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1.
Representative samples of the two fabric varieties of microcrystalline quartz, chalcedony and quartzine, from agates of different origin were investigated by transmission electron microscopy (TEM). Both varieties contain lamellar admixtures of the SiO2 mineral moganite. The transitions from quartz to moganite within the fabric differ in a characteristic way. Whereas in quartzine the gradient between the structures is steep, the transition is more continuous in chalcedony. The morphology of moganite in chalcedony and quartzine is determined by the (101)-face; in pure moganite from Gran Canaria it is governed by the (110)-face.  相似文献   

2.
Agates are natural hierarchicalal structural assemblies composed largely of silica minerals: chalcedony, quartz, and quartzine. Microstructural heterogeneities, most typical of those of agate-forming minerals, are characterized by variations in periods of helicoidal twisting of chalcedony and by micrometric rhythmic alternation of zones with a low refractive index within the matrix of normal chalcedony, quartz, and quartzine. A study of the degree of crystallization of chalcedony using the Barsanova-Yakovleva method showed that neither helicoidal twisting nor refractive index zoning brought about any significant disturbance in the chalcedony structure. Helicoidal twisting is a result of the stresses caused by substitution of Si4+ for Al3+ asymmetric with respect to chalcedony crystal. It is suggested that layering by the refractive index developed due to accumulation of small portions of gel consisting of high silica polymers at the front of crystal growth.  相似文献   

3.
The structural disparities that distinguish chalcedony from macrocrystalline quartz suggest that different crystallization mechanisms are operative during the growth of these two forms of silica. Although the paragenesis of chalcedony has provoked marked disagreement among researchers, a review of previous studies supports the idea that chalcedony can precipitate from slightly saturated aqueous solutions at relatively low temperatures (<100° C). These conditions for deposition suggest a model for chalcedony crystallization that involves the assembly of short-chain linear polymers via bridging silica monomers. This assembly occurs through a spiral growth mechanism activated by a screw dislocation withb=n/2 [110], wheren is an integer. The proposed model can account for a number of peculiarities that have been observed in chalcedony at the microstructural scale, such as: (1) the direction of fiber elongation along [110] rather than [001]; (2) the periodic twisting of chalcedony fibers about [110]; (3) the high density of Brazil twin composition planes; (4) the common intergrowth of moganite within chalcedony.  相似文献   

4.
Chalcedony from Brazilian agates, has been investigated by using transmission-electron microscopy, X-ray-diffraction, thermogravimetry and optical techniques. The quartz fibers of length-fast chalcedony are composed of submicroscopical polysynthetic, lamellar-twinned right- and lefthanded crystals, according to the Brazil law. This very narrow twinning causes 3 systems of diffuse diffraction streaks (corresponding to the three-fold symmetry) parallel to 〈10.1〉, very frequently possessing an intensity maximum at h±1/2, k, l±1/2. These extra reflections were detected both in electron- and X-ray-diffraction patterns. Wall-lining chalcedony is parallel fibrous consisting of smaller crystallites with a higher total water content (0.06±0.01 μm and 1.2±0.1 wt %) than spherulitic chalcedony in horizontal agate bands (ca. 0.1 μm and 0.7±0.1 wt%).  相似文献   

5.
We conducted hydrothermal flow-through experiments at 430 °C and 31 MPa to investigate the mechanism of silica precipitation on granite under crustal conditions. Two experiments were performed using different input solutions: a single-component Si solution, and a multi-component solution with minor Al, Na, and K. The degree of supersaturation with respect to quartz, Ω = CSi/CSi,Qtz,eq, where CSi and CSi,Qtz,eq indicate Si concentration in solutions and the solubility of quartz within water, respectively, decreased from 3-3.5 to <1.1 along the flow path. A variety of silica minerals formed during the experiments (opal-A, opal-C, chalcedony, and quartz), and their occurrences and modal abundances changed in response to Ω and the presence of additives in the solution.For near-equilibrium solutions (Ω < ∼1.2), silica precipitation occurred in a simple way in both experiments, being restricted to overgrowths on pre-existing quartz surfaces in the granite. At higher saturation levels (Ω > ∼1.2), silica minerals were deposited on other surfaces in addition to quartz. In the single-component experiment, the dominant silica minerals changed in the order of opal-A → opal-C → quartz with decreasing Si concentration along the flow path. In contrast, in the multi-component experiment, quartz and minor chalcedony formed throughout the entire reaction vessel. This finding indicates that impurities (Na, K, and Al) in the solutions inhibited the precipitation of opal and enhanced the direct nucleation of quartz. The systematic appearance of metastable silica minerals were examined by nucleation processes and macroscopic precipitation kinetics. Our experimental results indicate that different precipitation mechanisms yield contrasting textures, which in turn suggests that vein textures can be used as indicators of solution chemistry within the fracture.  相似文献   

6.
Silica occurs in abundance in a variety of hydrothermal samples from the Trans-Atlantic Geotraverse (TAG) hydrothermal mound, 26°N Mid-Atlantic Ridge. The water content, trace element chemistry, and mineralogy of crystalline silica from 15 different samples have been examined by vibrational spectroscopy and probe microanalysis. The samples are from: shallow subsurface ferric iron oxyhydroxide silica deposits (n=4), a fragment of an active white smoker chimney (n=1), anhydrite bearing hydrothermal breccias (n=2), pyrite silica breccias (n=3), and silicified wall rock breccias (n=5). Length-fast chalcedony occurs in association with variable quantities of ferric iron oxyhydroxides in hydrothermal breccias from the mound flanks, within shallower subsurface chert samples, and within white smoker chimney walls. Samples from the anhydrite zone contain textures which are suggestive of an origin involving replacement of anhydrite. Samples taken from TAG 1 and 5 from below the anhydrite zone contain no chalcedony. Instead they contain subhedral quartz crystals which show oscillatory zoning in aluminium. Two types of crystalline silica namely, type A and type B quartz, are defined on the basis of the infrared spectra in the OH region from 3200 cm−1 to 3600 cm−1. The type A quartz occurs beneath the anhydrite zone at TAG 1 and 5. We propose a model that relates specific varieties of crystalline silica to different thermal and chemical environments within the mound interior. Length-fast chalcedony occurs in an outer low temperature envelope across the top and sides of the mound. The common association between length-fast chalcedony and ferric iron oxyhydroxides suggests that chalcedony crystallization is favoured where catalysis by ferric iron can occur. The apparent suppression of fibrous silica at the expense of single quartz crystals with increasing depth is attributed to differing growth rates and degrees of supersaturation of silica-bearing solutions with increasing temperature within the mound. The transition from type A to type B single crystal growth is interpreted to occur at temperatures approaching ˜360 °C due to decreasing solubility of aluminium in quartz, so that aluminium is rendered unavailable for type A valence compensation. Received: 10 September 1998 / Accepted: 6 July 1999  相似文献   

7.
Crystalline rocks from breccias of the Ries basin, Germany, contain highly deformed quartz. Various planar deformation structures could be observed and classified into five different types: (1) Decorated planar elements, (2) Non-decorated planar elements, (3) Homogeneous lamellae, (4) Filled lamellae, (5) Planar fractures. All these structures are parallel to crystallographic planes: {10¯13}, {10¯12}, {10¯11}, {0001},{11¯21}, {11¯22}, {21¯31}, {51¯61}, {10¯10}. The most typical and most abundant planar structures are decorated and nondecorated planar elements parallel to {10¯13} and {10¯12}. Planar fractures are parallel to {0001} and {10¯11} and form at lower stress levels, probably earlier than the planar elements.Quartz containing planar elements, especially of the non-decorated type, has lower density, index of refraction and birefringence than normal quartz. This quartz is apparently a mixture of an amorphous phase and crystalline quartz, the amount of which can be calculated using average density or refractive index.Comparison of planar quartz structures found in tectonites and those produced artificially under static or dynamic high pressure conditions demonstrates that Ries quartz closely resembles deformed quartz recovered from shock wave experiments. The planar structures found in Ries quartz have been formed by shock wave actions with peak pressures in the 100–400 kbar range.Planar elements are explained to be traces of gliding processes during shock loading visible due to the fact that a high pressure phase (stishovite and/or a stishovite-like glass phase) has been produced along the glide planes. Upon pressure release most of the high pressure phase was transformed into an SiO2-glass (diaplectic glass).In comparison with experimental data the amount of residual crystalline quartz as well as type and orientation of planar structures in the quartz grains are clues to estimate the peak pressures responsible for these deformations. Shock waves with peak pressures exceeding about 400 kbar completely transform quartz into diaplectic SiO2-glass.  相似文献   

8.
The San José district is located in the northwest part of the Deseado massif and hosts a number of epithermal Ag–Au quartz veins of intermediate sulfidation style, including the Huevos Verdes vein system. Veins are hosted by andesitic rocks of the Bajo Pobre Formation and locally by rhyodacitic pyroclastic rocks of the Chon Aike Formation. New 40Ar/39Ar constraints on the age of host rocks and mineralization define Late Jurassic ages of 151.3 ± 0.7 Ma to 144.7 ± 0.1 Ma for volcanic rocks of the Bajo Pobre Formation and of 147.6 ± 1.1 Ma for the Chon Aike Formation. Illite ages of the Huevos Verdes vein system of 140.8 ± 0.2 and 140.5 ± 0.3 Ma are 4 m.y. younger than the volcanic host rock unit. These age dates are among the youngest reported for Jurassic volcanism in the Deseado massif and correlate well with the regional context of magmatic and hydrothermal activity. The Huevos Verdes vein system has a strike length of 2,000 m, with several ore shoots along strike. The vein consists of a pre-ore stage and three main ore stages. Early barren quartz and chalcedony are followed by a mottled quartz stage of coarse saccharoidal quartz with irregular streaks and discontinuous bands of sulfide-rich material. The banded quartz–sulfide stage consists of sulfide-rich bands alternating with bands of quartz and bands of chlorite ± illite. Late-stage sulfide-rich veinlets are associated with kaolinite gangue. Ore minerals are argentite and electrum, together with pyrite, sphalerite, galena, chalcopyrite, minor bornite, covellite, and ruby silver. Wall rock alteration is characterized by narrow (< 3 m) halos of illite and illite/smectite next to veins, grading outward into propylitic alteration. Gangue minerals are dominantly massive quartz intergrown with minor to accessory adularia. Epidote, illite, illite/smectite, and, preferentially at deeper levels, Fe-chlorite gangue indicate near-neutral pH hydrothermal fluids at temperatures of >220°C. Kaolinite occurring with the late sulfide-rich veinlet stage indicates pH < 4 and a temperature of <200°C. The Huevos Verdes system has an overall strike of 325°, dipping on average 65° NE. The orientations of individual ore shoots are controlled by vein strike and intersecting north-northwest-striking faults. We propose a structural model for the time of mineralization of the San José district, consisting of a conjugate shear pair of sinistral north-northwest- and dextral west-northwest-striking faults that correspond to R and R′ in the Riedel shear model and that are related to master faults (M) of north-northeast-strike. Veins of 315° strike can be interpreted as nearly pure extensional fractures (T). Variations in vein strike predict an induced sinistral shear component for strike directions of >315°, whereas strike directions of <315° are predicted with an induced dextral strike–slip movement. The components of the structural model appear to be present on a regional scale and are not restricted to the San José district.  相似文献   

9.
Mechanical deformation features in shocked biotites from crystalline rocks of the Ries crater are: kink bands, planar elements, and plastic lattice deformations as determined by X-ray investigations.Kink bands can be observed in micas of various pressure histories (stages 0, I, II and less frequently stage III of shock metamorphism). Kink bands in shocked micas are less symmetrical than kinks of static origin. Asymmetry increases with increasing dynamic pressures. Moreover, kink band width is sensitive against changing peak pressures. Distribution of kinked and undistorted micas within a rock permits to fix the shock front direction. Shock-induced kinks in micas are produced by various gliding processes in the cleavage plane (001).Planar elements seldom occur in biotites of shock stages II and III and have never been described in endogenic rocks. Up to now orientations of planar elements parallel to (111), (1¯11), (112) and (11¯2) have been determined. Planar elements are interpreted as planes of plastic lattice gliding. {[110]} is supposed to be the main gliding direction. In the same pressure region other plastic lattice deformations have been determined. They are orientated parallel to (001), (100) and (¯132) or (201) which results from single crystal X-ray investigations and may represent planes of plastic lattice gliding. The dependency of formation of gliding planes and gliding directions on increasing dynamic pressures will be discussed.  相似文献   

10.
The thermal behaviour of silica rocks upon heat treatment is dependent on the constituent minerals and petrographic texture types. These constituents can be shown to be mainly quartz in the form of two types of chalcedony (Length-fast (LF) chalcedony and Length-slow (LS) chalcedony, the latter also being termed quartzine) and moganite. Even though the thermal behaviour of LF-chalcedony is well understood, major uncertainties persist concerning the high-temperature behaviour of LS-chalcedony and moganite. We present here a comparative study of these three constituents of common silica rocks. Our results show that the chemical reaction is the same in all three, Si–OH + HO–Si → Si–O–Si + H2O, but that the reaction kinetics and activation temperatures are very different. LS-chalcedony begins to react from 200 °C upwards, that is at temperatures 50 °C below the ones observed in LF-chalcedony, and shows the fastest reaction kinetics of this ‘water’ loss. Chemically bound water (SiOH) in moganite is more stable at high temperatures and no specific activation temperature is necessary for triggering the temperature-induced ‘water’ loss. Moganite is also found to act as a stabilizer in silica rocks preventing them from temperature-induced fracturing. These findings have implications for the study of potential heat treatment temperatures of silica rocks (in industry and heritage studies), but they also shed light on the different structures of SiO2 minerals and the role of OH impurities therein.  相似文献   

11.
Uttarakhand geothermal area, located in the central belt of the Himalayan geothermal province, is one of the important high temperature geothermal fields in India. In this study, the chemical characteristics of the thermal waters are investigated to identify the main geochemical processes affecting the composition of thermal waters during its ascent toward the surface as well as to determine the subsurface temperature of the feeding reservoir. The thermal waters are mainly Ca–Mg–HCO3 type with moderate silica and TDS concentrations. Mineral saturation states calculated from PHREEQC geochemical code indicate that thermal waters are supersaturated with respect to calcite, dolomite, aragonite, chalcedony, quartz (SI > 0), and undersaturated with respect to gypsum, anhydrite, and amorphous silica (SI < 0). XRD study of the spring deposit samples fairly corroborates the predicted mineral saturation state of the thermal waters. Stable isotopes (δ18O, δ2H) data confirm the meteoric origin of the thermal waters with no oxygen-18 shift. The mixing phenomenon between thermal water with shallow ground water is substantiated using tritium (3H) and chemical data. The extent of dilution is quantified using tritium content of thermal springs and non-thermal waters. Classical geothermometers, mixing model, and multicomponent fluid geothermometry modeling (GeoT) have been applied to estimate the subsurface reservoir temperature. Among different classical geothermometers, only quartz geothermometer provide somewhat reliable estimation (96–140 °C) of the reservoir temperature. GeoT modeling results suggest that thermal waters have attained simultaneous equilibrium with respect to minerals like calcite, quartz, chalcedony, brucite, tridymite, cristobalite, talc, at the temperature 130 ± 5 °C which is in good agreement with the result obtained from the mixing model.  相似文献   

12.
The effect of Dauphiné twinning on plastic strain in quartz   总被引：1，自引：0，他引：1  
We present an electron backscatter diffraction analysis of five quartz porphyroclasts in a greenschist facies (T = 300–400°C) granitoid protomylonite from the Arolla unit of the NW Alps. Mechanical Dauphiné twinning developed pervasively during the incipient stage of deformation within two porphyroclasts oriented with a negative rhomb plane {z} almost orthogonal to the compression direction (z-twin orientation). Twinning was driven by the anisotropy in the elastic compliance of quartz and resulted in the alignment of the poles of the planes of the more compliant positive rhomb {r} nearly parallel to the compression direction (r-twin orientation). In contrast, we report the lack of twinning in two porphyroclasts already oriented with one of the {r} planes orthogonal to the compression direction. One twinned porphyroclast has been investigated with more detail. It shows the localization of much of the plastic strain into discrete r-twins as a consequence of the higher amount of elastic strain energy stored by r-twins in comparison to z-twins. The presence of Dauphiné twins induced a switch in the dominant active slip systems during plastic deformation, from basal <a> (regions without twinning) to {π} and {π′} <a> (pervasively twinned regions). Dynamic recrystallization is localized along an r-twin and occurred dominantly by progressive subgrain rotation, with a local component of bulging recrystallization. Part of the recrystallized grains underwent rigid-body rotation, approximately about the bulk vorticity axis, which accounts for the development of large misorientation angles. The recrystallized grain size piezometer for quartz yields differential stress of 100 MPa. The comparison of this palaeostress estimate with literature data suggests that mechanical Dauphiné twinning could have a potential use as palaeopiezometer in quartz-bearing rocks.  相似文献   

13.
We ask the question whether petrofabric data from anisotropy of magnetic susceptibility (AMS) analysis of deformed quartzites gives information about shape preferred orientation (SPO) or crystallographic preferred orientation (CPO) of quartz. Since quartz is diamagnetic and has a negative magnetic susceptibility, 11 samples of nearly pure quartzites with a negative magnetic susceptibility were chosen for this study. After performing AMS analysis, electron backscatter diffraction (EBSD) analysis was done in thin sections prepared parallel to the K1K3 plane of the AMS ellipsoid. Results show that in all the samples quartz SPO is sub-parallel to the orientation of the magnetic foliation. However, in most samples no clear correspondance is observed between quartz CPO and K1 (magnetic lineation) direction. This is contrary to the parallelism observed between K1 direction and orientation of quartz c-axis in the case of undeformed single quartz crystal. Pole figures of quartz indicate that quartz c-axis tends to be parallel to K1 direction only in the case where intracrystalline deformation of quartz is accommodated by prism <c> slip. It is therefore established that AMS investigation of quartz from deformed rocks gives information of SPO. Thus, it is concluded that petrofabric information of quartzite obtained from AMS is a manifestation of its shape anisotropy and not crystallographic preferred orientation.  相似文献   

14.
Chalcedony is a spatial arrangement of hydroxylated nanometre-sized α-quartz (SiO2) crystallites that are often found in association with the silica mineral moganite (SiO2). A supplementary Raman band at 501 cm−1 in the chalcedony spectrum, attributed to moganite, has been used for the evaluation of the quartz/moganite ratio in silica rocks. Its frequency lies at 503 cm−1 in sedimentary chalcedony, representing a 2 cm−1 difference with its position in pure moganite. We present a study of the 503 cm−1 band’s behaviour upon heat treatment, showing its gradual disappearance upon heating to temperatures above 300 °C. Infrared spectroscopic measurements of the silanole (SiOH) content in the samples as a function of annealing temperature show a good correlation between the disappearance of the 503 cm−1 Raman band and the decrease of structural hydroxyl. Thermogravimetric analyses reveal a significant weight loss that can be correlated with the decreasing of this Raman band. X-ray powder diffraction data suggest the moganite content in the samples to remain stable. We propose therefore the existence of a hitherto unknown Raman band at 503 cm−1 in chalcedony, assigned to ‘free’ Si–O vibrations of non-bridging Si–OH that oscillate with a higher natural frequency than bridging Si–O–Si (at 464 cm−1). A similar phenomenon was recently observed in the infrared spectra of chalcedony. The position of this Si–OH-related band is nearly the same as the Raman moganite band and the two bands may interfere. The actually observed Raman band in silica rocks might therefore be a convolution of a silanole and a moganite vibration. These findings have broad implications for future Raman spectroscopic studies of moganite, for the assessment of the quartz/moganite ratio, using this band, must take into account the contribution from silanole that are present in chalcedony and moganite.  相似文献   

15.
The structure of the microcrystalline silica varieties chalcedony, flint, moganite, opal-C and -CT is characterized by X-ray powder diffractometry and transmission electron microscopy (TEM). The role of impurities is investigated by infrared spectroscopy and chemical analysis. Microcrystalline opal, chalcedony and flint have a disordered intergrowth structure composed of cristobalite and tridymite domains in opal, and quartz and moganite domains in chalcedony and flint. Each constituent phase has different cell dimensions and symmetry. The main impurity is water which is enriched at the intergrowth interfaces. Density and refractive indices of microcrystalline silica depend on the water content.  相似文献   

16.
The Marl Slate, the English equivalent of the Kupferschiefer, has been studied with particular reference to the relationships between dolomitization and the origin of the metal sulphides. Dolomite occurs as: 1) discontinuous lenses of ferroan dolomicrite, 2) micronodules of finely crystalline dolospar in association with length-slow chalcedony and 3) discrete laminae of ferroan or non-ferroan dolospar. The ferroan dolomicrite has excess CaCO3, and is more abundant in the lower, sapropelic facies of the Marl Slate. It is considered to have formed by the penecontemporaneous alteration of calcium carbonate under hypersaline conditions. Small micronodules (typically about 0.3 mm in diameter) are also more abundant in the sapropelic Marl Slate. These frequently contain cores of length-slow chalcedony (quartzine) fibres and sometimes quartz megacrysts. Textural observations clearly indicate that this silica is of authigenic origin and the dolomite/chalcedony micronodules are interpreted as diagenetic replacements of a calcium sulphate mineral such as anhydrite. The discrete laminae of finely crystalline dolospar are often inter-laminated with calcite in the upper part of the Marl Slate. This dolomite is also calcium rich and represents a replacement, possibly of anhydrite, during a later phase of diagenesis. Metal sulphides occur in two distinct forms: as disseminated framboidal pyrite and as discrete lenses of pyrite, chalcopyrite, galena, sphalerite and rarer sulphides. The framboidal pyrite originated during early diagenesis by reaction of sulphide, produced by reduction of sulphate by organic material and micro-organisms, with iron also released in the reducing environment. The sulphide lenses are often in intimate association with dolospar, length-slow chalcedony and authigenic quartz megacrysts. This indicates that the lenses were produced during diagenesis by the reduction and replacement of calcium sulphate (anhydrite). Various sources, such as co-precipitation with dolomite precursors and the underlying Yellow Sands, may have supplied metals which were mobilized and transported by connate brines as diagenesis progressed.  相似文献   

17.
 The structure of the cristobalite-like polymorph of phosphorus oxynitride PON has been refined using neutron powder diffraction data. It is tetragonal, space group I&4macr;2d, Z=4. The four P–(O,N) distances are equal but the tetrahedron is compressed along c. In AX2 or ABX4 compounds, the tetragonal I&4macr;2d or I&4macr; structure is obtained when the average ratio of the cation to anion radius is below 1.186, whereas the tetragonal P41212 or orthorhombic C2221 structure is obtained at low temperatures for larger ratios. The cell parameters of this PON polymorph have been determined as a function of hydrostatic pressure by in situ angle dispersive X-ray powder diffraction in a diamond-anvil cell. Under truly hydrostatic pressure, a strong anisotropic behavior is observed with the c parameter being nearly incompressible. Very slight anisotropic stress strongly modifies the high-pressure behavior. According to the pressure-temperature conditions of treatment, three phases, cristobalite-, moganite-, and quartz-like, have been obtained by quenching experiments, and the P–T phase diagram of PON was derived. The high-pressure behavior of the α-quartz, moganite, and cristobalite-like polymorphs of PON and SiO2 is discussed. Received: 7 August 2000 / Accepted: 21 January 2001  相似文献   

18.
Electron microscopic examination of Namurian bedded cherts in North Wales (Great Britain) has revealed the existence of two dominant surface textures, granular and spongy, with a range of textures intermediate between the two. These textures can be used to differentiate between three principal lithologies within the cherts and between laminae and colour bands within the lithologies. A positive relationship is demonstrated between the presence of spongy texture and a high volume of impurities; there is a gradation, with increasing argillaceous/organic content, from granular through intermediate to spongy surfaces. It is suggested that this gradation reflects an increase in chalcedony over micro-crystalline quartz resulting from: (l) an increase in impurities; (2) an increase in the rate of precipitation of silica. The relationship between surface texture and type of lithology is considered to be consistent with the order of crystallisation for silica put forward by MILLOT (1960).  相似文献   

19.
Planar elements in quartz, produced by shock induced plastic deformation, have been investigated in four quartz-plagioclase veins contained in an amphibolite from the crystalline basement of the Ries Crater from the drill hole Nördlingen 1973.The crystallographic orientation of planar elements in quartz grains is similar in all four rocks ({10¯13} predominant, {0001} less frequent, {10¯12} and others still rarer), indicating an average shock pressure in the range between 150 and 200 kbar.The spatial density of planar elements as measured by the number of systems per shocked grain, the number of individual elements per shocked grain, or as ratio shocked: unshocked grains increases with increasing grain size. This grain size effect is supposed to be primarily a consequence of the heterogeneity of the stress field which produced a random distribution of local stress maxima and locally restricted areas of plastic quartz deformation in the rock. The probability that planar elements develop within one individual grain increases, therefore, with increasing grain size.In one leucosome in which the quartz grains were randomly oriented planar elements parallel to {10¯13} cluster in a stereographic projection within one belt. It is supposed that the pole of this belt indicates the direction in which the shock front passed through the rock.  相似文献   

20.
18O/16O, 34S/32S, and D/H ratios as well as vacuum-fusion H2O+ contents were measured for late Tertiary volcanic basaltic rocks ranging in composition from quartz tholeiites and alkali olivine basalts to melilite-bearing olivine nephelinites and for peridotite xenoliths from the Northern Hessian Depression of W.-Germany. Measured Oisotope ratios in both basalts and peridotites were corrected for variable degree of post-eruption, secondary alteration. The ranges and means of corrected 18O values ( SMOW) for the North Hessian lavas and peridotites are: (i) 8 tholeiites: ca. +6.1 to +7.3 (¯x=+6.6), (ii) 21 alkali olivine basalts: ca. +5.4 to +7.6 (¯x=+6.5), (iii) 19 nepheline basanites, limburgites, and olivine nephelinites: ca. +5.3 to +8.0 (¯x=+6.6), and (iv) 23 peridotites: +5.1 to 7.0 (¯x+6.0). The 34S values ( CDT) for the tholeiites range from –0.6 to +1.4 (¯x=–0.03) and for the alkali basalts range from +0.9 to +8.6 (¯x=+2.5). The approximate D value ( SMOW) of the pristine basalts and peridotites is estimated to have been ca. –90The quartz tholeiites appear to have had a different genetic history than the alkali basalts. Supported by chemical evidence, the 18O and 87Sr enrichment observed in the tholeiites suggests low crustal contamination of parental olivine tholeiite melts, derived from a depleted mantle source. The contamination by crustal partial melts may have occurred in granulitic lower crust during differentiation. By contrast the high 18O and 34S values observed for the alkali basalts and peridotites are best explained in terms of metasomatic alteration of the mantle source region by fluids enriched in 18O, K, and incompatible trace elements prior to partial melting. The 18O-K relationships for the peridotites indicate that the mantle beneath the Northern Hessian Depression has had a complex stable isotope history involving at least two distinct metasomatic events. The earlier event involved a CO2-rich fluid which modified 18O/16O ratios without altering the mineralogical character of the mantle peridotite. The second event involved an aqueous fluid, which mainly altered the clinopyroxene and introduced phlogopite (plus possibly apatite, carbonate, and amphibole). It superimposed an 18O and K enrichment upon a previously altered mantle.  相似文献   

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