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1.
Christian Desnoyers 《Earth and Planetary Science Letters》1980,47(2):223-234
The origin of olivine grains in C2 carbonaceous chondrites is a controversial topic: directly condensed material or detrital remnants of preexisting chondrules? This study shows that the Niger C2 meteorite is similar to Murchison but reveals several interesting features in relation to the origin of the olivine. Microprobe analysis of olivine (Si, Fe, Mg, Ca, Mn, Cr), glass and nickel-iron inclusions within the grains, and Fe-S-O phase as well as the relationships between the olivine grains in the aggregates, between the grains and the interstitial phyllosilicate matrix, between the inclusions and their host olivine grains, and the morphology of some aggregates all show that two populations of olivine coexist, probably crystallized from chondrule melts rather than by direct condensation from a solar nebula gas. The characteristics of the nickel-iron inclusions within the olivine suggest a magmatic chondrule-making stage from previously condensed materials. 相似文献
2.
《Earth and Planetary Science Letters》2006,241(1-2):32-46
This study examines the effects of natural shock metamorphism on fluid inclusions trapped in porous sedimentary target rocks and compares these results to previous experimental work on single crystal quartz. Samples of shock metamorphosed Coconino sandstone were collected from Barringer Meteorite Crater (Meteor Crater, Arizona) and classified based on their shock features into the six shock stages described by Kieffer [S.W. Kieffer, 1971. Shock metamorphism of the Coconino sandstone at Meteor Crater, Arizona, Journal of Geophysical Research 76, 5449-5473.]. The frequency of two-phase fluid inclusions decreases dramatically from unshocked samples of Coconino sandstone through shock stages 1a, 1b, and 2. No two-phase fluid inclusions were observed in shock stage 3 or 4 samples. However, the total number of grains containing fluid inclusions remains approximately the same for shock stages 1a–2, suggesting that two-phase fluid inclusions reequilibrated during impact to form single-phase inclusions. In shock stages 3 and 4, the total number of inclusions also decreases, indicating that at these higher shock pressures fluid inclusions are destroyed by plastic deformation and phase changes within the host mineral. Entrained quartz grains within a shock stage 5 sample contain two-phase inclusions, emphasizing the short duration of melting associated with the impact and the heterogeneous nature of impact processes. These results are similar to those observed in single-crystal experiments, although inclusions survive to slightly higher shock pressures in samples of naturally shocked Coconino sandstone. Results of this study suggest that the rarity of fluid inclusions in meteorites does not preclude the presence of fluids on meteorite parent bodies. Instead, fluid inclusions trapped during alteration events may have been destroyed due to shock processing. In addition, loss of fluids from inclusion vesicles along fractures and microcracks may lead to shock devolatilization, even in unsaturated target rocks. 相似文献
3.
T. G. Churikova B. V. Ivanov J. Eichelberger G. Wörner B. Browne P. Izbekov 《Journal of Volcanology and Seismology》2013,7(2):112-130
The data on the geochemistry of the rocks of Kizimen Volcano and results of microprobe studies of major and trace elements in plagioclase grains from acid lavas and basalt inclusions are presented. The characteristics of the Kizimen Volcano are the following: (1) basalt inclusions are abundant in acid lavas; (2) banded, mixed lavas occur; (3) the distribution curves of rare-earth elements of acidic lavas and basalt inclusions intersect; (4) Sr-Nd isotope systematics of the rocks and inclusions do not indicate mixture with crustal material; (5) plagioclase phenocrysts are of direct and reverse zonation; (6) olivine and hornblende, as well as acid and mafic plagioclases, coexist in the rocks. The studies revealed that the rocks are of a hybrid nature and originated in the course of repeated mixture of acid and mafic melts either with chemical and thermal interaction of melts or exclusively thermal ones. Study of the major- and trace-element distribution in zonal minerals provides an informative tool for understanding the history of the generation and evolution of melts in a magma chamber. 相似文献
4.
Robert B. Forbes 《Bulletin of Volcanology》1963,26(1):13-21
The basaltic rocks of the Hut Point area contain a diverse suite of inclusions. The xenolithic nature of many sedimentary, metamorphic and igneous inclusion types is clearly displayed, but previous studies have left considerable doubt as to whether the ultrabasic inclusions are noncognate xenoliths, and possible relicts from the mantle brought to the surface in a magma originating in this zone; or rather of cognate and/or cumulative origin. Olivine grains in the dunite and enstatite bearing peridotite inclusions commonly display undulose extinction and strain bands (translation laminae), and preferred indicatrix orientations expressed as B γ girdles. Enstatite frequently contains clinopyroxene exsolution laminae, and many grains are deformed and/or disrupted by shear zones. Weak B γ olivine fabrics have also been detected in the titanaugite peridotites and pyroxenites, but no evidence of post-crystallization deformation was detected. Titanaugite also occurs as phenocrysts (xenocrysts?) and in the groundmass of the host basalt chemical and optical data suggest that the titanaugite grains in the basalt and the inclusions are genetically related. These preliminary findings suggest that the titanaugite bearing inclusions may be of cognate origin, and that the dunite and enstatite bearing peridotite fragments are more likely noncognate xenoliths. Field and laboratory studies of this problem are continuing as supported by a grant from the National Science Foundation. 相似文献
5.
The origin of olivine grains isolated in the matrix of C2 carbonaceous chondrites is an important problem. If these grains are condensates from a solar nebular gas, they contain compositional, isotopic and physical features that further elucidate that process. If, however, they are grains released by the breakup of chondrules, then many important condensation features have been lost during the melting that took place to form chondrules.In evaluating these two possibilities, care must be taken to determine which inclusions in C2 meteorites are actual chondrules and which are aggregates of grains that have never undergone melting. The two main types of aggregates, pyroxene-rich and pyroxene-poor, are forty to fifty times more abundant than chondrules. Four scenarios are presented to account for the kinds of aggregates and isolated grains seen in the Murchison C2 meteorite. An analysis of these scenarios is made in light of olivine crystal morphology, comparison of composition of glass inclusions inside olivine grains with interstitial glass in true chondrules and size distributions of olivines, isolated, in aggregates and in chondrules.It is concluded that no scenario that includes a chondrule-making step can account for the observed population of isolated olivine grains. An origin by direct condensation, partial comminution, aggregation and accretion best accounts for the sizes and morphological features observed. 相似文献
6.
Plunge pool deposits from Australia's 'Top End' are considered as important archives of past monsoonal activity in the region. The available chronology of these deposits was so far based on thermoluminescence (TL) dating and indicated maximum flood magnitudes during the Last Glacial Maximum in contrast with more arid conditions as deduced from other archives of the region. This study revisits plunge pool deposits at Wangi Falls by applying multiple and single-grain Optically Stimulated Luminescence (OSL) dating of quartz and high-resolution gamma spectrometry, supported by radiocarbon dating of organic material. The aim is to reappraise the existing chronology and investigate if the deposits are affected by partial bleaching, post-depositional mixing and/or problems related to annual dose determination. The latter seems to have a minor impact on the ages at most. Equivalent Dose (De) distributions are broad, in particular for single grains, but apparently not result from partial bleaching or post-depositional mixing. Rather, microdosimetry caused by radiation hotspots in the sediment and zircon inclusions in the quartz grains is considered problematic for these sediments. The results presented here imply that the previous TL chronology overestimated the real deposition age of the sediments. 相似文献
7.
A fluid-inclusion study has been performed on quartzite nodules of stromboli volcano hosted by calc-alkaline lavas of both the Strombolicchio (200 ka) and Paleostromboli II (60 ka) periods. The nodules are mainly composed of quartz crystals with subordinate plagioclase and K-feldspar. Small interstitial minerals such as plagioclase, K-feldspar, clinopyroxene, biotite, and quartz are also found, together with glass. Muscovite, epidote and zircon occur as accessory minerals. Different quartzite nodules occur: (1) equigranular polygonal granoblastic quartzite nodules forming a polygonal texture with clear triple points; (2) inequigranular polygonal granoblastic quartzite nodules; and (3) break-up nodules with strongly resorbed quartz. These quartzites are restites from partial melting, involving felsic crustal rocks at the magma/wall rock contact. Restitic quartz re-crystallises at variable and generally high temperatures, leading to the formation of quartzites with different textures. Quartz grains contain five types of fluid inclusions distinguished on the basis of both fluid type and textural/phase relationships at room temperature. Type I are two-phase (liquid+vapour) CO 2-rich fluid inclusions. They are primary and subordinately pseudosecondary in origin and have undergone re-equilibration processes. Type II mono-phase/two-phase (vapour/liquid+vapour) CO 2-rich fluid inclusions are the most common and, based on their spatial distribution and shape, they can be divided into two subclasses: type IIa and type IIb. Type II inclusions are secondary or pseudosecondary and they are assumed to have formed after decrepitation of type I inclusions and cracking of the host quartz. Type III inclusions are mono-phase (vapour); they possibly contain CO 2 at very low density and surround the inner rims of quartz grains. Type IV two-phase silicate-melt inclusions contain glass±CO 2-rich fluid. Some of them are cogenetic with type II inclusions. Finally, type V two-phase (liquid+vapour) aqueous inclusions are both vapour-rich and liquid-rich aqueous inclusions. Microthermometric experiments were performed on both type I and II inclusions. Type I inclusions homogenise to liquid between 20 and 30.5 °C. Type IIa inclusions homogenise to vapour in the 24 to 30 °C range, with a maximum peak of frequency at 29 °C. Type IIb inclusions also homogenise to vapour between 14 and 25 °C. There appears to be no difference in homogenisation temperature distribution between the Strombolicchio and Paleostromboli II samples. The trapping pressures of the fluid inclusions have been obtained by combining the microthermometric data of the Strombolicchio and Paleostromboli II samples with the pressure–temperature–volume (i.e. density) characteristics for a pure CO 2 system. The data on the early inclusions (type I) suggest an important magma rest at a pressure of about 290 MPa (i.e. about 11-km depth). Type IIa CO 2 inclusions suggest that a second magma rest occurred at a pressure of about 100 MPa (i.e. about 3.5-km depth), whereas type IIb inclusions were trapped later at a shallower depth during the final magma upwelling. No pressure/depth differences seem to occur between the Strombolicchio and Paleostromboli II periods, indicating the same polybaric rests for the calc-alkaline magmas of Stromboli, despite their significantly different ages. This persistence in magma stagnation conditions from 200 to 60 ka suggests a similar plumbing system for the present-day Strombolian activity. 相似文献
8.
V. V. Anan’ev 《Journal of Volcanology and Seismology》2016,10(2):117-125
A microprobe study of Ol–An nodules (allivalites) as sampled from the basaltic andesites of Malyi Semyachik Volcano in Kamchatka revealed small inclusions of a mafic mineral in crystals of olivine and anorthite and in the intergranular mass. The mineral was later identified as spinel. A microprobe analysis of the spinel showed that the grains that were found in minerals are not zonal, with spinel inclusions of different chemical compositions occurring in a single crystal. In contrast to this situation, the spinel crystals in the intergranular basis are zonal and exhibit a reaction rim. The spinel compositions form a long and well-pronounced trend that occurs in the solvus zone of solid solutions that has not been adequately described in the literature. The existence of the resulting spinel trend in the solvus zone can be explained by an early capture of spinel grains by growing silicate crystals and by their rapid cooling after the eruption, which chilled the metastable solution. Spinel grains occasionally exhibited initial phases in the decomposition of a solid solution owing to their long residence in a cooling lava flow at temperatures below the solvus. It is supposed that these spinels were formed somewhat earlier or simultaneously with the crystallization of the silicate phase. This diversity in the compositions of non-zonal spinel can be explained by thermo diffusion that led to homogeneous compositions in the initially zonal spinel crystals after they had been included in the host mineral crystals. 相似文献
9.
Petrographic and chemical studies of the Qingzhen chondrite strongly suggest that it is the most highly unequilibrated (type 3) enstatite chondrite recognized so far. Qingzhen contains abundant, well-defined chondrules, some of which were incompletely molten during the chondrule formation process. The relict olivine grains within these chondrules contain dusty inclusions of almost pure metallic Fe, which appear to be the in-situ reduction product of the fayalitic component of the olivine. The reduction process presumably took place at the time of chondrule formation and the chondrule precursor material must have been more oxidized than average enstatite chondrite material. We believe that this oxidized material may have formed at the enstatite chondrite formation location in the solar nebula, provided fluctuations in the degree of oxidation of the nebular gas existed at such locations. Reheating of this material under more reducing conditions would lead to the observed reduction of the olivine. Igneous olivines within chondrules always contain detectable amounts of CaO, while relict olivines are essentially CaO-free. This seems to suggest that the relict olivines did not originate during a previous igneous process of chondrule formation and might represent condensation products from the early solar nebula. 相似文献
10.
LIU ZhengHong XU ZhongYuan & WANG KeYong College of Earth Science Jilin university Changchun China 《中国科学D辑(英文版)》2007,50(4):496-504
Polycrystalline quartz ribbons in high-grade metamorphic rocks from the Daqingshan region, are typi- cal microfabrics of, and provide information for, deep crust deformation and metamorphism. The quartz ribbons have straight boundaries and extend stably along gneissosity. They truncate other mineral grains in the rocks and may contain inclusions of such minerals that are lens-shaped and oriented. They frequently end into branching termination. Analysis fluid inclusions in polycrystalline quartz rib- bons reveal that the complex types of fluid inclusions are inhomogeneously distributed. They are ob- viously different from inclusions captured at granulite facies, in both fluid compositions and T-P esti- mations. Based on microfabric and fluid inclusion analysis, the polycrystalline quartz ribbons are suggested to be formed by SO2-rich fluids filling micro-fractures that are parallel to early gneissosity. The SO2 composition is derived from the deformed host rocks. The fluid phase has significant effects on the rheological characteristics, fracturing of rocks, and formation of quartz ribbons. 相似文献
11.
Polycrystalline quartz ribbons in high-grade metamorphic rocks from the Daqingshan region, are typical microfabrics of, and provide information for, deep crust deformation and metamorphism. The quartz ribbons have straight boundaries and extend stably along gneissosity. They truncate other mineral grains in the rocks and may contain inclusions of such minerals that are lens-shaped and oriented. They frequently end into branching termination. Analysis fluid inclusions in polycrystalline quartz ribbons reveal that the complex types of fluid inclusions are inhomogeneously distributed. They are obviously different from inclusions captured at granulite facies, in both fluid compositions and T-P estimations. Based on microfabric and fluid inclusion analysis, the polycrystalline quartz ribbons are suggested to be formed by SO2-rich fluids filling micro-fractures that are parallel to early gneissosity. The SO2 composition is derived from the deformed host rocks. The fluid phase has significant effects on the rheological characteristics, fracturing of rocks, and formation of quartz ribbons. 相似文献
12.
A Precambrian metadolerite dyke has two distinct types of remanence carriers; those with medium/high coercivities (unblocking fields of 20–120 mT) and those with low coercivities (unblocking fields of <15 mT). Optical examination reveals numerous submicron probably opaque inclusions in the plagioclase feldspar and also large opaque grains consisting of coarse oxidation-exsolution intergrowths of magnetite and ilmenite. All opaque phases have been examined using transmission electron microscopy together with microanalysis and electron diffraction. The submicron inclusions in the plagioclase are titanomagnetites(0 < x ≤ 0.14) with a size range between about 0.01 and 0.5 μm and axial ratios between 1 (equidimensional) and about 0.3. Many of these inclusions fall in the single-domain field but some are probably pseudo-single-domain. The large opaque grains contain almost pure magnetite and ilmenite and show no fine-scale exsolution; the magnetite regions of the intergrowths are of multidomain size and reveal multidomain structure under Lorentz electron microscopy. There are also some primary ilmenites containing very fine exsolved haematite, and there are very fine plates of ilmenite and very elongate needles of magnetite within the augite. Experiments on artificial samples containing very carefully prepared separates of plagioclase and large opaque grains show that the pure plagioclase acquires a remanence with unblocking fields of 20–140 mT and blocking temperatures of 390–590°C and the large opaque grains acquire a remanence with unblocking fields of less than 15 mT but a wide range of blocking temperatures up to about 570°C. It is concluded that the medium/high coercivity component of remanence in the rock is carried largely or possibly entirely by the submicron magnetites within the plagioclase and that the low coercivity component is carried largely or entirely by the multidomain magnetites in the large opaques. The contribution of the magnetite needles in the augite is uncertain as the rock does not contain any detectable component of remanence with the extremely high coercivities expected from their very elongate shape. 相似文献
13.
Besides Pb and U loss and mixing of crystals of different age, U gain is considered a possible cause of discordant U-Pb ages of zircons. However, whether U gain without new zircon growth occurs in nature had not been proven, so far. In order to test this possibility, two detrital zircon populations were studied for which the absence of later zircon overgrowth after deposition could be demonstrated. The samples were separated from a metaquartzite near a large pegmatite body and from metaquartzite inclusions found in the pegmatite (Martell Valley, Italian Alps). The distribution of neutron-induced fission tracks reveals distinct accumulation of U in the rims of more than 90% of the zircon grains of the inclusions (total U in the crystals: 540–850 ppm), whereas in the country rock only some of the grains show similar but weaker patterns (total U: 155–320 ppm). From the isotopic data and from additional U-Pb and Rb-Sr analyses of minerals and whole-rock samples of the pegmatite, the marginal accumulation and the higher concentration of U in the zircon grains of the inclusions are interpreted as the result of episodic U gain during the intrusion of the pegmatite and/or during a later metamorphism. From the concentration levels of common Pb, an addition of Pb - and possibly other elements - to the zircon grains is inferred. 相似文献
14.
JAN DOUMA 《Geophysical Prospecting》1989,37(7):831-849
Media containing aligned cracks or ellipsoidal inclusions as well as media consisting of sequences of isotropic layers show transverse isotropy with respect to elastic wave propagation. However, the transversely isotropic media which are equivalent to media containing aligned inclusions do not necessarily have to be representable by sequences of stable isotropic layers. These transversely isotropic media can be modelled by such sequences if - and only if - several stability conditions are satisfied. Important parameters determining whether these conditions are satisfied are the aspect ratio of the inclusions and the material filling the inclusions, the‘fluid’. An analytical expression describing the range of aspect ratios for which the constraints are satisfied can be derived. This expression (which is a good approximation for several crack models) and numerical calculations show that media containing water-filled inclusions can be represented by sequences of stable isotropic layers if the inclusions have aspect ratios less than 0.1. The limiting aspect ratio decreases for a decreasing ratio of the bulk modulus of the fluid to the shear modulus of the matrix material. Finally, media containing dry inclusions of any aspect ratio cannot be modelled by thin isotropic layering. These results depend only weakly on the crack density and on the matrix material. The representation of crack-induced anisotropy by layer-induced anisotropy can be used to classify crack-induced anisotropy and might be useful in the separation of the cause of anisotropy and the determination of the nature of the fluid. 相似文献
15.
V. V. Anan’ev 《Journal of Volcanology and Seismology》2011,5(6):399-408
Microprobe studies of unzoned plagioclases (An92–96) from crystal tuff of Mutnovskii Volcano and allivalite nodules of rocks from the Ksudach, Malyi Semyachik, and Golovnina
Volcanoes revealed small inclusions of a dark-colored mineral that was later identified as spinel. Microprobe analyses showed
that the grains are unzoned and spinel inclusions of different chemical compositions may occur in one plagioclase crystal.
The spinel compositions form a clear extended single trend corresponding to the solvus zone of a solid solution that has not
been described in the literature. The existence of this spinel trend in the solvus zone might have been due to early capturing
of spinel grains by growing plagioclase crystals and their rapid cooling soon after eruption, resulting in hardening of the
metastable solution. These spinels are supposed to form synchronously with plagioclase crystallization. The diversity of spinel
compositions is explained by thermo diffusive equalizing of originally zonal spinel crystals after they were captured by plagioclase
crystals or by their growth in crystallization haloes of anorthite. 相似文献
16.
J. J. Kasper‐Zubillaga G. Ortiz‐Zamora W. W. Dickinson J. Urrutia‐Fucugauchi A. M. Soler‐Arechalde 《地球表面变化过程与地形》2007,32(3):366-389
Textural, compositional, physical and geophysical determinations were carried out on 111 beach and dune sand samples from two areas in New Zealand: the Kapiti–Foxton coast sourced by terranes of andesite and greywackes and the Farewell Spit–Wharariki coast sourced by a wide variety of Paleozoic terranes. Our aim is to understand how long‐shore drift, beach width and source rock control the sedimentological and petrographic characteristics of beach and dune sands. Furthermore, this study shows the usefulness of specific minerals (quartz, plagioclase with magnetite inclusions, monomineralic opaque grains) to interpret the physical processes (fluvial discharges, long‐shore currents, winds) that distribute beach and dune sands in narrow and wide coastal plains. This was done by means of direct (grain size and modal analyses) and indirect (specific gravity, magnetic/non‐magnetic separations M/NM, magnetic susceptibility measurements, hysteresis loops) methods. Results are compared with beach sands from Hawaii, Oregon, the Spanish Mediterranean, Elba Island and Southern California. Compositionally, the Kapiti–Foxton sands are similar to first‐order immature sands, which retain their fluvial signature. This results from the high discharge of rivers and the narrow beaches that control the composition of the Kapiti–Foxton sands. The abundance of feldspar with magnetite inclusions controls the specific gravity of the Kapiti–Foxton sands due to their low content of opaque minerals and coarse grain size. Magnetic susceptibility of the sands is related mainly to the abundance of feldspars with Fe oxides, volcanic lithics and free‐opaque minerals. The Farewell Spit–Wharariki sands are slightly more mature than the Kapiti–Foxton sands. The composition of the Farewell Spit–Wharariki sands does not reflect accurately their provenance due to the prevalence of long‐shore drift, waves, little river input and a wide beach. Low abundance of feldspar with magnetite inclusions and free opaque grains produces poor correlations between specific gravity (Sg) and Fe oxide bearing minerals. The small correlation between opaque grains and M/NM may be related to grain size. The magnetic susceptibility of Farewell Spit–Wharariki sands is low due to the low content of grains with magnetite inclusions. Hysteresis and isothermal remnant magnetization (IRM) agree with the magnetic susceptibility values. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
17.
(1) The observed anomalies in meteoritic oxygen isotope compositions are not due to an incomplete mixing of several dust or gas-plus-dust components in the solar nebula. If they were, other elements would display similar anomalies. (The FUN inclusions in Allende appear to be exceptions to this premise.) (2) The anomalies must therefore stem from differing degrees of incomplete exchange of oxygen isotopes between the primordial gas and dust components of the nebula. The dust is more likely to have been the16O-enriched component. (3) Since the isotopic difference between dust and gas probably could not have been preserved if the dust was ever completely vaporized in the nebula, the Ca,Al-rich inclusions (CAI's) in carbonaceous chondrites are unlikely to be condensates, but instead are distillation residues. (4) If so, the observed depletion of super-refractory elements in the Group II CAI's cannot have been accomplished by fractional condensation in the solar nebula. (5) Then this depletion, and a number of other properties of the components of primitive meteoritic material, must be relics of pre-solar system fractionations among different populations of interstellar dust grains. 相似文献
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20.
John A. Wood 《Earth and Planetary Science Letters》1984,70(1):11-26
During the formation of the solar nebula interstellar grains were fallling into the nebula with velocities of the order of 10 km/s at the radial distance where the meteorites were to form. This kinetic energy is 20 times the amount of thermal energy needed to melt the grains. The grains were decelerated by aerodynamic drag in the nebula. Where grain-rich parcels of interstellar material fell into the nebula, heat generated by drag could not be radiated away because of the opacity imparted to the system by the grains, and high temperatures were reached. In this situation presolar aggregations of grains would melt to form chondrules. Many of the properties of chondrules (and also CAI's) are consistent with their formation by this means. The infall heating concept provides a new framework in which the formation and significance of chondritic meteorites can be understood. 相似文献