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1.
R chondrites are a distinct group of chondritic meteorites with unique mineralogical and chemical compositions. They contain various types of Al-rich objects [Ca,Al-rich inclusions (CAIs), Al-rich chondrules and fragments], whose mineralogical compositions and classifications were previously determined by Rout and Bischoff (2008). Here, we report on the bulk compositions of 126 such Al-rich objects determined by broad-beam electron probe microanalysis.Most of the CAIs, except a few, are significantly altered by complex nebular and/or parent body processes and the determination of their pristine composition is difficult. We found that the simple concentric spinel-rich inclusions have high Al2O3 (21–72 wt%) correlated with their high modal spinel. The subgroup of simple concentric spinel-rich CAIs have a high Al2O3 (21–57 wt%) and also sometimes high FeO (up to 36 wt%), due to a high hercynitic component in the spinel. One simple concentric spinel- and hibonite-rich CAI H030/L, identified as a HAL-type CAI by isotopic studies reported elsewhere, has a highly refractory composition (Al2O3～72 wt%). Most of the simple concentric spinel- and fassaite-rich CAIs have consistently high CaO (～2.5–17 wt%) compared to other simple concentric spinel-rich inclusions group, where only some inclusions have high CaO (up to 15 wt%). Simple concentric spinel- and Na,Al-alteration product-rich CAIs are heavily altered and have high Na2O (up to ～12.5 wt%).The three analyzed fassaite-rich spherules have high CaO and Al2O3, and complex spinel- and fassaite-rich CAIs have high CaO (up to 23 wt%) and SiO2 (up to 41 wt%). Most of the complex spinel- and plagioclase-rich CAIs are altered and contain high amounts of secondary oligoclase. However, a few are less affected by secondary alteration and these are characterized by relatively high CaO (up to 24 wt%) and Al2O3 (18–33 wt%); complex spinel and Na,Al-alteration product-rich CAIs are similar to the concentric spinel- and Na,Al-alteration product-rich CAIs. Due to Fe- and alkali-metasomatism, the vast majority of the inclusions in this subgroup were heavily altered, either in a nebular or parent body environment. As a result of this alteration, they contain high FeO and Na2O+K2O+Cl.Almost all inclusions have a Ca/Al-ratio below the solar ratio. This suggests that significant Ca/Al fractionation occurred during the formation of most CAIs, most probably due to disequilibrium condensation of spinel prior to melilite. However, a distillation process cannot be ruled out for some CAIs in producing the spinel enrichment. Some porous and fine grained CAIs may have been produced by agglomeration of refractory dust rich in spinel and fassaite. The HAL-type CAI, H030/L, most likely formed by distillation, similar to most of the HAL-type inclusions. Phase equilibrium analysis, in the CMAS system, shows that the fassaite-bearing spherules most likely formed by metastable crystallization and disequilibrium processes. Al-rich chondrules are characterized by >10 wt% Al2O3, and most of these also have high FeO and Na2O. Considering their bulk compositions, their precursors seem to have been a mixture of a ferromagnesian chondrule component rich in olivine and an anorthite–spinel–pyroxene–nepheline-rich CAI component. The mineral assemblages of some of the less altered Al-rich chondrules conform to those predicted by phase equilibrium studies.  相似文献   

2.
Experiments were conducted under canonical nebular conditions to see whether the chemical compositions of the various chondrule types can be derived from a single CI-like starting material by open-system melting and evaporation. Experimental charges, produced at 1580 °C and PH2 of 1.31×10−5 atm over 1 to 18 hours, consisted of only two phases, porphyritic olivine crystals in glass. Sulfur, metallic-iron and alkalis were completely evaporated in the first minutes of the experiments and subsequently the main evaporating liquid oxides were FeO and SiO2. Olivines from short runs (2-4 hours) have compositions of Fo83-Fo89, as in Type IIA chondrules, while longer experimental runs (12-18 hours) produce ∼Fo99 olivine, similar to Type IA chondrules. The concentration of CaO in both olivine (up to 0.6 wt.%) and glass, and their Mg#, increased with increasing heating duration. Natural chondrules also show increasing CaO with decreasing S, alkalis, FeO and SiO2. The similarities in bulk chemistry, mineralogy and textures between Type IIA and IA chondrules and the experimental charges demonstrate that these chondrules could have formed by the evaporation of CI precursors. The formation of silica-rich chondrules (IIB and IB) by evaporation requires a more pyroxene-rich precursor.Based on the FeO evaporation rates measured here, Type IIA and IA chondrules, were heated for at least ∼0.5 and ∼3.5 h, respectively, if formed at 1580 °C and PH2 of 1.31×10−5 atm. Type II chondrules may have experienced higher cooling-rates and less evaporation than Type I.The experimental charges experienced free evaporation and exhibited heavy isotopic enrichments in silicon, as well as zero concentrations of S, Na and K, which are not observed in natural chondrules. However, experiments on potassium-rich melts at the same pressure but in closed capsules showed less evaporation of K, and less K isotopic mass fractionation, than expected as a function of decreasing cooling rate. Thus the environment in which chondrules formed is as important as the kinetic processes they experienced. If chondrule formation occurred under conditions in which evaporated gases remained in the vicinity of the residual melts, the extent of evaporation would be reduced and back reaction between the gas and the melt could contribute to the suppression of isotopic mass fractionation. Hence chondrule formation could have involved evaporative loss without Rayleigh fractionation. Volatile-rich Type II and volatile-poor Type I chondrules may have formed in domains with high and low chondrule concentrations, and high partial pressures of lithophile elements, respectively.  相似文献   

3.
The paper is concerned with study of melt inclusions in minerals of ijolite xenoliths at Oldoinyo Lengai Volcano. Melt inclusions with different phase compositions occur in forsterite macrocrysts and in diopside, nepheline, fluorapatite, Ti-andradite, and Ti-magnetite crystals. Nepheline contains primary melt inclusions (silicate glass + gas-carbonate globule ± submicron globules ± sulfide globule ± daughter/trapped phases, represented by diopside, fluorapatite, Ti-andradite, and alumoakermanite). The gas-carbonate globule consists of a gas bubble surrounded by a fine-grained aggregate of Na-Ca-carbonates (nyerereite and gregoryite). Fluorapatite contains primary carbonate-rich melt inclusions in the core, which consist of nyerereite, gregoryite, thenardite, witherite, fluorite, villiaumite, and other phases. Their mineral composition is similar to natrocarbonatites. Primary melt inclusions (glass + gas bubble ± daughter phases) are rare in diopside and Ti-andradite. Diopside and forsterite have trails of secondary carbonate-rich inclusions. Besides the above minerals, these inclusions contain halite, sylvite, neighborite, Na-Ca-phosphate, alkali sulfates, and other rare phases. In addition, diopside contains sulfide inclusions (pyrrhotite ± chalcopy- rite ± djerfisherite ± galena ± pentlandite). The chemical compositions of silicate glasses in the melt inclusions vary widely. The glasses are characterized by high Na, K, and Fe contents and low Al contents. They have high total alkali contents (16–23 wt.% Na2O + K2O) and peralkalinity index [(Na + K)/Al] ranging from 1.1 to 7.6. The carbonate-rich inclusions in the ijolite minerals are enriched in Na, P, S, and Cl. The data obtained indicate that the parental melt in the intermediate chamber was heterogeneous and contained silicate, natrocarbonate, and sulfide components during the ijolite crystallization. According to heating experiments with melt inclusions, silicate-carbonate liquid immiscibility occurred at temperature over 580 °C.  相似文献   

4.
The abundance of metallic iron is highly variable in different kinds of chondrites. The precise mechanism by which metal fractionation occurred and its place in time relative to chondrule formation are unknown. As metallic iron is abundant in most Type I (FeO-poor) chondrules, determining under what conditions metal could form in chondrules is of great interest. Assuming chondrules were formed from low temperature nebular condensate, we heated an anhydrous CI-like material at 1580°C in conditions similar to those of the canonical nebula (PH2 = 1.3 × 10−5 atm). We reproduced many of the characteristics of Type IA and IIA chondrules but none of them contained any iron metal. In these experiments FeO was abundant in charges that were heated for as long as 6 h. At a lower temperature, 1350°C, dendritic/cellular metal crystallized from Fe-FeS melts during the evaporation of S. However, the silicate portion consisted of many relict grains and vesicles, not typical of chondrules.Evaporation experiments conducted at PH2 = 1 atm and 1565°C produced charges containing metallic iron both as melt droplets and inclusions in olivine, similar to those found in chondrules. Formation of iron in these experiments was primarily the result of desulfurization of FeS. With long heating times Fe° was lost by evaporation. Apart from some reduction of FeO by kerogen to make metal inclusions within olivine grains, reduction of FeO to make Fe° in these charges was not observed.This study shows that under canonical nebular conditions FeS and iron-metal are extremely volatile so that metal-rich Type I chondrules could not form by melting “CI.” Under these conditions FeO is lost predominantly by hydrogen stripping and, due to the relative low abundance of hydrogen at low pressures, remains in the melt for as long as 6 h. Conversely, at higher total pressures (1-atm H2) iron metal (produced mainly by the desulfurization of troilite) is less volatile and remains in the melt for longer times (at least 6 h). In addition, due to elevated pressures of hydrogen, FeO is stripped away much faster. These results suggest that chondrule formation occurred in environments with elevated pressures relative to the canonical nebula for iron metal to be present.  相似文献   

5.
Based on their mineralogy and petrography, ∼200 refractory inclusions studied in the unique carbonaceous chondrite, Acfer 094, can be divided into corundum-rich (0.5%), hibonite-rich (1.1%), grossite-rich (8.5%), compact and fluffy Type A (spinel-melilite-rich, 50.3%), pyroxene-anorthite-rich (7.4%), and Type C (pyroxene-anorthite-rich with igneous textures, 1.6%) Ca,Al-rich inclusions (CAIs), pyroxene-hibonite spherules (0.5%), and amoeboid olivine aggregates (AOAs, 30.2%). Melilite in some CAIs is replaced by spinel and Al-diopside and/or by anorthite, whereas spinel-pyroxene assemblages in CAIs and AOAs appear to be replaced by anorthite. Forsterite grains in several AOAs are replaced by low-Ca pyroxene. None of the CAIs or AOAs show evidence for Fe-alkali metasomatic or aqueous alteration. The mineralogy, textures, and bulk chemistry of most Acfer 094 refractory inclusions are consistent with their origin by gas-solid condensation and may reflect continuous interaction with SiO and Mg of the cooling nebula gas. It appears that only a few CAIs experienced subsequent melting. The Al-rich chondrules (ARCs; >10 wt% bulk Al2O3) consist of forsteritic olivine and low-Ca pyroxene phenocrysts, pigeonite, augite, anorthitic plagioclase, ± spinel, FeNi-metal, and crystalline mesostasis composed of plagioclase, augite and a silica phase. Most ARCs are spherical and mineralogically uniform, but some are irregular in shape and heterogeneous in mineralogy, with distinct ferromagnesian and aluminous domains. The ferromagnesian domains tend to form chondrule mantles, and are dominated by low-Ca pyroxene and forsteritic olivine, anorthitic mesostasis, and Fe,Ni-metal nodules. The aluminous domains are dominated by anorthite, high-Ca pyroxene and spinel, occasionally with inclusions of perovskite; have no or little FeNi-metal; and tend to form cores of the heterogeneous chondrules. The cores are enriched in bulk Ca and Al, and apparently formed from melting of CAI-like precursor material that did not mix completely with adjacent ferromagnesian melt. The inferred presence of CAI-like material among precursors for Al-rich chondrules is in apparent conflict with lack of evidence for melting of CAIs that occur outside chondrules, suggesting that these CAIs were largely absent from chondrule-forming region(s) at the time of chondrule formation. This may imply that there are several populations of CAIs in Acfer 094 and that mixing of “normal” CAIs that occur outside chondrules and chondrules that accreted into the Acfer 094 parent asteroid took place after chondrule formation. Alternatively, there may have been an overlap in the CAI- and chondrule-forming regions, where the least refractory CAIs were mixed with Fe-Mg chondrule precursors. This hypothesis is difficult to reconcile with the lack of evidence of melting of AOAs which represent aggregates of the least refractory CAIs and forsterite grains.  相似文献   

6.
In this study, we investigated Fe and Li isotope fractionation between mineral separates of olivine pheno- and xenocrysts (including one clinopyroxyene phenocryst) and their basaltic hosts. Samples were collected from the Canary Islands (Teneriffa, La Palma) and some German volcanic regions (Vogelsberg, Westerwald and Hegau). All investigated bulk samples fall in a tight range of Li and Fe isotope compositions (δ56Fewr = 0.06–0.17‰ and δ7Lima = 2.5–5.2‰, assuming δ7Li of the olivine-free matrix is virtually identical to that of the bulk sample for mass balance reasons). In contrast, olivine phenocrysts display highly variable, but generally light Fe and mostly light Li isotope compositions compared to their respective olivine-free basaltic matrix, which was considered to represent the melt (with δ56Feol = ? 0.24 to 0.14‰ and δ7Liol = ? 10.5 to + 6.5‰, respectively). Single olivine crystals from one sample display even a larger range of δ56Feol between ? 0.7 and + 0.1‰. One single clinopyroxene phenocryst displays the lightest Li isotope composition (δ7Licpx = ? 17.7‰), but no Fe isotope fractionation relative to melt. The olivine phenocrysts show variable Mg# and Ni (correlated in most cases) that range between 0.89 and 0.74 and between 300 and 3000 μg/g, respectively. These olivines likely grew by fractional crystallization in an evolving magma. One sample from the Vogelsberg volcano contained olivine xenocrysts (Mg# > 0.89 and Ni > 3000 μg/g), in addition to olivine phenocrysts. This sample displays the highest Li- and the second highest Fe-isotope fractionation between olivine and melt (Δ7Liol-melt = ? 13; Δ56Feol-melt = ? 0.29).Our data, i.e. the variable olivine- at constant whole rock and matrix isotope compositions, strongly indicate disequilibrium, i.e. kinetic Fe and Li isotope fractionation between olivine and melt (for Li also between cpx and melt) during fractional crystallization. Δ7Liol-melt is correlated with the Li partitioning between olivine and melt (i.e. with Liol/Limelt), indicating Li isotope fractionation due to preferential (faster) diffusion of 6Li into olivine during fractional crystallization. Olivine with low Δ7Liol-melt, also have low Δ56Feol-melt, indicating that Fe isotope fractionation is also driven by diffusion of isotopically light Fe into olivine, potentially, as Fe–Mg inter-diffusion. The lowest Δ56Feol-melt (? 0.40) was observed in a sample from Westerwald (Germany) with abundant magnetite, indicating relatively oxidizing conditions during magma differentiation. This may have enhanced equilibrium Fe isotope fractionation between olivine and melt or fine dispersed magnetite in the basalt matrix may have shifted its Fe isotope composition towards higher δ56Fe. The decoupling of Li- and Fe isotope fractionation in cpx is likely due to faster diffusion of Li relative to Fe in cpx, implying that the large investigated cpx phenocryst resided in the magma for only a short period of time which was sufficient for Li- but not for Fe diffusion. The absence of any equilibrium Fe isotope fractionation between the investigated cpx phenocryst and its basaltic host may be related to the similar Fe3 +/Fe2 + of cpx and melt. In contrast to cpx, the generally light Fe isotope composition of all investigated olivine separates implies the existence of equilibrium- (in addition to diffusion-driven) isotope fractionation between olivine and melt, on the order of 0.1‰.  相似文献   

7.
We report a study of the oxygen isotope ratios of chondrules and their constituent mineral grains from the Mokoia, oxidized CV3 chondrite. Bulk oxygen isotope ratios of 23 individual chondrules were determined by laser ablation fluorination, and oxygen isotope ratios of individual grains, mostly olivine, were obtained in situ on polished mounts using secondary ion mass spectrometry (SIMS). Our results can be compared with data obtained previously for the oxidized CV3 chondrite, Allende. Bulk oxygen isotope ratios of Mokoia chondrules form an array on an oxygen three-isotope plot that is subparallel to, and slightly displaced from, the CCAM (carbonaceous chondrite anhydrous minerals) line. The best-fit line for all CV3 chondrite chondrules has a slope of 0.99, and is displaced significantly (by δ17O ∼ −2.5‰) from the Young and Russell slope-one line for unaltered calcium-aluminum-rich inclusion (CAI) minerals. Oxygen isotope ratios of many bulk CAIs also lie on the CV-chondrule line, which is the most relevant oxygen isotope array for most CV chondrite components. Bulk oxygen isotope ratios of most chondrules in Mokoia have δ18O values around 0‰, and olivine grains in these chondrules have similar oxygen isotope ratios to their bulk values. In general, it appears that chondrule mesostases have higher δ18O values than olivines in the same chondrules. Our bulk chondrule data spread to lower δ18O values than any ferromagnesian chondrules that have been measured previously. Two chondrules with the lowest bulk δ18O values (−7.5‰ and −11.7‰) contain olivine grains that display an extremely wide range of oxygen isotope ratios, down to δ17O, δ18O around -50‰ in one chondrule. In these chondrules, there are no apparent relict grains, and essentially no relationships between olivine compositions, which are homogeneous, and oxygen isotopic compositions of individual grains. Heterogeneity of oxygen isotope ratios within these chondrules may be the result of incorporation of relict grains from objects such as amoeboid olivine aggregates, followed by solid-state chemical diffusion without concomitant oxygen equilibration. Alternatively, oxygen isotope exchange between an 16O-rich precursor and an 16O-poor gas may have taken place during chondrule formation, and these chondrules may represent partially equilibrated systems in which isotopic heterogeneities became frozen into the crystallizing olivine grains. If this is the case, we can infer that the earliest nebular solids from which chondrules formed had δ17O and δ18O values around -50‰, similar to those observed in refractory inclusions.  相似文献   

8.
Pegmatitic and other felsic rock pockets and dike-like intrusions are abundant in the South Kawishiwi Intrusion of the Duluth Complex, including the basal, Cu–Ni–PGE mineralized units. These occurrences are found as pockets, pods or as veins and contain abundant accessory apatite and quartz. Quartz hosts primary fluid inclusions as well as silicate melt inclusions. Combined microthermometry and Raman spectroscopy helped to determine the bulk composition of primary fluid inclusions that are CO2-rich (95 mol%) and contain small amounts of H2O (4.5 mol%), CH4 (0.4 mol%) and trace N2, respectively. This combined technique also made it possible to measure total homogenization temperatures of the inclusions (Thtot = ~ 225 ± 10 °C), otherwise not detectable during microthermometry. Silicate melt inclusions have been quenched to produce homogeneous glasses corresponding to the original melt. Composition of the entrapped melt is granitoid, peraluminous and is very poor in mafic components. We interpret the melt as a product of partial melting of the footwall rocks due to the contact effect of the South Kawishiwi Intrusion. The presence of CO2 in the vapor bubbles of the quenched melt inclusions and petrographic evidence suggest that the fluid and melt inclusion assemblages are coeval. The composition of the fluid and melt phase implies that the fluid originates from the mafic magma of the South Kawishiwi Intrusion and the fluid and melt phases coexisted as a heterogeneous melt–fluid system until entrapment of the inclusions.Coexistence of primary fluid and melt inclusions makes it possible to calculate a minimum entrapment pressure (~ 1.7 kbar) and thus estimate formation depth (~ 5.8 km) for the inclusions. Chlorine is suggested to behave compatibly in the silicate melt phase in the fluid–melt system represented by the inclusions, indicated by the high (up to 0.3%) Cl-concentrations of the silicate melt and CO2-rich nature of the fluid.Apatite halogen-contents provide further details on the behavior of Cl. Apatite in pegmatitic pockets often has elevated Cl-concentrations compared to troctolitic rocks, suggesting enrichment of Cl with progressive crystallization. Systematic trends of Cl-loss at some differentiated melt pockets suggest that in some places Cl exsolved into a fluid phase and migrated away from its source. The segregation of Cl from the melt is probably inhibited by the presence of CO2-rich fluids until the last stages of crystallization, increasing the potential for the development of late-stage saline brines.Platinum-group minerals are often present in microcracks in silicate minerals, in late-stage differentiated sulfide veinlets and in association with chlorapatite, indicating the potential role of Cl-bearing fluids in the final distribution of PGEs.  相似文献   

9.
The ultramafic member of the Variscan Ślęża Ophiolite (SW Poland) consists of heavily serpentinised, refractory harzburgites. Those located down to 1.5 km below paleo-Moho contain scarce grains or aggregates of olivine, clinopyroxene and spinel. Non-serpentine phases occur in various assemblages: M1—olivine (Fo 90.2–91.0%, NiO 0.38–0.47 wt.%) and rounded or amaeboidal aluminous chromite, rimmed by Al poor chromite and magnetite; M2—olivine (Fo 90.5–91.5, NiO 0.32–0.44 wt.%), olivine with magnetite inclusions (Fo 87.1–92.5, NiO 0.01–0.68 wt.%), rounded, cleavaged clinopyroxene I (Mg# 91.1–93.2, Al2O3 3.00–4.00 wt.%, Cr2O3 1.00–1.40 wt.%) and elongated clinopyroxene II and clinopyroxene from symplectites with magnetite (Mg# = 92.2–94.1, Al2O3 2.20–3.20 wt.% and Cr2O3 0.80–1.20 wt.%). Clinopyroxene is depleted in REEs relative to chondrite. The M3 assemblage consists of intergrown olivine (Fo 90.8–92.7, NiO 0.20–0.38 wt.%) and clinopyroxene (Mg# = 96.0–98.1, Al2O3 0.00–1.00 wt.% and Cr2O3 0.20–0.60 wt.%).The M1 assemblage contains chromite which records greenschist-facies metamorphism. Textural relationships and chemical composition of clinopyroxene occurring in the M2 assemblage are similar to those formed in oceanic spreading centres by LREE depleted basaltic melt percolation. Olivine occurring in M1 assemblage and part of that from M2 have composition typical of residual olivine from the abyssal harzburgites and of olivine formed in those rocks by melt percolation. The olivine with magnetite inclusions (M2 assemblage) and that from M3 record later deserpentinization event, which supposedly produced also M3 clinopyroxene. The non-serpentine phases from the Ślęża ophiolite mantle member, albeit very poorly preserved, document depleted basaltic melt percolation in the Variscan oceanic spreading centre.  相似文献   

10.
The iron-rich olivine end-member, fayalite, occurs in the matrix, chondrules, Ca-Al-rich inclusions (CAIs), silicate aggregates, and dark inclusions in the Kaba and Mokoia oxidized CV3 chondrites. In most occurrences, fayalite is associated with magnetite and troilite. To help constrain the origin of the fayalite (Fa98-100), we measured oxygen and silicon isotopic compositions and Mn-Cr systematics in fayalite from two petrographic settings of the Kaba meteorite. One setting consists of big fayalite laths embedded in the matrix and radiating from a core of fine-grained magnetite and sulfide, while the other setting consists of small fayalite-magnetite-sulfide assemblages within or at the surface of Type I barred or porphyritic olivine chondrules. Oxygen in the big fayalite laths and small chondrule fayalites falls on the terrestrial fractionation line, and is distinct from that in chondrule forsterites, which are enriched in 16O (Δ17O = ∼−4‰). Oxygen in the big fayalite laths may be isotopically heavier than that in chondrule fayalites. Silicon isotopes suggest that forsterite is ∼1‰/amu heavier than adjacent fayalite within Kaba chondrules. However, we were unable to confirm large silicon isotopic differences among fayalites reported previously. The Mn-Cr data for big Kaba fayalites give an initial 53Mn/55Mn ratio of (2.07 ± 0.17) × 10−6, consistent with literature results on Mokoia chondrule fayalites. The combined data suggest that fayalites in both petrographic settings formed at about the same time, ∼9.7 Ma after the formation of CAIs. Our data indicate that those fayalite-magnetite-troilite assemblages replacing metal inside and around chondrules formed by aqueous alteration on the meteorite parent body. The formation site and mechanism for the big fayalite laths is less clear, but the petrographic setting indicates that they did not form in situ. None of the models that have been suggested for formation of these fayalites is entirely satisfactory.  相似文献   

11.
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   

12.
The Hetaoping skarn type Pb–Zn deposit is located in the Baoshan–Narong–Dongzhi block metallogenic belt (BND belt), a belt between the Tengchong terrane and the Lanping basin. The deposit is hosted by marble of the upper Cambrian Hetaoping Formation and there are no outcrops of plutonic rocks present. This deposit is one of two large Pb–Zn deposits recently discovered in the BND belt. The Hetaoping deposit is a high Mn skarn. Four types of fluid inclusions were recognized in quartz from the deposit: vapor-rich inclusions (Type I), liquid-rich inclusions (Type II), pure vapor inclusions (Type III), and pure fluid inclusions (Type IV). The coexistence of Type I and Type III inclusions in Stage I (pre-ore stage) and Stage II (main ore stage) shows evidence of fluid boiling. Quartz-hosted fluid inclusions (Stage I and Stage II) display high homogenization temperatures and salinities (134–315 °C; 3.7–18.6 wt% NaCl equivalent) but calcite-hosted fluid inclusions in Stage III (post-ore stage) record lower homogenization temperatures and salinities (85–214 °C; 0.5–5.4 wt% NaCl equivalent). These data suggest a possible mixing between primary magmatic water and meteoric water. Based on chromatography data, the fluid inclusions in quartz contain abundant CO2 and O2 and subordinate CO, CH4 and C2H2 + C2H4, suggesting an oxidizing environment. Based on their Na/K and Cl/SO4 ratios, fluids contained in fluid inclusions are similar to volcanic spring waters. The low Na/K ratios (0.40–1.34) of the ore-forming fluids may have resulted from interaction with a deep alkaline intermediate-acid intrusion. Hydrogen and oxygen isotope determinations on quartz from different ore stages show low δ18O and δD values relative to VSMOW (−4.3‰ to 2.3‰; −109‰ to −91‰), indicating that the ore-forming fluids were diluted by external fluid sources as the skarn system cooled. Overall, geological and geochemical interpretations suggest that the Hetaoping deposit is a distal manganese skarn Pb–Zn deposit related to concealed intrusions.  相似文献   

13.
Electron probe microanalysis and microscopy is a widely used modern analytical technique primarily for quantifying chemical compositions of solid materials and for mapping or imaging elemental distributions or surface morphology of samples at micrometer or nanometer-scale. This technique uses an electromagnetic lens-focused electron beam, generated from an electron gun, to bombard a sample. When the electron beam interacts with the sample, signals such as secondary electron, backscattered electron and characteristic X-ray are generated from the interaction volume. These signals are then examined by detectors to acquire chemical and imaging information of the sample. A unique part of an electron probe is that it is equipped with multiple WDS spectrometers of X-ray and each spectrometer with multiple diffracting crystals in order to analyze multiple elements simultaneously. An electron probe is capable of analyzing almost all elements (from Be to U) with a spatial resolution at or below micrometer scale and a detection limit down to a few ppm.Mineral inclusions in chromite from the Wafangdian kimberlite, Liaoning Province, China were used to demonstrate the applications of electron probe microanalysis and microscopy technique in characterizing minerals associated with ore deposits, specifically, in this paper, minerals associated with diamond deposit. Chemical analysis and SE and BSE imaging show that mineral inclusions in chromite include anhydrous silicates, hydrous silicates, carbonates, and sulfides, occurring as discrete or single mineral inclusions or composite multiple mineral inclusions. The chromite–olivine pair poses a serious problem in analysis of Cr in olivine using electron probe. Secondary fluorescence of Cr in chromite by Fe in olivine drastically increases the apparent Cr2O3 content of an olivine inclusion in a chromite. From the chemical compositions obtained using electron probe, formation temperatures and pressures of chromite and its mineral inclusions calculated using applicable geothermobarometers are from 46 kbar and 980 °C to 53 kbar and 1130 °C, which are within the stability field of diamond, thus Cr-rich chromite is a useful indication mineral for exploration of kimberlite and diamond deposit. A composite inclusion in chromite composed of silicate and carbonate minerals has a bulk composition of 33.2 wt.% SiO2, 2.5 wt.% Al2O3, 22.0 wt.% MgO, 7.5 wt.% CaO, 2.5 wt.% BaO, 0.8 wt.% K2O, 25.5 wt.% CO2, and 0.8 wt.% H2O, similar to the chemical composition of the Wafangdian kimberlite, suggesting that it is trapped kimberlitic magma.  相似文献   

14.
We measured major, minor, and trace-element compositions for eleven Al-rich chondrules from unequilibrated ordinary chondrites to investigate the relationships between Al-rich chondrules, ferromagnesian chondrules, Ca-, Al-rich inclusions (CAIs), and amoeboid olivine aggregates (AOAs). Phase equilibrium considerations show that, for the most part, mineral assemblages in Al-rich chondrules are those expected from melts of the observed compositions. The diversity of mineral assemblages and Al-rich chondrule types arises mainly from the fact that the array of compositions spans both the spinel-saturated anorthite-forsterite reaction curve and a thermal divide defined by where the anorthite-forsterite join crosses the reaction curve. The reaction curve accounts for the two principal varieties of Al-rich chondrule, plagioclase-phyric and olivine-phyric, with or without aluminous spinel. The thermal divide influences the subsequent evolution of each variety. A third variety of Al-rich chondrule contains abundant sodium-rich glass; trace-element fractionation patterns suggest that these glassy Al-rich chondrules could have been derived from the other two by extensive alteration of plagioclase to nepheline followed by remelting. The bulk compositions of Al-rich chondrules (except sodium-rich ones) are intermediate in a volatility sense between ferromagnesian chondrules and type C CAIs. The combined trend of bulk compositions for CAIs, Al-rich chondrules, and ferromagnesian chondrules mirrors, but does not exactly match, the trend predicted from equilibrium condensation at PT ∼ 10-3 atm; the observed trend does not match the trend found for evaporation from a liquid of chondritic composition. We thus infer that the bulk compositions of the precursors to CAIs, Al-rich chondrules, were ferromagnesian chondrules were controlled primarily by vapor-solid reactions (condensation or sublimation) in the solar nebula. Some Al-rich chondrules are consistent with an origin by melting of a compound CAI-ferromagnesian chondrule hybrid; others cannot be so explained. Any hybrid model is restricted by the constraint that the CAI precursor consisted dominantly of pyroxene + plagioclase + spinel; melilite cannot have been a significant component. Amoeboid olivine aggregates also have the inferred mineralogical characteristics of Al-rich chondrule precursors—they are mixtures of olivine with plagioclase-spinel-pyroxene-rich CAIs—but the few measured bulk compositions are more olivine-rich than those of Al-rich chondrules.  相似文献   

15.
《Gondwana Research》2016,29(4):1391-1414
Experiments on the origin of the Udachnaya-East kimberlite (UEK) have been performed using a Kawai-type multianvil apparatus at 3–6.5 GPa and 900–1500 °C. The studied composition represents exceptionally fresh Group-I kimberlite containing (wt.%): SiO2 = 25.9, TiO2 = 1.8, Al2O3 = 2.8, FeO = 9.0, MgO = 30.1, CaO = 12.7, Na2O = 3.4, K2O = 1.3, P2O5 = 1.0, Cl = 0.9, CO2 = 9.9, and H2O = 0.5. The super-solidus assemblage consists of melt, olivine (Ol), Ca-rich (26.0–30.2 wt.% CaO) garnet (Gt), Al-spinel (Sp), perovskite (Pv), a CaCO3 phase (calcite or aragonite), and apatite. The low pressure assemblage (3–4 GPa) also includes clinopyroxene. The apparent solidus was established between 900 and 1000 °C at 6.5 GPa. At 6.5 GPa and 900 °C Na–Ca carbonate with molar ratio of (Na + K)/Ca ≈ 0.44 was observed. The UEK did not achieve complete melting even at 1500 °C and 6.5 GPa, due to excess xenogenic Ol in the starting material. In the studied P–T range, the melt has a Ca-carbonatite composition (Ca# = molar Ca/(Ca + Mg) ratio = 0.62–0.84) with high alkali and Cl contents (7.3–11.4 wt.% Na2O, 2.8–6.7 wt.% K2O, 1.6–3.4 wt.% Cl). The K, Na and Cl contents and Ca# decrease with temperature. It is argued that the primary kimberlite melt at depths > 200 km was an essentially carbonatitic (< 5 wt.% SiO2), but evolved toward a carbonate–silicate composition (up to 15–20 wt.% SiO2) during ascent. The absence of orthopyroxene among the run products indicates that xenogenic orthopyroxene was preferentially dissolved into the kimberlite melt. The obtained subliquidus phase assemblage (Ol + Sp + Pv + Ca-rich Gt) at P–T conditions of the UEK source region, i.e. where melt was in the last equilibrium with source rock before magma ascent, differs from the Opx-bearing peridotitic mineral assemblage of the UEK source region. This difference can be ascribed to the loss of substantial amounts of CO2 from the kimberlite magma at shallow depths, as indicated by both petrological and experimental data. Our study implies that alkali-carbonatite melt would be a liquid phase within mantle plumes generated at the core–mantle boundary or shallower levels of the mantle, enhancing the ascent velocity of the plumes. We conclude that the long-term activity of a rising hot mantle plume and associated carbonatite melt (i.e. kimberlite melt) causes thermo-mechanical erosion of the subcontinental lithosphere mantle (SCLM) roots and creates hot and deformed metasomatic regions in the lower parts of the SCLM, which corresponds to depths constrained by P–T estimates of sheared Gt-peridotite xenoliths. The sheared Gt-peridotites undoubtedly represent samples of these regions.  相似文献   

16.
Major and minor element bulk compositions of 90 individual chondrules and 16 compound chondrule sets in unequilibrated (type 3) H-group chondrites were determined in polished thin sections by broad beam electron probe analysis and the chondrules were classified petrographically into six textural types (barred olivine, porphyritic olivine, porphyritic pyroxene, barred pyroxene, radiating pyroxene, fine-grained). Although analyses of individual chondrules scatter widely, the mean composition of each textural type (except barred pyroxene) is rather distinct, as verified by discriminant function analysis. Al2O3, TiO2 and Na3O are correlated in chondrules, but Al2O3 and CaO do not correlate. Compound chondrule sets were found to consist almost entirely of chondrules or partial chondrules of similar texture and composition.The data suggest that composition played a conspicuous role in producing the observed textures of chondrules, though other factors such as cooling rates and degrees of supercooling prior to nucleation were also important. If compound chondrules formed and joined when they were still molten or plastic, then the data suggest that chondrules of each textural type could have formed together in space or time. The correlation of Al2O3 and TiO2 with Na2O and not with CaO appears to rule out formation of chondrules by direct equilibrium condensation from the nebula. We conclude that the most reasonable model for formation of the majority of chondrules is that they originated from mixtures of differing fractions of high-, intermediate- and low-temperature nebular condensates that underwent melting in space. A small percentage of chondrules might have formed by impacts in meteorite parent-body regoliths.  相似文献   

17.
The Dexing deposit, located in the Circum-Pacific ore belt, is the largest porphyry copper deposit in eastern China. It is composed of 3 separate plutons, which host three mines: Tongchang, Fujiawu and Zhushahong mines. The porphyritic granodiorite samples studied in this investigation were collected from the Tongchang ore-forming pluton of this giant deposit. This paper presents electron microprobe analyses of biotite, apatite, amphibole, plagioclase, potassium feldspar and rehomogenized glassy melt inclusions from the Tongchang porphyritic granodiorites. Petrographic observations of the samples are consistent with portions of the granodioritic magma represented by our samples being overprinted by potassic hydrothermal fluid which variably altered these minerals.All of the studied micas are Mg-rich biotites. The biotites are separated into altered magmatic and secondary types based on their petrographic and geochemical characteristics. The phlogopite components of the secondary biotites are typically higher than those of the altered magmatic biotites, and the XMg values of all biotites correlate negatively with Cl contents, consistent with the Mg–Cl avoidance principle. The XMg values also correlate negatively with (K2O + Na2O + BaO), FeO and TiO2 for both generations of biotites. The calculated log (fH2O/fHCl) values (for 690 K) of the coexisting potassic fluids, which are determined from the altered magmatic biotite compositions, range from 4.43 to 4.67, and are very similar to those of other major porphyry deposits. However, the log(fH2O/fHF) and log(fHF/fHCl) values for the same batch of hydrothermal fluids are significant higher and lower than those of these other porphyry deposits, respectively.The Cl concentrations of amphiboles and melt inclusions range from 0.18 to 0.32 wt.% and 0.15 to 0.44 wt.%, respectively. Most apatites trapped in biotite and plagioclase phenocrysts display a bimodal Cl distribution: 0.19 to 1.35 wt.% and 1.48 to 3.73 wt.%. Similarly, the S contents of the apatite also show a distinct bimodal distribution reflecting the effects of variable anhydrite saturation during evolution of the Tongchang melt and variable dissolution of anhydrite by saline aqueous fluids. The Cl contents of the apatites from the Tongchang system are typically higher than those of other studied porphyry deposits. Furthermore, the Cl contents of the melt inclusions are at or very near the Cl saturation levels (0.36 to 0.46 wt.% at 850 °C and 50 MPa and 0.42 to 0.54 wt.% at 850 °C and 200 MPa) for these melt compositions at shallow crustal pressures. These findings suggest that the area of the granodioritic magma represented by our samples, and perhaps the bulk of the Tongchang granodioritic magma was rich in Cl. The melt inclusion compositions are consistent with a high-salinity, hydrosaline liquid being exsolved directly from the granodioritic melt directly. This high-salinity hydrosaline liquid was likely very efficient at dissolving, transporting and precipitating ore metals in the mineralizing magmatic–hydrothermal system.  相似文献   

18.
The behavior of Na and K during evaporation from chondrule composition melts was studied using a vacuum furnace. Though Na is the less volatile of the two as an element, it is lost more rapidly than K from silicate melts. Mass fractionation of K isotopes was measured by ion microprobe and Rayleigh fractionation is observed for vacuum evaporation (10−5 atm). With higher pressures of air, the K loss rate decreases but with increasing hydrogen pressure, K is lost more rapidly. δ41K decreases with higher pressures, because of back reaction between melt and K in the gas. With long heating duration, the release of light K condensed within the furnace leads to interaction with the K-depleted melt and a further reduction of δ41K. Natural chondrules differ in some ways from our experimental residues. Some (especially type IIA) have superchondritic Na and K, despite their assumed formation in nebular hydrogen, which enhances volatile loss, and chondrules do not show K isotopic fractionation. Type I chondrules in Semarkona (LL3.0) either plot on our evaporation trend, or are depleted in K but slightly enriched in Na, relative to K. In Bishunpur (LL3.1), type I chondrules are mostly K-depleted but moderately to strongly enriched in Na. In petrologic type 3.2 to 3.4 chondrites they are enriched in both K and Na, like type II chondrules. The alkali contents suggest type I chondrules experienced evaporation and subsequent metasomatism. Their normal δ41K values suggest closed-system evaporation of a chondritic precursor in a gas with relatively high K pressures due to vaporization of dust accompanying chondrule precursor aggregates. Type II chondrules are volatile-rich, as well as unfractionated in K isotopes. They probably formed in a gas with higher pK than in the case of type I chondrules, due to heating of a more dust-rich parcel of gas.  相似文献   

19.
Due to their common occurrence in various types of chondrites, igneous rims formed on pre-existing chondrules throughout chondrule-forming regions of the solar nebula. Although the peak temperatures are thought to reach similar values to those achieved during chondrule formation events, the heating duration in chondrule rim formation has not been well defined. We determined the two-dimensional chemical and oxygen isotopic distributions in an igneous rim of a chondrule within the Northwest Africa 3118 CV3oxA chondrite with sub-micrometer resolution using secondary ion mass spectrometry and scanning electron microscopy. The igneous rim experienced aqueous alteration on the CV parent body. The aqueous alteration resulted in precipitation of the secondary FeO-rich olivine (Fa40–49) and slightly disturbed the Fe-Mg distribution in the MgO-rich olivine phenocrysts (Fa11–22) at about a 1 μm scale. However, no oxygen isotopic disturbances were observed at a scale greater than 100 nm. The MgO-rich olivine, a primary phase of igneous rim formation, has δ17O = −6 ± 3‰ and δ18O = −1 ± 4‰, and some grains contain extreme 16O-rich areas (δ17O, δ18O = ∼−30‰) nearly 10 μm across. We detected oxygen isotopic migration of approximately 1 μm at the boundaries of the extreme 16O-rich areas. Using oxygen self-diffusivity in olivine, the heating time of the igneous rim formation could have continued from several hours to several days at near liquidus temperatures (∼2000 K) in the solar nebula suggesting that the rim formed by a similar flash heating event that formed the chondrules.  相似文献   

20.
The Shapinggou porphyry Mo deposit, one of the largest Mo deposits in Asia, is located in the Dabie Orogen, Central China. Hydrothermal alteration and mineralization at Shapinggou can be divided into four stages, i.e., stage 1 ore-barren quartz veins with intense silicification, followed by stage 2 quartz-molybdenite veins associated with potassic alteration, stage 3 quartz-polymetallic sulfide veins related to phyllic alteration, and stage 4 ore-barren quartz ± calcite ± pyrite veins with weak propylitization. Hydrothermal quartz mainly contains three types of fluid inclusions, namely, two-phase liquid-rich (type I), two- or three-phase gas-rich CO2-bearing (type II) and halite-bearing (type III) inclusions. The last two types of fluid inclusions are absent in stages 1 and 4. Type I inclusions in the silicic zone (stage 1) display homogenization temperatures of 340 to 550 °C, with salinities of 7.9–16.9 wt.% NaCl equivalent. Type II and coexisting type III inclusions in the potassic zone (stage 2), which hosts the main Mo orebodies, have homogenization temperatures of 240–440 °C and 240–450 °C, with salinities of 34.1–50.9 and 0.1–7.4 wt.% NaCl equivalent, respectively. Type II and coexisting type III inclusions in the phyllic zone (stage 3) display homogenization temperatures of 250–345 °C and 220–315 °C, with salinities of 0.2–6.5 and 32.9–39.3 wt.% NaCl equivalent, respectively. Type I inclusions in the propylitization zone (stage 4) display homogenization temperatures of 170 to 330 °C, with salinities lower than 6.5 wt.% NaCl equivalent. The abundant CO2-rich and coexisting halite-bearing fluid inclusion assemblages in the potassic and phyllic zones highlight the significance of intensive fluid boiling of a NaCl–CO2–H2O system in deep environments (up to 2.3 kbar) for giant porphyry Mo mineralization. Hydrogen and oxygen isotopic compositions indicate that ore-fluids were gradually evolved from magmatic to meteoric in origin. Sulfur and lead isotopes suggest that the ore-forming materials at Shapinggou are magmatic in origin. Re–Os dating of molybdenite gives a well-defined 187Re/187Os isochron with an age of 112.7 ± 1.8 Ma, suggesting a post-collisional setting.  相似文献   

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