On the origin of isolated olivine grains in type 2 carbonaceous chondrites

The origin of olivine grains isolated in the matrix of C2 carbonaceous chondrites is an important problem. If these grains are condensates from a solar nebular gas, they contain compositional, isotopic and physical features that further elucidate that process. If, however, they are grains released by the breakup of chondrules, then many important condensation features have been lost during the melting that took place to form chondrules.In evaluating these two possibilities, care must be taken to determine which inclusions in C2 meteorites are actual chondrules and which are aggregates of grains that have never undergone melting. The two main types of aggregates, pyroxene-rich and pyroxene-poor, are forty to fifty times more abundant than chondrules. Four scenarios are presented to account for the kinds of aggregates and isolated grains seen in the Murchison C2 meteorite. An analysis of these scenarios is made in light of olivine crystal morphology, comparison of composition of glass inclusions inside olivine grains with interstitial glass in true chondrules and size distributions of olivines, isolated, in aggregates and in chondrules.It is concluded that no scenario that includes a chondrule-making step can account for the observed population of isolated olivine grains. An origin by direct condensation, partial comminution, aggregation and accretion best accounts for the sizes and morphological features observed.     

Textural evidence bearing on the origin of isolated olivine crystals in C2 carbonaceous chondrites

In some cases the mechanical competence of chondrules in carbonaceous chondrites has been reduced by alteration of their mesostasis glass to friable phyllosilicate, providing a mechanism by which euhedral olivines can be separated from chondrules. Morphological features of isolated olivine grains found in carbonaceous chondrites are similar to those of olivine phenocrysts in chondrules. These observations suggest that the isolated olivine grains formed in chondrules, by crystallization from a liquid, rather than by condensation from a vapor.     

A new approach to decipher the origin of the carbonaceous chondrite fission krypton and xenon

It is suggested that the carbonaceous chondrite fission krypton and xenon, as measured in the primitive meteorites, may have been produced by nuclear fission induced by CNO flare particles in the few-MeV/nucleon energy range on very heavy target elements such as Au, Hg, Tl, Pb, and Bi. It is speculatively proposed that the locale of this process has been the T-Tauri phase of our sun.     

The Adhi Kot breccia and implications for the origin of chondrules and silica-rich clasts in enstatite chondrites

The Adhi Kot EH4 enstatite chondrite breccia consists of silica-rich clasts (12+mn; 5 vol.%), chondrule-rich clasts (55+mn; 10 vol.%) and matrix (35+mn; 10 vol.%). The silica-rich clasts are a new kind of enstatite chondritic material, which contains more cristobalite (18–28 wt.%) than enstatite (12–14 wt.%), as well as abundant niningerite and troilite. The bulk atomic Mg/Si ratios of the clasts (0.22–0.40) are much lower than the average for enstatite chondrites (0.79). Kamacite and martensite (with 8–11 wt.% Ni and a martensitic structure) occur in all three breccia components. The clasts have kamacite-rich rims, and kamacite-rich aggregates occur in the matrix.A unidirectional change in the ambient pS₂/pO₂ ratio in the region of the solar nebula where Adhi Kot agglomerated can explain many of the breccia's petrologic features. If this region initially had a very high pS₂/pO₂ ratio in a gas of non-cosmic composition, sulfurization of enstatite and metallic Fe (e.g., MgSiO₃ + 2Fe + C + 3H₂S = MgS + SiO₂ + 2FeS + H₂O + CH₄) may have occurred, producing abundant niningerite, free silica and troilite at the expense of enstatite and metallic Fe. The Ni content of the residual metal would have increased, perhaps to ～ 8–10 wt.%. The silica-rich clasts agglomerated under these conditions; a significant fraction of the originally produced niningerite was lost (perhaps by aerodynamic size-sorting processes), lowering the clasts' bulk Mg/Si ratios.The pS₂/pO₂ ratio then decreased (perhaps because of infusion of additional H₂O) and sulfurization of metallic Fe and enstatite ceased. The chondrule-rich clasts agglomerated under these conditions, acquiring little free silica and niningerite. An episode of chondrule formation occurred at this time (by melting millimeter-sized agglomerates of this relatively silica-poor enstatite chondrite material and concomitant fractionation of an immiscible liquid of metallic Fe,Ni and sulfide). The chondrule-rich clasts agglomerated many such chondrules. Subsequently, the matrix agglomerated, acquiring the few remaining chondrules. Kamacite-rich aggregates formed, after the cessation of metal sulfurization, and agglomerated with the matrix. The kamacite-rich clast rims were acquired at this time.The components of Adhi Kot accreted to the EH chondrite parent body, where the breccia was assembled, buried beneath additional accreting material, and metamorphosed at temperatures of ? 700°C. Impact-excavation of the breccia and deposition onto the surface caused the formation of martensite from taenite inside the clasts and the matrix. At the surface, impact-melting produced an albite glass spherule, which was incorporated into the matrix. However, the absence of solar-wind-implanted rare gases in bulk Adhi Kot indicates that the breccia spent little time in a regolith.     

The composition and origin of large microporphyritic chondrules in the Manych (L-3) chondrite

The majority (26/37) of the largest chondrules (d ≥ 1400 μm) exposed in a thin section of the Manych chondrite are more or less rounded fragments of microporphyry, most of which contain from 50 to 80 vol.% olivine. Modal and phase analyses were used to calculate the approximate bulk compositions of nine such chondrules. Six vary modestly around the mean composition of L-group chondrites less most of their metal and troilite and are thought to have formed by bulk melting of L-group material with loss of an immiscible Fe-Ni-S liquid. Two other chondrules, which are olivine-rich and Na- and Si-poor, formed in the same way but with some loss of volatile constituents to a vapor phase. The ninth chondrule, an olivine-poor microporphyry, may be a non-representative sample of a coarser microporphyritic rock.Comparison of these microporphyritic chondrules with the products of controlled cooling experiments and with chemically similar olivine microporphyry in the St. Mesmin chondrite (LL-breccia) suggests that the microporphyritic chondrules are fragments of magmatic rocks which crystallized from masses of liquid no less than 10 cm across.     

The trace element chemistry of CaS in enstatite chondrites and some implications regarding its origin

Samples of the mineral oldhamite (CaS) were extracted from enstatite chondrites and analyzed by INAA. Prior to extraction, the petrologic setting of the grains was studied microscopically and their minor element contents determined by microprobe analyses. Minor element contents of CaS are known to vary and correlate with petrologic type, indicating secondary redistribution during metamorphism. For this reason, samples were chosen that displayed a range of minor element contents. The trace element contents determined in this study follow a similar pattern. The most primitive samples of CaS studied, contain virtually the entire inventory of the host meteorite's LREE and Eu plus 30–50% of the HREE inventory. In less primitive samples the LREE are less enriched although Eu remains highly concentrated in CaS. Several other elements, including lithophiles (Ba, Cs, Cr, Hf and Sc) and chalcophiles (Sb and Zn) are most enriched in more primitive CaS. The high concentrations of refractory elements, several of which have a tendency to form sulfides at high temperatures in a gas slightly more reducing than solar, lend support to the suggestion that CaS originated at high temperatures in a reduced region of the nebula. The high concentrations of volatile Cs, Sb and Zn indicate that with decreasing temperature CaS continued to interact with the nebular gas, which therefore must have had a low oxygen fugacity at low temperatures.     

Ca, Al-rich inclusions in Yamato 791717 (CO3) carbonaceous chondrite: Formation and alteration

In three polished thin sections of Yamato 791717 (CO3). fifty-five Ca, Al-rich inclusions were found, which include two hibonite-bearing, eight melilite-rich and forty-five spinel-pyroxene inclusions. Based on the petrography and mineral chemistry of the inclusions, it is proposed that the melilite-rich inclusions and spinel-pyroxene inclusions condensed in the solar nebula, and the hibonite-bearing inclusions crystallized from melts. The heavy alteration of the inclusions in Yamato 791717, which probably took place under a very oxidizing condition in the solar nebular, is also confirmed. Project supported by the National Natural Science Foundation of China (Grant No. 49673200). and by the Japanese Society for Promotion of Sciences (JSPS).     

Compositions of droplet chondrules in the Manych (L-3) chondrite and the origin of chondrules

Expanded beam microprobe analyses of 18 drop-formed chondrules and 5 irregular masses of devitrified glass in the Manych chondrite show trends and ranges of chemical variation similar to those reported previously for large microporphyritic chondrules in this meteorite. These variations are inconsistent with differentiation of chondrules by crystal-liquid fractionation or separation of immiscible silicate and Fe-Ni-S liquids at various oxygen fugacities. They appear to reflect non-representative sampling of microporphyritic precursor rocks texturally and mineralogically similar to, but in some cases coarser than, the microporphyritic chondrules in Manych. About half of the droplet chondrules and devitrified glasses also bear evidence of more or less vapor-liquid fractionation.The chemical and petrographic properties of Manych chondrules are best explained by a genetic model which entails: (1) melting of extended masses of chondritic material (≥10 cm across); (2) extraction of immiscible Fe-Ni-S liquids; (3) crystallization of the remaining silicate liquids to form microporphyritic rocks; and (4) fragmentation of these rocks to produce microporphyritic chondrules or, with remelting, droplet chondrules. The initial melting may have been caused by either impact or solar heating, but fragmentation and remelting of the microporphyritic precursor rocks were most likely caused by impact.     

The geochemical behavior of refractory noble metals and lithophile trace elements in refractory inclusions in carbonaceous chondrites

Recent models of Ca, Al-rich inclusion (CAI) petrogenesis suggest that refractory inclusions may be residues of interstellar dust aggregates that were incompletely evaporated and partially melted in the solar nebula. These models, and the recent availability of new thermodynamic data, have led us to re-examine the traditional interpretation that lithophile refractory trace elements (LRTE) condensed as oxides in solid solution in refractory major condensates, while refractory noble metals (RNM) condensed as micron-sized nuggets of Pt-metal alloys. Calculations of LRTE-RNM alloy stability fields under nebular oxygen fugacities and partitioning experiments lead us to conclude that: (1) Ti, Zr, Nb, Hf, U, and Ta form stable alloys with RNM under nebular conditions; (2) the observation that metallic Zr, Nb, and Ta occur in some Pt-metal nuggets and grains is explained by the stability of these LRTE-RNM alloys under normal nebular oxygen fugacities; (3) metallic Ti, Hf, and U may also occur in some nuggets; (4) the lanthanides, the other actinides (Th, Pu), and Y do not form stable alloys, and thus probably do not occur alloyed with RNM; and (5) the partitioning of U (but not Th, Pu, or the REE) into RNM is a novel actinide and REE/actinide fractionation mechanism that is based on metal/silicate fractionation (rather than on the relative volatility of their oxides).We propose that micron-sized Pt-metal nuggets formed from smaller grains of RNM alloys and compounds during the evaporation and melting of primitive dust aggregates. This process would have been enhanced by: (1) the possibility that the RNM were present as compounds (especially with As and S) as well as metallic alloys in interstellar dust and in some primitive meteoritical material, since they often exhibit non-siderophile behavior; and (2) the fluxing of volatiles through CAI's during distillation. Microscopic nuggets are common in melilite chondrules, indicating that residence in a slowly-cooled silicate melt may have favored their formation. Cation diffusivity and variations in localf_O2 can explain why metallic LRTE-bearing nuggets are not common in CAI's (despite the relative stability of LRTE-RNM alloys). We propose that the lithophile component of Fremdlinge is enriched in super-refractory elements, and that Group II CAI's formed from Fremdlinge-poor dust. We interpret the Group II REE fractionation as a pre-solar event, and predict that Nd/Sm dating will yield an age greater than the canonical age of the solar system. If metal/silicate fractionation in a cold solar nebula can explain Group II REE patterns, the possibility that Group II CAI's are also distillation residues cannot be excluded.     

I-Xe age and trapped Xe components of the Murray (C-2) chondrite

A neutron-irradiated bulk sample of the Murray (C-2) carbonaceous chondrite was etched with H₂O₂ and then divided into colloidal and non-colloidal fractions. The H₂O₂ treatment removed ～80% of the trapped Xe and greatly increased variations in the¹²⁹Xe/¹³²Xe ratio measured in stepwise heating. The colloid showed very little excess¹²⁹Xe, but the anti-colloid gave a fairly good I-Xe correlation corresponding to formation 3.7 ± 2.1 m.y. after Bjurböle.Variations in the trapped Xe component were also observed; most notably the 550°C anti-colloid fraction has large deficiencies relative to AVCC at the heavy isotopes. A tentative decomposition suggests U-Xe, a “primitive” trapped component, as the dominant component with minor contributions from H-Xe, L-Xe, and S-Xe (s-process nucleosynthesis). The identification of U-Xe rests primarily on the agreement of themeasured¹³⁴Xe/¹³⁶Xe ratio with U-Xe. This observation lends support to proposals for such a “primitive” trapped Xe component and demonstrates that at least some carbonaceous chondrite phases sampled a xenon reservoir nearly devoid of H-Xe.     

Composition and origin of clasts and inclusions in the Abee enstatite chondrite breccia

The concentrations of 25 major, minor and trace elements have been determined in four clasts, a metal-rich inclusion and two dark metal-poor inclusions from the Abee enstatite chondrite. The clasts are heterogeneous, displaying 2-fold enrichments or depletions in some elements. The data suggest that there are two generations of metal, one with low, the other with high concentrations of refractory siderophiles. The other elemental patterns can be understood in terms of variations in the abundance of major minerals. We infer that Sc and Mn are located largely in the niningerite ((Fe,Mg)S), V in the troilite (FeS) and rare earth elements in the oldhamite (CaS).Heterogeneities among the clasts are probably primary, resulting from the accretion-agglomeration process, although shock processes in a regolithic setting remain a possibility provided that they were followed by a period of metamorphism sufficient to erase petrologic evidence.In the dark inclusions the concentrations of the rare earths, Eu excepted, are 4 × higher than mean EH levels; this infers enhanced amounts of CaS. The dark inclusions are low in siderophiles, Sc, Mn, K, Na and Al, implying low amounts of metal, niningerite and feldspar. The origin of the dark inclusions is unclear; they do not appear to be the result of a simple, single-stage process.     

Composition of metal in type III carbonaceous chondrites and its relevance to the source-assignment of lunar metal

The metal in seven Type III carbonaceous chondrites has been measured for concentrations of Ni, Co and Cr. Cobalt in kamacite is 3.2 to 5.5 times greater than in taenite on composite grains containing both phases. No correlation was found between the metal compositions and sub-type classification. Ni and Co contents of kamacite from several of the Type III's studied fall outside of the range for these elements in bulk meteoritic metal and are relevant to the assignment of a meteoritic vs a non-meteoritic origin for lunar metal particles in the fines and breccias.     

The Blithfield meteorite and the origin of sulfide-rich,metal-poor clasts and inclusions in brecciated enstatite chondrites

Blithfield (EL6) is one of five known enstatite chondrite breccias. It consists of troilite-rich clasts (35 ± 5vol.%) embedded in an extremely metallic Fe,Ni-rich matrix (65 ± 5 vol.%) that contains metal nodules up to 17 mm in size. Clasts and matrix agglomerated independently in the solar nebula under conditions of high and lowpS₂/pO₂ ratios, respectively. The matrix accreted to an EL chondrite planetesimal and was metamorphosed to～ 1000°C, above the FeNiS eutectic; chondrule textures were obliterated. The S-rich eutectic melt was lost from the matrix. The matrix material was buried to a depth of at least 3 km; accreting troilite-rich material was incorporated into the matrix as clasts. The breccia cooled through～ 500°C at 1000–10,000°C/Myr. After cooling below～ 500°C, Blithfield was quenched, possibly by impact excavation from depth and deposition onto the surface.Clasts or inclusions that are enriched in sulfide and depleted in metallic Fe,Ni are common in brecciated enstatite chondrites. Variations in thepS₂/pO₂ ratio in the nebular regions where these materials formed may explain many of their petrologic properties. The silica-rich clasts in Adhi Kot (EH4) formed at very highpS₂/pO₂ratios(> 10²⁷); niningerite, free silica and troilite were produced from the sulfurization of enstatite and metallic Fe. The troilite-rich clasts in Blithfield and Atlanta (EL6) as well as the troilite-rich regions of the Hvittis (EL6) matrix formed at somewhat lowerpS₂/pO₂ ratios where sulfurization of metalic Fe produced troilite. The Ni content of the residual metal increased, forming some metal of martensitic composition. The dark inclusions in Abee (EH 4), which contain up to 9 wt.% oldhamite, formed at highpS₂/pO₂ ratios in the presence of an additional Ca-rich component.     

Petrogenesis of complex veins in the Chantonnay (L6f) chondrite

Two cross-cutting veins in the Chantonnay (L6f) chondrite illustrate different patterns of fractionation of total chondritic shock melts. The earlier vein, which is dark-colored and bears abundant host rock xenoliths, is strongly reduced and sodium-poor relative to the bulk meteorite. It resembles and may be cogenetic with melt pockets in Chantonnay. The later vein, which is lighter-colored and somewhat vesicular, lacks evidence of either Na loss or reduction but shows modest internal differentiation. Its metal and total iron contents (26.5 wt.%) are higher than normal for L-group chondrites.The trend of chemical fractionation recorded in the earlier Chantonnay vein resembles that reported for chondrules in ordinary chondrites, suggesting that chemical variations among chondrules in part reflect variations among their parental shock melts.     

Oxygen isotope compositions of quartz grains (4—16μm) from Chinese eolian deposits and their implications for provenance

In recent years, the researches of inland Asia aridification since late Cenozoic have attracted much attention in the paleoclimate community. Rea et al.[1] studied a 12 Ma eolian record in North Pacific and associated it with the aridification of northwes…     

Homologous temperature of olivine: Implications for creep of the upper mantle and fabric transitions in olivine

The homologues temperature of a crystalline material is defined as T/T_m, where T is temperature and T_m is the melting(solidus) temperature in Kelvin. It has been widely used to compare the creep strength of crystalline materials. The melting temperature of olivine system,(Mg,Fe)_2SiO_4, decreases with increasing iron content and water content, and increases with confining pressure. At high pressure, phase transition will lead to a sharp change in the melting curve of olivine. After calibrating previous melting experiments on fayalite(Fe_2SiO_4), the triple point of fayalite-Fe_2SiO_4 spinel-liquid is determined to be at 6.4 GPa and 1793 K. Using the generalized means, the solidus and liquidus of dry olivine are described as a function of iron content and pressure up to 6.4 GPa. The change of T/T_m of olivine with depth allows us to compare the strength of the upper mantle with different thermal states and olivine composition. The transition from semi-brittle to ductile deformation in the upper mantle occurs at a depth where T/T_m of olivine equals 0.5. The lithospheric mantle beneath cratons shows much smaller T/T_m of olivine than orogens and extensional basins until the lithosphere-asthenosphere boundary where T/T_m 0.66, suggesting a stronger lithosphere beneath cratons. In addition, T/T_m is used to analyze deformation experiments on olivine. The results indicate that the effect of water on fabric transitions in olivine is closely related with pressure. The hydrogen-weakening effect and its relationship with T/T_m of olivine need further investigation. Below 6.4 GPa(200 km), T/T_m of olivine controls the transition of dislocation glide from [100] slip to [001] slip. Under the strain rate of 10~(-12)–10~(-15) s~(-1) and low stress in the upper mantle, the [100](010) slip system(A-type fabric) becomes dominant when T/T_m 0.55–0.60. When T/T_m 0.55–0.60, [001] slip is easier and low T/T_m favors the operation of [001](100) slip system(C-type fabric). This is consistent with the widely observed A-type olivine fabric in naturally deformed peridotites, and the C-type olivine fabric in peridotites that experienced deep subduction in ultrahigh-pressure metamorphic terranes. However, the B-type fabric will develop under high stress and relatively low T/T_m. Therefore, the homologues temperature of olivine established a bridge to extrapolate deformation experiments to rheology of the upper mantle. Seismic anisotropy of the upper mantle beneath cratons should be simulated using a four-layer model with the relic A-type fabric in the upper lithospheric mantle, the B-type fabric in the middle layer, the newly formed A- or B-type fabric near the lithosphere-asthenosphere boundary, and the asthenosphere dominated by diffusion creep below the Lehmann discontinuity. Knowledge about transition mechanisms of olivine fabrics is critical for tracing the water distribution and mantle flow from seismic anisotropy.     

Cooling rates in the shock veins of chondrites: constraints on the (Mg, Fe)2SiO4 polymorph transformations

The occurrence of γ-phase, a high-pressure polymorph of olivine (α-phase), in the shock veins of Sixiangkou chondrite was due to a greater cooling rate (> 10 000°C·s^-1) in the veins. Because γ-phase partially reverted to β-phase and no back-transformation from β-phase to α-phase took place, the shock veins of Peace River chondrite with a cooling rate of 1 000–2 000δC·s^-1 contain a great amount of β-phase. In the shock veins of Mbale chondrite with a cooling rate of <500°C·s^-1, the majority of γ-phase reverted to α-phase. The heat dissipation in shock veins took place after a stage of shock compression of chondrite parent body, and the parent body was broken into fragmental pieces. Cooling rate in the shock veins constrained the back-transformations of (Mg, Fe)₂SiO₄ high-pressure polymorphs. Project of Chen and Xie supported by the National Natural Science Foundation of China (Grant No. 496720981, Natural Science Foundation of Guangdong Province (Grant No. 960500), and the Science Foundation of Academia Sinica for the returned scholars.     

Empirical evidence for a celestial origin of the climate oscillations and its implications

We investigate whether or not the decadal and multi-decadal climate oscillations have an astronomical origin. Several global surface temperature records since 1850 and records deduced from the orbits of the planets present very similar power spectra. Eleven frequencies with period between 5 and 100 years closely correspond in the two records. Among them, large climate oscillations with peak-to-trough amplitude of about 0.1 and 0.25°C, and periods of about 20 and 60 years, respectively, are synchronized to the orbital periods of Jupiter and Saturn. Schwabe and Hale solar cycles are also visible in the temperature records. A 9.1-year cycle is synchronized to the Moon's orbital cycles. A phenomenological model based on these astronomical cycles can be used to well reconstruct the temperature oscillations since 1850 and to make partial forecasts for the 21st century. It is found that at least 60% of the global warming observed since 1970 has been induced by the combined effect of the above natural climate oscillations. The partial forecast indicates that climate may stabilize or cool until 2030–2040. Possible physical mechanisms are qualitatively discussed with an emphasis on the phenomenon of collective synchronization of coupled oscillators.     

Oxygen isotopic heterogeneities,their petrological correlations,and implications for melt origins of chondrules in unequilibrated ordinary chondrites

Ten whole chondrules separated from the Dhajala (H3, 4), Hallingeberg (L3), and Semarkona (LL3) chondrites were individually analyzed for bulk element composition by instrumental neutron activation with half of each chondrule subsequently sacrificed for oxygen isotopic analysis and half retained for petrographic and electron microprobe analysis. On a three-isotope plot (δ¹⁷O vs. δ¹⁸O), the chondrules neither cluster near their respective chondrite hosts nor in the vicinities of previously recognized chondrite group averages. Instead, they define a trend resolvable into mixing and fractionation components but dominated by mixing in a manner similar to that previously observed for clasts from the LL3 chondrite ALHA76004. Covariations of chondrule isotopic mixing and fractionation parameters with petrological parameters were sought by two-variable linear least-squares regression analyses. However, the only two isotopic/petrological correlations significant at the 95% confidence level were δ¹⁷O vs. total bulk Fe (r = ?0.68) and mixing parameter,m¹⁸, vs. bulk weight ratio (CaO + Al₂O₃)/MgO (r = +0.67). Other correlations of apparent statistical significance were found by treating the chondrules as separate porphyritic (3 porphyritic olivine-pyroxene, 1 porphyritic olivine, 1 barred olivine) and non-porphyritic (4 radial pyroxene, 1 granular pyroxene/cryptocrystalline) textural subgroups. The reliability of the trends, based on so few samples, is not clear but the results at least indicate that possible existence of distinct isotopic/petrological subgroups of chondrules should be further investigated. Absence of certain isotopic/petrological trends expected as condensation effects argues against direct nebular condensation as the dominant process of chondrule formation. Instead, a model involving melting of heterogeneous solids, followed by various degrees of liquid/gas exchange, is favored. In any case, chondrule oxygen isotopic evolution was dominated by two-component mixing; fractional vaporization was, at most, a second-order effect. In addition to chondrules, parent bodies of unequilibrated ordinary chondrites must have also incorporated a¹⁶O-rich component which might have been fine-grained “matrix”.