Olivine Textures Research Articles

Formation of metal and silicate globules in Gujba: a new Bencubbin-like meteorite fall

Gujba is a coarse-grained meteorite fall composed of 41 vol% large kamacite globules, 20 vol% large light-colored silicate globules with cryptocrystalline, barred pyroxene and barred olivine textures, 39 vol% dark-colored, silicate-rich matrix, and rare refractory inclusions. Gujba resembles Bencubbin and Weatherford in texture, oxygen-isotopic composition and in having high bulk δ 15N values (∼+685‰). The 3He cosmic-ray exposure age of Gujba (26 ± 7 Ma) is essentially identical to that of Bencubbin, suggesting that they were both reduced to meter-size fragments in the same parent-body collision. The Gujba metal globules exhibit metal-troilite quench textures and vary in their abundances of troilite and volatile siderophile elements. We suggest that the metal globules formed as liquid droplets either via condensation in an impact-generated vapor plume or by evaporation of preexisting metal particles in a plume. The lower the abundance of volatile elements in the metal globules, the higher the globule quench temperature. We infer that the large silicate globules also formed from completely molten droplets; their low volatile-element abundances indicate that they also formed at high temperatures, probably by processes analogous to those that formed the metal globules. The coarse-grained Bencubbin-Weatherford-Gujba meteorites may represent a depositional component from the vapor cloud enriched in coarse and dense particles. A second class of Bencubbin-like meteorites (represented by Hammadah al Hamra 237 and QUE 94411) may be a finer fraction derived from the same vapor cloud.

Seismic, Structural and Petrological Models of the Subcrustal Lithosphere in Southern Germany: A Quantitative Revaluation

—Anisotropy in the subcontinental lithosphere becomes increasingly important, because it is observed in many seismic studies especially for P n -waves. Typical rocks of the uppermost mantle are peridotites, which predominantly exhibit a pronounced elastic anisotropy. This anisotropy is mainly caused by the anisotropic elastic properties and the lattice preferred orientation (here referred to as texture) of olivine. To evaluate the elastic anisotropy of peridotites from the subcontinental lithosphere, specimens of the Northern Hessian Depression (Germany) and the Balmuccia Ultramafic Massif (Northern Italy) have been used. They comprise four olivine texture types, which are characteristic for olivine textures observed worldwide. The bulk rock elastic properties have been calculated using olivine and orthopyroxene textures, their single-crystal elastic constants at ambient pressure/temperature conditions and their volume fraction. Clinopyroxene and spinel are assumed to be randomly distributed. The effect of four different orientations of the foliation within the uppermost mantle has been evaluated, since this orientation is usually unknown.¶Two of the olivine textures have a pronounced azimuthal dependence of compressional waves when a horizontal foliation within the uppermost mantle is presumed. These variations cause significant azimuthal variations of the P-wave reflections coefficients at the Moho. Primarily, we predict a significant azimuthal dependence of the critical points where the reflected amplitude increases from approximately 15% to 95%. Possibly, these azimuthal variations can be detected by seismic reflection measurements carried out at earth surface.¶The remaining two texture types only manifest a small directional dependence. When anisotropy of compressional waves is observed in seismic studies, these latter types can only be of subordinate importance. However, all of the peridotites investigated are able to explain the seismically observed azimuthal variations of compressional waves when a vertical foliation is proposed. This ambiguity can be substantially reduced when shear waves (S-waves) are considered. The directional distribution of S-wave velocities and of the S-wave splitting exhibits characteristic patterns for the different olivine texture types. This could be used to discriminate between different texture types and orientations of the foliation within the uppermost mantle. A fundamental requirement for a more comprehensive interpretation is the availability of detailed S-wave observations. The maximum S-wave splitting in the peridotites investigated coincides with the maximum of the faster (leading) S-wave. This may be of importance to detect S-wave splitting in future seismic studies.

The Relationship between Olivine Textures and Seismic Anisotropy in a Database of Upper Mantle Rocks

odern texture analysis methods developed by H.J.Bunge have been applied to typical olivine textures from a database of 125 samples. The database represents the first attempt to have a statistical view of textures in rock forming minerals from the Earth's upper mantle. The relationship between texture strength measured by the J index and calculated seismic anisotropy reveals a trend of increasing seismic anisotropy with increasing texture strength for three texture types. These texture types are a single crystal, [100] fibre and [010] fibre types which represent 95% of the database. These texture types represent a family of textures with very similar orientation distribution functions (O.D.F.) skeletons. A fourth texture type is a very strong [001] fibre texture which has a different O.D.F. skeleton. The seismic anisotropy is constant with increasing texture strength for the [001] fibre texture. The whole database is characterised by mean values of J index, P-wave and S-wave anisotropy of 9.5, 11.6% and 9.6%, suggesting that strong olivine textures and seismic anisotropy are a statistical characteristic of the Earth's upper mantle sampled by the database.

An experimental study of trace element partitioning between olivine, orthopyroxene and melt in chondrules: equilibrium values and kinetic effects

The concentrations of 32 elements (major, minor, REE, Ba, Hf, Sc, V, Co, Ni, Sr, Y, Zr, Nb, Ge, Th and U) were measured using an ion microprobe in olivine and orthopyroxene, and in coexisting glass in the run products produced in isothermal and dynamic crystallization experiments on chemical compositions that produce porphyritic olivine, radial pyroxene and barred olivine textures. Cooling rates in the dynamic crystallization experiments ranged from 1°C/h to 2191°C/h. The mineral/melt partition coefficients ( D) calculated from the measured concentrations for both olivine and orthopyroxene showed very little change between equilibrium experiments and dynamic experiments with cooling rates of up to 100°C/h. The data appear to define reliable equilibrium D values. Olivine D's range from 2 × 10 −5 for U to ∼ 10 for Ni, and orthopyroxene D's range from 2 × 10 −5 for U to ∼ 8 for Mg. There are regular relationships between the ionic radius, the valence of the trace element and the partition coefficients in olivine and orthopyroxene in all experiments. While there are some differences in the D values between olivine and orthopyroxene, they follow very similar trends. For experiments at high cooling rates there is an increase in the measured concentrations for incompatible trace elements in the crystals which yields apparent partition coefficients greater by up to 3 orders of magnitude from the equilibrium values obtained for isothermal equilibrium and slow cooling experiments. In contrast, compatible trace element D values are only slightly sensitive to cooling rate, typically varying by less than a factor of 2. SEM studies showed that melt inclusions were common in orthopyroxenes. It is shown that the increase in apparent D values for pyroxene observed with high cooling rates can be quantitatively explained by trapping of melt inclusions in crystals with equilibrium D values. Incorporation of melt inclusions may also quantitatively explain the increases in apparent D values in olivine at high cooling rates. However, no inclusions of melt were identified in olivine, even at the fastest cooling rate. If the high apparent D values obtained for olivine at high cooling rates were due to incorporation of trace elements in the lattice and not due to entrapment of melt, it would require incorporation of elements independent of valence and ionic radius.

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.

Formation of olivine textures in pallasites and thermal history of pallasites in their parent body

An analysis of the textures of pallasites has been made using data concerning the kinetic and rheological properties of silicates and metals. Pallasites containing rounded olivines (e.g., the Springwater and Brehnam pallasites) have been heated to above the solidus temperature of the metallic iron phases, ∼ 1270 K. The rounded olivines of grain size 0.5–1.0 cm observed in the Springwater pallasites were formed between 1270 and 1370 K. On the other hand, those of grain size 0.5–1.0 mm as found in the Brehnam pallasites may have been heated to above 1370 K; however, the duration of heating at such high temperatures must have been less than 5 × 10 3 y. Pallasites containing angular olivines with microscopically rounded corners (e.g., the Eagle Station, Dora pallasites) have suffered shock events fracturing the olivine grains, which may correspond to collisions during the accretional stage of the parent body, and experienced successive annealing during cooling from a temperature between 1150 and 1270 K.

Komatiites: An early precambrian phenomenon

Using the presently accepted definition, komatiites are ultramafic volcanic rocks characterised by bladed and/or skeletal olivine texture (spinifex texture). Where textural evidence is absent,the geochemical definition is equivocal and as a result there is considerable ambiguity, particularly among the basaltic types. If the added control of Al 2O 3/TiO 2 ⩽ 25 is added to the definition, this eliminates rocks of the boninite group and also the low-Ti basalts found in some ophiolite complexes. The bulk of komatiites occur in the Archaean and the most obvious explanation is that the eruption of high (up to 30%) MgO melts was controlled by an enhanced geothermal gradient. There is some evidence to suggest that the high CaO/Al 2O 3 ratio characteristic of some komatiites (e.g. Barberton) is found only in those greenstone belts older than 3.0 b.y.

Experimental reproduction of textures of chondrules

The textures of chondrules have been reproduced by crystallizing melts of three different compositions at 1 atm with cooling rates ranging from 400 to 20°C/min under 10 −9 to 10 −12 atm P O 2 . A porphyritic olivine texture has been formed from a melt of olivine-rich composition ( SiO 2 = 45 wt.% ), a barred-olivine texture from melt of intermediate composition ( SiO 2 = 47 wt.% ), and radial-olivine texture from melt of pyroxene-rich composition ( SiO 2 = 57 wt.% ). The cooling rate for producing barred olivine is most restricted; the rate ranges from 120 to 50°C/min. Other textures can be formed with wider ranges of cooling rate. The results of the experiments indicate that some of the major types of textures of chondrules can be formed with cooling rate of about 100°C/min. With this cooling rate, the texture varies depending on the composition of melt.

Olivine recrystallization textures

SummaryMosaic olivine textures, with straight triple grain boundaries meeting at angles of 120°, are common in dunites. This is a well-known feature of annealed metals, and is the result of grain-boundary migration during recrystallization in which the system tends toward a state of minimum interfacial energies. Geometrically similar textures are present at the boundaries between such mosaic grains and strain-banded relict olivine grains. The unstrained mosaic grains make angular projections into the strained grains exactly at the junctions between the deformation bands. These are interpreted to be triple junctions with the two differently oriented bands acting as separate grains, and thus also due to recrystallization.