Preferred Orientation Of Olivine Research Articles

Seismic and thermo-compositional insights into the uppermost mantle beneath the Northern Andes magmatic arc

The mantle wedge beneath the Northern Andes magmatic arc is expected to be latitudinally heterogeneous as suggested by the highly variable composition of recovered xenoliths, a discontinuous pattern of low seismic velocity zones, and unusually high Vp/Vs ratios below volcanic centers. Such heterogeneity might be controlling the development of the modern arc which currently shows a narrow northern volcanic segment between ∼4° and 6°N (<75 km wide), a volcanic gap of hundreds of km in its central portion (∼2°-4°N), and a wider southern volcanic expression between ∼2°S and 2°N (>120 km wide). Through an analysis of Pn and Sn inter-station wave speed estimates, the seismic structure dissimilarity and latitudinal thermo-compositional insights were constrained within the uppermost mantle beneath the arc. Results, integrating wave speed estimates, anisotropy, and thermo-compositional inferences obtained by wave velocity comparison using physical properties of minerals and phase equilibria modeling, suggest a seismically slower structure, higher anisotropy, and warmer conditions beneath the northern region (>4°N), whereas a faster, less anisotropic, and colder mantle towards the south (<2°N). We interpret the warmer and higher degree of anisotropy in the northern uppermost mantle as influenced by the Caldas tear, prompting hot mantle influx. Beneath the gap region, seismic speeds are similar to those in the north, yet a colder thermal state is suggested. We interpret the colder temperatures and the abrupt reduction in surface volcanism as reflecting an absence, or a reduced amount of magma pounding within the Moho vicinity. The southern upper mantle is seismically faster and colder compared to the northern portions of the arc. We interpret the fast velocities, and a wider volcanic expression on the surface, as influenced by the Carnegie ridge interaction prompting a shallower subduction with respect to the north. Our wave speed estimate and thermo-compositional inferences draw an increase in temperature and seismic anisotropy in the uppermost mantle from south to north along the Northern Andes magmatic arc. The main controlling factor of the general anisotropy below the entire arc seems to be the preferred orientation of olivine and pyroxene rather than an aligned melt fabric.

UNIAXIAL COMPRESSIVE STRENGTH, DRY UNIT WEIGHT AND FRACTURE PATTERNS OF ULTRABASIC ROCKS IN OTHRYS MOUNTAIN (CENTRAL GREECE): CORRELATIONS AND EVALUATION

Ultrabasic rocks, taken from the Othrys mt. (Central Greece), are studied in this paper. The structural geology and tectonics of the study area are described. Uniaxial Compressive Strength (UCS) and dry unit weight (γ) values are calculated and fracture angles are measured. The results are statistically assessed and empirical relationships (exponential equations) between UCS and γ are presented for the ultrabasic roch, divided in Peridotites, Serpentinised Peridotites and Serpentinites. Due to the low correlation coefficient of the Serpentinised Peridotites, it was decided that the Serpentinised Peridotites and Serpentinites should be examined together. The correlation coefficient of the combined category is much better than the separate ones. Furthermore, this paper demonstrates that the majority of the ultrabasic rocks tested were breaking at one angle (φ°), which mainly fluctuated between 75° and 90°. The fracture angles correlate with previously recognised geological (mainly tectonic) structures. The observed deviations are due to pétrographie variety, structural complexity, preferred orientation of olivine and orthopyroxene and internal imprinted tectonic deformation.

Preferred Mineral Orientation by Recrystallization under non-Hydrostatic Stress and Geotectonic Stress in the Upper Mantle

Significant elastic anisotropy of olivine single crystal is well known. The velocity of P waves has a maximum in the direction of crystallographic a-axis (identical to optical-acoustic Z-axis). It is also known that dunite and peridotite which are found at the central core of an orogenic zone have a strong tendency to preferred orientation of olivine. The preferred orientation of olivine and elastic anisotropy of dunite in the tectonic zone of Japan are measured optically and acoustically. The latter is carried out using the ultrasonic wave at high pressures. The direction dependence of P wave velocity is expressed by V, = a + b cos2 8, where the direction of 8 = 0 is found to be nearly equal to the direction of lineation which coincides statistically with the direction of a-axis concentration. Recently the existence of anisotropy of the uppermoFt mantle came to be convincing by analysing the results of seismic refraction profiles at the ocean bottom. The P wave velocity can be expressed empirically as V, = a + b cos2 8 where 0 is the azimuth to the perpendicular to the ridge direction. Here the observed seismic anisotropy is attributed to the regional preferred orientation of constituent minerals of rocks composing the upper mantle. The mechanism to produce preferred orientation is known to be either a recrystallization due to elastic anisotropy of minerals or a non-elastic effect such as gliding. In olivine the translation gliding in (010) plane is known. However, it does not seem that such a pure translation gliding can give rise to the significant preferred lattice orientation under stress. Let the preferred orientation of olivine aggregates be due to recrystallization. The stable mineral orientation is specified by the minimum condition of a certain thermodynamic function 4. It must be consistent with the stress and strain continuities at the grain boundary. According to our theory (1) and (2) 4 is represented by a sum of: (a) A function dependent upon a grain shape (interface configuration between grains) (first order). It plays a significant role in mineral orientation of tectonite. However, it is insensitive to the elastic anisotropy. (b) A linear function of deviatric stress component (second order). (c) A quadratic function of stress component (second order). Terms (b) and (c) are related to the stored energy and potential due to deformation and are dependent upon the stress-strain relation, while (a) is independent of stressstrain relation. The expression of C#J is shown in Table 1. Under high hydrostatic pressures greater than lOkb, term (b) becomes predominant. In the actual mineral aggregates, the grain boundary is not a plane, and the surface integral expression of 4 is necessary. To find the condition C#J = min the variational problem for the relative angle between the axis of anisotropy and the axis of external

Shear-wave splitting analysis of the upper mantle at the Niigata-Kobe Tectonic Zone with the data of the Joint Seismic Observations at NKTZ

AbstractWe conducted seismic observations with a spatially high density seismic network at the Niigata-Kobe Tectonic Zone, central Japan. The seismic network was used for the analysis of shear-wave splitting. Large lateral variations were found in the polarization direction data: the northern part of the research area yields polarization directions ofNW-SE (Region A), the central part of the research area with the polarization direction of NNE-SSW (Region B), the eastern part of the research area with the polarization direction of NE-SW (Region C), and the southern part of the research area with the polarization direction of E-W (Region D). The polarization directions in Regions B and C could be explained by the preferred orientation of olivine caused by the flow of the subducting Philippine Sea plate. However, the cause of anisotropic region which was related to the heterogeneous structure was also plausible. The polarization direction in Region A might be related to the flow caused by both of the subducting Philippine Sea and Pacific slabs. The polarization direction at the Region D could not be produced by the flow in the wedge and might be related to an anisotropic region beneath the slab. The lateral variation of the polarization direction does not support a model that the NKTZ is a collision region.

Converted waves reveal a thick and layered tectosphere beneath the Kalahari super-craton

Thick and high-velocity roots are generally observed beneath the Archean cratons. Inside these high-velocity keels, velocity contrasts are detected neither by surface nor by body waves tomographies. We present here evidences based on the S-to-P and P-to-S converted waves for the existence of an irregularly stratified and thick keel beneath the Kalahari super-craton. To satisfy surface wave data and S-to-P conversions, the velocity model should have beneath the Moho a ∼ 160 km thick anisotropic structure with vertical slow axis and decreasing anisotropic parameters with depth. Such a structure may stem from the preferred orientation of olivine [100] axis in the horizontal plane under shearing deformation. This structure, together with the ∼ 100 km thick layer underlying it, forms a ∼ 300 km thick continental root beneath the cratons. Inside this root, the P and S velocities should be higher on average respectively by an amount of 6% and 4% than the AK135 velocity model. Beneath ∼ 300–350 km depth, a low velocity zone is clearly detected that may be either the remainder of large magma reservoirs related to cratonic flood basalts or a melted silicate layer created by the transformation, just above the 410-km deep discontinuity, of wadsleyite assembly to olivine assembly.

岩石組織・鉱物の選択配向と弾性波速度異方性

Velocity anisotropy and preferred mineral orientation in crustal and mantle rocks are briefly reviewed in this paper. While elastic-wave velocity anisotropy has been measured in the laboratory at high pressure and temperature, it is also calculated theoretically from the preferred orientation of rock-forming minerals and their elastic constants. Laboratory measurements generally agree well with theoretical calculations by using the Voigt averaging method. This consistency has demonstrated that the preferred orientation of olivine dominantly controls velocity anisotropy in mantle rocks. For crustal rocks, biotite, muscovite and hornblende are major mineral components that cause strong anisotropy. Seismic observations of S-wave polarization anisotropy are also reviewed briefly in relation to the preferred mineral orientation in the lithosphere.

Mantle anisotropy beneath northwest Pacific subduction zones

To assess the location, strength, and orientation of seismic anisotropy in the southern Kuril, Japan, and Izu‐Bonin subduction zones, we analyzed shear wave splitting in local S phases and teleseismic SKS phases. Fast directions from these phases are roughly parallel to the direction of absolute Pacific plate motion (∼WNW) in Izu‐Bonin, roughly parallel to the strike of the trench (∼NNE) in central Honshu, and roughly N in the southern Kuril back arc in the vicinity of Sakhalin Island. Assuming that the orientation and strength of anisotropy does not vary with depth, modeling of splitting times from local S and teleseismic phases recorded at Sakhalin Island requires that the maximum depth of anisotropy lies between 480 km and 950 km. In contrast, splitting times for local S phases from events in Izu‐Bonin rule out anisotropy in the transition zone. All the data are consistent with a model in which the lower transition zone (520–660 km) and lower mantle are largely isotropic and in which anisotropy occurs intermittently in the upper transition zone (410–520 km), possibly due to preferred orientation of β spinel. Assuming that splitting in the upper mantle is produced by preferred orientation of olivine, the observed fast directions indicate that the geometry of back arc strain varies systematically between subduction zones. The relationship of fast directions and plate motions suggests that the subducting slab exerts significant control on back arc flow, but that flow correlated with upper plate deformation must also exist.

On the Lehmann discontinuity

The Lehmann discontinuity (∼200km) in the upper mantle is proposed to result from a change in the preferred orientation of olivine as a result of a change in deformation mechanism: in the shallow upper mantle, deformation is due to dislocation creep and results in an anisotropic structure, whereas in the deeper upper mantle, diffusion creep becomes dominant due to a smaller activation volume, resulting in an isotropic structure. For anisotropy due to dislocation‐glide corresponding to a horizontal flow, the resultant anisotropy will yield VSH&gt;VSV and VPH&gt;vPV. Thus, for waves propagating nearly vertically (e.g., ScS waves), there will be discontinuous jumps in the S‐ and P‐wave velocities at this discontinuity. In this model, the depth of the Lehmann discontinuity depends on the deformation conditions such as stress level, geotherm, water fugacity and grain‐size, giving a natural explanation for its observed variation with tectonic setting.

The role of recrystallization in the preferred orientation of olivine

The effects of recrystallization on the preferred orientation in olivine have been studied. The recrystallization mechanisms studied include grain growth, the deformation-induced grain boundary migration (DIGBM) and subgrain rotation (SGR). Grain boundary migration is involved in the first two, but not in SGR. Significant preferred orientation was found only in the latter two where dislocations are involved. The combined microstructural and fabric (preferred orientation) observations indicate that the grain boundary migration produces strong preferred orientation when the driving force is the dislocation energy but not when the grain boundary energy is the driving force. The nature of preferred orientation due to dislocation-related recrystallization (DIGBM or SGR) was found to depend on the mechanism of recrystallization. The DIGBM produces preferred orientation in which the low dislocation density grains with unfavourable orientation for easy slip dominate, thereby significantly altering the preferred orientation formed by dislocation glide. In contrast, SGR does not essentially alter the deformation fabric, and therefore the fabric associated with SGR recrystallization is related to the geometry of flow. The present results demonstrate that the existence of seismic anisotropy in the upper mantle is strong evidence for the dislocation mechanism(s) of deformation, and suggest that care must be exercised in applying the results of laboratory fabric studies made for a particular mechanism of recrystallization to the Earth's interior where the dominant mechanism(s) of dynamic recrystallization can be different.

Recently formed elastic anisotropy and petrological models for the continental subcrustal lithosphere in southern Germany

This paper advances new evidence for elastic anisotropy in the continental subcrustal lithosphere in southern Germany. The range of petrological models compatible with the observed azimuthal variation of seismic P-wave velocity is explored. The azimuthal distribution of amplitudes of mantle phases and the observed increase of P velocity with depth both indicate a continuation of anisotropy with depth together with an increase of preferred orientation. Even depletion of the upper mantle in basaltic components, as suggested by mantle xenoliths from various parts of Germany, cannot explain the velocity-depth and azimuthal amplitude observations without an increase of anisotropy with depth. Preferred orientation of olivine is the most likely mechanism for the observed phenomena. Its fast a-axis at the Moho level is directed towards N22.5°E. The b-axis is also required to be horizontal; i.e., the b-plane, one of the preferred glide planes of olivine, is vertical, with a strike of N22.5°E. Therefore, this preferred glide plane of olivine practically coincides with the plane of maximum horizontal shear stress deduced from fault-plane solutions of earthquakes in western Germany. This is a strong indication that the preferred orientation of olivine is formed in the recent West European crustal stress field leaking into the upper mantle. The distribution of velocities to a depth of at least 50 km requires slight horizontal rotation of the a-axis with depth by ∼ 10° towards N32°E, and a change in the modal composition towards a depletion increasing with depth compatible with the composition of mantle xenoliths from western Germany. Further experiments are needed to substantiate this suggestion, which could lead to a better understanding of the interaction of crustal and upper-mantle stress-strain fields.

Seismic anisotropy in an ophiolite peridotite: Application to oceanic upper mantle

Compressional velocity Vp to 8 kbar and 275°C in a homogeneous, 0.5% porosity, little‐altered harzburgite from the Antalya ophiolite complex in Turkey was measured along the three principal kinematic axes as defined from the preferred orientations of olivine and enstatite. The observed Vp anisotropy was 0.7 km/s for pressures P from 2 to 8 kbar and temperatures T from 25° to 275°C. The seismic anisotropy at 250°C and 5 kbar (15‐km depth) calculated from the single‐crystal elastic constants and the fabric data were within 3% of the observed velocities in the core directions. Measured values of the P and T derivatives of Vp for the three cores are 14 m/s kbar and −0.6 m/s °C, which are in reasonable agreement with calculated average values from single‐crystal data. These data result in a critical thermal gradient of 7.5°C/km. A model of oceanic crust and uppermost mantle (constructed from the structural data in this ophiolite complex) permits an estimation of the azimuthal seismic anisotropy in the uppermost mantle beneath this paleocrust under assumed conditions of 250°C and 15‐km depth. The result is Δ Vp = 0.3 km/s with Vp = 8.46 and 8.16 km/s normal and parallel to the ridge, respectively. The azimuthal shear velocity anisotropy is 0.18 km/s. Measurements of Vp are also presented for three lherzolite cores from Salt Lake Crater, Hawaii. The T derivatives of Vp for these cores agree with those obtained for the harzburgite; however, the P derivatives are consistently higher. The higher values of the P derivatives for the lherzolite sample are considered to result from its higher porosity.

Mechanisms of preferred orientations of olivine in tectonite peridotite: Comment and reply

Mechanisms of preferred orientations of olivine in tectonite peridotite: Comment and reply

Mechanisms of preferred orientations of olivine in tectonite peridotite: Comment and reply

Mechanisms of preferred orientations of olivine in tectonite peridotite: Comment and reply

Structural analysis of olivine in pallasitic meteorites: Deformation in planetary interiors

Pallasitic meteorites, as examples of samples derived from deep within their parent bodies, can provide information about conditions that existed within planetary interiors. Olivine crystal shape can be correlated with past deformation. Pallasites containing rounded olivines are free of kink bands and other evidence of plastic flow. Pallasites containing angular crystals, on the other hand, show a clear correlation between features of physical deformation, such as olivine crystal shape, size, degree of fracturing, and percentage of kink banded crystals. Pencil glide on the system {0kl} [100] is the predominant mechanism of plastic flow. Glide on this system in olivine indicates deformation at low strain rates and moderate to high temperatures. This occurred in the parent body while the pallasites were still deeply buried. According to petrofabric analyses, the parent fabrics of all pallasites studied are free of preferred orientations of olivine. Certain grains containing kink bands, however, do show preferred alignments. Kink banding in the pallasites is by a mechanism that is distinct from the shock-induced kinking of chondrites, achondrites, and mesosiderites.

The relationship between preferred orientation of olivine in dunite and the tectonic environment [discussion

Discussion, by R. N. Brothers, in which evidence is presented supporting the hypothesis that the orientation of olivine crystals in the peridotite of Dun mountain, New Zealand, is attributable to semisolid intrusion. In reply, the various postulates are critically analyzed by Battey, who favors the theory that the olivines were oriented in the regional orogenic stress pattern.

The relationship between preferred orientation of olivine in dunite and the tectonic environment

A definite relationship is found between the fold axes, the olivine orientation, and the joint pattern of the peridotite in the Dun mountain area, New Zealand. Analysis of the olivine orientation relative to the tectonic setting of the Nelson peridotite belt leads to the conclusion that in this case it is related to the axes of regional folding and is probably produced by the folding forces. The c-axis of the olivine may be oriented parallel to the axis of folding, and the b-axis may lie in the axial plane of upright folds, though it may also tend to form, with the a-axis, a girdle in the a-c tectonic plane.