Olivine Crystallographic Preferred Orientation Research Articles

Dehydration breakdown of antigorite and the formation of B-type olivine CPO

Peridotite formed by contact metamorphism and dehydration breakdown of an antigorite schist from the Happo area, central Japan shows a strong olivine crystallographic preferred orientation (Ol CPO). The lack of mesoscale deformation structures associated with the intrusion and the lack of microstructural evidence for plastic deformation of neoblastic grains suggest that olivine CPO in this area did not form as a result of solid-state deformation. Instead, the good correspondence between the original antigorite orientation and the orientation of the newly formed olivine implies the CPO formed by topotactic growth of the olivine after antigorite. Ol CPO is likely to develop by a similar process in subduction zone environments where foliated serpentinite is dragged down to depths where antigorite is no longer stable. The Happo Ol CPO has a strong a-axis concentration perpendicular to the lineation and within the foliation—commonly referred to as B-type Ol CPO. Seismic fast directions parallel to the ocean trench are observed in many convergent margins and are consistent with the presence of B-type Ol CPO in the mantle wedge of these regions. Experimental work has shown that B-type CPO can form by dislocation creep under hydrous conditions at relatively high stresses. There are, however, several discrepancies between the characteristics of natural and laboratory samples with B-type Ol CPO. (1) The formation conditions (stress and temperature) of some natural examples with B-type CPO fall outside those predicted by experiments. (2) In deformation experiments, slip in the crystallographic c-axis direction is important but has not been observed in natural examples of B-type CPO. (3) Experimental work suggests the presence of H2O and either high shear stress or relatively low temperatures are essential for the formation of B-type CPO. These conditions are most likely to be achieved close to subduction boundaries, but these regions are also associated with serpentinization, which prevents strong olivine CPO patterns from forming. We show B-type Ol CPO can form as a result of static topotactic growth of olivine after high-temperature breakdown of foliated serpentinite. These results resolve the discrepancies between experimental and natural examples of B-type CPO and show the need to rethink the formation process of olivine CPO in convergent margins. Topotactic growth of olivine after antigorite can account for the inferred distribution of B-type Ol CPO in the mantle wedge more successfully than dislocation creep.

Shear wave anisotropy beneath the volcanic front in South Kyushu area, Japan: Development of C-type olivine CPO under H2O-rich conditions

[1] Shear wave splitting from local intermediate-depth earthquakes is investigated to detect the anisotropic structure in the mantle wedge beneath the South Kyushu area, Japan. We observed shear wave splitting with NEE-SWW to NWW-SEE polarization directions and delay times of 0.04–0.63 s. Trench-normal shear wave polarization anisotropy with a delay time greater than 0.3 s probably overlaps in the high VP/VS region at a depth of 100–150 km beneath the volcanic front. Model calculations for the cause of the anisotropy suggest that the development of “C-type” olivine crystallographic preferred orientation (CPO) with a trench-parallel b axis concentration and a trench-normal a axis concentration best reproduces observations compared to A-, B-, or E-type olivine CPOs and antigorite CPO. We conclude that a thick anisotropic layer, approximately 50 km, is formed by concentration of interstitial fluid in peridotite. We briefly discuss the mechanism carrying water to the deep mantle wedge. A possible explanation is that interstitial fluids in the mantle wedge are effectively released because of a decrease in dihedral angles of olivine-fluid interfaces at 150 km depth. Our results imply a close relation between the high density of explosive volcanoes in the South Kyushu area and the underlying water-rich mantle.

The influence of hydrous phases on the microstructure and seismic properties of a hydrated mantle rock

To better understand the microstructural evolution of a “serpentinized” mantle rock and the influence of various hydrous phases on the seismic properties of the mantle wedge, we have conducted the detailed microstructural analyses of a sample of tremolite–chlorite–antigorite schist collected from the Moses Rock dike (central part of the Colorado Plateau). We performed differential effective media (DEM) modelling to study the effect of three hydrous phases forming two-phase aggregates with olivine, considering the crystallographic preferred orientation (CPO) of each phase and the shape ratio of the hydrous phases. We have demonstrated that in a partially serpentinized peridotite, the olivine CPO characteristic of [100](010) dislocation glide is still preserved, and the high-temperature asthenospheric flow is preserved with a foliation normal to that of antigorite schist. The transformation of olivine into antigorite occurs predominantly (~75%) by the relationship (100)ol || (001)atg with [001]ol || [010]atg, with the (010)ol || (001)atg and [001]ol || [010]atg relationship observed in areas of weak antigorite CPO. Chlorite results from the phase transformation of olivine in a relatively static environment, as shown by the correlation between the olivine–chlorite CPOs with (100)ol || (100)ch, (010)ol || (001)ch and (001)ol || (010)ch. The fluid percolation that caused the localized metasomatism and partial hydration of the mantle occurred possibly along trans-lithospheric shear zones. The presence of chlorite induces the most important drop on the P-wave velocities and may help to explain some local low velocities in the fore-arc mantle wedges, but is unlikely to be of global importance due to its very high Vp/Vs ratio~1.9. On the other hand, antigorite is the only phase that causes important modification on the propagation directions of P and S-waves, and the only phase to explain the polarization of the fastest shear waves parallel to the subduction trench.

Development of A-type olivine fabric in water-rich deep upper mantle

Water controls the activity of slip systems in olivine resulting in various types of olivine crystallographic preferred orientation (i.e., fabric) in mantle rocks. The A-type olivine fabric is the most commonly observed olivine fabric in natural peridotites. Development of A-type olivine fabric (developed by the (010)[100] slip system) is known to be limited to the water-poor conditions of the shallow upper mantle (<200km depth). We have performed simple-shear deformation experiments of olivine at 7.2–11.1GPa and 1400–1770 K. Here we show that A-type olivine fabric was developed under water-rich conditions (>2130ppm H/Si in olivine), while B-type fabric (by the (010)[001] slip system) was observed under moderately wet conditions (750–2130ppm H/Si). Developments of C-type (by the (100)[001] slip system) fabric was limited to water-poor conditions (<220ppm H/Si). We found that monotonic decrease in the seismic anisotropy VSH/VSV (the ratio of horizontally and vertically polarized shear waves) with depth in the global one-dimensional models is well explained by the olivine fabrics developed in the horizontal flow of a water-poor mantle. Only A-type olivine fabric can explain the vertical mantle flow which associates the seismic anisotropy of VSH/VSV<1 in the deep upper mantle (>200km depth). A strong anomaly of VSH/VSV<1 observed in the deep upper mantle beneath the East Pacific Rise is well explained by the distribution of water-rich regions (in which A-type olivine fabric is dominantly developed) in the deep upper mantle and upwelling mantle flows.

Pressure‐ and stress‐induced fabric transition in olivine from peridotites in the Western Gneiss Region (Norway): implications for mantle seismic anisotropy

AbstractThe effects of pressure, water and differential stress on the crystallographic preferred orientation (CPO) of olivine were investigated by an integrated study on 13 peridotite samples from four areas in the Western Gneiss Region (WGR) of the Norwegian Caledonides. The highly depleted dunites are mantle residues after Archean melt extraction, while the garnet peridotites were formed by refertilization of the dunites via melt percolation in the Proterozoic. These orogenic peridotites were tectonically emplaced into Proterozoic gneisses and metasedimentary rocks of the WGR and were recrystallized under high‐pressure (HP) to ultrahigh‐pressure (UHP) conditions when the Baltic Shield was subducted beneath Laurentia during the 420–400 Ma Scandian orogeny. The CPOs of olivine, enstatite and diopside were measured using the electron backscatter diffraction technique. The weakly deformed garnet wehrlites and dunites from the HP central WGR (the Gurskøy, Almklovdalen and Tafjord areas) show the A‐type olivine fabric defined by the [100] axis subparallel to the stretching lineation and the (010) plane subparallel to the foliation, which may be a relict fabric formed at high temperature and low strain rate in the mantle wedge prior to subduction. The [001](010) fabric (B‐type) is observed in strongly sheared HP garnet peridotites and dunites from the Almklovdalen and Tafjord areas, suggesting the dominant activation of [001](010) slip system at high stress and high strain rate. Recrystallized fine olivine grains may develop a fabric different from porphyroclasts and record a gradual fabric transition. The olivine CPO in garnet lherzolites from Otrøy in the northernmost WGR is characterized by the concentration of [001] axes parallel to the stretching lineation and [100] axes normal to the foliation, indicating a prevailing [001](100) slip (C‐type fabric) at P–T conditions of &gt;6 GPa and 850–950 °C. Despite very different deformation microstructures, Fourier transform infrared analysis reveals very low water contents (&lt;13 ppm H2O by the Paterson calibration) in olivine for both coarse porphyroclasts and recrystallized small grains, which is consistent with the high‐degree partial melting of the peridotites during the Archean. Therefore, a combination of UHP and low temperature plays a more important role than water in promoting the C‐type olivine fabric in a dry, cold and deeply subducted continental slab. With increasing subduction depths, the C‐type olivine fabric will predominate over the A‐type fabric and make the maximum P‐wave velocity and shear wave splitting of peridotites normal to the foliation. However, the stress‐induced fabric transition in olivine from the A‐type to the B‐type may be localized in shear zones and not distinguishable in seismic anisotropy observations.

Numerical modelling of the upper-mantle anisotropy beneath a migrating strike-slip plate boundary: the San Andreas Fault system

We performed forward modelling of seismic anisotropy beneath a migrating strike-slip plate boundary to: (1) test if such a geodynamic context might explain teleseismic shear wave splitting data in the vicinity of the central part of the San Andreas Fault and (2) constrain the power of such data to unravel vertical and lateral variations in deformation patterns in the upper mantle. The modelling involves five steps: (1) thermomechanical modelling, using a finite-element code, of the deformation field, (2) viscoplastic self-consistent modelling of the resulting olivine and pyroxene crystal preferred orientations, (3) calculation of the elasticity tensors for different domains of the finite elements (FEs) model, (4) forward modelling of seismic wave propagation through the model using ray theory, finite-frequency theory and a full-wave approach and (5) performing splitting measurements on the synthetic seismograms. SKS splitting data in central California are best fitted by a model with a hotter geotherm within 60 km of the plate boundary accounting for the opening of an asthenospheric window due to the northward migration of the Mendocino Triple Junction. The westward motion of the plate boundary cannot however explain the rotation of fast polarization directions east of the San Andreas Fault in central California. Comparison between modelled and measured individual shear wave splitting also implies that the homogeneity of the two-layer models accounting for the observations in the vicinity of the San Andreas Fault indicates a sharp transition between lithospheric and asthenospheric deformations beneath this plate boundary. The ability of different synthetic approaches to localize horizontally and vertically the plate boundary-related deformation differs significantly. Splitting in ray theory synthetics closely follow variations in olivine crystallographic preferred orientations in the model. In contrast, splitting analysis on full-wave synthetics, which should be more representative of actual long-period SKS waves, results in smooth apparent lateral variations of anisotropy; the exact location and width of the plate boundary may only be retrieved by comparing fast-polarization profiles obtained using a multichannel analysis on waves with different periods.

Superplasticity in hydrous melt-bearing dunite: Implications for shear localization in Earth’s upper mantle

Deformation experiments on hydrous melt-bearing dunite (olivine+4vol% orthopyroxene+4vol% clinopyroxene with less than 2.5vol% of the melt phase) were conducted at pressures of 1.3–5.7GPa and temperatures of 1270–1490K in order to explore the effect of intergranular fluids on the plastic flow of olivine in Earth’s upper mantle. The strain rate was proportional to steady-state creep strength to the 2.1 power, and the creep strength markedly increased with increase in grain size. Developments of the crystallographic preferred orientation of olivine and flattening of olivine grains were hardly observed even after 33–55% shortening of the samples. These observations show that grain boundary sliding (GBS) dominated the deformation of olivine (i.e., superplasticity). The creep strength of hydrous melt-bearing dunite was 2–5 times lower than that of melt-free dunite. The dependence of creep rate on melt fraction is known to be expressed empirically as ε̇(ϕ)=ε̇(0)exp(αϕ), where α is a constant and ϕ is the melt fraction. The experimentally obtained value of α was in the range of 150–230, corresponding to 5–7 times the reported values for the olivine–basalt system at 0.3GPa (i.e., creep strength of dunite was efficiently reduced by the hydrous melt). Superplasticity is the dominant creep mechanism of olivine in fluid-bearing fine-grained peridotites under low-temperature and high-stress conditions (i.e., peridotite shear zones in the upper mantle). Superplasticity induced by geological fluids would play an important role in the shear localization (and thus initiation of subduction) in the upper mantle.

Change of olivine a-axis alignment induced by water: Origin of seismic anisotropy in subduction zones

The effects of water on the crystallographic preferred orientation (CPO) of olivine aggregates were investigated through simple-shear deformation experiments under asthenospheric upper mantle conditions (pressure=2.1–5.2GPa, temperature=1490–1830K) using a deformation-DIA apparatus. Formation of the A-type olivine fabric (developed by the (010)[100] slip system) was observed under water-depleted conditions (COH<650ppm H/Si in olivine), while B-type fabric (by the (010)[001] slip system) or a B-type-like fabric (by the (010)[h0l] slip system) were predominantly formed under water-rich conditions (>1000ppm H/Si). In comparison with fabrics of anhydrous olivine (≤111ppm H/Si), those of olivine having higher water contents (≥625ppm H/Si in olivine) showed stronger anisotropic properties (e.g., P-wave anisotropy, S-wave polarization anisotropy, and the ratio of horizontally and vertically polarized shear waves). The water-induced olivine CPO transition from A-type to B-type(-like) fabric accompanies a change in the alignment of the seismic fast a-axes, resulting in flow-parallel and flow-perpendicular shear wave splitting under water-depleted and water-rich conditions, respectively. The rotation of the fast direction of shear-wave splitting across an arc, which is observed in many subduction zones, is well explained by the likely bimodal water distribution along the mantle wedge (i.e., water-rich in fore-arc and water-depleted in back-arc regions) and the developments of two different types of olivine fabrics (i.e., B-type(-like) fabric in fore-arc and A-type fabric in back-arc regions).

Crystal preferred orientation in peridotite ultramylonites deformed by grain size sensitive creep, Étang de Lers, Pyrenees, France

In naturally deformed upper mantle rocks a strong olivine crystallographic preferred orientation (CPO) occurs in rocks with grain sizes larger than about 15 μm. Finer grained peridotites tend to have weak to random olivine CPO. The different types of olivine CPO are usually interpreted in relation to the dominant deformation mechanisms: with a strong CPO indicating dislocation creep and a random CPO indicating grain size sensitive (GSS) mechanisms involving grain boundary sliding and diffusion creep. Here we report the occurrence of a weak but systematic olivine CPO in ultra-fine-grained (0.5–10 μm) ultramylonites from the Étang de Lers lherzolite, Pyrenees, France. The microstructures, with elongated grains and mixed distribution of phases in the ultramylonites indicate deformation by dominant GSS creep. In theory, an olivine CPO may develop in rocks deformed by GSS creep during dislocation accommodated boundary sliding or phase boundary controlled diffusion creep in pyroxene-rich rocks. Alternatively, the CPO may be a relict texture preserved because the elongated grain shapes limit rotation during grain boundary sliding. Our observations confirm theoretical and experimental studies, which predict that systematic CPO and an associated seismic anisotropy can occur in naturally deformed upper mantle rocks during deformation by grain boundary sliding and diffusion creep.

Obliteration of olivine crystallographic preferred orientation patterns in subduction-related antigorite-bearing mantle peridotite: an example from the Higashi–Akaishi body, SW Japan

Abstract Large parts of the mantle wedge near subduction boundaries are likely to be hydrated and contain antigorite. This mineral is acoustically highly anisotropic and potentially has a strong influence on seismic properties of the wedge. The Higashi–Akaishi body of SW Japan is an exhumed sliver of partially serpentinized forearc mantle, ideal for studying the effects of antigorite on the development of tectonic fabrics in the mantle. Samples with less than 1% antigorite show strong B-type olivine crystallographic preferred orientation (CPO) patterns. In contrast, samples with &gt;10% antigorite deformed during the same tectonic event show much weaker olivine CPO patterns lacking the flow-normal a -axis concentration. These microstructural data suggest that the development of antigorite during deformation weakens olivine CPO due to phase boundary slip and associated rigid-body rotation of olivine grains. Antigorite and similar sheet silicates are likely to be present to some extent in the mantle wedge of all convergent margins. Our results suggest that even if this amount is only a few percent, strong olivine CPO is unlikely to develop and any pre-existing CPO is likely to be destroyed. Under these conditions, olivine CPO is unlikely to contribute significantly to seismic anisotropy in the mantle wedge.

A microstructural imprint of melt impregnation in slow spreading lithosphere: Olivine‐rich troctolites from the Atlantis Massif, Mid‐Atlantic Ridge, 30°N, IODP Hole U1309D

The 1415 m deep IODP Hole U1309D (Mid‐Atlantic Ridge, 30°N) is the second deepest hole drilled into slow spreading oceanic lithosphere. The recovered section comprises essentially gabbroic rocks, with a large range of compositions. The most primitive end‐members of the gabbroic sequence, herein referred to as olivine‐rich troctolites (ol &gt; ∼70%), have textures and geochemical compositions intermediate between that of mantle peridotites and primitive cumulates, indicative of melt impregnation processes. We carried out a detailed microstructural study to further characterize the petrogenetic processes leading to their formation, as well as discuss their mode of emplacement and relationship with neighboring mantle lithosphere. In olivine‐rich troctolites, olivines range from coarse‐grained subhedral crystals, commonly containing well‐developed subgrains, to medium‐grained rounded crystals with fewer or no substructures. They are embedded in large, undeformed pyroxene and plagioclase poikiloblasts. Olivine substructures reveal dislocation creep that is consistent with activation of the main high‐temperature slip systems, dominantly (010)[100]. Olivine crystallographic preferred orientation is very weak but generally shows a relatively stronger, uncommon [001] concentration. These unusual olivine fabrics are interpreted as resulting from melt impregnation of a previously deformed olivine matrix: the solid olivine framework is disrupted by olivine corrosion along grain and subgrain boundaries, and the high‐temperature plastic fabric is modified in a liquid‐dominated regime. Based on mineral composition and fabrics and in comparison with what is observed in impregnated mantle rocks elsewhere, we posit that olivine represents relicts of mantle peridotites disaggregated by large melt influx, although the mantle origin of olivine is not unequivocally demonstrated yet. Whatever the initial lithology, impregnation by large volumes of melt has strongly modified the original composition and microstructure. If the mantle origin hypothesis is correct, the original olivine fabric could have been efficiently weakened by dunitization prior to disruption of the olivine framework by melt impregnation. Incorporation, at the base of the lithosphere, of small slivers of impregnated dunite into gabbroic sections, trapped between successive igneous units, may be a common mechanism of lower crustal accretion at slow spreading ridges. Extensive melt‐rock interaction processes are expected to contribute significantly to the final chemical composition of erupted lavas.

Upper mantle seismic anisotropy resulting from pressure-induced slip transition in olivine

Seismic anisotropy in the oceanic lithosphere results from flow-induced crystallographic preferred orientation of dry olivine during lithosphere creation. Recent experiments, however, showed that high water activity changes the flow mechanisms of olivine and hence the crystallographic preferred orientation, better explaining the seismic anisotropy in the mantle wedge above subduction zones. Whether changes in the crystallographic preferred orientation of olivine are unique to the effects of water has become controversial and is critical to resolve. Here we report low-stress, high-strain experiments on typical dry mantle rock at high pressures and temperatures, showing that at ∼3 GPa, pressure induces the same profound transition in olivine crystallographic preferred orientation that is produced by high water activity at lower pressure. One important consequence for global tectonics is that alignment of fast seismic waves parallel to trenches beneath subducting slabs probably reflects entrainment of sub-lithospheric mantle in the direction of subduction, rather than trench-parallel flow as interpreted at present. From the large variety of crystallographic preferred orientations in olivine in both experiments and natural rocks, we infer that in addition to the pressure-induced changes in olivine slip systems implied here, there are probably further changes in slip systems at higher pressure and temperature. Seismic anisotropy in Earth’s oceanic lithosphere and in the mantle wedge above subduction zones is associated with crystallographic preferred orientations of olivine. Experiments at high pressure and temperature suggest that a pressure of ∼3 GPa can induce the same changes in the crystal structure of olivine as high water activity at lower pressures.

Wrench faults down to the asthenosphere: geological and geophysical evidence and thermomechanical effects

Abstract We review a set of geological and geophysical observations that strongly support a coherent deformation of the entire lithosphere in major intracontinental wrench faults. Tectonic studies of wrench faults eroded down to the middle to lower crust show that, even in cases in which the lower to middle crust is partially melted, strain remains localized (although less efficiently) in transcurrent shear zones. Seismic profiling as well as seismic tomography and magnetotelluric soundings provide strong argument in favour of major wrench faults crosscutting the Moho and deforming the upper mantle. Pn velocity anisotropy, shear-wave splitting and electric conductivity anisotropy measurements over major wrench faults and in transpressional domains support that a wrench fault fabric exists over most or even the entire lithosphere thickness. These seismic and electrical anisotropies are generated by a crystallographic preferred orientation of olivine and pyroxenes developed in the mantle during the fault activity, which is frozen in the lithospheric mantle when the deformation stops. The preservation of such a ‘wrench fault type’ fabric within the upper mantle may have major effects on the subsequent tectonothermal behaviour of continents, because olivine is mechanically and thermally anisotropic. Indeed, the association of numerical models and laboratory data on textured mantle rocks strongly suggests that the orogenic continental lithosphere is an anisotropic medium with regards to its stiffness and to heat diffusion. This anisotropy may explain the frequent reactivation, at the continents scale, of ancient lithospheric-scale wrench faults and transpressional belts during subsequent tectonic events.