Recrystallization Of Olivine Research Articles

Ultramafic rocks of a fracture-zone ophiolite, North Cascades, Washington

The Ingalls Complex was deformed in a Late Jurassic oceanic fracture zone. An unusually diverse group of ultramafic tectonites comprise three units in this ophiolite. Unit 1 consists mostly of poorly to moderately foliated harzburgite and dunite characterized by porphyroclastic textures. Irregular-shaped and tabular dunite bodies probably represent intrusive bodies or residues of partial melting. Voluminous Unit 2 consists mostly of poorly to strongly foliated Iherzolite and clinopyroxene-bearing harzburgite, plagioclase peridotite is present locally. Olivine and enstatite generally define equigranular mosaics or weakly porphyroclastic textures. Clinopyroxene, however, in some samples displays only weak deformation, compositional zoning, simple (growth?) twins and interstitial, commonly poikilitic texture. Clinopyroxene and plagioclase in these samples probably formed from a melt after recrystallization of olivine and enstatite, indicating that these Iherzolites are impregnated peridotites. Other Iherzolites and clinopyroxene-bearing harzburgites may represent weakly depleted mantle. Pods of metagabbroic gneiss within Unit 2 probably are small intrusions that were deformed as they cooled. Unit 3 represents a major high-temperature ( 700 °≥ 900 ° C ) shear zone that separates Units 1 and 2, and consists of strongly foliated, commonly mylonitic Iherzolites and hornblende peridotites. The latter are the most strongly foliated ultramafites, and olivine in them records stresses as high as 275 MPa. The abundance of hornblende implies a genetic relationship between mylonitization and the hydration and metasomatism necessary to form such rocks from Iherzolites. Mineral chemistry and geothermometry are typical of mantle tectonites in many ophiolites and oceanic fracture zones. There is a particularly strong similarity between the spinels in the Ingalls Complex and the spinels from the Owen and Vema fracture zones. Hornblende in Unit 3 ranges from edenite to edenitic hornblende. Calculated temperatures from CPX-OPX pairs in Unit 2 range from 950 ° to 1000 ° C. These data are best explained by horizontal and vertical displacements in oceanic fracture zones which may juxtapose lithosphère formed at different structural levels and with different petrologic histories. Unit 1 represents a block of upper mantle which probably experienced convective upwelling and partial melting beneath a spreading ridge near its intersection with a fracture zone. During convective upwelling at least parts of the upper mantle represented by Unit 2 were impregnated by a melt. The impregnated Iherzolite may be accounted for by the edge effect at a fracture zone or reduced melt production at a fracture zone due to ribbon spreading. The mylonite zone of Unit 3 reworks Iherzolites and harzburgites which were originally part of the block represented by Unit 2 and probably records deformation in the active segment of the fracture zone. These observations document the complexities that may occur in ultramafic rocks within oceanic fracture zones. They also suggest that impregnation, metasomatism, and high stresses are important in these zones.

Subgrain rotation and dynamic recrystallization of olivine, upper mantle diapirism, and extension of the basin-and-range province

On the basis of an abrupt change of olivine grain size at certain depths in the upper mantle beneath the Basin-and-Range province in the western U.S.A., Mercier (1980b) proposed that subgrain rotation (SGR) recrystallization had taken place above the grain size discontinuity, and grain boundary migration (GBM) recrystallization at depths greater than the discontinuity. The rotation of subgrains and the characteristics of dynamically recrystallized neoblasts of olivine were analyzed in a series of dunite samples deformed experimentally to compressive strains of about 15–60%. Whereas the rotation angle between adjacent subgrains increases with strain, the mean rotation angle did not exceed 2° even in the most heavily deformed samples. Rotation angles between kink bands, which formed at all experimental conditions, also increase with strain; at the highest strains rotations of up to 110° are not uncommon. At low strains the dominant rotation axis between kinks is [001] and less frequently [ovw]; at high strains [uvw] rotation axes are common. Neoblasts did form mainly on old grain boundaries and less frequently within old grains. Rotation axes between intragranular neoblasts and their hosts are of the type [uvw] and the rotation angles are always large, features which seem to be inconsistent with SGR recrystallization and suggest GBM recrystallization. Neoblasts may have formed in highly strained regions of old grains where slip occurs with [uvw] rotation axes. The olivine textures and fabrics of lherzolite nodules originating in the uppermost mantle above the grain size discontinuity are similar to those of nodules from the Dreiser Weiher, Germany (Mercier, 1980b). A study of Dreiser Weiher samples shows, however, that they have features difficult to reconcile with the SGR mechanism. It is proposed here that GBM recrystallization occurred throughout a rising upper mantle diapir beneath the Basin-and-Range extension zone. As the upward flow crossed the regional lithosphere-asthenosphere boundary at about 65 km depth, it diverged causing flow velocities to decrease abruptly and the strain rate dropped approximately by an order of magnitude. As a consequence the differential stress decreased by about a factor of two and the olivine grain size increased by a factor of two.

On the inter-relationship between grain size sensitive creep and dynamic recrystallization of olivine : Zeuch, D H TectonophysicsV93, N1–2, 20 March 1983, P151–168

On the inter-relationship between grain size sensitive creep and dynamic recrystallization of olivine : Zeuch, D H TectonophysicsV93, N1–2, 20 March 1983, P151–168

On the inter-relationship between grain size sensitive creep and dynamic recrystallization of olivine

The recent theoretical results of Twiss (1976) and Goetze (1978) suggest that at high temperatures and sufficiently high stresses the creep behavior of dry olivine should be dominated by either nonlinear diffusion accommodated grain-boundary sliding or nonlinear Coble creep mechanisms. This would result following the production of a fine grain-size by dynamic recrystallization. For the high-temperature experimental work performed by Karato et al. (1982) dry single crystals of olivine were almost totally recrystallized during creep, and temperature changing experiments were performed on the resulting dynamically recrystallizing polycrystalline aggregates. However, the activation energy for creep determined by Karato et al. (1982) was far higher than that predicted by the models of Twiss (1976) or Goetze (1978), although the conditions required for operation of at least the model of Twiss (1976) apparently were satisfied. The data for the highly recrystallized specimens from the higher stress, lower temperature experiments of Zeuch and Green (1979) and Zeuch (1980) are in good agreement with the results of Karato et al. (1982). These latter experiments were conducted under conditions where either the model of Twiss (1976) or Goetze (1978) should have been applicable. I tentatively conclude that although fine grain sizes were produced in the experiments of Karato et al. (1982), Zeuch and Green (1979) and Zeuch (1980) by dynamic recrystallization, there is no evidence for a transition to grain-boundary diffusional creep mechanisms at high or low stresses despite the predictions of Twiss (1976) or Goetze (1978). Instead, deformation is dominated by dislocation movement with recovery by dynamic recrystallization.

Surface-wave propagation in an ocean basin with an anisotropic upper mantle: numerical modelling

Summary. The characteristics of surface-wave propagation in ocean basins are examined numerically for models with two types of anisotropic alignment in the upper mantle: one resulting from glide-plane slip in olivine with horizontal or vertical slip-planes, and the other from syntectonic recrystallization of olivine in a zone of horizontal shear. Glide-plane slip can cause highly anomalous inclined-Rayleigh particle-motion in the third-generalized mode (corresponding to the isotropic second-Rayleigh mode). The amplitude of this anomaly is rather insensitive to details of the structure. Syntectonic recrystallization can cause an anomalous combination of inclined-and tilted-Rayleigh motion in all modes. The variation with period of the amplitude of the anomaly in the fundamental mode can indicate the approximate depth to the anisotropic layer. In both types of alignment, the sense of tilt and the inclination varies with direction of propagation in a manner characteristic of the structural symmetry.

Surface-wave propagation in an ocean basin with an anisotropic upper mantle: observations of polarization anomalies

Summary. Observations of surface waves crossing ocean basins indicate that, at least in some cases, a better interpretation of what was previously considered to be the simultaneous arrival of fundamental Love and second Rayleigh modes, with similar group velocities, is the coupled motion of the second-generalized mode surface wave propagating in an anisotropic upper-mantle. This mode displays the polarization anomalies expected from anisotropic alignments caused by syntectonic recrystallization of olivine in a zone of horizontal shear. The polarity of the pattern of anomalies, found in waves crossing the Pacific Basin, suggests that the lithosphere is dragging the asthenosphere.

Textural and chemical variations of olivine and chrome spinel in the East Dover ultramafic bodies, south-central Vermont

Regional metamorphism of upper-greenschist to epidote-amphibolite facies produced variations of olivine and chrome spinel textures and compositions within the East Dover ultramafic bodies that can be attributed to differing intensities of recrystallization of the ultramafic rocks. Olivine shows a progressive change from highly strained grains in the least recrystallized rocks to relatively strain-free grains in the most recrystallized rocks. Fo content of olivine bears a direct relationship to texture: with increase in degree of recrystallization, olivine changes composition from Fo 90 to Fo 97 . Chrome spinel grains are ubiquitously rimmed by opaque spinels. Variations in thickness of these rims and shape of the translucent chrome spinel cores occur in a regular manner. Thickest rims occur around the most anhedral cores, and these are found in the most strongly recrystallized rocks. Euhedral chrome spinels with thin opaque spinel rims are found in the least recrystallized rocks. With increasing recrystallization, Mg/(Mg + Fe) and A1/(A1 + Cr) ratios increase in the spinel cores. Zones of intensity of recrystallization indicated by both olivine and spinel textures and compositions are geographically concentric with the map outlines of the ultramafic bodies. Observed textural and compositional variations suggest a reaction involving simultaneous serpentinization and recrystallization of olivine in the presence of an oxidizing fluid. Partial buffering of this fluid (which originated in the country rocks) and subsequent development of a substantial f o 2 gradient between the margins and cores of the larger ultramafic bodies resulted in a more nearly complete reaction at the margins than in the cores of the bodies. This reaction need not have occurred as a single event, and equilibrium was only maintained on the scale of hand specimens ( f o 2 , olivine composition) to fractions of mineral grains (spinel composition).

The Lodran meteorite and its relationship to the ureilites

SummaryLodran is a unique meteorite consisting of roughly equal amounts of metal, olivine, and pyroxene with minor amounts of sulphide, chromite, phosphide, chrome-diopside, and a new phase with a composition close to (K,Na)AlSi5O12. Zähringer reported planetary-type rare gases in both the metal and silicates, suggesting a primitive nature. The pyroxene composition is Fs13.8with little variation. Olivine composition averages Fa12.6, but varies at least ±20 % both among grains and zoned within single grains; only the Fe-rich olivine is in equilibrium with the pyroxene. The metal probably cooled rapidly (700 K/Myr) at high temperatures and slower (30 K/Myr) at lower temperatures. Two compositional populations of chromite are found.A model for the formation of Lodran includes three steps: Formation of large olivine, pyroxene, and metal grains, with the trapping of small olivine inclusions in pyroxene and pyroxene in olivine. Equilibration and recrystallization of olivine, pyroxene, and metal, loss of alkalis and Ca; this probably occurred in a parent-body setting. And incorporation of reducing materials and mild reheating sufficient to produce the zoning in the olivine but not enough to re-equilibrate the pyroxene.Phase compositions and rare-gas concentrations in ureilites are similar to those in Lodran. In some respects Lodran appears to be a metal-rich ureilite, but the higher Fe/(Fe+Mg) ratios in the latter (Fa 21 olivine) suggest origin on separate parent bodies.The Harvard University meteorite is a mesosiderite and not closely related to Lodran.

Classification of textures and fabrics of peridotite xenoliths from South African kimberlites

A previous classification of textures and fabrics in peridotite xenoliths from kimberlites is summarized and extended by addition of two new types: the secondary tabular type which results from recrystallization following an intense flowage; the fluidal mosaic type, with distinctive tectonic stripes which could involve superplasticity. The coarse granular and tabular types, previously defined, are strongly recrystallized and recovered; they could represent a mantle that is rigid or deforming at slow strain rates. The porphyroclastic and mosaic types would correspond to peridotites flowing with a faster strain rate at the time they were extracted by the kimberlite magma. The textures and fabrics of these two highly deformed types have been generated by plastic flow; olivine recrystallization has been minor in the former type and complete in the latter. These data and interpretations argue for a non-steady-state origin for the kink in the pyroxene geotherm proposed by other authors.

Petrofabric studies of eclogite, spinel-Websterite, and spinel-lherzolite Xenoliths from kimberlite-bearing breccia pipes in southeastern Utah and northeastern Arizona

Textural evidence, clinopyroxene fabric, and mineral assemblages of eclogite nodules from the kimberlite-bearing breccia pipes of the east Colorado plateau are consistent with an origin by metamorphic recrystallization of predominantly basic igneous rocks. The eclogites are divided into three fabric groups exhibiting linear, planar, and random fabrics. Clinopyroxenes follow similar orientation rules as amphiboles in metamorphosed crustal tectonites. Partial recrystallization of olivine and extensive postcrystalline strain in the pyroxenes of the lherzolites and the websterites are believed to be due to a modification of the original fabric under crustal conditions. The inferred structural history of the ultramafic nodules puts various constraints on petrologic models attempting to explain their origin.

Syntectonic Recrystallization of Olivine and Modes of Flow in the Upper Mantle

Olivine in dunite, peridotite, and olivine powder has been recrystallized syntectonically during compression tests at temperatures from 950°C to 1350°C, confining pressures from 5 to 30 kb, and strain rates from 10 −3 to 10 −8 /sec. For intragranular recrystallization of the dunite, the orientations of the new grains are controlled by the host orientation. For inter-granular recrystallization and total recrystallization of the dunite and of olivine powder, the orientations of the grains are controlled dominantly by the stress. The fabric produced over the entire range of physical conditions is a [010] maximum parallel to σ 1 and [001] and [100] girdles in the σ 2 = σ 3 plane. With few exceptions, and allowances for the experimental requirement that σ 2 = σ 3 , fabrics in olivine tectonites are similar to those produced experimentally. Such probable upper mantle source materials as ultramafic nodules in diamond-bearing kimberlites and in under-saturated basalts and “Alpine-type” peridotites show these same preferred orientations. We propose, therefore, that Syntectonic recrystallization is an important or dominant mode of flow in the upper mantle at temperatures above about 500°C. The implications of this conclusion, as they bear on seismicity and seismic anisotropy, are considered in light of the two most seriously regarded hypotheses advanced to explain motions of spreading plates of the lithosphere.

Origin of ultramafic rocks, their tectonic setting and history: A contribution to the discussion of the paper “the origin of ultramafic and ultrabasic rocks” by P.J. Wyllie

A subdivision of the orogenic peridotites into ophiolitic (or truly Alpine-type) and “root-zone” peridotites is proposed, and their differences in tectonic setting and metamorphic facies are discussed. Some current hypotheses for the origin and mode of emplacement of the orogenic peridotites are briefly reviewed. It is shown next how various microstructural approaches, such as interpretative petrotectonic studies and the less commonly employed comparative petrofabric analysis of peridotites and their country-rocks, may be used to unravel the igneous, metamorphic and tectonic histories of ultramafic rocks in general. Criteria are established by which igneous flow fabrics, tectonic deformation fabrics and piezocrystalline fabrics of olivine may be distinguished. Special attention is paid to the piezocrystalline shape and lattice fabrics of olivine, deduced by Hartman and Den Tex (1964) through application of W.B. Kamb's theory of thermodynamic equilibrium orientation to the orthorhombic crystal lattice of olivine. The petrofabric habit of olivine and its role in shaping foliations or schistosities in metamorphic peridotites is emphasized. In the second part brief summaries are presented of the fabric and metamorphic data concerning three types of peridotite: (1) a “root-zone” peridotite, (2) an ophiolitic peridotite, and (3) some Iherzolitic nodules from alkali-olivine basalts, while their fabric and metamorphic histories are compared and contrasted. It is shown that they are all more of less complex metamorphic tectonites, having suffered several solid-state deformations and metamorphic events that are largely lacking in their host rocks, thus indicating that they were emplaced in the predominantly solid state. The garnetiferous “root-zone” peridotite of Alpe Arami (Swiss Pennine Alps) has a principal γ-olivine fabric perpendicular to the primary layering and first axial plane schistosity. It developed under eclogite- and granulite facies conditions, and is indicative of piezocrystallization in the absence of a liquid capable of dissolving olivine. After its solid emplacement it has also suffered Alpine tectonization and amphibolite facies rétrogradation. The ophiolitic lherzolite of the French Pyrenees has a principal α-olivine fabric perpendicular to the primary layering and first axial plane schistosity. This is also suggestive of piezocrystallization but in the presence of a liquid capable of dissolving olivine. The Iherzolite was then emplaced, aided by explosive gas release but, except for some fracture cleavages, it was not affected by penetrative Alpine deformation or metamorphism. The Iherzolitic nodules from Dreiser Weiher (F.R.Germany) possess a simple fabric, somewhat similar to that of the Pyrenean Iherzolite, whereas those from the Plateau de Velay in France have a much more complex fabric that may be derived from a “Dreiser Weiher fabric” by solid-state deformation and recrystallization of olivine and orthopyroxene crystals. It is concluded that solid derivation from upper mantle material is the most likely origin for the three types of peridotites, and that their impoverishment in eclogitic or basaltic constituents by partial fusion is improbable on account of their fabric and metamorphic history.