformed peridotites with it. The remaining peridotite anneals after the period of extensive metasomatism and recrystallizes to become coarse-grained peridotite again.
... The axes of orthopyroxene from sample 8156 produce numerous dispersed maxima on the pole figure, which makes unambiguous interpretation difficult. The observed fabric pattern is closest to the ABC type, which was established in some experiments on the deformation of orthopyroxene [54]. ...
We provide results of a comprehensive mineralogical and microstructural study of relict lherzolites of the main ore field and fresh rocks from a deep structural borehole drilled in the south-eastern part of the Kempirsay massif. Olivine and orthopyroxene from lherzolites contain numerous pieces of evidence of material redistribution at different scales caused mainly by solid-state processes, such as plastic flow of mantle, syntectonic recrystallization, and annealing. The results of deformation-induced processes at the submicron scale are recorded by optical and electronic microscopy. In olivine, the plastic deformation caused segregation of impurities at structural defects. As a result, abundant tiny rods of newly formed Cr-spinels occurred inside its grains. Moreover, in enstatite, deformation caused partial or complete chemical decomposition with exsolution of diopside, pargasite and spinel lamellae up to the formation of a “fibrous” structure. In other cases, it provided partial or complete recrystallization to form new phases of enstatite-2, forsterite, diopside, pargasite, and spinel. Petrographic observations are validated by geochemical data, i.e., regularly decreasing concentrations of minor elements in neoblasts compared to large grains (porphyroclasts). Further redistribution of spinel grains with the formation of chromitite bodies is witnessed by their permanent association with the most mobile phase of the upper mantle, i.e., olivine, which is the only mineral that remains stable under the intense plastic flow. An increased concentration of Cr-spinel grains during formation of massive chromitites could appear under conditions close to pressure sintering, as evidenced by stressed textures of ores and an increased grain size compared to disseminated chromitites. The formation of unique chromitite deposits is associated with integration of numerous disparate podiform bodies into “ore bunches” due to the tectonic impact in the shear-compression regime. This was most likely associated with transition of the rifting (spreading) regime to that of the upper mantle of the fore-arc basin.
... Compared to the (100)[001], which is considered the main activated slip system in orthopyroxene (Soustelle et al., 2009;Tommasi et al., 2008;Skemer et al., 2006;Xu et al., 2006;Ishii & Sawaguchi, 2002;Vauchez and Garrido, 2001;Mainprice et al., 2000;Christensen and Lundquist, 1982), the (100)[010] has not been frequently reported in literature. However, rare natural occurrences (Jung et al., 2010;Kohlstedt & Vander Sande, 1973;Nazè et al., 1987;Skrotzki, 1990Skrotzki, , 1994, deformation experiments (Nazè et al., 1987;Raleigh et al., 1971), room temperature indentations of enstatite (Van Duysen et al., 1985), and numerical simulations (Jahn & Martonak, 2008), suggest that orthopyroxene can deform and rotate around the [001] until the (100) tends to be parallel to the XY foliation plane and the Burgers vector b [010] becomes parallel to the stretching lineation, as observed in our orthopyroxene porphyroclasts. ...
Peridotites of Ulten Zone (Eastern Alps, N Italy) show a transition from coarse protogranular spinel lherzolites to fine‐grained amphibole + garnet peridotites, recorded by the crystallization of garnet coronas around spinel. Pyroxenite veins, transposed along the peridotite foliation, show a similar metamorphic evolution from coarse‐grained (garnet‐free) websterites to fine‐grained garnet websterites. In both peridotites and websterites, garnet previously exsolved from porphyroclastic high‐temperature pyroxenes and later crystallized along the foliation. This evolution has been interpreted to reflect cooling and pressure increase of websterites and host peridotites from spinel‐ to garnet‐facies conditions. Microstructures and crystallographic orientation data indicate that the re‐equilibration of garnet websterites in the garnet stability field occurred during deformation. Porphyroclastic pyroxenes have been interpreted to deform by dislocation glide and creep. In particular, TEM observations indicate the activation of the (100)[010] slip system in orthopyroxene. Core‐and‐mantle microstructures also suggest that dislocation creep was aided by subgrain rotation recrystallization, leading to the formation of neoblastic pyroxenes. These recrystallized grains deformed by diffusion‐accommodated grain boundary sliding, as indicated by the occurrence of quadruple junctions and straight, aligned grain boundaries. The transition from dislocation creep to diffusion creep in websterites was accompanied by the crystallization of garnet along foliation, which triggered the pinning of the recrystallized matrix and stabilized the fine‐grained microtexture for diffusion creep, promoting rheological weakening. Garnet websterites of Ulten Zone thus offer a unique opportunity to investigate the effects of reaction softening during the corner flow in the supra‐subduction lithospheric mantle induced by the descending slab.
... Water content in olivine ranges from 4 to 19 ppm with an average of 11 ppm H 2 O by weight (Table 3). FTIR spectra of orthopyroxene are rather homogeneous and characterized by major O-H absorption bands at 3596, 3520 and 3420 cm − 1 (Fig. 6), which are in accordance with the typical bands in orthopyroxene from mantle xenoliths (Xia et al., 2010;Jung et al., 2010;Wang et al., 2014;Demouchy et al., 2015;Zhu et al., 2017). Water contents are varying from 53 to 194 ppm with an average of 121 ppm H 2 O by weight in orthopyroxenes (Table 3). ...
... the classification from Jung et al. (2010). AC-type, characterized by the alignment of [100] axis perpendicular to the XY-plane and the alignment of [001] axis subparallel to the X-direction or the formation of girdles within the XY-plane, is shown in most of samples (Fig. 7). ...
SKS wave splitting measurements revealed strong and complex seismic anisotropy in the upper mantle beneath the southeastern Tibetan Plateau. To better understand lithospheric deformation and upper mantle anisotropy observed in this region, we performed studies on microstructures, seismic anisotropy and chemical compositions of the mantle peridotite xenoliths collected from Maguan, Yunnan Province, in the SE Tibetan Plateau. The mantle xenoliths show two types of olivine crystal preferred orientation (CPO), AG–type and A–type. AG–type fabric is characterized by a concentration of [010]-axis normal to the foliation and a large circle girdle of the [100] and [001] axes in foliation. A–type fabric shows point concentrations of [100] and [010] axes subparallel to lineation and subnormal to foliation, respectively. Orthopyroxene and clinopyroxene align their [001]-axis sub-parallel to the [100]-axis of olivine. Seismic anisotropy calculated from the fabric of xenoliths displays moderate to strong polarization anisotropy (AVs) ranging from 3.1% to 7.0%, with 4.6% on average. Based on the seismic properties of xenoliths, we propose that, in SE Tibetan Plateau (south of 26°N), the SKS wave splitting is attributed to melt-enhanced anisotropy with a vertical structural frame lithosphere. Under this structural configuration, the large delay time could be expected with the fast wave polarization direction (FPD) in E–W direction. Our study also supports the model of double anisotropic layers in this area. The fast orientation of lower layer caused by lithospheric mantle anisotropy is in E–W direction, while the upper one with the NNW–SSE FPD is related to the frozen anisotropy in the middle-lower crust.
