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1.
Glass from the West Clearwater Lake hypervelocity impact crater contains numerous spheroids, 10 to 500 μm across, which appear to have formed at high temperatures as fluids immiscible in the enclosing melt. The spheroids are distinguished from small, normal, largely void gas vesicles, which are also present, by being completely filled in all cases; by having fillings which vary in composition from spheroid to spheroid, even between spheroids in close association; and by indications that the present fillings are representative of the contents present before the matrix melt chilled. Most of the spheroids are classified petrographically into three types. Type I, the most numerous, includes all spheroids>100 μm and are filled with uncommon pale brown to yellow montmorillonites with an unusual structure intermediate between dioctahedral montmorillonite and saponite. Type II, brown and green, are filled with Fe-rich montmorillonites, while Type III are aluminia-rich montmorillonites crystallized into mica-like sheaves. Rare, small spheroids are filled with calcite or silica. In a few cases one spheroid encloses another of similar or different type. Electron microprobe analyses indicate that with few exceptions Types I and III spheroids belong to a Mg series of montmorillonites in which the main chemical variation is the substitution of Mg for Al. A second Fe-K series includes Type II and a few Type I spheroids and shows substitution of Fe by Al, relatively high K2O and, in the alumina-rich members, low SiO2. The close association of spheroids with deformed, embayed lechatelierite inclusions indicates that they formed while the latter were liquid, i.e. at temperatures above 1700°C, as rapidly moving impact melt engulfed highly shocked inclusions of quartz-bearing country rock. The preservation of spheroids in the West Clearwater Lake glass is attributed mainly to the position of the glass masses within the breccias lining the crater floor. It is considered that the glass in this location did not achieve free flight but, as part of a large mass, cooled relatively slowly through the high temperature regime in which the spheroids were generated, and then, when detached, chilled rapidly to preserve a record of this transient stage in their history.  相似文献   

2.
Reduction spheroids are small-scale, biogenic, redox-controlled, metal enrichments that occur within red beds globally. This study provides the first analysis of the compositionally unique reduction spheroids of the Tumblagooda Sandstone. The work aims to account for their composition and consequently improve existing models for reduction spheroids generally, which presently fail to account for the mineralogy of the Tumblagooda Sandstone reduction spheroids. Interstitial areas between detrital grains contained in the cores of these reduction spheroids are dominated by microplaty haematite, in addition to minor amounts of svanbergite, gorceixite, anatase, uraninite, monazite and illite. The haematite-rich composition, along with an absence of base metal phases and the vanadiferous mica roscoelite, makes these reduction spheroids notable in comparison to other global reduction spheroid occurrences. Analyses of illite crystallinity provide values for samples of the Tumblagooda Sandstone host rock corresponding to heating temperatures of ca 200°C. Consequently, while Tumblagooda Sandstone reduction spheroids formed via the typical metabolic processes of dissimilatory metal-reducing bacteria, the combination of a unique mineralogy and illite crystallinity analysis provides evidence of more complex late-stage heating and reoxidation. This has not previously been recognised in other reduction spheroids and therefore expands the existing model for reduction spheroid genesis by also considering the potential for late-stage alteration. As such, future reduction spheroid studies should consider the potential impact of post-formation modification, particularly where they are to be used as evidence of ancient microbial processes; such as in the search for early evidence of life in the geological record on Earth or other planets. Additionally, because of their potential for modification, reduction spheroids serve as a record of the redox history of red beds and their study could provide insights into the evolution of redox conditions within a given red bed during its diagenesis. Finally, this paper also provides insights into the relatively understudied diagenetic history of the Tumblagooda Sandstone; supplying the first reliable and narrow constraints on its thermal history. This has important implications for the thermal history of the Carnarvon Basin and its petroleum prospectivity more broadly.  相似文献   

3.
Microstructures recently reported from an Archaean sedimentary succession (ca. 3.0 Ga) in the Mount Goldsworthy–Mount Grant area in the northeastern Pilbara Craton meet the criteria for compelling evidence of biogenicity [Sugitani, K., Grey, K., Allwood, A., Nagaoka, T., Mimura, K., Minami, M., Marshall, C.P., Van Kranendonk, M.J., Walter, M.R., 2007. Diverse microstructures from Archaean chert from the Mount Goldsworthy–Mount Grant area, Pilbara Craton, Western Australia: microfossils, dubiofossil, or pseudofossils. Precambrian Res. 158, 228–262]. The structures are morphologically diverse. Although they were tentatively classified into five major morphological types (thread-like, film-like, small (<15 μm) and large (>15 μm) spheroidal, and spindle-like), the possible taxonomic significance of these groups was not discussed. Building on our earlier analysis, we focus on the morphology of the larger spheroids, as well as presenting further evidence relating spindles and several bizarre forms, and attempt to group them taxonomically and adduce additional evidence for their biogenicity.Taphonomic features were identified in each of the various morphological groups, but the range of morphological diversity of the spheroids cannot be attributed solely to taphonomic alteration. Four subdivisions of spheroids are proposed: (1) simple single-walled spheroids, (2) thin-walled spheroids having a diffuse envelope, (3) thick-walled spheroids, and (4) spheroids having an extensively folded wall. Simple single-walled spheroids, 15–60 μm in diameter, with various wall textures but commonly lacking envelopes or appendages form the dominant subgroup. Other complex morphologies are present and include aligned or associated bodies of thin-walled spheroids with diffuse envelopes, and spindle-like structures containing inner spheroidal bodies. The degree of morphological complexity and associations between structures suggest the presence of reproductive phases. If correct, this implies that the early Earth (ca. 3.0 Ga) showed a higher level of biodiversity than is currently postulated.  相似文献   

4.
Atomic absorption analyses of 25 metallic spheroids from Barringer Meteorite Crater have been carried out for Fe, Ni, Co and Cu. In addition, electron microprobe analyses of 58 impactite metallic particles have been carried out for Fe, Ni and Co from four different impactite samples. The normalized Ni, Co and Cu contents of the spheroids were from 13–22 per cent, 0.8–1.3 per cent and 260–430 μg/g, respectively. These figures represent enrichment factors of 2–3 in the spheroids compared to analyses of the bulk meteorite. The Co/Ni and Cu/Ni ratios in the spheroids are close to the respective ratios in the bulk meteorite. This suggests that the spheroids were formed by preferential removal of iron by oxidation from a chemically homogeneous liquid.The impactite metallic particles had Ni contents from 10 to 95 per cent and Co contents from 0·3 to 4 per cent. The Co contents of these particles showed a positive correlation with Ni up to 60 per cent Ni and a negative correlation beyond 60 per cent Ni. Reaction of the impactite metallic particles with SiO2 of the target can explain these variations. Our findings show that extensive chemical reaction between projectile and target occurs at impact.  相似文献   

5.
Chert spheroids are distinctive, early‐diagenetic features that occur in bedded siliceous deposits spanning the Phanerozoic. These features are distinct in structure and genesis from similar, concentrically banded ‘wood‐grain’ or ‘onion‐skin’ chert nodules from carbonate successions. In the Miocene Monterey Formation of California (USA), chert spheroids are irregular, concentrically banded nodules, which formed by a unique version of brittle differential compaction that results from the contrasting physical properties of chert and diatomite. During shortening, there is brittle fracture of diatomite around, and horizontally away from, the convex surface of strain‐resistant chert nodules. Unlike most older siliceous deposits, the Monterey Formation still preserves all stages of silica diagenesis, thus retaining textural, mineralogical and geochemical features key to unravelling the origin of chert spheroids and other enigmatic chert structures. Chert spheroids found in opal‐A diatomite form individual nodules composed of alternating bands of impure opal‐CT chert and pure opal‐CT or chalcedony. With increased burial diagenesis, surrounding diatomite transforms to bedded porcelanite or chert, and spheroids no longer form discrete nodules, yet still display characteristic concentric bands of pure and impure microcrystalline quartz and chalcedony. Petrographic observations show that the purer silica bands are composed of void‐filling cement that precipitated in curved dilational fractures, and do not reflect geochemical growth‐banding in the manner of Liesegang phenomena invoked to explain concentrically banded chert nodules in limestone. Chertification of bedded siliceous sediment can occur more shallowly (< 100 m) and rapidly (< 1 Myr) than the bulk silica phase transitions forming porcelanite or siliceous shale in the Monterey Formation and other hemipelagic/pelagic siliceous deposits. Early diagenesis is indicated by physical properties, deformational style and oxygen‐isotopic composition of chert spheroids. Early‐formed cherts formed by pore‐filling impregnation of the purest primary diatomaceous beds, along permeable fractures and in calcareous–siliceous strata.  相似文献   

6.
Spheroids of some m size are described from the lead-zinc-ore of Tynagh, Ireland. The main element composition of the spheroids is Si, Zn, S, Al, K, Fe with some Ca and Mg in order of decreasing amount. In most spheroids which are rich in Zn-and poor in Fe-content an almost constant ratio of (Fe+Zn):S and Si:Al:K gave reason to assume the mineral phases sphalerite with various iron content and phyllosilicates as the main constituents. It is suggested that the spheroids were formed by similar processes Degens et al. (1972) described for recent formation of sphalerite globules in the Lake Kivu.  相似文献   

7.
Many kinds of ichnofossil Zoophycos occur commonly in the carbonate rocks of Pennsylvanian to Cisuralian Taiyuan Formation in North China. In this study, carbonate microbodies types were identified in four differently-colored fillings of Zoophycos using scanning electron microscopy(SEM) and energy dispersive spectroscopy(EDS). Based on the morphologic characteristics, these carbonate microbodies can be divided into three groups, i.e., spheroids,framboids and rhabditiforms. According to the structural features of surface and individual or aggregate morphologies,the three groups can be further subdivided into thirteen types:(1) smooth spheroids;(2) spheroids with tiny thorns;(3)spheroids with a finely granulated surface;(4) spheroids with a flocculent surface;(5) spheroids with a vermiform surface;(6) framboid monomers;(7) framboid colonies;(8) linear smooth rhabditiform bodies;(9) smooth rhabditiform bodies with expanding ends;(10) biserial rhabditiform bodies;(11) spiral rhabditiform bodies;(12) thorny rhabditiform bodies; and(13) branched rhabditiform bodies. This paper not only describes the morphology, composition and occurrence of the various carbonate microbodies, but also discusses their possible microbial genesis, as follows:(1)carbonate spherical microbodies most likely were generated after globular bacterial cells had been fully displaced by minerals;(2) framboid monomers and colonies corresponding to the morphology of biogenic strawberry(or raspberry)pyrite, with their appearance and internal structure possibly inheriting the morphology of microbial cells, were indirectly generated by some microenvironmental changes due to microbial activity;(3) the morphological features,size, occurrences and preservation of filamentous and rhabditiform microbodies indicate that they may be biogenic structures, and possibly mineralized microbial fossils; and(4) some kind of symbiotic relationship exists between microbial action and the Zoophycos trace-makers. Besides, the differently-colored fillings of Zoophycos are most likely closely related to differences in the composition of microbial taxa, which in turn reflect different microenvironmental conditions.  相似文献   

8.
The Ulundi Formation of the lowermost Fig Tree Group in the type area in the Barberton Mountain Land consists of a sequence of quartz—sericite schists, banded cherts and tuffaceous rocks up to 40 m thick. It overlies talc—carbonate schists assigned to the upper part of the Onverwacht Group. In the cherts, which in parts represent the sulfide facies of a banded-iron formation, silicified spheroids have been observed which appear to be diagenetically altered, locally zeolitized accretionary lapilli. Portions of the cherts formed through silicification of fine-grained felsic tuffs and contain abundant stylolite seams along which 4% of the rock thickness has been removed by intrastratal solution during deformation.A supposedly organogenic spheroidal structure, Ramsaysphera, has been previously described from the upper part of the Ulundi Formation. The spheroids, with diameters of up to 8 mm, occur in a distinct bed of up to at least 1.6 m thickness and can be traced in outcrop over an area which originally covered 17 × 30 km. The bed was deposited in fairly still water of several 100 m depth. The spheroids accumulated with only little finer-grained detrital material, the interstices between them being filled by cherty silica and minor amounts of dolomite. Up to 65% of the spheroids consist of tectonically elongated, originally round fragments of a micro-felsitic groundmass with remnants of phenocrysts. The majority of the remaining spheroids consists of concentrically banded bodies of siliceous material with minor amounts of chlorite and sericite, as well as small dolomite crystals. These spheroids represent highly altered granules of volcanic glass deposited from volcanic ash clouds through a body of water. The prototype of Ramsay-sphera also belongs to this group of spheroids, and there is little microscopic or macroscopic evidence for its explanation as an organic structure.  相似文献   

9.
The problem of the origin of the Witwatersrand gold-uranium deposit is considered. The fluvial and explosion hydrothermal origin of pyrite spheroids and Au-bearing pseudoconglomerate is denied. Evidence for the primary character of spheroids and the autochthonous crystallization of pyrite under endogenic conditions is furnished. The association of gold with carbonaceous rocks is interpreted in terms of the nuclear magnetic physicochemical phenomena. It is concluded that the Witwatersrand deposit is a hydrothermal metasomatic deposit.  相似文献   

10.
There is very little understanding of tellurium (Te) distribution and behaviour in sedimentary rocks. A suite of 15 samples of reduction spheroids (centimetre‐scale pale spheroids in otherwise red rock), including samples from eight localities in Triassic red beds across the British Isles, were mapped for Te using Laser Ablation–Inductively Coupled Plasma–Mass Spectrometry. Almost all showed enrichment in Te in the cores of the spheroids relative to background red bed concentrations, by up to four orders of magnitude. Some were also enriched over background in gold and/or mercury. In one case, discrete telluride minerals were recorded. The data show that Te is mobile and can be concentrated in low‐temperature sedimentary environments, controlled by redox variations. The consistency in enrichment across widely separate localities implies that the enrichment is a normal aspect of red bed diagenesis and so likely to be controlled by a ubiquitous process, such as microbial activity.  相似文献   

11.
Spheroidal dolomite cements are a pervasive and distinctive feature in five exposed lithologies of different geological age in Kuwait. The spheroids range in diameter from ～5–350 μm, have concentric zones and nuclei of fluid inclusions with or without a radial fabric, and compositions varying from near stoichiometric to Ca56 Mg44. All spheroids appear to be primary in origin with no evidence of replacement. Some concentric zones show selective calcitization and/or leaching. Most of these features are confined to within 30 m of the surface. Consequently, spheroidal dolomites recognizable at depth in ancient sequences may be indicative of a local unconformity. The dolomite distributions are areally correlative with known areas of hydrocarbon seepage, the latter being confined to prominent oil field anticlinal trends. It is proposed that the dolomitization was caused by groundwaters carrying hydrocarbons to the surface, which were then oxidized to carbon dioxide. The gas bubbles may have acted as nuclei for spheroid growth with a possible inducement by bacteria which have an affinity for hydrocarbons. The implication is that the regional distribution of spheroidal dolomites may be an indicator of a potential reservoir facies at depth.  相似文献   

12.
作者认为地球莫霍球面体形状经历了一个从6.5亿年的"倒梨形体"到2.5亿年的"哑铃形体"再到现在的"正梨形体"的演变;提出了地旋幔流——大陆被动漂移的观点,认为自6.5亿年以来,由于地球旋转方向变化和地球自转速率减慢等因素的影响,地幔发生了大规模由北向南、自西向东的幔流运动,从而使地球莫霍球面体发生由"倒梨形体"向"正梨形体"的变化。  相似文献   

13.
一九八一年,我们在内蒙古阿拉善左旗北部地区进行岩体调查时,在巴音诺尔公复式花岗岩体中发现一些球状体(图版Ⅰ—1),单个球体直径为10—14厘米(图版Ⅰ—3),  相似文献   

14.
Apollo 14 regolith breccia 14076, long known to be uniquely endowed with high-alumina, silica-poor (HASP) material of evaporation-residue origin, has been found to contain a diverse suite of complementary condensates, dubbed GASP (gas-associated spheroidal precipitates). GASP occurs in two forms: as glassy or extremely fine grained quenched-melt spheroids, mostly less than 5 μm across; and as quenched textured clasts up to 200 μm across. In two of the clasts, origin by aggregation of spheroidal GASP is confirmed by the presence of relict spheroids. GASP is distinctively depleted in the same refractory major oxides that are characteristically enriched in HASP: Al2O3 and CaO. Among the larger GASP spheroids, Al2O3 is seldom >1 wt%; among the clasts, excluding two instances of apparent contamination by Na- and K-rich substrate-derived melt, bulk Al2O3 averages 0.3 wt%. Depletion of Al2O3 and CaO is also manifested by pyroxene compositions in some clasts; e.g., in the largest clast, En82Wo0.45 with 0.07 wt% Al2O3. Although GASP bulk compositions are nearly pure SiO2 + MgO + FeO, they are nonetheless highly diverse. Spheroid compositions range in mg from 7 to 84 mol%, and in FeO/SiO2 (weight ratio) from 0.002 to 0.67. Bulk compositions and textures of many GASP spheroids suggest that liquid immiscibility occurred prior to quenching; implying that these materials were, some time after condensation, at temperatures of ∼1680 °C. Textural evidence for immiscibility includes lobate boundaries between silicic and mafic domains, and a general tendency for quenched mafic silicates to be concentrated into a few limited patches rather than evenly dispersed. The parent melt of the largest clast’s pyroxene is inferred to have formed as a partial melt within the parent aggregation of GASP matter, compositionally near the pyroxene + cristobalite + melt eutectic and thus at ∼1500 °C. A few GASP spheroids show possible signs of in-flight collision-coalescence, but aggregation of the much larger clasts probably took place in mushy puddles on the lunar surface. Little mixing took place between these GASP puddles and the related HASP, probably because GASP condensation did not commence until after an intermediate stage during which, while neither net evaporation nor net condensation took place, expansion of the vapor cloud carried the eventual GASP matter well apart from the HASP. Considering the characteristic length-scale of lunar regolith mixing, the concentration of both GASP and HASP into this single unique regolith sample (14076) is most consistent with a parent crater size (diameter) of 10-100 km. I speculate that the 14076 regolith may have been unusually situated, almost directly uprange from an unusually oblique large impact. Mercurian analogs of the 14076 impact condensates may have significant implications for remote sensing.  相似文献   

15.
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16.
A new class of figures of equilibrium for a rotating gravitating fluid located inside a gravitating ring or torus is studied. These figures form a family of sequences of generalized oblate spheroids, in which there is for any value of the tidal parameter α in the interval 0 ≤ \(0 \leqslant \frac{\alpha }{{\pi G\rho }} \leqslant 0.1867\) ≤ 0.1867 a sequence of spheroids with oblatenesses emin (α) ≤ e ≤ emax (α). A series of classicalMaclaurin spheroids from a sphere to a flat disk is obtained for α = 0. At intermediate values 0 < α ≤ αmax, there are two limiting non-rotating spheroids in each sequence. When α = αmax, the sequence degenerates into a single non-rotating spheroid with ecr ≈ 0.9600, corresponding to the maximum oblateness of E7 elliptical galaxies. The second part of the paper considers the influence of rings of dark matter on the dynamics of elliptical galaxies. It is proposed that the equilibrium of an oblate isolated non-rotating galaxy is unstable, and it cannot be supported purely by anisotropy of the stellar velocity dispersion. A ring of dark matter can stabilize a weakly rotating galaxy, supplementing standard dynamical models for such stellar systems. In order for a galaxy to acquire appreciable oblateness, the mass of the ring must be an order of magnitude higher than the mass of the galaxy itself, consistent with the ratios of the masses of dark and baryonic matter in the Universe. The influence of massive external rings could shed light on the existence of galaxies with the critical oblateness E7.  相似文献   

17.
A sequence of oxidate facies sediments associated with a cupriferous massive sulphide deposit in the Solomon Islands is the product of precipitation at an exhalative fluid/seawater interface. A siliceous sinter bed within the sediments contains magnesioferrite spheroids having a distinctive framboid texture. Scanning electron micrographs of the spheroids show an internal structure of parallel rods composed of sub-spherical microcrysts. The formation of the magnesioferrite framboid texture is ascribed to the coagulation of magnetic iron hydroxide gel particles due to magnetic attraction but facilitated by the presence of a strong electrolyte. By analogy a simple mechanism to account for pyrite framboid formation is proposed. This mechanism requires primary iron sulphide particels to be attracted to one another because of the ferrimagnetic properties of a precursor FeS polymorph or alternatively by Van der Waals forces accentuated by the presence of charged ions in a strong electrolyte. Ordering of resultant microcrysts is mainly a close-packing effect which produces robust aggregates resistant to deformation during subsequent diagenesis.  相似文献   

18.
Doklady Earth Sciences - Tungstenite of unusual shapes (spheroids, rings, threads, spirals, etc.) was found in scheelite of gold-bearing quartz–carbonate–pyrite–chalcopyrite...  相似文献   

19.
ABSTRACT Spheroids in the size range of 30–200 nm coat sand grains on sediments around the island of Vulcano. They occur as single individuals, as ball-shaped clumps resembling biological colonies with radiating chains of bodies, and eventually merge into sheets in which the individual nannobacteria are no longer evident. Rarely, they appear to bore into the glass grains . Older rocks are composed mainly of closely packed nanospheroids. These spheroids are believed to be nannobacterial cells based on their resemblance to nannobacteria found in other surficial environments, and their occurrence in colony-like groups. If so, nannobacteria play a very important role in the destruction of volcanic materials on Vulcano.  相似文献   

20.
Mineralogy and geochemistry of reduction spheroids in red beds   总被引：1，自引：0，他引：1  
Summary The mineralogy and geochemistry of reduction spheroids from continental and marine red beds of Europe, North America and Oman were investigated. Reduction spheroids are spheroidal, isolated reduction sites in hematitic rocks. They consist of a mineralized core (0.1 to 5 cm diameter) and a hematite-dissolution halo (1 to 20 cm diameter). Irrespective of origin and age of host rocks, all reduction spheroid cores show a very similar mineralogy dominated by the vanadian mica roscoelite and a similar pattern of element enrichment relative to their host rocks dominated by V and U. Element enrichments in most reduction spheroids are very similar to those of sandstone-hosted vanadium-uranium deposits except for a lack of a molybdenum enrichment. Isotopically light sulfide of in-situ, low-temperature origin is an indication for the involvement of bacterial sulfate reduction during reduction spheroid formation.
Mineralogie und Geochemie von Reduktionshöfen in Rotsedimenten
Zusammenfassung Reduktionshöfe aus kontinentalen und marinen Rotsedimenten von Europa, Nordamerika und aus dem Oman wurden geochemisch und mineralogisch untersucht. Reduktionshöfe sind kugelsymmetrische, isolierte Reduktionszentren in hämatithaltigen Gesteinen. Sie bestehen aus einem mineralisierten Kern von 0.1 bis 5 cm Durchmesser welcher von einem infolge Hämatitlösung gebleichten Hof von 1 bis 20 cm Durchmesser umgeben ist. Reduktionshöfe zeigen unabhängig von ihrer Herkunft und vom Alter des Nebengesteins immer eine sehr ähnliche Mineralogie der mineralisierten Kerne mit dominierendem Roscoelith sowie eine ähnliche Charakteristik der Elementanreicherung relativ zum Muttergestein. Die Elementanreicherung wird von V und U dominiert und gleicht jener von Sandstein-Typ Vanadium-Uran-Lagerstätten mit Ausnahme des Fehlens einer Molybdänanreicherung. In-situ unter Tieftemperaturbedingungen produziertes, isotopisch leichtes Sulfid ist ein Hinweis darauf, dass bakterielle Sulfatreduktion bei der Bildung der Reduktionshöfe eine Rolle spielte.

With 7 Figures  相似文献   

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