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The two textural varieties of olivine-rich Allende inclusions (rimmed and unrimmed olivine aggregates) consist primarily of a porous, fine-grained mafic constituent (inclusion matrix) that differs from the opaque meteorite matrix of CV3 chondrites by being relatively depleted in sulfides, metal grains, and (perhaps) carbonaceous material. Olivine is the most abundant mineral in Allende inclusion matrix; clinopyroxene, nepheline, sodalite, and Ti-Al-pyroxene occur in lesser amounts. Olivine in unrimmed olivine aggregates (Type 1A inclusions) is ferrous and has a narrow compositional range (Fo50–65). Olivine in rimmed olivine aggregates (Type 1B inclusions) is, on average, more magnesian, with a wider compositional range (Fo53–96). Olivine grains in the granular rims of Type 1B inclusions are zoned, with magnesian cores (Fo>80) and ferrous rinds (Fo<70). Ferrous olivines (Fo<65) in both varieties of inclusions commonly contain significant amounts of Al2O3 (as much as ~0.7 wt%), CaO (as much as ~0.4 wt%), and TiO2 (as much as ~0.2 wt%), refractory elements that probably occur in submicroscopic inclusions of Ca,Al,Ti-rich glass (rather than in the olivine crystal structure). Defocussed beam analyses of Allende matrix materials demonstrate that: (1) inclusion matrix in Type 1A inclusions is more enriched in olivine and FeO than inclusion matrix in the cores of Type 1B inclusions; (2) opaque matrix materials are depleted in feldspathoids and enriched in sulfides and metal grains relative to inclusion matrix; (3) the bulk compositions of Type 1A and Type 1B inclusions overlap; and (4) excluding sulfides and metal, the bulk compositions of Allende matrix materials cluster in a complementary pattern around the bulk composition of C1 chondrites.Inclusion matrix and meteorite matrix in Allende and other CV3 chondrites are probably relatively primitive nebular material, but a careful evaluation of the equilibrium condensation model suggests that these matrix materials do not consist of crystalline phases that formed under equilibrium conditions in a relatively cool gas of solar composition. Allende inclusion matrix is interpreted as an aggregate of condensates that formed under relatively oxidizing, non-equilibrium conditions from supercooled, supersaturated vapors produced during the vaporization of interstellar dust by aerodynamic drag heating in the solar nebula; CV3 meteorite matrix contains, in addition, a proportion of interstellar material that was heated (but not vaporized) in the nebula. Granular olivine in rimmed olivine aggregates may have formed during the recrystallization and incipient melting of aggregates of inclusion matrix in the nebula. The mineral chemistry of matrix olivine in Allende seems to have been established by three different processes: non-equilibrium vapor → solid condensation; recrystallization and partial melting in the nebula; and equilibration (without textural homogenization) in the meteorite parent body. 相似文献
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Ronald E. Cohen Alan S. Kornacki John A. Wood 《Geochimica et cosmochimica acta》1983,47(10):1739-1757
All objects >100 μm in apparent diameter in five polished thin sections of the Mokoia CV3 chondrite were studied and classified. Number and volume percentages and mean apparent size of each type of chondrule and inclusion were determined. Three major types of olivine chondrules were observed: igneous chondrules, recrystallized chondrules, and chondrules that appear to be accretional aggregates. Coarse-grained CAI's have igneous textures and mineral parageneses, while fine-grained CAI's are aggregates containing varying proportions of Al-rich concentric objects, Ca-rich chaotic material, and inclusion matrix. Chondrules and refractory inclusions in Mokoia and Allende are broadly similar in texture and mineral chemistry, but Mokoia refractory inclusions contain phyllosilicates rather than feldspathoids, and melilite-rich CAI's are more abundant in Allende.We think that most CAI's formed during the metamorphism, partial melting, and incomplete distillation of primitive dust aggregates when they were heated in the solar nebula. In the process, Ca-rich melt appears to have been physically separated from Al-rich residues, producing the observed fractionation of Ca from Al into distinct constituents of CAI's. Some CAI's may be aggregates of devitrified, amorphous metastable condensates. Inclusion matrix may have condensed from silicate-rich vapors produced during distillation. Mokoia inclusion matrix contains phyllosilicates that are probably primitive nebular material. 相似文献
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A detailed mineralogic and chemical study of Willy, a very large (150 μm diameter) Fremdling from the Allende CAI 5241, was performed and compared to other Fremdlinge from Allende CAI's 5241 and TS-34 in an attempt to understand the nature and mode of formation of these exotic and complex objects. Willy is composed primarily of V-rich magnetite, V-rich fassaite, and Ni-Fe metal containing Co and Pt. Minor phases include an Fe-Ni-sulfide, V-rich spinel, scheelite (the first reported occurrence in a meteorite), and Cl-apatite. Phases found in trace amounts include nuggets of Os-Ru-Re metal, molybdenite, an unidentified Fe-, Mg-molybdate, and diopside. The Fremdling is concentrically zoned and contains a complex porous core of magnetite, metal, sulfide, scheelite, and other minor phases surrounded by a compact mantle of magnetite with minor apatite. The mantle is surrounded by a dense rim composed of fassaite with minor spinel that appears to be a typical occurrence around oxide-containing Fremdlinge. At the boundary between the fassaite-rim and the magnetite-mantle of Willy is a thin zone (<20 μm wide) of an apparent reaction assemblage consisting of V-rich MgAl2O4, FeAl2O4, and a third V-rich spinel, possibly FeV2O4, in intimate intergrowth.From the observed chemistry and texture, a multistage sequence of formation of Willy, possibly occurring in the solar nebula and involving major changes in T and , can be deduced. The first phases that may have formed in the interior are magnetite and an Fe or Ca tungstate. Refractory metal nuggets and sulfide were introduced after this stage followed by two stages of Ni-Fe formation during which Pt was dissolved in the metal. This was followed by formation of the magnetite mantle, introduction of apatite and possible alteration of ferberite to scheelite. Finally, the V-rich fassaite rim formed, accompanied or followed by reaction forming the complex Fe-, V-rich spinels. All of these steps preceded introduction of the Fremdling into the CAI precursor which in turn appears to have occurred prior to formation of spinel and the major silicate phases in the CAI.The concentric mineral zoning and texture of Willy may indicate that it is one of the few Fremdlinge that was not substantially recrystallized after having been captured in the CAI. It thus may represent the precursor material for many of the other Fremdlinge, since numerous Fremdlinge studied exhibit many of the mineral and chemical features observed in Willy. The co-existence of magnetite, sulfide, and Ni-Fe metal in the interior of Willy and the co-existence of metal, hercynite spinel and magnetite at the rimmantle boundary suggests that the maximum temperature at which Willy could have existed for long periods of time in its present state was about 500–600°C. Similarly, the existence of highly heterogeneous V-, Ti-rich fassaite rims around Willy and other Fremdlinge implies rapid cooling rates for the CAI's. 相似文献
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Many Type B CaAl-rich inclusions (CAI's) in the Allende carbonaceous chondrite contain two types of spinel structures, “framboids” and “palisades.” Framboids are clumps of spinel grains generally <100 μm across. Experimental studies (Wark and Lovering, 1982) show that they probably formed in situ by solid state growth processes. Palisades are texturally different and consist of ovoid shells of spinel grains that appear in thin sections as long arcs or rings with diameters ranging from ～50 μm up to 2 cm. No in situ formation process seems able to explain the variety of sizes and morphologies of palisades nor the different compositions and textures of the enclosed and enclosing materials. We therefore suggest that palisades are the spinel rims of smaller, earlier-formed Type B CAI's that were incorporated into other CAI material in various ways—by capture into liquid drops, by solid condensate overgrowths and by the partial melting or welding of agglomerates containing the bodies. As some Type B bodies have been found inside Type A host material it appears that in at least some regions, and probably generally, Type A CAI's formed after Type B. We propose that Type B CAI's are residues from the heating and incomplete evaporation of interstellar dust during the accretion of the protosolar nebula, and that Type A CAI's are later condensates from completely evaporated dust whose exotic 16O-rich component became more diluted in the gas phase. 相似文献
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Peridotite xenoliths from Grenada,Lesser Antilles Island Arc 总被引：2，自引：2，他引：0
Ultramafic xenoliths comprising harzburgite, lherzolite (reacted harzburgite) and spinel-rich dunite, occur in alkali olivine basalts (M series) of Grenada in the Lesser Antilles island arc. Textures are protogranular, porphyroclastic and granular; the latter are restricted to dunites and areas of the harzburgites/lherzolites where interaction with host magma has occurred. Primary mineralogy comprises olivine, orthopyroxene, clinopyroxene, and spinel. Harzburgites are residual from a fractional partial melting event totaling ~22%. Infiltration of harzburgite by (and reaction with) basalt has produced: a wehrlite, with partial dissolution of primary spinel, an increase in the oxygen fugacity (ƒO2) from primary values 1–2 log ƒO2 units above the fayalite-magnetite-quartz (FMQ) buffer, to 2–2.5 log units above the buffer; reaction of orthopyroxene to form patches of intergrown olivine and clinopyroxene, and bronzite andesite glass (60 wt%, SiO2 18–20 wt% Al2O3 and 3–4 wt% Na2O) with flat to light rare earth element-depleted, chondrite-normalized abundances. Refertilisation of the mantle by reacting melts, producing a clinopyroxene-rich lithology, may form a source of ankaramitic (high-Ca) arc basalts.Editorial responsibility: T.L. Grove 相似文献
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Robert N. Clayton Glenn J. MacPherson Ian D. Hutcheon Andrew M. Davis Lawrence Grossman Toshiko K. Mayeda Carol Molini-Velsko John M. Allen Ahmed El Goresy 《Geochimica et cosmochimica acta》1984,48(3):535-548
We have discovered two FUN inclusions, CG-14 and TE, among a group of five forsterite-rich inclusions in Allende, two of which are described for the first time herein. All five consist of euhedral forsterite and spinel crystals poikilitically enclosed by fassaite. Forsterite and spinel are usually segregated from one another, sometimes into a spinel-rich mantle and a forsterite-rich core. Some inclusions contain vesicles, indicating that they were once molten. The crystallization sequence inferred from textures is: spinel, forsterite, fassaite and, finally, Mg-rich melilite. One concentrically-zoned inclusion contains melilite in its mantle whose composition lies on the opposite side of the liquidus minimum in the melilite binary from that in its core. This suggests that segregation of forsterite from spinel in all of these inclusions could be due to volatilization of MgO and SiO2 relative to Al2O3 and CaO from the outsides of droplets. CG-14 is relatively uniformly enriched in refractory elements relative to Cl chondrites by a factor similar to that for Ca-, Al-rich coarse-grained inclusions except for Ca, Al and Hf which are unusually low. No Ce anomaly such as found in FUN inclusions Cl and HAL is present in CG-14. Whole-rock samples of CG-14 and TE are more strongly mass-fractionated in oxygen relative to “normal” Allende inclusions than the FUN inclusion EK 1-4-1 and less so than Cl. Relative to bulk Allende, both inclusions have strongly massfractionated magnesium and silicon and 25Mg excesses or deficits of 24Mg or 26Mg. CG-14 has a 29Si excess or a deficit of 28Si or 30Si. Volatilization loss cannot be responsible for the magnesium and silicon isotope fractionations, as this would require prohibitively large mass loss from these magnesium-rich inclusions. The remarkable similarity in textures between FUN and non-FUN inclusions implies similar thermal histories, arguing against different rates of evaporative loss of major elements. Sputtering alone may be insufficient to account for the magnitude and direction of oxygen isotope fractionation in FUN inclusions. 相似文献
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Olga V Pravdivtseva Alexander N Krot Alexander P Meshik Klaus Keil 《Geochimica et cosmochimica acta》2003,67(24):5011-5026
Complex I-Xe and mineralogical studies have been performed on four heavily-altered Allende fine-grained spinel-rich Ca, Al-rich inclusions (CAIs) and four Allende dark inclusions (DIs) showing various degrees of iron-alkali metasomatic alteration. The CAIs are largely composed of Fe-rich spinel, Al-diopside, and secondary nepheline and sodalite. The DIs consist of chondrules and Allende-like matrix composed of lath-shaped fayalitic olivine, nepheline, sodalite, and Ca, Fe-rich pyroxene ± andradite ± FeNi-sulfide nodules. Chondrule phenocrysts are extensively or completely replaced by fayalitic olivine, nepheline, and sodalite; metal nodules are replaced by FeNi-sulfides, andradite and Ca, Fe-rich pyroxenes. The chondrules and matrices are crosscut by Ca, Fe-rich pyroxene ± FeNi-sulfide ± fayalitic olivine veins. DIs are surrounded by continuous Ca-rich rims composed of andradite, wollastonite, kirschsteinite, and Ca, Fe-rich pyroxenes, whereas the outer portions of the inclusions are depleted in Ca.Three CAIs yield well-defined I-Xe isochrons with ages 3.1 ± 0.2, 3.0 ± 0.2 and 3.7 ± 0.2 Ma younger than the Shallowater internal standard (4566 ± 2 Ma). Similar release profiles suggest the same iodine carrier (most probably sodalite) for all four CAIs. The Allende DIs yield I-Xe ages from 0.8 ± 0.3 to 1.9 ± 0.2 Ma older than Shallowater. Based on the petrographic observations, we infer that the DIs experienced at least two-stage alteration. During an early stage of the alteration, which took place in an asteroidal setting, but not in the current location of the DIs, chondrule silicates were replaced by secondary fayalitic olivine, nepheline, and sodalite. Calcium lost from the chondrules was redeposited as Ca, Fe-rich pyroxene veins and Ca, Fe-rich pyroxene ± andradite nodules in the matrix. The second stage of alteration resulted in mobilization of Ca from the DIs and its re-deposition as Ca-rich rims composed of Ca, Fe-rich pyroxenes, andradite, and wollastonite, around the DIs. We interpret I-Xe ages of the DIs as time of their alteration prior incorporation into Allende. The younger I-Xe ages of the fine-grained spinel-rich CAIs may reflect hydrothermal alteration of the Allende host, which could have occurred contemporaneously with the second stage of alteration of the Allende DIs. The lack of evidence for the disturbance of I-Xe system in the Allende DIs may suggest that fluid responsible for the alteration of the Allende CAIs was in equilibrium with the I- and Xe-bearing phases of the DIs. 相似文献
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A.E. Beswick 《Geochimica et cosmochimica acta》1973,37(2):183-208
K and Rb distributions between aqueous alkali chloride vapour phase (0.7 molar) and coexisting phlogopites and sanidines have been investigated in the range 500 to 800°C at 2000 kg/cm2 total pressure.Complete solid solution of RbMg3AlSi3O10(OH)2 in KMg3AlSi3O10(OH)2 exists at and above 700°C. At 500°C a possible miscibility gap between approximately 0.2 and 0.6 mole fraction of the Rb end-member is indicated.Only limited solid solution of Rb AlSi3O8 in KAlSi3O8 has been found at all temperatures investigated.Distribution coefficients, expressed as in solid/(Rb/K) in vapour, are appreciably temperature-dependent but at each temperature are independent of composition for low Rb end-member mole fractions in the solids. The determined values and their approximate Rb end-member mole fraction () ranges of constancy are summarized as follows: (°C)
| (°C)T | ||||
| 500 | 0–0.2 | 0–0.07 | ||
| 700 | 0–0.2 | 0–0.1 | ||
| 800 | 0–0.2 | 0–0.1 |
| Adularia (Or98) | 25.6 | 350–700 | |
| Albite (Ab97, Ab99) | 21.3 | 350–800 | |
| Anorthite (An96) | 26.2 | 350–800 |
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