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1.
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2.
Berman’s (1983) activity-composition model for CaO-MgO-Al2O3-SiO2 liquids is used to calculate the change in bulk chemical and isotopic composition during simultaneous cooling, evaporation, and crystallization of droplets having the compositions of reasonable condensate precursors of Types A and B refractory inclusions in CV3 chondrites. The degree of evaporation of MgO and SiO2, calculated to be faithfully recorded in chemical and isotopic zoning of individual melilite crystals, is directly proportional to evaporation rate, which is a sensitive function of PH2, and inversely proportional to the droplet radius and cooling rate. When the precursors are partially melted in pure hydrogen at peak temperatures in the vicinity of the initial crystallization temperature of melilite, their bulk chemical compositions evolve into the composition fields of refractory inclusions, mass-fractionated isotopic compositions of Mg, Si, and O are produced that are in the range of the isotopic compositions of natural inclusions, and melilite zoning profiles result that are similar to those observed in real inclusions. For droplets of radius 0.25 cm evaporating at PH2 = 10−6 bar, precursors containing 8 to 13 wt.% MgO and 20 to 23% SiO2 evolve into objects similar to compact Type A inclusions at cooling rates of 2 to 12 K/h, depending on the precise starting composition. Precursors containing 13 to 14 wt.% MgO and 23 to 26% SiO2 evolve into objects with the characteristics of Type B1 inclusions at cooling rates of 1.5 to 3 K/h. The relatively SiO2-poor members of the Type B2 group can be produced from precursors containing 14 to 16 wt.% MgO and 27 to 33% SiO2 at cooling rates of <1 K/h. Type B2’s containing 27 to 35 wt.% SiO2 and <12% MgO require precursors with higher SiO2/MgO ratios at MgO > 15% than are found on any condensation curve. The characteristics of fluffy Type A inclusions, including their reversely zoned melilite, can only be understood in the context of this model if they contain relict melilite.  相似文献   

3.
Compact type A (CTA) inclusions are one of the major types of coarse-grained refractory inclusions found in carbonaceous chondrites. They have not been studied in a systematic fashion, leading to some uncertainties and unproven assumptions about their origin. To address this situation, we studied a total of eight CTAs from Allende, Efremovka and Axtell by scanning electron-microscopic and electron and ion-microprobe techniques. These inclusions are very melilite-rich, ranging from ∼60 vol% to nearly monomineralic. Also present are Mg–Al spinel (5–20%), perovskite (trace–∼3%) and, in some samples, Ti-rich (∼17 wt% TiO2tot) fassaite (trace–∼20%), and rhönite (≤1%). Melilite compositions are mostly between Åk15 and Åk40. Chondrite-normalized REE abundance patterns for melilite (flat at ∼10 × CI with positive Eu anomalies) and fassaite (slight HREE enrichment relative to LREE and negative Eu anomalies) are like those for their counterparts in once-molten type B inclusions. The patterns for rhönite have positive slopes from La through Lu and abundances <10 × CI for La and 35–60 × CI for Lu. Features of CTAs that suggest that they were once molten include: rounded inclusion shapes; positively correlated Sc and V abundances in fassaite; radially oriented melilite laths at inclusion rims; and the distribution of trace elements among the phases. Fractional crystallization models show that, with one exception, the REE contents of perovskite and fassaite arose by crystallization of these phases from late, residual liquids that would have resulted from prior crystallization of the observed proportions of melilite and spinel from liquids having the bulk compositions of the inclusions. One Allende CTA (TS32), however, has several features (irregular shape, reversely zoned melilite, fassaite REE contents) that are not readily explained by crystallization from a melt. This inclusion may have undergone little melting and may be dominated by relict grains.  相似文献   

4.
Lightly altered Al-rich inclusions in amoeboid olivine aggregates have cores containing primary melilite + fassaite + spinel + perovskite and no secondary alteration products. In moderately altered inclusions, whose cores now contain only fassaite + spinel + perovskite, melilite was replaced by a fine-grained mixture of grossular + anorthite + feldspathoids and perovskite was partially replaced by ilmenite. In heavily altered inclusions, fassaite has been replaced by a mixture of phyllosilicates + ilmenite and the remaining primary phases are spinel ± perovskite. In very heavily altered inclusions, no primary phases remain, the spinel having reacted to form either phyllosilicates or a mixture of olivine + feldspathoids. This sequence of alteration reactions may reflect successively lower solar nebular equilibration temperatures. During alteration, SiO2, Na2O, K2O, FeO, Cr2O3, H2O and Cl were introduced into the inclusions and CaO was lost. MgO may have been lost during the melilite reaction and added during formation of phyllosilicates. Electron microprobe analyses indicate that the phyllosilicates are a mixture of Na-rich phlogopite and chlorite or Alrich serpentine. Thermodynamic calculations suggest that, at a solar nebular water fugacity of 10−6, Na-rich phlogopite could have formed from fassaite at ~470 K and chlorite from Na-rich phlogopite at ~328 K. Olivine mantling Al-rich inclusions is not serpentinized, suggesting that these objects stopped equilibrating with the nebular gas above 274 K.  相似文献   

5.
The coarse-grained, igneous, anorthite-rich (Type C) CAIs from Allende studied (100, 160, 6-1-72, 3529-40, CG5, ABC, TS26, and 93) have diverse textures and mineralogies, suggesting complex nebular and asteroidal formation histories. CAIs 100, 160, 6-1-72, and 3529-40 consist of Al,Ti-diopside (fassaite; 13-23 wt% Al2O3, 2-14 wt% TiO2), Na-bearing åkermanitic melilite (0.1-0.4 wt% Na2O; Åk30-75), spinel, and fine-grained (∼5-10 μm) anorthite groundmass. Most of the fassaite and melilite grains have “lacy” textures characterized by the presence of abundant rounded and prismatic inclusions of anorthite ∼5-10 μm in size. Lacy melilite is pseudomorphed to varying degrees by grossular, monticellite, and pure forsterite or wollastonite. CAI 6-1-72 contains a relict Type B CAI-like portion composed of polycrystalline gehlenitic melilite (Åk10-40), fassaite, spinel, perovskite, and platinum-group element nuggets; the Type B-like material is overgrown by lacy melilite and fassaite. Some melilite and fassaite grains in CAIs 100 and 160 are texturally similar to those in the Type B portion of 6-1-72. CAIs ABC and TS26 contain relict chondrule fragments composed of forsteritic olivine and low-Ca pyroxene; CAI 93 is overgrown by a coarse-grained igneous rim of pigeonite, augite, and anorthitic plagioclase. These three CAIs contain very sodium-rich åkermanitic melilite (0.4-0.6 wt% Na2O; Åk63-74) and Cr-bearing Al,Ti-diopside (up to 1.6 wt% Cr2O3, 1-23 wt% Al2O, 0.5-7 wt% TiO2). Melilite and anorthite in the Allende Type C CAI peripheries are replaced by nepheline and sodalite, which are crosscut by andradite-bearing veins; spinel is enriched in FeO. The CAI fragment CG5 is texturally and mineralogically distinct from other Allende Type Cs. It is anorthite-poor and very rich in spinel poikilitically enclosed by Na-free gehlenitic melilite (Åk20-30), fassaite, and anorthite; neither melilite nor pyroxene have lacy textures; secondary minerals are absent. The Al-rich chondrules 3655b-2 and 3510-7 contain aluminum-rich and ferromagnesian portions. The Al-rich portions consist of anorthitic plagioclase, Al-rich low-Ca pyroxene, and Cr-bearing spinel; the ferromagnesium portions consist of fosteritic olivine, low-Ca pyroxene, and opaque nodules.We conclude that Type C CAIs 100, 160, 6-1-72, and 3529-40 formed by melting of coarse-grained Type B-like CAIs which experienced either extensive replacement of melilite and spinel mainly by anorthite and diopside (traces of secondary Na-bearing minerals, e.g., nepheline or sodalite, might have formed as well), or addition of silica and sodium during the melting event. CG5 could have formed by melting of fine-grained spinel-melilite CAI with melilite and spinel partially replaced anorthite and diopside. CAIs ABC, 93, and TS-26 experienced melting in the chondrule-forming regions with addition of chondrule-like material, such as forsteritic olivine, low-Ca pyroxene, and high-Ca pyroxene. Anorthite-rich chondrules formed by melting of the Al-rich (Type C CAI-like) precursors mixed with ferromagnesian, Type I chondrule-like precursors. The Allende Type C CAIs and Al-rich chondrules experienced fluid-assisted thermal metamorphism, which resulted in pseudomorphic replacement of melilite and anorthite by grossular, monticellite, and forsterite (100, 160, 6-1-72, 3592-40) or by grossular, monticellite, and wollastonite (ABC, 93, TS-26). The pseudomorphic replacement was followed or accompanied by iron-alkali metasomatic alteration resulting in replacement of melilite and anorthite by nepheline and sodalite, enrichment of spinel in FeO, and precipitation of salite-hedenbergite pyroxenes, wollastonite, and andradite in fractures and pores in and around CAIs.  相似文献   

6.
The equilibrium crystallization sequence at 1 atmosphere in air of a melt corresponding in composition to the average composition of Type B Ca-Al-rich inclusions from the Allende meteorite is: spinel (1550°C) → melilite (1400°C; Åk22) → anorthite (1260°C) → Ti-Al-rich clinopyroxene (1230°C; “Ti-fassaite”). The melilite becomes increasingly åkermanitic with decreasing temperature. The pyroxene is similar in composition to fassaites from Type B inclusions. Preliminary results suggest that the crystallization sequence is similar at oxygen fugacities near the iron-wüstite buffer.The results of these experiments have been integrated with available phase equilibrium data in the system CaO-MgO-Al2O3-SiO2TiO2 and a phase diagram for predicting the crystallization sequences of liquids with compositions of coarse-grained Ca-Al-rich inclusions has been developed.Available bulk compositions of coarse-grained inclusions form a well-defined trend in terms of major elements, extending from Type A and Bl inclusions near the spinel-melilite join to more pyroxene-rich Type B2 inclusions. The trend deviates from the expected sequence of solid condensates from a nebular gas at P = 10?3 atm if pure diopside is assumed to be the clinopyroxene that condenses. The Type A-B1 end of the trend is similar in composition to calculated equilibrium condensates at 1202–1227°C and the trend as a whole parallels the sequence of condensates expected from diopside condensation at ~ 1170°C. The trend is consistent to first order with the condensation of solid Ti-rich fassaite in place of pure diopside at higher temperatures than those at which pure diopside is predicted to condense. Partially molten condensates may be likely in this case or if the nebular pressure is higher than 10?3 atm.  相似文献   

7.
There are two types of white, coarse-grained, Ca-Al-rich inclusions in Allende. Type A inclusions contain 80–85 per cent melilite, 15–20 per cent spinel, 1–2 per cent perovskite and rare plagioclase, hibonite, wollastonite and grossularite. Clinopyroxene, if present, is restricted to thin rims around inclusions or cavities in their interiors. Type B inclusions contain 35–60 per cent pyroxene, 15–30 per cent spinel, 5–25 per cent plagioclase and 5–20 per cent melilite. The coarse pyroxene crystals in Type B's contain >15 per cent Al2O3 and >1.8 per cent Ti, some of which is trivalent. Type A pyroxenes contain <9 per cent Al2O3 and <0.7 per cent Ti.Electron microprobe analyses of 600 melilite, 39 pyroxene, 35 plagioelase, 33 spinel and 20 perovskite grains were performed in 16 Type A, 1 intermediate and 9 Type B inclusions in Allende and 1 Type A in Grosnaja. Melilite composition histograms from individual Type A inclusions are usually peaked between Ak10 and Ak30 and are 15–20 mole % wide while those from Type B inclusions are broader, unpeaked and displaced to higher åkermanite contents. Most pyroxenes contain < 1 per cent FeO. All plagioclase is An 98 to An 100. Spinel is almost pure MgAl2O4. Perovskite contains small (< 1 per cent) but significant amounts of Mg, Al, Fe, Y, Zr and Nb.Inferred bulk chemical compositions of Type A inclusions are rather close to those expected for high-temperature condensates. Those of Type B inclusions suggest slightly lower temperatures but their Ca/Al ratio seems less than the Type A's, indicating that the Type B's may not be their direct descendants. Some textural features suggest that the inclusions are primordial solid condensetes while others indicate that they may have been melted after condensation. Fragmentation and metamorphism may have also occurred after condensation.  相似文献   

8.
Interdiffusion coefficients of Al + Al vs. Mg + Si in the gehlenite–åkermanite system of melilite were determined by coupled annealing of synthesized end-member single crystals. The observed diffusion coefficients for a couple-annealed sample vary for about 2 orders of magnitude, showing strong dependence on the gehlenite–åkermanite composition: diffusion coefficient observed at 1350 °C, for example, is 3 × 10?13 cm2 s?1 at 5 mol% åkermanite composition (Ak5), increases to 2 × 10?11 cm2 s?1 at Ak80, and then decreases to 1 × 10?12 cm2 s?1 at Ak95. The diffusion coefficient–temperature relation indicates high activation energy of diffusion of about 420 kJ mol?1 for gehlenite-rich melilite. The observed diffusion coefficient–composition relation may be explained by a combination of (1) the diffusion coefficient–melting temperature relation (Flynn's rule) and (2) the feasibility of local charge compensation, which can possibly be maintained more easily in the intermediate chemical composition. The high activation energy value for gehlenitic melilite appears to correspond to the complex diffusion mechanism. The observed highly variable diffusion coefficients suggest that gehlenite–åkermanite zoning in the melilite crystals in Ca, Al-rich inclusions in the carbonaceous meteorites may provide a sensitive indicator for the thermal history of the inclusions.  相似文献   

9.
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   

10.
The oxygen isotopic micro-distributions within and among minerals in a coarse-grained Ca, Al-rich inclusion (CAI), 7R-19-1 from the Allende meteorite, were measured by in situ using secondary ion mass spectrometry (SIMS). All values of O isotopic ratios in 7R-19-1 minerals fall along the carbonaceous chondrite anhydrous mineral mixing (CCAM) line on a δ17OSMOW vs. δ18OSMOW plot. Major refractory minerals (spinel, fassaite and melilite) in 7R-19-1 showed large negative anomalies of Δ17O in the order, spinel (−21‰) > 16O-rich melilite (∼−18‰) > fassaite (−15 to +1‰) > 16O-poor melilite (−8 to +2‰). However, the lower limit values of Δ17O are similar at about −21‰, a value commonly observed in CAIs. The similarity in the extreme values of the isotope anomaly anomalies suggests that crystallization of all CAIs started from an 16O enrichment of 21‰ (Δ17O) relative to terrestrial values. The order of the O isotopic anomalies observed for 7R-19-1, except for 16O-poor melilite, is parallel to the crystallization sequence determined by experiment from CAI liquid (Stolper, 1982), indicating that the O isotopic exchange in 7R-19-1 occurred between CAI melt and surrounding gas while 7R-19-1 was crystallizing from the 16O enriched CAI liquid (∼−21‰ in Δ17O) in the 16O-poor solar nebula. However, the a single crystallization sequence during the cooling stage cannot explain the existence of 16O-poor melilite. The presence of 16O-poor melilite suggests that multiple heating events occurred during CAI formation. The sharp contact between 16O-rich and 16O-poor melilite crystals and within 16O-rich melilite indicates that these multiple heatings occurred quickly. Based on the O isotopic and chemical compositions, fassaite crystals were aggregates of relic crystals formed from CAI melt whichthat have had various O isotopic compositions from the remelting processes. The results of intra-mineral distributions of O isotopes also support multiple heating events during CAI formation.  相似文献   

11.
The clinopyroxene in coarse-grained refractory inclusions contains significant amounts of Ti and V, two elements that are multivalent over the range of temperatures and oxygen fugacities under which the inclusions formed. The Ti3+/Ti4+ ratios and the valence of V of these pyroxenes are valuable recorders of nebular conditions. The former can be calculated stoichiometrically from electron probe analyses, but only for relatively Ti-rich grains (i.e., >∼4 wt% ). For Ti-poor pyroxene, and for measurement of V valence, another technique is needed. We have, for the first time, applied K-edge X-ray absorption near edge structure (XANES) spectroscopy to the measurement of Ti and V valence in meteoritic clinopyroxene in refractory inclusions. Use of MicroXANES, a microbeam technique with high (μm-scale) spatial resolution, allowed measurement of Ti and V valence along traverses across (a) Ti-, V-rich “spikes” in pyroxene in Type B1 inclusions; (b) typical grains in a Type B2 inclusion; and (c) the pyroxene layer of the Wark-Lovering rim sequence on the outsides of two inclusions. Measurements of Ti3+/(Ti3+ + Ti4+), or Ti3+/Titot, by XANES agree with values calculated from electron probe analyses to within ∼0.1, or ∼2σ. The results show that Ti3+/Titot increases sharply at the spikes, from 0.46 ± 0.03 to 0.86 ± 0.06, but the V valence, or V2+/(V2+ + V3+), does not change, with V2+ ≈ V3+. We found that pyroxene in both Types B1 and B2 inclusions has Ti3+/Titot and V2+/Vtot ratios between 0.4 and 0.7, except for the spikes. These values indicate, to first order, formation at similar, highly reducing oxygen fugacities that are consistent with a solar gas. The pyroxene in the rim on an Allende fluffy Type A coarse-grained refractory inclusion, TS24, has an average Ti3+/Titot of 0.51 ± 0.08 and an average V2+/Vtot of 0.61 ± 0.06, determined by XANES. These values are within the range of those of pyroxene in the interiors of inclusions, indicating that the rims also formed under highly reducing conditions. Measurements of Ti3+/Titot of pyroxene in the rim of a Leoville compact Type A inclusion, 144A, by both XANES and electron probe give a wide range of results. Of our 72 XANES analyses of this rim, 66% have Ti3+/Titot of 0.40-0.71, and the remaining analyses range from 0 to 0.38. In data from Simon et al. [Simon J. I., Young E. D., Russell S. S., Tonui E. K., Dyl K. A., and Manning C. E. (2005) A short timescale for changing oxygen fugacity in the solar nebula revealed by high-resolution 26Al-26Mg dating of CAI rims. Earth Planet. Sci. Lett.238, 272-283.] for this sample, 7 electron probe analyses yield calculated Ti3+/Titot values that are positive and 15 do not. In the probe analyses that have no calculated Ti3+, Ca contents are anticorrelated and Al contents directly correlated with the total cations per 6 oxygens, and the data fall along trends calculated for addition of 1-7% spinel to pyroxene. It appears likely that electron probe analyses of pure pyroxene spots have Ti3+/Titot values that are typical of refractory inclusions, in agreement with the majority of the XANES results. The average of the XANES data for 144A, 0.41 ± 0.14, is within error of that for TS24. The rim of 144A probably formed under reducing conditions like those expected for a solar gas, and was later heterogeneously altered, resulting in an uneven distribution of secondary, FeO-, Ti-bearing alteration products in the rim, and accounting for the measurements with low Ti3+/Titot values.  相似文献   

12.
Electron and ion-probe microanalysis have been used to obtain zoning profiles for major and trace elements in olivine phenocrysts from a high-magnesian andesite from Shodo-Shima island, southwest Japan. This rock was previously thought to represent undifferentiated, primary magma. Some crystals have unzoned cores, while others show cores which are reversely zoned with respect to Mg/ (Mg+Fe), Ni, Mn and Cr. In addition, some Ni profiles show a normally zoned hump at the most central portions of the reversely zoned crystals. All crystals show normally zoned rims. The Li concentrations are constant throughout the cores of all crystals studied, but rise sharply, by a factor of up to at least six, in the rims. The Ca and Co concentrations are essentially constant throughout all the crystals. Mechanisms for producing the observed zoning profiles are discussed, and it is concluded that the reverse zoning was produced by the introduction of crystals into a less differentiated magma than that in which they grew. The reversely zoned crystals could therefore represent xenocrysts which were introduced into an undifferentiated magma, or phenocrysts introduced into a more primitive magma by a magma mixing process. The Ni profiles are used to estimate the residence time of these crystals in the more primitive magma. The following trace element partition co-efficients have been estimated for the olivine-groundmass system in this rock: D Ni=16; D Mn=1.1; D Co=4.2; D Ca =0.02; D Ti=0.005; D V=0.05; D Sc=0.2; D Na=0.0002. Studies of trace element zoning will become increasingly important as the new generation of trace element microprobes become available but a larger database of experimentally determined values for trace element partition coefficients and diffusion coefficients in crystals and magmas, and a better understanding of other disequilibrium processes are required to fully exploit the new data.  相似文献   

13.
Type B CAIs are subdivided into B1s, with well-developed melilite mantles, and B2s, with randomly distributed melilite. Despite intensive study, the origin of the characteristic melilite mantle of the B1s remains unclear. Recently, we proposed that formation of the melilite mantle is caused by depletion of the droplet surface in volatile magnesium and silicon due to higher evaporation rates of volatile species compared to their slow diffusion rates in the melt, thus making possible crystallization of melilite at the edge of the CAI first, followed by its crystallization in the central parts at lower temperatures. Here, we present the results of an experimental study that aimed to reproduce the texture observed in natural Type B CAIs. First, we experimentally determined crystallization temperatures of melilite for three melt compositions, which, combined with literature data, allowed us to find a simple relationship between the melt composition, crystallization temperature, and composition of first crystallizing melilite. Second, we conducted a series of evaporation and cooling experiments exposing CAI-like melts to gas mixtures with different oxygen fugacities (fO2). Cooling of the molten droplets in gases with logfO2?IW-4 resulted in crystallization of randomly distributed melilite, while under more reducing conditions, melilite mantles have been formed. Chemical profiles through samples quenched right before melilite started to crystallize showed no chemical gradients in samples exposed to relatively oxidizing gases (logfO2?IW-4), while the near-surface parts of the samples exposed to very reducing gases (logfO2?IW-7) were depleted in volatile MgO and SiO2, and enriched in refractory Al2O3. Using these experimental results and the fact that the evaporation rate of magnesium and silicon from CAI-like melts is proportional to , we estimate that Type B1 CAIs could be formed by evaporation of a partially molten precursor in a gas of solar composition with . Type B2 CAIs could form by slower evaporation of the same precursors in the same gas with .  相似文献   

14.
The petrography and mineral chemistry of 110 Ca-, Al-rich inclusions (CAIs) and 9 Ca- and/or Al-rich amoeboid olivine aggregates (AOAs) from the Ningqiang carbonaceous chondrite are reported. These CAIs are referred to as hibonite-bearing and hibonite-free melilite-spinel-rich (Type A), and spinel-pyroxene inclusions. Melilite is more gehlenitic in the hibonite-bearing Type As than in the other two types, and all of them vary within a range of Åk0-30. Modal compositions of the three types of CAIs overlap with each other, and make up a continuum with wide ranges of melilite: spinel: diopside. The diopside occurs as rims on the CAIs or their individual concentric objects. The 9 AOAs contain spinel ± diopside ± anorthite in the centers of the aggregates; the spinel grains rimmed by diopside in the centers are similar to the spinel-pyroxene inclusions. Bulk compositions of these CAIs vary along the condensation trajectory, with the hibonite-bearing Type As plotting at the beginning followed by hibonite-free Type As then by spinel-pyroxene inclusions as temperature decreases. Bulk compositions of the AOAs are close to the lowest temperature condensation trajectory. Except for a few with compact textures, most of the Type As and spinel-pyroxene inclusions are fluffy aggregates, probably pristine vapor-solid condensates of the nebula.The bulk compositions of the Type As appear to overlap with the range of most melilite-Ti-Al-clinopyroxene-rich (Type B) inclusions. Hence, crystallization of liquids produced by melting the Type As can form Type B inclusions, without significant evaporative loss of MgO or SiO2. A few Type Bs have bulk compositions deviating from the range of their proposed precursors, and may have suffered significant evaporation, as suggested in previous studies.  相似文献   

15.
Calcium- and aluminum-rich inclusions (CAIs), occurring in chondritic meteorites and considered the oldest materials in the solar system, can provide critical information about the environment and time scale of creation of planetary materials. However, interpretation of the trace element and isotope compositions of CAIs, particularly the light elements Li, Be, and B, is hampered by the lack of constraint on melilite-melt and spinel-melt partition coefficients. We determined melilite-melt and spinel-melt partition coefficients for 21 elements by performing controlled cooling rate (2 °C/h) experiments at 1 atmosphere pressure in sealed platinum capsules using a synthetic type B CAI melt. Trace element concentrations were measured by secondary ion mass spectrometry (SIMS) and/or laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Melilites vary only slightly in composition, ranging from Åk31-43. Results for the partitioning of trace elements between melilite and melt in three experiments and between spinel and melt in two experiments show that partition coefficients are independent of trace element concentration, are in good agreement for different analytical techniques (SIMS and LA-ICP-MS), and are in agreement with previous measurements in the literature. Partition coefficients between intermediate composition melilites and CAI melt are the following: Li, 0.5; Be, 1.0; B, 0.22; Rb, 0.012; Sr, 0.68; Zr, 0.004; Nb, 0.003; Cs, 0.002; Ba, 0.018; La, 0.056; Nd, 0.065; Sm, 0.073; Eu, 0.67; Er, 0.037; Yb, 0.018; Hf, 0.001; Ta, 0.003; Pb, 0.15; U, 0.001; Th, 0.002. Site size energetics analysis is used to assess isovalent partitioning into the different cation sites. The Young’s modulus deduced from +2 cations partitioning into the melilite X site agrees well with the bulk modulus of melilite based on X-ray diffraction methods. The changes in light element partitioning as melilite composition varies are predicted and used in several models of fractional crystallization to evaluate if the observed Li, Be, and B systematics in Allende CAI 3529-41 are consistent with crystallization from a melt. Models of crystallization agree reasonably well with observed light element variations in areas previously interpreted to be unperturbed by secondary processes [Chaussidon, M., Robert, F., McKeegan, K.D., 2006. Li and B isotopic variations in an Allende CAI: Evidence for the in situ decay of short-lived 10Be and for the possible presence of the short-lived nuclide 7Be in the early solar system. Geochim. Cosmochim. Acta70, 224-245], indicating that the trends of light elements could reflect fractional crystallization of a melt. In contrast, areas interpreted to have been affected by alteration processes are not consistent with crystallization models.  相似文献   

16.
Refractory Ti-bearing minerals in the calcium-, aluminum-rich inclusion (CAI) Inti, recovered from the comet 81P/Wild 2 sample, were examined using analytical (scanning) transmission electron microscopy (STEM) methods including imaging, nanodiffraction, energy-dispersive spectroscopy (EDX) and electron energy loss spectroscopy (EELS). Inti fassaite (Ca(Mg,Ti,Al)(Si,Al)2O6) was found to have a Ti3+/Ti4+ ratio of 2.0 ± 0.2, consistent with fassaite in other solar system CAIs. The oxygen fugacity (logfO2) of formation estimated from this ratio, assuming equilibration among phases at 1509 K, is −19.4 ± 1.3. This value is near the canonical solar nebula value (−18.1 ± 0.3) and in close agreement with that reported for fassaite-bearing Allende CAIs (−19.8 ± 0.9) by other researchers using the same assumptions. Nanocrystals of osbornite (Ti(V)N), 2–40 nm in diameter, are embedded as inclusions within gehlenite, spinel and diopside in Inti. Vanadium is heterogeneously distributed within some osbornite crystals. Compositions range from pure TiN to Ti0.36V0.64N. The possible presence of oxide and carbide in solid solution with the osbornite was evaluated. The osbornite may contain O, but C is not present at detectable levels. The presence of osbornite, likely a refractory early condensate, together with the other refractory minerals in Inti, indicates that the parent comet contains solids that condensed closer to the proto-sun than the distance at which the parent comet itself accreted. The estimated oxygen fugacity and the reported isotopic and chemical compositions are consistent with Inti originating in the inner solar system like other meteoritic CAIs. These results provide insight for evaluating the validity of models of radial mass transport dynamics in the early solar system. The oxidation environments inferred for the Inti mineral assemblage are inconsistent with an X-wind formation scenario. In contrast, radial mixing models that allow accretion of components from different heliocentric distances can satisfy the observations from the cometary CAI Inti.  相似文献   

17.
The article describes the thermal metamorphism of siliceous carbonate rocks near the dolerite intrusive body in Eastern Siberia. The mineral associations at the immediate contact with dolerite are the following: wollastonite+rankinite, rankinite+spurrite (+melilite?), spurrite+melilite+merwinite+calcite and merwinite+monticellite+melilite+calcite. The melilite in these associations is usually unzoned; its composition being essentially gehlenitic. During the regressive stage of contact metamorphism new akermanite-rich melilite and calcite were formed by replacement of merwinite and earlier gehlenitic melilite through participation of CO2. The newly forming melilite grains have sharp compositional zoning. The origin of zoning was connected with the fall of temperature and decrease of the mole fraction of CO2 in the fluid equilibrated with the minerals.  相似文献   

18.
Sixty-eight refractory inclusions and fragments were found in two polished thin sections of the Sahara 97159 EH3 chondrite, indicative of the highest abundance of refractory inclusions (22/cm2, or 0.06 vol.%) in enstatite chondrites studied to date. All of the inclusions are intensely altered, mainly producing feldspathoids and albite, CaO depletion and minor Ti-rich compounds, such as Ti-sulfides. The alteration assemblages and FeO-poor spinel suggest that the reactions took place under reducing and SiO2-rich conditions. This is consistent with the redox state of the host enstatite chondrite. The presence of Ti sulfides and low FeO alteration phases distinguishes alteration of E chondrite refractory inclusions from that of carbonaceous and ordinary chondrites.Most of the inclusions are referred to as Type A-like (35) and spinel-rich (26), respectively. Assuming melilite has been altered, these inclusions could be analogues of individual concentrically zoned objects of fluffy melilite-spinel-rich (Type A) and spinel-pyroxene-rich inclusions from carbonaceous chondrites such as the Ningqiang (CV anomalous) and Y 81020 (CO3) chondrites. Two inclusions consist mainly of Ca-pyroxene, fine-grained alteration products (feldspathoids and albite) and spinel. They are probably altered fragments of Ca-pyroxene-plagioclase-rich (Type C) inclusions, assuming all plagioclase has been altered to produce the fine-grained groundmass. Five other inclusions are hibonite and/or corundum bearing, similar to those reported in carbonaceous chondrites. Abundance ratios of various types of the inclusions from Sahara 97159 are similar to those from Ningqiang and Y 81020.Most of the observations, including mineral assemblages, mineral chemistry, texture, bulk compositions, O isotopic compositions and REE patterns, of the Sahara inclusions suggest a common reservoir of refractory inclusions in enstatite, ordinary and carbonaceous chondrites. The apparent differences, such as absence of melilite and anorthite, rare Wark-Lovering rim and small size, can be explained by intense alteration due to large change of postformation environment of these inclusions, size sorting and collision during transfer. Hence, these differences are not inconsistent with the common reservoir model. Refractory inclusions in non-carbonaceous chondrites may put additional constraints on origins of refractory inclusions, and provide hints for a spatial relationship of their host meteorites.  相似文献   

19.
Lawsonite eclogites are crucial to decipher material recycling along a cold geotherm into the deep Earth and orogenic geodynamics at convergent margins. However, their tectono‐metamorphic role and record especially at ultrahigh‐pressure (UHP) conditions are poorly known due to rare exposure in orogenic belts. In a ~4 km long cross‐section in Muzhaerte, China, at the western termination of the HP‐UHP metamorphic belt of western Tianshan, metabasite blocks contain omphacite and lawsonite inclusions in porphyroblastic garnet, although matrix assemblages have been significantly affected by overprinting at shallower structural levels. Two types of lawsonite eclogites occur in different parts of the section and are distinguished based on inclusion assemblages in garnet: Type 1 (UHP) with the peak equilibrium assemblage garnet+omphacite±jadeite+lawsonite+rutile+coesite±chlorite±glaucophane and Type 2 (HP) with the assemblage garnet+omphacite±diopside+lawsonite+titanite+quartz±actinolite±chlorite+glaucophane. Pristine coesite and lawsonite and their pseudomorphs in Type 1 are present in the mantle domains of zoned garnet, indicative of a coesite‐lawsonite eclogite facies. Regardless of grain size and zoning profiles, garnet with Type 1 inclusions systematically shows higher Mg and lower Ca contents than Type 2 (prp4–25grs13–24 and prp1–8grs20–45 respectively). Phase equilibria modelling indicates that the low‐Ca garnet core and mantle of Type 1 formed at UHP conditions and that there was a major difference in peak pressures (i.e., maximum return depth) between the two types (2.8–3.2 GPa at 480–590°C and 1.3–1.85 GPa at 390–500°C respectively). Scattered exposures of Type 1 lawsonite eclogite is scatteredly exposed in the north of the Muzhaerte section with a structural thickness of ~1 km, whereas Type 2 occurs throughout the rest of the section. We conclude from this regular distribution that they were derived from two contrasting units that formed along two different geothermal systems (150–200°C/GPa for the northern UHP unit and 200–300°C/GPa for the southern HP unit), with subsequent stacking of UHP and HP slices at a kilometre scale.  相似文献   

20.
Allende “fluffy” Type A's (FTA's) are a distinct sub-group of Ca-, Al-rich inclusions whose primary mineral assemblage consists of Al-rich melilite (Åk 0–33), spinel that is commonly very V-rich, perovskite and, frequently, hibonite. Some contain relatively coarse-grained melilite (up to 1.5 mm) that is intensely kink-banded and commonly reversely-zoned, hibonite and V-rich spinel. Others contain much finer-grained and strain-free melilite (?50 μm) and have not been found to contain hibonite or V-rich spinel. Some FTA's contain both coarser- and finer-grained melilite and textural relationships indicate that the latter is replacing the former. FTA's are characterized by extremely irregular shapes and 60–75 volume per cent of fine-grained, secondary alteration products. Many are aggregates of innumerable nodules, each of which is surrounded by a Wark-Lovering-type rim sequence. These nodules are frequently separated from one another by matrix-like clastic rim material. Other FTA's do not have nodular structure. Structural and mineralogical characteristics of their Wark-Lovering rims suggest that FTA's did not achieve their shapes by deformation of a liquid or a hot, plastic solid. In contrast to those in Type B inclusions, formation of reverse zoning in the coarser-grained melilite crystals in FTA's cannot be understood in terms of crystallization from a liquid but are readily explainable by condensation from a solar nebular gas during a period of falling pressure. Further evidence against a liquid origin is the wide range of spinel compositions within individual coarser-grained FTA's. The fact that the reversely-zoned melilite crystals cannot have been produced in any kind of sublimation or distillation process precludes formation of these inclusions as volatilization residues. FTA's are aggregates in some of which are preserved vapor-solid condensate grains that formed at high temperature in the solar nebula.  相似文献   

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