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1.
The coexisting pyroxenes and olivines from nine British andIcelandic Tertiary porphyritic acid glasses have been separatedand analysed chemically. Eight new analyses of augite and ferroaugites,one of orthopyroxene, and five of iron-rich olivines are presented,together with their optical properties. The trend of crystallizationof the ferroaugites is discussed. New optical determinativecurves for these ferroaugites and the iron-rich olivines arepresented. The relationship of the pyroxenes and olivines totheir analysed residual glasses is considered.  相似文献   

2.
Pyroxenes and olivines from the trough bands in the Upper Zone (UZa) of Skaergaard Intrusion have been investigated, together with previously analysed pyroxenes (Brown, 1957; Brown and Vincent, 1963) and olivines from the Layered Series ferrodiorites. The electron microprobe, electron microscope, and analytical electron microscope EMMA-4 were used. Results show a striking difference between the cumulus and intercumulus trends of the trough-band pyroxenes. The cumulus trend follows that of the main Layered Series whereas the intercumulus trend shows a shrinking of the miscibility gap together with great enrichment in the Fs molecule, the miscibility gap being symmetrical about ～Wo24. The shrinking appears to be a function of the different crystallisation conditions in the intercumulus liquid which was closed off in “cells” from the main mass of supernatant liquid. Enrichment in the Fs molecule is due to the much lower crystallisation temperatures of the intercumulus pyroxenes. Iron enrichment is also reflected in the intercumulus olivines. For the cumulus trend, Brown's calcium-poor pyroxene trend (1957) has been extended into more iron-rich parts of the pyroxene quadrilateral, well after olivine has reappeared and subsequent to the increase in calcium of the ferroaugites. The subsolidus trend for pyroxenes in the Fs-rich region has also been established.  相似文献   

3.
Fragments of igneous rocks, glasses and minerals comprise 25 per cent of the studied sample of the Luna 20 soil. Basalt fragments in the Luna 20 soil are similar to basalts from the mare regions of the Moon—in that they are characterized by the presence of iron-rich olivines and pyroxenes. On the basis of the FeO contents of plagioclases, it appears possible to distinguish between the plagioclase of the mare and highland regions of the Moon. Other igneous rock fragments are anorthosite, gabbroic anorthosite and anorthositic gabbro. The most abundant rock type (75 per cent of the sample) is microbreceia. One third of the fragments of microbreccia have undergone thermal metamorphism resulting in the homogenization of phases and the development of poikioblastic and hornfelsic textures. Excluding the basalt fragments, the dominant minerals in the Luna 20 soil are anorthite (An93–98), magnesium-rich orthopyroxenes, intermediate clinopyroxenes and olivine (< Fa50). Chemically, the Luna 20 and Apollo 16 soil samples are similar, but the Luna 20 soil is slightly depleted in aluminum and calcium and enriched in iron and magnesium relative to the Apollo 16 soils. The slight difference in bulk chemistry of the two soils may be a result of the presence of a minor amount of mare material in the Luna 20 soil and its apparent absence in the Apollo 16 soils.  相似文献   

4.
The olivine phenocrysts of four basalts (12004, 12008, 12009 and 12022) are concentrically zoned and have core compositions about as magnesian as experimentally produced liquidas olivines, features which suggest fractional crystallization and absence of Fe-Mg reequilibration. In the magnesium- and olivine-rich granular basalt 12035, the olivines are either unzoned or are zoned toward adjacent grains and have compositions more iron-rich than either cumulus olivines or liquidus olivines (should the rock represent the composition of a melt), features which suggest extensive Fe-Mg re-equilibration.  相似文献   

5.
路凤香 Tayl.  LA 《地球科学》1990,15(6):617-627
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6.
Primitive magmas provide critical information on mantle sources, but most Martian meteorites crystallized from fractionated melts. An olivine-phyric shergottite, Yamato 980459 (Y-980459), has been interpreted to represent a primary melt, because its olivine megacrysts have magnesian cores (Fo84-86) that appear to be in equilibrium with the Y-980459 whole-rock composition based on Fe-Mg partitioning. However, crystal size distribution (CSD) plots for Y-980459 olivines show a size gap, suggesting a cumulus origin for some megacrysts. Because melting experiments using the Y-980459 whole-rock composition have been used to infer the thermal structure and volatile contents of the Martian mantle, the interpretation that this rock is primitive should be scrutinized.We report major, minor and trace element compositions of Y-980459 olivines and compare them with results from melting experiments (both hydrous and anhydrous) and thermodynamic calculations. Cores of the olivine megacrysts have major and minor element contents identical to those of the most magnesian olivines from the experiments, but they differ slightly from those of thermodynamic calculations. This is probably because the Y-980459 whole-rock composition lies near the limit of the range of liquids used to calibrate these models. The megacryst cores (Fo80-85) exhibit minor and trace element (Mn-Ni-Co-Cr-V) characteristics distinct from other olivines (megacryst rims and groundmass olivines, Fo < 80), implying that the megacryst cores crystallized under more reduced conditions (∼IW + 1).Y-980459 contains pyroxenes with orthopyroxene cores mantled by pigeonite and augite. We also found some reversely zoned pyroxenes that have augite cores (low-Mg#) mantled by orthopyroxenes (high-Mg#), although they are uncommon. These reversely zoned pyroxenes are interpreted to have grown initially as atoll-like crystals with later crystallization filling in the hollow centers, implying disequilibrium crystallization at a moderate cooling rate (3-7 °C/h). The calculated REE pattern of a melt in equilibrium with normally zoned pyroxene is parallel to those of glass and the Y-980459 whole-rock as well as other depleted olivine-phyric shergottites, suggesting that Y-980459 was derived from a depleted mantle reservoir.Considering the CSD patterns of Y-980459 olivines, we propose that the olivine megacrysts are cumulus crystals which probably formed in a feeder conduit by continuous melt replenishment, and the parent melt composition was indistinguishable from the Y-980459 whole-rock with 0-2 wt% of H2O and 0-5 wt% of CO2. The final magma pulse entrained these cumulus olivines and then crystallized groundmass olivines and pyroxenes. Although Y-980459 contains small amounts of cumulus olivine (<∼6 vol%), we conclude that the Y-980459 whole-rock composition closely approximates a Martian primary melt composition.  相似文献   

7.
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8.
Petrographic and chemical criteria indicate that the overwhelming majority of olivines in kimberlites are probably cognate phenocrysts. The implied low volume of xenocryst olivines requires that primitive kimberlite magmas are highly ultrabasic liquids. Two chemically distinctive olivine populations are present in all of the kimberlites studied. The dominant olivine population, which includes large rounded olivines and smaller euhedral crystals, is Mg-rich relative to late-stage rim compositions. It is characterized by a range in 100 Mg/(Mg + Fe) and uniform Ni concentration, reflecting Rayleigh-type crystallization during magma evolution. The most Mg-rich of these olivines are considered to be similiar to those in the mantle source rocks. The second compositional population, generally very subordinate, though markedly more abundant in the megacrystrich Monastery kimberlite, is Fe-rich relative to rim compositions. This group of olivines crystallized from evolved liquids in equilibrium with iron-rich megacrysts, both entrained by the kimberlite magma during ascent. Differences between the chemical fields of Fe-rich olivines in Group I and Group II kimberlites point to relatively deeper derivation of the latter suite. Olivine chemistry can be used to characterize kimberlite magma sub-types, and may prove to be a useful tool for evaluating the diamond potential of kimberlites.  相似文献   

9.
The mineralogical and chemical characteristics of the fine-grained matrix (< or = 3 micrometers) of the unique primitive carbonaceous chondrite Acfer 094 have been investigated in detail by scanning electron microscopy (SEM) and analytical transmission electron microscopy (ATEM). Generally, the fine-grained matrix represents a highly unequilibrated assemblage of an amorphous material, small forsteritic olivines (200-300 nm), low Ca-pyroxenes (300-400 nm), and Fe,Ni-sulfides (100-300 nm). The matrix is basically unaffected by secondary processes. Only minor amounts of serpentine and ferrihydrite, as products of hydrous alteration, are present. Texturally, the amorphous material acts as a groundmass to olivines, pyroxenes, and sulfides, mostly exhibiting rounded or elongated morphologies. Only very few clastic mineral grains have been found. The texture and chemical composition of the amorphous material are consistent with an origin by disequilibrium condensation in either the cooling solar nebula or a circumstellar environment. As such, the amorphous material may be considered as a possible precursor of matrix materials in other types of chondrites. The non-clastic matrix olivines (Fo98-99) and pyroxenes (En97-100) are suggested to have formed either by condensation in the solar nebula under highly oxidizing conditions or by recrystallization from the amorphous material. The formation of these grains by fragmentation of chondrule components is unlikely due to chemical and microstructural reasons. Rapid cooling caused the observed intergrowths of clino/orthoenstatite in the Mg-rich matrix pyroxenes. Although some similarities exist comparing the fine-grained matrix of Acfer 094 with the matrices of the unequilibrated CO3 chondrite ALHA77307 and the unique type 3 chondrite Kakangari, Acfer 094 remains unique. Since it contains the highest measured concentrations of circumstellar SiC and the second highest of diamond (highest is Orgueil), it seems reasonable to suggested that at least parts of the amorphous material in the fine-grained matrix may be of circumstellar origin.  相似文献   

10.
长白山天池火山岩浆演化——来自主矿物成分的证据   总被引：10，自引：6，他引：10  
长白山天池火山岩的主要矿物成分为橄榄石、辉石、长石和铁钛氧化物，经电子探针测试，其成分变化特征明显，自造盾阶段一造锥阶段一全新世喷发，橄榄石中铁含量增加，一直达到铁橄榄石的端元组分；单斜辉石中铁质和钠质含量增加；长石则从基性的拉长石向含钾、钠渐高的歪长石和透长石演化；铁钛氧化物含量则明显减少。三阶段斑晶矿物成分的变化反映了天池火山近200万年来岩浆演化的完整系列，符合分离结晶作用的演化趋势。  相似文献   

11.
Petrologic studies were made on the fine-grained matrices of type 3 ordinary chondrites of the lowest petrologic subtype. The matrix minerals, in order of abundance, are olivine (Fo99 to Fo9), enstatite or bronzite, augite or subcalcic augite, albite, Fe-Ni metal, troilite, magnetite, spinel (MgAl2O4), chromite, and calcite. Fe- and Mg-rich fluffy particles and albite-like particles are also major constituents. The chemical compositions of olivine and pyroxenes vary within and among the chondrites and are in gross disequilibrium, showing that the matrix materials were hardly heated after their formation. Textural relationships indicate that magnesian olivine was formed after Ca-pyroxene, followed by intermediate to iron-rich olivine. Intermediate olivine was formed from enstatite and metallic iron under relatively oxidizing conditions. The observations indicate that matrices of chondrites are neither the fragments of chondrules nor the precursors of chondrules. They were mostly the products of condensation and reaction among solids and/or between solids and the ambient gas mostly at low temperatures, and thus they contain records of primitive processes in the nebula. In order to explain the presence of olivines more iron-rich than Fo50, the presence of free SiO2 or a high activity of SiO2 in the gas is necessary, which was not shown in previous thermochemical calculations. Mineral assemblages of matrix minerals of chondrites of different chemical groups differ systematically according to oxidation state of the parental meteorites, indicating that they were formed at different oxygen fugacities. The rims of chondrules, and surrounding matrix materials, must have accreted onto chondrules during turbulent movements of the nebula.  相似文献   

12.
The chemical composition of the pyroxenes and olivines of 12 basaltic rocks and 5 lherzolite nodules was determined quantitatively by electron micro-probe analysis. The composition of the pyroxenes depends on the type of basalt in which they occur. Tholeiitic basalts with normative quartz contain three pyroxenes: orthorombic pyroxenes, pigeonites and augites. All pyroxene phases are zoned and do not show any exsolution. Their Ti and Al contents (Ca-Tschermaks and Ti-augite molecules) are small. All pyroxene phases were formed under disequilibrium with each other and with the melt because of rapid quenching. The sequence of crystallization: orthopyroxene—pigeonite—augite could be established by their Cr content.The alkali olivine basalts undersatured in SiO2 and the olivine nephelinites are characterized by Ti and Al-rich clinopyroxenes. The distribution of Ti and Al in the pyroxenes of the alkali olivine basalts shows a differentiation trend from the cores of the phenocrysts to their outer zones and to the crystals of the ground mass. Thereby the Ca-Tschermaks molecule is being replaced more and more by the Ti-augite molecule. The Ti content of the pyroxenes of the olivine nephelinites decreases in the last stage of differentiation because simultaneously increasing amounts of titaniferous magnetite crystallize.The pyroxenes of lherzolite peridotite nodules are characterized by high Al and low Ti contents which differ according to the type of basalt (alkali olivine basalt or olivine nephelinite) in which the nodules occur. The homogeneous distribution of the elements within the single grains indicates crystallization under equlibrium conditions. The conditions of their formation are comparable to those of Al-pyroxene peridotites in the upper mantle. The composition of pyroxenes of early accumulates of alkali basaltic melts differ from those of peridotite nodules. Therefore lherzolite nodules can be taken as residues of deeper peridotite masses.  相似文献   

13.
The rates of production of21Ne and22Ne in spallation reactions, both due to solar as well as galactic cosmic rays, in some major meteoritic minerals, e.g. olivines, feldspars and pyroxenes, are calculated using their energy spectra and excitation functions. The production profiles of21Ne and22Ne due to galactic cosmic rays, and the22Ne/21Ne ratio depend upon the size of the meteoroid. The22Ne/21Ne ratio is very sensitive to the abundance of sodium and consequently its depth profile is distinctly different in feldspars, the ratio increasing with depth rather than decreasing as in pyroxenes and olivines. In the near-surface regions, up to a depth of 2cm, production due to solar flare protons dominates, giving rise to a steep gradient in isotopic production as well as in the22Ne/21Ne ratio. Composite production profiles are given and compared with measurements in some meteorites.  相似文献   

14.
 Iron-rich olivines (Fa100∼Fa70Fo30) experimentally altered in alkaline and acidic aqueous oxidizing environment were examined in transmission electron microscope to determine the precipitate phases and their crystallographic relationships to the host. The planar zones consist of precipitates of laihunite and hematite in alkaline aqueous oxidizing environment while hematite and amorphous silica in acidic aqueous oxidizing environment. The directions of planar zones produced in alkaline aqueous oxidizing environment are parallel to (100)∼(001) of olivine dependent on the composition of the host olivines, while those produced in acidic aqueous oxidizing environment are always parallel to (001) of olivine independent from the composition of the host olivine. These differences are interpreted by the dissolution kinetics of olivine. The present experimental results explain why the planar zones found in oxidized iron-rich olivines in carbonaceous chondrite are parallel to (100) of olivine while those in terrestrial olivines are parallel to (001) of olivine. That is, the difference is attributed to the pH of the alteration environment. Received: August 19, 1996 / Revised, accepted: February 14, 1997  相似文献   

15.
Many carbonaceous chondrites contain discrete olivine fragments that have been considered to be primitive material, i.e. direct condensates from the solar nebula or pre-solar system material. Olivine occurring in chondrules and as isolated grains in C3(0) chondrites has been characterized chemically and petrographically. Type I chondrules contain homogeneous forsterite grains that exhibit a negative correlation between FeO and CaO. Type II chondrules contain zoned fayalite olivines in which FeO is positively correlated with CaO and MnO. The isolated olivines in C3(0) chondrites form two compositional populations identical to olivines in the two types of porphyritic olivine chondrules in the same meteorites. Isolated olivines contain trapped melt inclusions similar in composition to glassy mesostasis between olivines in chondrules. Such glasses can be produced by fractional crystallization of olivine and minor spinel in the parent chondrule melts if plagioclase does not nucleate. The isolated olivine grains are apparently clastic fragments of chondrules. Some similarities between olivines in C3(0), C2, and Cl chondrites may suggest that olivine grains in all these meteorites crystallized from chondrule melts.  相似文献   

16.
Measured were the abundance and distribution of nitrogen in glasses of glass inclusions in olivines of CV3, CO3, CR, C4, CH3, and LL chondritic meteorites by means of the 14N(d, p)15N nuclear reaction. Similar to what was observed with carbon, nitrogen is present in low concentrations (<20 ppm) in the structure of olivines but can by stored in variable amounts in glasses of glass inclusions. These primitive glasses, characterized by a Si-Al-Ca-rich composition, have highly variable nitrogen contents (30 to 1500 ppm) and highly inhomogeneous nitrogen distribution. Nitrogen contents are independent of the chemical composition of the glasses. The heterogeneous distribution is a common feature of all studied inclusions, as is evidenced by the variable contents of nitrogen in glass inclusions occurring in the same olivine grain. Nitrogen heterogeneity is suggestive of trapping of solid nitrogen carrier phases during formation of the constituents of chondrules. However, part of the originally trapped nitrogen appears to have been lost, possibly, by ulterior oxidation and subsequent transformation into volatile species.  相似文献   

17.
The ferroaugites, inverted ferrowollastonites and the brown and green ferrohedenbergites from the Upper Zone (UZb and UZc) of the Skaergaard intrusion (Brown and Vincent, 1963) have been studied with the electron microprobe, and where necessary, with the electron microscope. The cloudy “inclusions” in the inverted ferrowollastonite (Woss) of 4471 are established to be strain fields associated with stacking faults, dislocations and sub-grain boundaries. The green pyroxenes of 1881 have undoubtedly inverted from Woss, as both major and minor element chemistry show. The orientation of the tie-line joining coexisting Ca-rich and Ca-poor pyroxenes has also been established for this part of the quadrilateral, together with the Fe-Mg values at which the 4471 inverted Woss would project on to Brown and Vincent's (1963) trend line for Ca-rich pyroxenes. These Fe-Mg values are the same as those of the 1881 brown ferrohedenbergites (Hedss). The subsolidus cooling history of the inverted Woss has been examined in the light of the present data. It is proposed that a Woss of solidus composition Wo39 may either (a) react to a two-phase assemblage of Hedss (composition Wo42.5) + metastable clinoferrosilite, or (b) invert metastably to a Hedss of the same composition. For specimen 4471, these two types of subsolidus behaviour may occur in different crystals within the same large mosaic-patterned grain. The proposed model is consistent with difficulty in nucleation of clinoferrosilitic lamellae, combined with the sluggishness of reactions at low temperatures for these Fe-rich compositions. In both case (a) and (b), inversion to Hedss (with or without the formation of mosaic texture) precedes exsolution of clinoferrosilite. The two final subsolidus compositions for the host are ～Wo46 and ～Wo42, for types (a) and (b) respectively, and the final subsolidus composition of the lamellae is Wo0-Wo2. The brown and green pyroxenes of 4330 show distinct differences in chemistry, the green being richer in Si, and depleted in Al and Ti relative to the brown. The 4330 green pyroxenes are poorer in Mn, and richer in Na, compared to the green inverted Woss. The green colour in these UZc pyroxenes may be due to the drop in Ti content relative to brown pyroxenes.  相似文献   

18.
Glass inclusions in olivines of the Renazzo, El Djouf 001, and Acfer 182 CR-type chondrites are chemically divers and can be classified into Al-rich, Al-poor, and Na-rich types. The chemical properties of the glasses are independent of the occurrence of the olivine (isolated or part of an aggregate or chondrule) and its composition. The glasses are silica-saturated (Al-rich) or oversaturated (Al-poor, 24% normative quartz). All glasses have chondritic CaO/Al2O3 ratios, unfractionated CI-normalized abundances of refractory trace elements and are depleted in moderately volatile and volatile elements. Thus the glasses are likely to be of a primitive condensate origin whose chemical composition has been established before chondrule formation and accretion, rather then the product of either crystal fractionation from chondrule melts or part melting of chondrules. Rare Na-rich glasses give evidence for elemental exchange between the glass and a vapor phase. Because they have Al2O3 contents and trace element abundances very similar to those of the Al-rich glasses, they likely were derived from the latter by Ca exchange (for Na) with the nebula. Elemental exchange reactions also have affected practically all olivines (e.g., exchange of Mg of olivine for Fe2+, Mn2+, and Cr3+). Glasses formed contemporaneously with the host olivine. As the most likely process for growing nonskeletal olivines from a vapor we consider the VLS (vapor-liquid-solid) growth process, or liquid-phase epitaxy. Glasses are the possible remnants of the liquid interface between growing crystal and the vapor. Such liquids can form stably or metastably in regions with enhanced oxygen fugacity as compared to that of a nebula of solar composition.  相似文献   

19.
Clinopyroxenes from various lithologic units in a single tholeiitic dolerite intrusion 50 m thick were analysed with the electron microprobe. Microphenocrysts of augite in the chilled marginal rock have the least scattered CaMgFe ratios, around Ca38Mg50Fe12, and lowest Ti/Al ratios, less than 1/4, of the augites examined; they crystallized prior to emplacement. The augites which crystallized in situ are more Fe-rich and have higher Ti/Al ratios, close to 1/2. Pigeonite started crystallizing later than augite and formed rims to augite crystals. Continuous zoning from augite to subcalcic augite as was previously reported by Benson (1944) for the same dolerite is not observed. Subcaloic ferroaugites, however, link augite and ferropigeonite in the outermost margins of pyroxenes in the later segregations. The small scale differentiation in situ gave rise to globules, veins, and pegmatoids, in which sector-zoning of augite is well developed. The {100} sectors are enriched in MgSiO3, FeSiO3, and MnSiO3 components relative to the {010} and the {110} sectors, and are accordingly depleted in other components. The protosite theory of sector-zoning formation proposed by Nakamura (1973) is further developed and the environment which favours the development of sector-zoning is discussed. The formation of augite rims around quartz-rich xenoliths and the compositional characteristics of the augites concerned are due to local high silica activities. Comments are made concerning some lunar pyroxenes in the light of the present study.  相似文献   

20.
The angrites are a small and heterogeneous group of achondritic meteorites with highly unusual chemical and mineralogical features. The abundant presence of glasses in D'Orbigny makes this rock a unique member of the angrite group. Glasses fill open spaces, form pockets, and occur as inclusions in olivines. Their physical settings exclude an incorporation from an external source. Major and trace element (rare earth elements [REE], Li, B, Be, transition elements, N and C) contents of these glasses and host olivines were measured combining laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), secondary-ion mass spectrometry (SIMS), Nuclear Reaction Analysis (NRA), and EMP techniques. Based on the major element composition, glasses filling voids could represent either a melt formed by melting an angritic rock or a melt from which angrites could have crystallized. Trace element contents of these glasses strongly indicate a direct link to the D'Orbigny bulk meteorite. They are incompatible with the formation of the glasses by partial melting of a chondritic source rock or by shock melting. The refractory elements (e.g., Al, Ti, Ca) have about 10 × CI abundances with CaO/TiO2 and FeO/MnO ratios being approximately chondritic. Trace element abundances in the glasses appear to be governed by volatility and suggest that the refractory elements in the source had chondritic relative abundances. Although the glasses (and the whole rock) lack volatile elements such as Na and K, they are rich in some moderately volatile elements such as B, V, Mn, Fe (all with close to CI abundances), and Li (about 3-5 × CI). These elements likely were added to the glass in a sub-solidus metasomatic elemental exchange event. We have identified a novel mechanism for alteration of glass and rock compositions based on an exchange of Al and Sc for Fe and other moderately volatile elements in addition to the well-known metasomatic exchange reactions (e.g., Ca-Na and Mg-Fe).Because glass inclusions in olivine were partly shielded from the metasomatic events by the host crystal, their chemical composition is believed to be closer to the original composition than that of any other glasses. The relative trace element abundances in glasses of glass inclusions in olivine and glass pockets are also unfractionated and at the 10 to 20 × CI level. These glasses are chemically similar to the common void-filling glasses but show a much wider compositional variation. Inclusion glasses demonstrate that at least olivine grew with the help of a liquid. In analogy to olivines in carbonaceous chondrites, initial formation could also have been a vapor-liquid-solid condensation process. At that time, the glass had a purely refractory composition. This composition, however, was severely altered by the metasomatic addition of large amounts of FeO and other moderately volatile elements. The presence of volatile elements such as carbon and nitrogen in glasses of glass inclusions is another feature that appears to give these glasses a link with those hosted by olivines of carbonaceous chondrites. All these features point to an origin from a vapor with relative abundances of condensable elements similar to those in the solar nebula.  相似文献   

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