从稀土元素和同位素比值看分异的陨石

根据分异型陨石的稀土元因丰度模式和年代学研究,对如何解释母体分异过程,介绍目前的研究成果。一、前言在获得原始太阳系行星形成初期事件及其时间信息方面,陨石是很珍贵的样品。陨石分为原始型陨石—球粒陨石和分异型陨石两大类。其中,分异型陨石组的石质陨石—     

普通球粒陨石的物理和岩石学性质及其分类参数

不同球粒陨石群的物理和岩石学性质,包括球粒的平均大小、球粒结构类型、复合球粒、带火成边球粒及含硫化物的比例、化学组成及矿物学特征等可用以划分球粒陨石的化学-岩石类型和小行星类型,这些性质提供了不同球粒陨石群有用的分类参数及其形成环境的信息.由于不同球粒陨石群的△17O与日心距离存在有相关关系,因此,依据不同球粒陨石群形...     

陨石学及天体化学研究某些新进展

陨石是来自含气体-尘粒的太阳早期星云盘凝聚和吸积的原始物质,大多数原始物质因吸积后的作用过程而改变（如月球、地球及火星样品）,但有一些却完整的保存下来（如球粒陨石或球粒陨石中的难熔包体）。这些原始的物质通常依据同位素丰度特征来识别,依据其矿物-岩石学特征和成因可将已知的陨石划分许多更小的类型。陨石学及天体化学的新近进展包括：新近识别的陨石群;发现新类型球粒陨石及行星际尘粒中发现前太阳和星云组分;利用短寿命放射性核素完善了早期太阳系年代学;洞察宇宙化学丰度、分馏作用及星云源区及通过次生母体的作用过程阐释星云和前星云的记录。本文概述了早期太阳系内从星云到陨石的演化过程。依据这些资料,对早期太阳系所经历的多种核合成的输入、瞬时加热事件与星云动力学有一些新的认识,以及认识到小星子和行星体系的演化比以前预期的更快速。     

我国普通球粒陨石岩石学,化学组成及分类的研究

对69个普通球粒陨石进行了岩石学及化学组成的研究,在此基础上提出橄榄石成分(mol％Fa)-铁纹石中钴含量两维分类参数,69个球粒陨石包括25个H、20个L、17个LL、2个介于H与L之间及5个介于L与LL之间的类型。根据矿物学的分类参数及化学组成的研究,普通球粒陨石母体至少有5个,即H、H/L、L、L/LL及LL,而不是3个(H、L及LL)。每群球粒陨石的不同岩石类型之间化学成分无重大的变化,表明球粒陨石的变质作用是在封闭体系的条件下发生的。本文给出了普通球粒陨石不同化学群和不同岩石类型的平均化学成分。     

太阳系~(53)Mn-~(53)Cr同位素体系

短周期放射性核素的初始丰度和分布情况,已成为陨石学和天体化学的重要研究领域之一。已有研究证实地外天体中53 Cr的放射性母体为53 Mn。53 Mn的半衰期为3.7±0.4Ma,可对太阳系形成之后的20Ma内发生的事件进行精确定年。本文系统总结了已报道的碳质球粒陨石、普通球粒陨石、顽辉石球粒陨石和分异陨石中的53 Mn-53 Cr同位素体系数据,依据55 Mn/52 Cr值和53 Cr异常探讨了太阳系形成时53 Mn和53 Cr的初始分布情况、太阳系初始的53 Mn/55 Mn值,讨论了陨石中普遍存在的54 Cr/52 Cr值异常和碳质球粒陨石全岩的54 Cr和53 Cr异常值之间的正相关关系对53 Mn-53 Cr体系定年影响。     

车里雅宾斯克(Chelyabinsk)陨石的岩石矿物特征与冲击变质作用

2013年2月15日,俄罗斯车里雅宾斯克(Chelyabinsk)发生了伴随罕见的空中爆炸的大规模陨石雨事件。本文对3块代表不同冲击变质程度的车里雅宾斯克陨石碎块进行了研究。它们都具有部分熔壳,其中1块仅出现碎裂,1块含有冲击熔融细脉,1块基本由冲击熔融囊和冲击熔脉组成。冲击变质程度最低的样品,代表了该陨石母体小行星的原始岩石矿物学特征:即具有粗粒的岩石结构和均一的矿物化学组成,但仍保留一些残余球粒,表明受到了明显的热变质作用,其岩石类型可划分为5型。铁镁质硅酸盐高的Fe O含量(橄榄石Fa:27.9mol%~28.2mol%,辉石Fs值:23.3mol%~23.7mol%)、以及较低的Fe-Ni金属含量,表明其化学群属于低铁低金属的LL群。我们所分析的样品与前人报导的结果相似,未发现不同岩性的岩屑,表明车里雅宾斯克陨石的原始岩矿特征较为均一。3块陨石碎块中,随着冲击程度的增强,其冲击变质特征依次表现为硅酸盐矿物的破碎、熔长石化更为普遍、陨硫铁与铁镍合金共熔、硅酸盐熔脉的形成、铬铁矿与长石共熔、以及大量熔融囊的发育等。但是,在冲击熔融囊和熔脉中,以及相邻围岩中均未发现高压矿物相。熔脉中的橄榄石晶屑和相邻围岩的橄榄石颗粒表现为化学成分的不均一,在背散射电子图像中呈不同灰度的结构。这与其他强烈冲击变质陨石中橄榄石的林伍德石或瓦茨利石相变相似。该陨石中林伍德石或瓦茨利石的缺失很可能是由于强烈撞击后高温产生的退变质。这也表明车里雅宾斯克陨石的母体小行星可能遭受了非常强烈的撞击事件。     

南极格罗夫山H群和L群普通球粒陨石对比研究

通过对15块南极格罗夫山普通球粒陨石(中国第19次和第22次南极科考回收)进行岩石学、矿物学分析,为其进一步研究提供重要基础数据。研究表明这些陨石均为平衡型陨石,其中有5块为H群,其余为L群。除GRV 051869和GRV 021491经历较强冲击变质作用、具有S4型的冲击变质程度外,其他陨石的冲击变质作用轻微,主要集中在S1和S2型。这些陨石的风化程度普遍较轻,仅GRV052076达到了W3型,其他为W1和W2型。主要矿物成分分析表明,组成H群和L群的最初始星云物质可能是相同的,陨石的主要差别是由于两个群陨石各自所处环境的不同所造成。L群平均球粒半径大于H群球粒半径,可能为球粒形成过程中星云温度变化不均一或者不同类型球粒分不同时期形成。另外,研究表明橄榄石中的Ca含量可以作为一个反映陨石热历史的有用指标。     

普通球粒陨石NWA15004岩石矿物学特征及球粒类型研究

普通球粒陨石是目前发现数量最多的陨石,对认识早期太阳星云演化和太阳系物质成分具有重要的意义。Northwest Africa (NWA) 15004是一块非洲西北部新发现的普通球粒陨石。本次研究使用光学显微镜、电子探针以及扫描电镜等分析仪器对该陨石进行详细的岩石学、矿物学及球粒特征研究。结果表明该陨石球粒轮廓较为模糊,基质重结晶明显,橄榄石平均Fa值为25.4 mol%(PMD为2.65%),低钙辉石的平均Fs值21.3 mol%(PMD为3.95%),硅酸盐矿物化学成分较为均一,根据岩相学及矿物学特征将其划分为L5型普通球粒陨石。橄榄石和辉石颗粒发育波状消光和面状破裂,且观察到有熔融囊的出现,表明该陨石受到S3以上的冲击变质作用。球粒的成因和形成的星云环境需要准确的球粒类型划分,球粒按结构类型分类较多,但其化学成分均一,该陨石所有球粒的橄榄石辉石的Mg^#约为74.5,均为Ⅱ型富铁球粒,结合“CIPW标准”计算基质化学成分均为A5型球粒。利用共生单斜辉石和斜方辉石矿物对成分特征计算得到NWA 15004陨石热变质平衡温度为814℃,说明该陨石母体经历了较高程度热变...     

根据40Ar/39Ar年龄谱探讨陨石冲击事件的分期

⁴⁰Ar/³⁹Ar年龄谱是研究陨石冲击事件的重要资料。根据对55块陨石⁴⁰Ar/³⁹Ar冲击年龄和陨石暴露年龄的分析,发现陨石的冲击年龄与陨石类型之间存在对应关系。据此,将陨石冲击事件划分为九期。其中3900-4000Ma、470-540Ma和小于65Ma是陨石母体的三个重要演化阶段。阶段Ⅰ、Ⅱ和Ⅲ(冲击年龄大于30亿年)主要涉及高钙型无球粒陨石。所有球粒陨石的冲击年龄均小于30亿年。陨石暴露年龄因类型而异,铁陨石最大,石铁陨石次之,石陨石最小。     

8块新发现的西北非球粒陨石的矿物岩石学特征

为了解不同种类小行星母体的起源与演化信息,选取8块近期在西北非地区发现的未经过详细研究的球粒陨石,利用扫描电子显微镜观察其显微结构,利用能谱仪及电子探针测试样品的成分。结果显示,NWA 7613与NWA 8340为CV3_(oxA)型陨石,另外5块普通球粒陨石的热变质程度变化更为广泛且球粒中橄榄石铁含量更高。NWA 7613(LL3)球粒中橄榄石CaO含量稍高(0.08%~0.24%),高于平衡型普通球粒陨石(小于0.05%)。NWA 6468(R4)与普通球粒陨石具有相似的岩相结构,但不发育铁镍金属,且橄榄石铁含量(Fa_(35.9~42.1))及镍含量(平均含0.23%)更高,是强氧化环境下的产物。NWA 7251(Limpact melt)具有特殊的火成结构,是大规模灾难性撞击事件产物,橄榄石铁指数(Fa_(21.4~26.7))与L型球粒陨石的橄榄石成分一致,但CaO含量(0.16%~0.31%)高于平衡型普通球粒陨石。     

Initiation of plate tectonics in the Hadean: Eclogitization triggered by the ABEL Bombardment

When plate tectonics began on the Earth has been long debated and here we argue this topic based on the records of Earth-Moon geology and asteroid belt to conclude that the onset of plate tectonics was during the middle Hadean(4.37-4.20 Ga). The trigger of the initiation of plate tectonics is the ABEL Bombardment, which delivered oceanic and atmospheric components on a completely dry reductive Earth, originally comprised of enstatite chondrite-like materials. Through the accretion of volatiles, shock metamorphism processed with vaporization of both CI chondrite and supracrustal rocks at the bombarded location, and significant recrystallization went through under wet conditions, caused considerable eclogitization in the primordial continents composed of felsic upper crust of 21 km thick anorthosite, and 50 km or even thicker KREEP lower crust. Eclogitization must have yielded a powerful slab-pull force to initiate plate tectonics in the middle Hadean. Another important factor is the size of the bombardment. By creating Pacific Ocean class crater by 1000 km across impactor, rigid plate operating stagnant lid tectonics since the early Hadean was severely destroyed, and oceanic lithosphere was generated to have bi-modal lithosphere on the Earth to enable the operation of plate tectonics.Considering the importance of the ABEL Bombardment event which initiated plate tectonics including the appearance of ocean and atmosphere, we propose that the Hadean Eon can be subdivided into three periods:(1) early Hadean(4.57-4.37 Ga),(2) middle Hadean(4.37-4.20 Ga), and(3) late Hadean(4.20-4.00 Ga).     

Pb-Pb dating constraints on the accretion and cooling history of chondrites

We have analyzed the Pb isotopic compositions of whole-rocks and various components (CAIs, chondrules, and/or mineral separates) of two carbonaceous chondrites, Allende (CV3) and Murchison (CM2), and nine ordinary chondrites, Sainte Marguerite (H4), Nadiabondi and Forest City (H5), Kernouvé (H6), Bjurböle (L/LL4), Elenovka and Ausson (L5), Tuxtuac (LL5), and Saint-Séverin (LL6) by MC-ICP-MS. Three CAI fractions from Allende define an isochron with an age of 4568.1 ± 9.4 Ma (MSWD = 0.08) and plot on the same isochron as fragments of the Efremovka inclusion E60 analyzed by Amelin et al. [Amelin, Y., Krot, A. N., Hutcheon, I. D., and Ulyanov, A. A. (2002a). Lead isotopic ages of chondrules and calcium-aluminum-rich inclusions. Science297, 1679-1683]. When these two groups of samples are combined, the isochron yields an age of 4568.5 ± 0.5 (MSWD = 0.90), which is our best estimate of the age of the Solar System. Chondrules and pyroxene-olivine fractions from the ordinary chondrites yield ages that reflect the blocking of Pb isotope equilibration with the nebular gas. The combination of these ages with the corresponding metamorphic phosphate ages provides constraints on the thermal history of the different chondrite parent bodies. Among the H chondrites, Sainte Marguerite cooled to below ∼1100 K within a few My at 4565 Ma and to ∼800 K at 4563 Ma. Nadiabondi appears to have experienced a slightly more protracted cooling history with the corresponding interval lasting from 4559 to 4556 Ma. The data from Forest City and Kernouvé show evidence of late-stage perturbation with resulting U/Pb fractionation. Likewise, Pb isotopes in Tuxtuac (LL5) record a cooling history lasting from ∼4555 to 4544 Ma, which may indicate that the cooling history for the LL parent body was more prolonged than for the H parent body. We suggest a thermal evolution model for the growth of the planetary bodies based on the release of radiogenic heat from ²⁶Al and ⁶⁰Fe. This model incorporates the accretion rate, which determines the time at which the radiogenic heat becomes efficiently trapped, and the terminal size of the parent body, which controls its overall thermal inertia. The parent bodies of carbonaceous chondrites, which show little indication of metamorphic transformation, collect cooler nebular material at a relatively late stage. Small asteroids of ∼10-50 km radius accreting within 1-3 My could be the parent bodies of H and LL chondrites. The parent body of the L chondrites is likely to be a larger asteroid (r > 100 km) or possibly the product of collisions of smaller planetary bodies.     

K–Ar ages of meteorites: Clues to parent-body thermal histories

Whereas most radiometric chronometers give formation ages of individual meteorites >4.5 Ga ago, the K–Ar chronometer rarely gives times of meteorite formation. Instead, K–Ar ages obtained by the ³⁹Ar–⁴⁰Ar technique span the entire age of the solar system and typically measure the diverse thermal histories of meteorites or their parent objects, as produced by internal parent body metamorphism or impact heating. This paper briefly explains the Ar–Ar dating technique. It then reviews Ar–Ar ages of several different types of meteorites, representing at least 16 different parent bodies, and discusses the likely thermal histories these ages represent. Ar–Ar ages of ordinary (H, L, and LL) chondrites, R chondrites, and enstatite meteorites yield cooling times following internal parent body metamorphism extending over ∼200 Ma after parent body formation, consistent with parent bodies of ∼100 km diameter. For a suite of H-chondrites, Ar–Ar and U–Pb ages anti-correlate with the degree of metamorphism, consistent with increasing metamorphic temperatures and longer cooling times at greater depths within the parent body. In contrast, acapulcoites–lodranites, although metamorphosed to higher temperatures than chondrites, give Ar–Ar ages which cluster tightly at ∼4.51 Ga. Ar–Ar ages of silicate from IAB iron meteorites give a continual distribution across ∼4.53–4.32 Ga, whereas silicate from IIE iron meteorites give Ar–Ar ages of either ∼4.5 Ga or ∼3.7 Ga. Both of these parent bodies suffered early, intense collisional heating and mixing. Comparison of Ar–Ar and I–Xe ages for silicate from three other iron meteorites also suggests very early collisional heating and mixing. Most mesosiderites show Ar–Ar ages of ∼3.9 Ga, and their significantly sloped age spectra and Ar diffusion properties, as well as Ni diffusion profiles in metal, indicate very deep burial after collisional mixing and cooling at a very slow rate of ∼0.2 °C/Ma. Ar–Ar ages of a large number of brecciated eucrites range over ∼3.4–4.1 Ga, similar to ages of many lunar highland rocks. These ages on both bodies were reset by large impact heating events, possibly initiated by movements of the giant planets. Many impact-heated chondrites show impact-reset Ar–Ar ages of either >3.5 Ga or <1.0 Ga, and generally only chondrites show these younger ages. The younger ages may represent orbital evolution times in the asteroid belt prior to ejection into Earth-crossing orbits. Among martian meteorites, Ar–Ar ages of nakhlites are similar to ages obtained from other radiometric chronometers, but apparent Ar–Ar ages of younger shergottites are almost always older than igneous crystallization ages, because of the presence of excess (parentless) ⁴⁰Ar. This excess ⁴⁰Ar derives from shock-implanted martian atmosphere or from radiogenic ⁴⁰Ar inherited from the melt. Differences between meteorite ages obtained from other chronometers (e.g., I–Xe and U–Pb) and the oldest measured Ar–Ar ages are consistent with previous suggestions that the ⁴⁰K decay parameters in common use are incorrect and that the K–Ar age of a 4500 Ma meteorite should be possibly increased, but by no more than ∼20 Ma.     

Formation of the Bencubbin polymict meteoritic breccia

The Bencubbin meteorite is a polymict breccia consisting of a host fraction of ~60% metal and ~40% ferromagnesian silicates and a selection of carbonaceous, ordinary and ‘enstatite’ chondritic clasts. Concentrations of 27 elements were determined by neutron activation in replicate samples of the host silicates and the ordinary and carbonaceous chondritic clasts; 12 elements were determined in the host metal. Compositional data for the ordinary chondrite clast indicate a classification of LL4 ± 1. Refractory element data for the carbonaceous chondrite clast indicate that it belongs to the CI-CM-CO clan; its volatile element abundances are intermediate between those of CM and CO chondrites. Abundances of nonvolatile elements in the silicate host are similar to those in the carbonaceous chondrite clast and in CM chondrites; the rare earths are unfractionated. We conclude that it is not achondritic as previously designated, but chondritic and that it is probably related to the CI-CM-CO clan; its volatile abundances are lower than those in CO chondrites. Oxygen isotope data are consistent with these classifications. Host metal in Bencubbin and in the closely related Weatherford meteorite has low abundances of moderately volatile siderophiles; among iron meteorite groups its nearest relative is group IIIF.We suggest that Bencubbin and Weatherford formed as a result of an impact event on a carbonaceous chondrite regolith. The impact generated an ‘instant magma’ that trapped and surrounded regolithic clasts to form the polymict breccia. The parent of this ‘magma’ was probably the regolith itself, perhaps mainly consisting of the so-called ‘enstatite’ chondrite materials. Accretion of such a variety of materials to a small parent body was probably only possible in the asteroid belt.     

Cumberland Falls chondritic inclusions—II. Trace element contents of forsterite chondrites and meteorites of similar redox state

Mineralogic study of black inclusions in the Cumberland Falls enstatite achondrite revealed that they constitute a highly unequilibrated chondritic suite distinct from other chondrite groups. This highly shocked suite, the forsterite (F) chondrites, exhibits mineralogic trends apparently produced during primary nebular condensation and accretion over a broad redox range. We analyzed these samples and possibly related meteorites for Ag, As, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Sb, Se, Te, Tl, U and Zn, trace elements known to yield important genetic information. The results demonstrate the compositional coherence and distinctiveness of the F chondrite suite relative to other chondrites. The Antarctic aubrite, ALH A78113, may include more F chondrite material. Trace element contents do not vary with mineral compositions hence do not reflect redox variations during formation of F chondrite parental matter. Trace element mobilization—during secondary heating episodes in the F chondrite parent or during its disruptive collision with the enstatite meteorite parent body—is not detectable. Chemical trends in F chondrites apparently reflect primary nebular processes. Cosmochemical fractionation of lithophiles from siderophiles and chalcophiles occurred at moderately high temperatures, certainly higher than those existing during formation of primitive carbonaceous, enstatite and ordinary chondrites of petrologic type ≤3.     

Asteroid (4) Vesta II: Exploring a geologically and geochemically complex world with the Dawn Mission

More than 200 years after its discovery, asteroid (4) Vesta is thought to be the parent body for the howardite, eucrite and diogenite (HED) meteorites. The Dawn spacecraft spent ∼14 months in orbit around this largest, intact differentiated asteroid to study its internal structure, geology, mineralogy and chemistry. Carrying a suite of instruments that included two framing cameras, a visible-near infrared spectrometer, and a gamma-ray and neutron detector, coupled with radio tracking for gravity, Dawn revealed a geologically and geochemically complex world. A constrained core size of ∼110–130 km radius is consistent with predictions based on differentiation models for the HED meteorite parent body. Hubble Space Telescope observations had already shown that Vesta is scarred by a south polar basin comparable in diameter to that of the asteroid itself. Dawn showed that the south polar Rheasilvia basin dominates the asteroid, with a central uplift that rivals the large shield volcanoes of the Solar System in height. An older basin, Veneneia, partially underlies Rheasilvia. A series of graben-like equatorial and northern troughs were created during these massive impact events 1–2 Ga ago. These events also resurfaced much of the southern hemisphere and exposed deeper-seated diogenitic lithologies. Although the mineralogy and geochemistry vary across the surface for rock-forming elements and minerals, the range is small, suggesting that impact processes have efficiently homogenized the surface of Vesta at scales observed by the instruments on the Dawn spacecraft. The distribution of hydrogen is correlated with surface age, which likely results from the admixture of exogenic carbonaceous chondrites with Vesta's basaltic surface. Clasts of such material are observed within the surficial howardite meteorites in our collections. Dawn significantly strengthened the link between (4) Vesta and the HED meteorites, but the pervasive mixing, lack of a convincing and widespread detection of olivine, and poorly-constrained lateral and vertical extents of units leaves unanswered the central question of whether Vesta once had a magma ocean. Dawn is continuing its mission to the presumed ice-rich asteroid (1) Ceres.     

The Omolon pallasite: Chemical composition,mineralogy, and genetic implications

The chemical composition of mineral components of the Omolon pallasite was determined by neutron-activation. Six types of olivines were distinguished. Four types differ in the abundance of Co relative to Ni of CI chondrites. The fifth and sixth types were distinguished on the basis of REE distribution in them. Both last types are variably enriched in LREE relative to CI chondrites. In terms of Ca content relative to CI chondrite, these six types are subdivided into two groups: low-calcium and high-calcium. The difference in Ca contents can be caused by different cooling rate of the precursor of these olivines. The distribution pattern of siderophile elements in the pallasite metal indicates that a metallic phase experienced chemical transformations since the time of its formation. The analysis of chemical composition of accessory minerals showed that: (1) HREE are accumulated in tridymite; (2) troilite and daubreelite were formed under different temperature conditions; (3) magnetite is the mineral of the outer zone of melting crust. Four fragments with anomalous contents of lithophile elements were found in the pallasites and studied. The unusual chemical composition of phases and high degree of HREE fractionation in the fragments suggest their formation at high temperatures at the early stage of the Solar system evolution. It is assumed that the Omolon pallasite was formed as impact-brecciated mixture of the asteroid core (with composition close to IIIAB group of iron meteorites) and mantle olivine from incompletely differentiated parent body of chondrite composition.     

Origin of fossil meteorites from Ordovician limestone in Sweden

Based on the analysis of data in [1, 2] on the concentrations of noble gases and the cosmic ray exposure age (CREA) of chromite grains in fossil meteorites, it was demonstrated in [3] that the distributions of gas concentrations and cosmic ray exposure ages can be explained under the assumption of the fall of a single meteorite in the form of a meteorite shower in southern Sweden less than 0.2 Ma after the catastrophic destruction of the parental body (asteroid) of L chondrites in space at approximately 470 Ma. This assumption differs from the conclusion in [1, 2, 4] about the long-lasting (for 1–2 Ma) delivery of L chondrites to the Earth, with the intensity of the flux of this material one to two orders of magnitude greater than now. The analysis of newly obtained data on samples from the Brunflo fossil meteorite [5] corroborates the hypothesis of a meteorite shower produced by the fall of a single meteorite. The possible reason for the detected correlations between the cosmic ray exposure ages of meteorites and the masses of the samples with the ²⁰Ne concentrations can be the occurrence of Ne of anomalous isotopic composition in the meteorites.