铁陨石及中铁陨石的稀有气体同位素丰度和放射性核素活度

测定了两个铁陨石及两个中铁陨石的稀有气体同位素丰度和＾１０Ｂｅ、＾２６Ａｌ和＾３６Ｃｌ的放射性核素活度。在所研究的４个陨石中,东乌珠穆沁旗（类型Ⅰ）、渭源（类型Ⅲ）中铁陨石及宁波铁陨石（ⅣＡ）是在中国回收的,另一个无结构铁陨石Ｒａｆｒｕｔｉ是于１８８６年在瑞士发现的。根据＾１０Ｂｅ－＾２１Ｎｅ、＾２６Ａｌ－＾２１Ｎｅ和＾３６Ｃｌ－＾３６Ａｒ计算了两个铁陨石的产率及宇宙射线暴露年龄。其中Ｒａｆｒｕｔ     

A search for primordial atmospheric-like argon in an iron meteorite

He and Ar were extracted from the Weekeroo Station octahedrite in six heating steps. The Ar isotopic ratios were searched for any indication of an indigenous component with atmospheric-like values. Negative results were obtained.     

Nedagolla,a remelted iron meteorite

The Nedagolla meteorite was recognized by Axon to be a rare example of an iron which has been preterrestrially reheated to the point of melting. The dendrite secondary arms are spaced 200 μm apart, implying that Nedagolla solidified and cooled at ~0·02°C/sec. The presence of (Fe, Cr)_1-xS inclusions precipitated during cooling in the interdendritic regions and evidence of solute redistribution of Ni, Cr, Co, Si and P are consistent with this cooling rate. Such a rate indicates that Nedagolla cooled very near the surface of its parent ‘body’. Secondary microstructural features including the presence of isothermal taenite and minute phosphide precipitates, which have formed from the dissolution of primary phosphide material, indicate a later reheating to about 750°C for a period of several hours.     

Orthopyroxene-olivine assemblages in diogenites and mesosiderites

Diogenites contain equilibrated orthopyroxene-olivine assemblages. Mn is very regularly partitioned between olivine and orthopyroxene in pallasites, diogenites and synthetic eucrite melts, with an $\text{FeO}\text{MnO}$ partition ratio for olivine versus orthopyroxene of 1.6 by weight over a very wide range of FeO contents. In contrast to diogenites, Fe and Mn are not regularly partitioned between the olivine and orthopyroxene of mesosiderites and these minerals were not in equilibrium. Mesosiderite olivine differs from diogenite olivine in $\text{Fe}\text{Mn}$ and $\text{Ca}\text{Mn}$ ratios. Lack of olivine-orthopyroxene equilibrium suggests that olivine in mesosiderites was derived not from a pyroxenite component analogous to diogenites but from dunites.     

Systematic compositional variations in the Cape York iron meteorite

Concentrations of Re, Ir and Au are nearly constant within individual masses of the Cape York IIIAB iron meteorite, but differences between masses can be as large as a factor of 2, the extremes being Savik (5.1 μg/g Ir) and Agpalilik (2.7 μg/g Ir). The S concentration shows a still larger range from 13 mg/g in Agpalilik to 1.4 mg/g in Savik. A relatively large compositional hiatus between Dog and Agpalilik probably reflects inadequate sampling of the original material.Concentrations of Ir vary by ~10% and Au by ~3% between the ends of an 85-cm section from the Agpalilik mass of Cape York, but other sections through Agpalilik show smaller variations. These concentration ranges are much larger than expected from radial crystallization of a moderately large (radius 10 s of km) core. These variations in the Agpalilik mass may reflect dendritic crystallization, or they may have resulted from the process that produced the large concentration range among the Cape York masses.Large gradients in Re and Ir and small gradients in Ni and Au were also observed in samples within 2 cm of a large (100 cm³) troilite nodule. These gradients may reflect rapidly changing solid/liquid distribution coefficients during the final crystallization of S-rich liquid.The compositional trends among the various masses can either be explained by mixing of disparate end members followed by diffusive homogenization on a scale of m, or by dendritic crystallization on the ceiling of the IIIAB magma chamber. The mixing of a solid similar in composition to Savik with a liquid in equilibrium with this solid yields a good match to the observed trends, in which case Agpalilik consists of a mixture of 64% liquid and 36% solids. The bulk S content of the IIIAB core is calculated to be 14 mg/g on the basis of this model.     

Mineralogy of silicate inclusions in the Elga IIE iron meteorite

The petrography, mineral modal data and major and trace element compositions of 15 silicate inclusions in the Elga iron meteorite (chemical group IIE) show that these inclusions represent chemically homogeneous zoned objects with highly variable structures, reflecting the sequence of crystallization of a silicate melt during cooling of the metal host. The outer zones of inclusions at the interface with their metal host have a relatively medium-grained hypocrystalline texture formed mainly by Cr-diopside and merrillite crystals embedded in high-silica glass, whereas the central zones have a fine-grained hypocrystalline texture. Merrillite appears first on the liquidus in the outer zones of the silicate inclusions. Na and REE concentrations in merrillite from the outer zones of inclusions suggest that it may have crystallized as α-merrillite in the temperature range of 1300–1700°С. Merrillite tends to preferentially accumulate Eu without Sr. Therefore, strongly fractionated REE patterns are not associated with prolonged differentiation of the silicate melt source but depend on crystallization conditions of Н-chondrite droplets in a metallic matrix. The systematic decrease in Mg# with increasing Fe/Mn in bronzite may indicate partial reduction of iron during crystallization of the inclusion melt. The modal and bulk compositions of silicate inclusions in the Elga meteorite, as well as the chemical composition of phases are consistent with the model equilibrium crystallization of a melt, corresponding to 25% partial melting of H-chondrite, and the crystallizing liquidus phase, merrillite, and subsequent quenching at about 1090°С. Despite a high alkali content of the average weighted bulk inclusion composition, La/Hf and Rb/Th fall within the field of H chondrites, suggesting their common source. Our results reveal that silicate inclusions in the Elga (IIE) iron meteorite originated by mixing of two impact melts, ordinary chondrite and Ni-rich iron with а IIE composition, which were produced by impact event under near-surface conditions on a partially differentiated parent asteroid.     

Thermal history of mesosiderites revisited

The results of a fission track study of uranium-rich merrillite crystals in a number of mesosiderites are clearly incompatible with cooling rates as low as ~ 0.1°C/Myr which have previously been inferred for these meteorites. Classical metallographic techniques thus appear to give erroneous results when applied to this type of meteorites. There appears to be no reason to advocate exotic scenarios to explain the thermal history of mesosiderites.     

Native silicon and iron silicides in the Dhofar 280 lunar meteorite

The Dhofar 280 lunar highland meteorite is the first one in which native silicon was identified in association with iron silicides. This association is surrounded by silicate material enriched in Si, Na, K, and S and occurs within an impact-melt matrix. Compared to the meteorite matrix, the objects with native Si and the silicate material around them show high Al-normalized concentrations of volatile elements and/or elements with low sensitivity to oxygen but are not any significantly enriched in refractory lithophile elements. Some lithophile elements (V, U, Sm, Eu, and Yb) seem to be contained in reduced forms, and this predetermines REE proportions atypical of lunar rocks and a very low Th/U ratio. The admixture of siderophile elements (Ni, Co, Ge, and Sb) suggests that the Si-bearing objects were contaminated with meteorite material and were produced by the impact reworking of lunar rocks. The high concentrations of volatile elements suggest that the genesis of these objects could be related to the condensation of silicate vapor generated during meteorite impacts. The reduction of silicon and other elements could take place in an impact vapor cloud, with the subsequent condensation of these elements together with volatile components. On the other hand, condensates of silicate vapor could be reduced by impact reworking of impact breccias. Impact-induced vaporization and condensation seem not to play any significant role in forming the composition of the lunar crust, but the contents of the products of such processes can be locally relatively high. The greatest amounts of silicate vapor were generated during significant impact events. For example, more than 70% of the total mass of lunar material evaporated in the course of impact events should have resulted from the collision of the Moon with a cosmic body that produced the Moon??s largest South Pole-Aitken basin.     

Olivines and olivine coronas in mesosiderites

Olivines and their surrounding coronas in mesosiderites have been studied texturally and compositionally by optical and microprobe methods. Most olivine is compositionally homogeneous but some is irregularly zoned. It ranges from Fo_58–92 and shows no consistent pattern of distribution within and between mesosiderites. Olivine occurs as large single crystals or as partially recrystallized mineral clasts, except for two lithic clasts. One is in Emery, the other in Vaca Muerta, and they are both shock-modified olivine orthopyroxenites. $\text{FeO}\text{MnO}$ ratios in olivine exhibit a variety of differing trends and range from 22–46, most commonly 35–40. These values are lower than those in olivine from diogenites sensu stricto (45–50) and have therefore experienced a different history. Some of the olivine clasts could have coexisted with some of the large orthopyroxene clasts as equilibrium assemblages, but some could not. Much of the olivine may be derived from mesosiderite olivine orthopyroxenites, which differ from diogenites sensu stricto. More magnesian olivine may be a residue from one or more source rocks, with varying degrees of melting. These events probably occurred in a highly evolved and differentiated parent body.Fine-grained coronas surround olivine, except for those in impact-melt group mesosiderites (Simondium, Hainholz, Pinnaroo) and those without tridymite in their matrices (Bondoc, Veramin). Coronas consist largely of orthopyroxene, plagioclase, clinopyroxene, chromite, merrillite and ilmenite and are similar to the matrix, but lack metal and tridymite. Coronas contain abundant orthopyroxene but are unusually rich in chromite (up to 7%) and merrillite (up to 20%). The outer parts of the corona grade into the matrix, but have little or no metal and tridymite. Texturally the innermost part of the corona can be divided into three stages of development: I Radiating acicular; II Intermediate; III Granular. Stage I is the result of the greatest disequilibrium between olivine and matrix orthopyroxene and Stage III has the least disequilibrium. Coronas are the result of the reaction olivine + tridymite = orthopyroxene, probably because FeO (and MgO) diffuse from olivine to tridymite in the matrix. Absence of metal and concentration of chromite in the corona are probably the result of an FeO potential gradient away from the olivine. Merrillite concentrations are a result of P₂O₅ migration into the corona but are controlled by the availability of calcic pyroxene, or possibly plagioclase. Although the coronas are texturally similar to terrestrial and lunar counterparts, they are unique and represent different kinds of reactions marked by a large degree of intra-corona diffusion under dry conditions. Opaque oxide-silicate-metal buffer assemblages yield apparent equilibration conditions of about 840°C and fO₂ near 10^?20. Poikiloblastic pyroxene textures in some coronas suggest a closing of reaction systems between 900 and 1000°C and such systems may record a higher temperature stage of development.     

Nucleogenic noble gas components in the Cape York iron meteorite

We report data on neutron capture products of the secondary cosmic ray component, the inferred proton and neutron fluences, and the identification of double beta decay of ⁸²Se in heavily shielded samples of the Cape York iron meteorite. One purpose of this study is to develop a new chronometer for cosmic ray exposure, based on the nuclides ¹²⁹I (16 My half-life) and ¹²⁹Xe from low energy cosmic ray reactions on Te. The abundance ratio of these two nuclides permits the determination of an (effective) exposure age of 93 ± 16 My, which represents the first exposure age datum of Cape York. The very small concentrations of spallogenic ³⁸Ar = 6.5 × 10⁻¹⁰ cm³ STP/g in the metal and troilite (per g Fe) document the heavily shielded locations of our sample. An excess of ¹²⁹Xe in the troilite is shown to be entirely due to the decay of cosmic-ray-produced ¹²⁹I. On the other hand, an inclusion in the troilite reveals the presence of ¹²⁹Xe from extinct ¹²⁹I and documents its ~4.5 Gy formation age. Mono-isotopic excess of ⁸²Kr is identified as due to ββ-decay of ⁸²Se with an inferred half-life of 1.0 × 10¹⁰ y. This represents the first ββ-decay product observed in a meteorite.     

Transmission electron microscopy on iron monosulfide varieties from the Suizhou meteorite

Transmission electron microscopy on the iron monosulfide (FeS) varieties from the Suizhou meteorite (Hubei, China) reveals the intergrowth of primary hexagonal 2C troilite and minor monoclinic 4C pyrrhotite (SG: F2/d) phases as nanometer-scale domain microstructure. In addition, anti-phase domain boundaries are found to present in the 2C troilite superstructure with the displacement vector 1/4[001]_2C, which is expected to form during the translational symmetry breaking during cooling from higher symmetry, high-temperature modification of the NiAs-type (SG: P6₃/mmc) structure. Furthermore, 60° rotation twinning about the pseudo-hexagonal c-axis is observed in the 4C pyrrhotite superstructure, which may result from rotation symmetry reduction induced by the ordered arrangements of metal vacancies through solid-state transformation during further cooling. All the above microstructural characteristics are discussed with consideration to the thermal metamorphism history experienced by the Suizhou meteorite.     

Analytical electron microscopy study of the plessite structure in the Carlton iron meteorite

In order to gain a better understanding of the formation of plessite in iron meteorites, various electron optical techniques were employed to study the range of plessite structures observed in the Carlton fine octahedrite. Compositional and structural studies of twins in clear taenite and the cloudy zone were made. Transmission electron microscopy studies of martensitic and duplex α + γ plessite regions show the presence of γ-taenite rods, 10–200 nm wide, in an α-kamacite matrix. Scanning transmission electron microscope X-ray analyses showed Ni contents in the y rods of $\text{≥43}\text{wt}\text{\%}$ and Ni contents in the a matrix of 3 wt% Ni. The reaction path involves the decomposition of $\text{α}{}_2$ martensite into $\text{α + γ}$ and these reactions occur below 200°C and possibly below 100°C. Apparently the formation of plessite is intimately related to the formation of martensite and the further decomposition of martensite during the cooling history of the meteorite. It is quite probable that the martensite decomposition reaction has occurred in a large number of iron meteorites and is responsible for many of the observed plessite structures.     

Regolith history of the aubritic meteorite parent body revealed by neutron capture effects on Sm and Gd isotopes

Enstatite achondrites (aubrites) when compared to other stone meteorites have unusually long cosmic-ray exposure (CRE) ages. We report here the ¹⁵⁰Sm/¹⁴⁹Sm and ¹⁵⁸Gd/¹⁵⁷Gd ratios in six different structural phases, i.e., light and dark (shocked) grains and in matrix materials of Pesyanoe, in three different fragments from Pena Blanca Spring, and in one from Norton County, Shallowater, and Khor Temiki, to investigate the regolith history on the aubrite parent body. The results from phases components of Pesyanoe confirm earlier reported evidence for regolith irradiation of several aubrites. The inferred neutron fluences for six Pesyanoe separates vary between (2.13 and 2.82) × 10¹⁶ n cm⁻². The fluences also significantly exceed those expected from cosmic-ray irradiation during transit to Earth and approach those observed in the lunar regolith. These observations confirm that the brecciated Pesyanoe meteorite, which contains solar wind (SW) gases only in dark phases, was processed in a regolith and that structural phases were differentially irradiated before compaction. On the other hand, in some aubrites (Mt. Egerton, Shallowater, Pena Blanca Spring, Norton County) neutron capture effects may entirely be due to space irradiation.     

A dendritic solidification model to explain Ge-Ni variations in iron meteorite chemical groups

Segregation during solidification is responsible for the secondary fractionation of trace elements in iron meteorite chemical groups. This study examines the consequences of dendritic segregation on the Ge-Ni fractionation in iron meteorite chemical groups. Solidification experiments and computer simulations of the dendritic solidification process indicate that the effect of P on the partitioning behavior of Ge and the effect of solid state diffusion on segregation are both important in understanding the observed Ge-Ni correlations. The Ge-Ni concentration trends predicted by the dendritic solidification model agree well with the observed variations.     

Electron microprobe analysis of some minor- and accessory components in mesosiderites

Summary Analyses of chromite, ilmenite, whitlockite, farringtonite, schreibersite, and tridymite from four mesosiderite specimen were made using electron microprobe techniques. These minerals are chemically homogeneous with the exception of chromite, which exhibits slight zoning and grain to grain compositional variation in the same specimen. It can be assumed that schreibersite, troilite, and phosphates formed by solid state nucleation from -NiFe or solid state reactions between mixed metal and silicate fragments at temperatures of approximately 600–700°C during a stage of thermal metamorphism. Chromite was associated with the silicate fraction before metal and silicate fragments have mixed. It is shown that the results of this work are consistent with the theory of mesosiderite genesis evolved byPowell (1969, 1971).

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Elektronenstrahlmikroanalyse einiger Nebenbestandteile und Akzessorien in Mesosideriten

Zusammenfassung In der vorliegenden Arbeit wird über Mikrosondenanalysen von Chromit, Ilmenit, Whitlockit, Farringtonit sowie Schreibersit und Troilit in vier Mesosideriten berichtet. Mit Ausnahme von Chromit, der leichten Zonarbau und geringe Unterschiede in der Zusammensetzung in derselben Probe zeigt, sind alle Minerale homogen. Für die Bildung von Schreibersit, Troilit und den Phosphaten können Ausscheidungsreaktionen im festen Zustand aus -NiFe oder Reaktionen — ebenfalls im festen Zustand — zwischen metallischen und silikatischen Fragmenten in einem Gemenge bei Temperaturen von etwa 600–700°C während einer thermischen Metamorphose angenommen werden. Chromit war schon vor der Mischung der silikatischen und metallischen Anteile mit dem Silikat assoziiert. Die Ergebnisse stehen mit einer bereits vorliegenden Theorie über die Entwicklung der Mesosiderite vonPowell (1969, 1971) in gutem Einklang.

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With 3 Figures

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Dedicated to Professor Dr.F. Hecht on the occasion of his 70th birthday.     

Copper stable isotopes as tracers of metal-sulphide segregation and fractional crystallisation processes on iron meteorite parent bodies

We report high precision Cu isotope data coupled with Cu concentration measurements for metal, troilite and silicate fractions separated from magmatic and non-magmatic iron meteorites, analysed for Fe isotopes (δ⁵⁷Fe; permil deviation in ⁵⁷Fe/⁵⁴Fe relative to the pure iron standard IRMM-014) in an earlier study (Williams et al., 2006). The Cu isotope compositions (δ⁶⁵Cu; permil deviation in ⁶⁵Cu/⁶³Cu relative to the pure copper standard NIST 976) of both metals (δ⁶⁵Cu_M) and sulphides (δ⁶⁵Cu_FeS) span much wider ranges (−9.30 to 0.99‰ and −8.90 to 0.63‰, respectively) than reported previously. Metal-troilite fractionation factors (Δ⁶⁵Cu_M-FeS = δ⁶⁵Cu_M − δ⁶⁵Cu_FeS) are variable, ranging from −0.07 to 5.28‰, and cannot be explained by equilibrium stable isotope fractionation coupled with either mixing or reservoir effects, i.e. differences in the relative proportions of metal and sulphide in the meteorites. Strong negative correlations exist between troilite Cu and Fe (δ⁵⁷Fe_FeS) isotope compositions and between metal-troilite Cu and Fe (Δ⁵⁷Fe_M-FeS) isotope fractionation factors, for both magmatic and non-magmatic irons, which suggests that similar processes control isotopic variations in both systems. Clear linear arrays between δ⁶⁵Cu_FeS and δ⁵⁷Fe_FeS and calculated Cu metal-sulphide partition coefficients (D_Cu = [Cu]_metal/[Cu]_FeS) are also present. A strong negative correlation exists between Δ⁵⁷Fe_M-FeS and D_Cu; a more diffuse positive array is defined by Δ⁶⁵Cu_M-FeS and D_Cu. The value of D_Cu can be used to approximate the degree of Cu concentration equilibrium as experimental studies constrain the range of D_Cu between Fe metal and FeS at equilibrium to be in the range of 0.05-0.2; D_Cu values for the magmatic and non-magmatic irons studied here range from 0.34 to 1.11 and from 0.04 to 0.87, respectively. The irons with low D_Cu values (closer to Cu concentration equilibrium) display the largest Δ⁵⁷Fe_M-FeS and the lowest Δ⁶⁵Cu_M-FeS values, whereas the converse is observed in the irons with large values D_Cu that deviate most from Cu concentration equilibrium. The magnitudes of Cu and Fe isotope fractionation between metal and FeS in the most equilibrated samples are similar: 0.25 and 0.32‰/amu, respectively. As proposed in an earlier study (Williams et al., 2006) the range in Δ⁵⁷Fe_M-FeS values can be explained by incomplete Fe isotope equilibrium between metal and sulphide during cooling, where the most rapidly-cooled samples are furthest from isotopic equilibrium and display the smallest Δ⁵⁷Fe_M-FeS and largest D_Cu values. The range in Δ⁶⁵Cu_M-FeS, however, reflects the combined effects of partial isotopic equilibrium overprinting an initial kinetic signature produced by the diffusion of Cu from metal into exsolving sulphides and the faster diffusion of the lighter isotope. In this scenario, newly-exsolved sulphides initially have low Cu contents (i.e. high D_Cu) and extremely light δ⁶⁵Cu_FeS values; with progressive equilibrium and fractional crystallisation the Cu contents of the sulphides increase as their isotopic composition becomes less extreme and closer to the metal value. The correlation between Δ⁶⁵Cu_M-FeS and Δ⁵⁷Fe_M-FeS is therefore a product of the superimposed effects of kinetic fractionation of Cu and incomplete equilibrium between metal and sulphide for both isotope systems during cooling. The correlations between Δ⁶⁵Cu_M-FeS and Δ⁵⁷Fe_M-FeS are defined by both magmatic and non-magmatic irons record fractional crystallisation and cooling of metallic melts on their respective parent bodies as sulphur and chalcophile elements become excluded from crystallised solid iron and concentrated in the residual melt. Fractional crystallisation processes at shallow levels have been implicated in the two main classes of models for the origin of the non-magmatic iron meteorites; at (i) shallow levels in impact melt models and (ii) at much deeper levels in models where the non-magmatic irons represent metallic melts that crystallised within the interior of a disrupted and re-aggregated parent body. The presence of non-magmatic irons with a range of Fe and Cu isotope compositions, some of which record near-complete isotopic equilibrium implies crystallisation at a range of cooling rates and depths, which is most consistent with cooling within the interior of a meteorite parent body. Our data therefore lend support to models where the non-magmatic irons are metallic melts that crystallised in the interior of re-aggregated, partially differentiated parent bodies.     

Metamorphic reactions in mesosiderites: origin of abundant phosphate and silica

The high modal abundances of merrillite [Ca₃(PO₄)₂] and tridymite in most mesosiderites are not the result of igneous fractionation but are attributed to redox reactions between silicates and P-bearing Fe-Ni metal within a limited T-fO₂ range at low pressure. The Emery mesosiderite is the most tridymite- and merrillite-rich mesosiderite so it is used as the model for this study. Examination of reactions in the system CaO-SiO₂-MgSiO₃-Fe-P-O indicate that essentially all of the present phosphorus in Emery should have been dissolved in the metallic portion (calculated to have contained 0.65 wt% P originally), and that it largely reacted to form phosphate. The thermodynamic calculations predict that the reactions would have occurred between 970°C, log fO₂ = ?16.5 and 1030°C, log fO₂ = ?15.0 for the range of phase compositions in Emery. A narrower range of conditions is expected for other mesosiderites. Phosphide (schreibersite) formed only later at temperatures < 600°C during slow cooling. The recalculated amounts of dissolved P and S in the metallic portion of Emery reduce the temperature of the metal liquidus to < 1350°C and the solidus to < 800°C. Thus mixing of liquid metal with cold silicates near the parent body's surface would not have resulted in substantial melting of the silicates but would have resulted in their metamorphism, which is consistent with the textural relationships observed in Emery. This scenario of redox reactions and redistributions of components between metal and silicates represents a new insight into the complexities of mesosiderite processing that helps unravel the mesosiderite history and recalculate their original components.     

早更新世晚期松花江水系袭夺：地球化学和沉积学记录

水系演化重建是恢复区域构造历史及古环境变化的重要方法。松花江作为中国七大水系之一,其水系演化的相关研究目前还比较薄弱,尤其是第四纪松花江中- 上游是否存在流向反转尚无准确结论。沉积物是河流地质过程的直接产物,是水系演化研究的关键。为此,本研究对哈尔滨荒山钻孔岩芯(HS)进行了磁化率、古地磁和元素地球化学组成分析。结果表明,岩芯62. 3 m(0. 94 Ma B. P. )处,沉积物的岩性、磁化率及元素地球化学组成均发生显著变化。岩芯沉积物岩性在62. 3 m上下发生明显变化,是河流沉积两个沉积旋回的转折点。62. 3 m以下地层的磁化率极低,基本为0,且变化幅度极低,元素地球化学组成则急剧波动,并表现出与现在松花江哈尔滨段下游水系(岔林河、蚂蜒河和牡丹江)相近的地球化学组成;然而,62. 3 m以上地层的磁化率突然升高(99. 673×10-8 m3 kg-1),并具有周期性的高低变化。元素组成呈现稳定的小幅变化,并表现出与拉林河、嫩江和松花江吉林段相近的地球化学组成。0. 94 Ma B. P. 沉积物物源的变化表明河流流向发生了显著变化,松花江中上游河段流向曾在此时发生反转。地球化学记录的水系反转也得到了河流阶地地貌和沉积学证据(砾石的粒径和排列方向)的支持。早更新世早- 中期,以佳- 依分水岭为界,松花江中上游(肇源- 依兰河段)河流自东向西流入松嫩湖盆,松花江下游向东流经三江平原;早更新世晚期,受构造- 地貌- 气候耦合作用的影响,佳- 依分水岭持续抬升,而三江平原一侧不断下沉,佳- 依分水岭两侧河流发生溯源侵蚀,最终导致佳- 依分水岭在0. 94 Ma B. P. 被切穿,松花江中上游水系被下游水系所袭夺,河流流向发生反向,自西向东流经依佳峡谷进入三江平原,现代松花江水系逐渐建立。本研究有助于我们加深对松嫩平原水系演化的理解,为探索东亚水系演化的一致性和区域差异性提供研究证实,同时对松花江流域自然资源禀赋的调查乃至国土空间开发利用具有重要实践意义。     

The Paleoclimatic Variation Records of Carbonate and Iron Oxides in the Weinan Loess Section

The contents and distributions of CaCO3,Fe2O3.FeO and free Fe2O3 in the Weinan loess section of Shaanxi Province of China were investigated through dense sampling.The results show that the contents of CaCO3 and the ration of Fe2O3/FeO may be chosen as proxy in dices for the precipitation and temperature changes in the formation time of the strata,respectively.According to these geochemical indices,six stages of plaeoclimate evolution are proposed in this region since 142 ka B.P.,and secondary climate changes are discussed as well based on the curves of geochemical indices.     

Petrogenesis of silicate inclusions in the Weekeroo Station IIE iron meteorite: differentiation,remelting, and dynamic mixing

The Weekeroo Station IIE iron meteorite contains a variety of felsic and mafic inclusions enclosed in an FeNi-metal host. Petrographic, EMP, and SIMS data suggest that the petrogenesis of the silicates was complex, and included differentiation, remelting, FeO-reduction, and dynamic mixing of phases.Differentiation produced a variety of olivine-free inclusion assemblages, ranging from pyroxene + plagioclase + tridymite with peritectic compositions, to coarse orthopyroxene, to plagioclase + tridymite and its glassy equivalent. Individual phases have similar trace-element abundances and patterns, despite large variations in inclusion textures, modes, and bulk compositions, probably as a result of mechanical separation of pre-existing phases in an impact event that dynamically mixed silicates with the metallic host. Trace-element data imply that augite and plagioclase grains in different inclusions crystallized from the same precursor melt, characterized by relatively unfractionated REE abundances of ∼20–30 × CI-chondrites except for a negative Eu anomaly. Such a precursor melt could have been produced by ∼2–5% equilibrium partial melting of an H-chondrite silicate protolith, or by higher degrees of partial melting involving subsequent fractional crystallization. Glass appears to have formed by the remelting of pre-existing plagioclase and orthopyroxene, in a process that involved either disequilibrium or substantial melting of these phases. During remelting, silicate melt reacted with the FeNi-metal host, and FeO was reduced to Fe-metal. Following remelting and metal-silicate mixing, inclusions apparently cooled at different rates in a near-surface setting on the parent body; glass- or pigeonite-bearing inclusions cooled more rapidly (≥2.5°C/hr between 1000–850°C) than pigeonite-free, largely crystalline inclusions.The results of this study point to two likely models for forming IIE iron meteorites, both involving collision between an FeNi-metal impactor and either a differentiated or undifferentiated silicate-rich target of H-chondrite affinity. Each model has difficulties and it is possible that both are required to explain the diverse IIE group.