岩庄陨石橄榄石的冲击效应

岩庄陨石在太空中经受过极高能的冲击碰撞、陨石中橄榄石出现的冲击效应包括:面状裂隙、位错、镶嵌状结构、砾化结构、固态转变以及熔融重结晶等。其中首次找到了天然击变玻璃,并发现两种产状的林伍德石。研究证实岩庄陨石经受的冲击波峰压高达100GPa,温度峰值大于2000℃。     

六个中国普通球粒陨石的组成和各相元素分布

本文运用INAA测定了6个中国普通球粒陨石磁性相21个元素和非磁相20个元素的含量。5个H群球粒陨石的组成没有显示出明显的差异,但其元素丰度明显地不同于另一个L群的肇东球粒陨石。相对于CI,磁性相中Ga归一化的元素丰度可以作为普通球粒陨石分群的指示剂。根据普通球粒陨石各相元素丰度建立的模式成分和组成与全岩分析结果一致。     

南极格罗夫山陨石GRV053645的岩石矿物学特征

南极格罗夫山陨石GRV053645是一块非常原始的普通球粒陨石,它很好地保存有太阳星云凝聚和增生的相关信息,对研究太阳星云的起源意义重大。本文描述了这块陨石的岩石矿物学特征,包括球粒结构类型、基质不透明、橄榄石和低钙辉石具有明显的成分环带等。根据Fe-Ni合金含量、球粒平均直径大小和橄榄石平均Fa值,表明这块陨石属于低铁群(L)。利用橄榄石Fa的PMD值与岩石类型亚类的相关性,其岩石类型应为3.4亚类。该陨石冲击变质程度和风化等级都比较低,分别为S2和W1。     

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...     

天门山震旦/寒武系界线上地外撞击事件痕迹

从湘西北天门山震旦—寒武系界线上发现的冲击砾裂、冲击角砾、冲击岩脉、微球粒撞击玻粒(microirghi-zites)、微球粒玻璃陨石(microtektites),特别是陨石碎屑残迹现象,可能表明:在震旦—寒武系界线上曾经发生过地外撞击的事件。     

荷叶塘陨石:一个L3型普通球粒陨石的岩石学和地球化学特征

荷叶塘为一块我国降落的原始3型普通球粒陨石,因此具有重要研究意义。本文对荷叶塘陨石光薄片及粉末样品的岩石学、矿物学和全岩组成地球化学特征进行研究,为这块陨石的深入研究提供重要基础数据。研究表明荷叶塘陨石具L3型陨石岩石学特征,具典型的球粒陨石结构,球粒清晰,球粒结构类型多,基质重结晶程度低,组成模式为:球粒80vol%,金属和硫化物含量为5vol%,基质15vol%。矿物化学成分表明,该陨石球粒以Ⅰ型(贫铁型)球粒为主,橄榄石Fa0.41-34.1(PMD=51),低钙辉石Fs1.82-27.2(PMD=88),Wo0.18-3.13(PMD=103),铁纹石中Co含量平均0.62%(PMD=20),矿物成份不均一程度高,橄榄石矿物结晶颗粒内部化学成分变化大,呈正环带分布,与岩浆型结晶顺序一致,球粒与基质及间隙物成分明显不同,表现为不同物质来源。化学成分全岩分析结果显示,荷叶塘陨石亲石、亲铁元素含量均为L型陨石特征。依据以上岩石矿物学和化学组成特征,依照陨石亚分类参数,将其类型划分为L3.4型普通球粒陨石。冲击变质程度S2,风化程度W1。研究结果表明荷叶塘陨石为一块受后期水、热蚀变和风化影响较少的原始类型陨石。组成矿物成分极不均一,在矿物晶体内部,球粒内部及球粒与基质间均有明显变化。     

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。     

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。     

普通球粒陨石中磷酸盐矿物的冲击变质特征

普通球粒陨石中的磷酸盐矿物主要有2种,一种是无水磷酸钙——白磷钙石(Ca3[PO4]2),另一种是含氯的磷酸钙——氯磷灰石(Ca5[PO4]3Cl)。我国陨落的L群寺巷口陨石和随州陨石中这2种磷酸盐矿物都有产出,其中以白磷钙石较为常见,而在H群的岩庄陨石中仅见有白磷钙石。我们的研究发     

动态高压下陨石矿物的冲击熔融和分离结晶及其地球化学意义

应用近代微束矿物学分析测试技术,对在中国陨落的随州、寺巷口和岩庄等三块球粒陨石中矿物的冲击熔融和分离结晶作用进行了较系统的研究。查明存在有硅酸盐单矿物熔体、硅酸盐矿物混合熔体和全岩熔体三种组成不同的冲击熔体。研究结果显示:(1)三种熔融相与未熔相之间在主成分和微量元素组成上没有明显差别,说明它们是原地熔融的产物,但在较大的冲击熔块中,也发现冲击熔融作用引发了一部分元素,如亲铁元素、硒元素和轻重稀土元素的分异现象;(2)查明Al2O3、Cr2O3、Na2O和CaO等优先进入从冲击熔体结晶的辉石常压相或辉石的高压相——镁铁榴石(属地幔过渡带矿物)中;(3)Al元素能进入阿基莫石(即辉石的钛铁矿结构相,属下地幔矿物)中,以固熔体形式稳定下来。ELNES的测定查明,其氧化铁组分中Fe3+/∑Fe的比值高达0.67(3);(4)陨石全岩熔体中硅酸盐相与金属-硫化物相之间为完全不混熔,后者以孤立的共结体团块产于硅酸盐熔体之中,除Zn、Na、Cr、Co和Cu在硫化物相中和Na在金属相中有明显富集外,其他元素的浓度则无明显变化;(5)在寺巷口陨石熔脉的金属-硫化物共结团块内发现了FeNi金属-硫化铁-磁铁矿组合,进一步证实了S和O等轻元素可以加入到以Fe-Ni金属为主要组成的地核成分中去;(6)在岩庄陨石的FeNi金属-硫化铁共结体团块的硫化铁内发现了Na、Mn和Fe的磷酸盐矿物小球体,说明P和Na、Mn等元素也能成为地核的组成元素。以上研究在行星演化、地幔矿物学和地球化学,以及陨石学研究上均有着重要的科学意义。     

Chemical and physical studies of type 3 chondrites—IV: Annealing studies of a type 3.4 ordinary chondrite and the metamorphic history of meteorites

Samples of a type 3.4 chondrite have been annealed at 400–1000°C for 1–200 hours, their thermoluminescence properties determined and analyzed for K, Na, Mn, Sc and Ca by instrumental neutron activation analysis. After annealing at ?900°C, the samples showed a 50% decrease in TL sensitivity, while after annealing at 1000°C it fell to 0.1-0.01 times its unannealed value and loss of Na and K occurred. The TL and compositional changes resemble those observed for the equilibrated Kernouve chondrite after similar annealing treatments, except that the sharp TL decrease, and element loss, occurred at ~ 1100°C; this difference is presumably due to petrographic differences in the feldspar of the two meteorites. The temperature and the width of the TL peak showed a discontinuous increase after annealing at 800°C; peak temperature jumped from 130 to 200°C and peak width increased from 90 to 150°C. The activation energies for these TL changes are 7–10 kcal/mole. Similar increases in the TL peak temperature have been reported in TL studies of Amelia, VA, albite, where they were associated with the low to high-temperature transformation. However, the activation energy for the transformation is ~80 kcal/mole. These changes in TL emission characteristics resemble trends observed in type 3 ordinary chondrites and it is suggested that type 3.3–3.5 chondrites have a low-feldspar as TL phosphor and > 3.5 have high-feldspar as the phosphor. Thermoluminescence therefore provides a means of palaeothermometry for type 3 ordinary chondrites.     

天然冲击球粒陨石的化学组成及冲击效应

本文运用电子探针、ＩＮＡＡ方法研究了两块强烈冲击的中国普通球粒陨石的矿物组成、化学组成、冲击熔融相，非溶融相和磁性金属相微量元素丰度，结合稀有气体含量和母体冲击特征，讨论了它们的冲击效应和母体热历史，证明了母体热变质作用叠加了冲击效应，冲击效应增加了陨石矿物组成平衡程度，提高了母体的岩石类型，但冲击热效应对陨石中非气体挥发性元素含量及化学组成没有明显的影响，说明冲击热效应对陨石中非气体挥发性元素含     

Chemical and physical studies of type 3 chondrites—I: Metamorphism related studies of Antarctic and other type 3 ordinary chondrites

Thermoluminescence sensitivity measurements have been made on 18 unequilibrated ordinary chondrites; 12 finds from Antarctica, 5 non-Antarctic finds and 1 fall. The TL sensitivities of these meteorites, normalized to Dhajala, range from 0.034 (St. Mary's County) to 2.3 (Allan Hills A78084), and, based primarily on these data, petrologic type assignments range from 3.3 (St. Mary's County) to 3.9 (Allan Hills A78084). Although the very low levels of metamorphism experienced by types 3.0 to ～3.4 evidently cause large changes in TL sensitivity, the new data demonstrate that they are unable to cause any appreciable homogenization of silicate compositions. We have therefore slightly revised the silicate heterogeneity ranges corresponding to the lower petrologic types.We have discovered that the temperature of maximum TL emission and the broadness of the major TL peak, vary systematically with TL sensitivity; as TL increases these parameters first decrease and then increase. Several mechanisms which could account, partially or completely, for the relationship between TL sensitivity and metamorphism are discussed. Those which involve the formation of feldspar—the TL phosphor in equilibrated meteorites—seem to be consistent with the trends in peak temperature and peak width since experiments on terrestrial albite show that the TL peak broadens and moves to higher temperatures as the stable form changes from the low (ordered) state to the high (disordered) state. (The post-metamorphism equilibration temperature of type ～3.5 meteorites would then correspond to the transformation temperature for the high to low form of meteorite feldspar.) Other factors which may be involved are obscuration of the TL by carbonaceous material, changes in the composition of the phosphor and changes in the identity of the phosphor.     

Chemical and physical studies of type 3 chondrites—VIII: Thermoluminescence and metamorphism in the CO chondrites

The thermoluminescence properties of nine CO chondrites have been measured. With the exception of Colony and Allan Hills A77307 (ALHA 77307), whose maximum induced TL emission is at approximately 350°C, CO chondrites exhibit two TL peaks, one at 124 ± 7°C (130°C peak) and one at 252 ± 7°C (250°C peak). The 130°C peak shows a 100-fold range in TL sensitivity (0.99 ± 0.21 for Isna to 0.010 ± 0.004 for Colony), and correlates with various metamorphism-related phenomena, such as silicate heterogeneity, metal composition and McSween's metamorphic subtypes. The peak at 250°C does not show these correlations and, Colony excepted, varies little throughout the class (0.3 to 0.07, Colony 0.018 ± 0.004). Mineral separation experiments, and a series of annealing experiments on Isna, suggest that the TL properties for CO chondrites reflect the presence of feldspar in two forms, (1) a form produced during metamorphism, and analogous to the dominant form of feldspar in type 3 ordinary chondrites, and (2) a primary, metamorphism-independent form, perhaps associated with the amoeboid inclusions. If this interpretation is correct, then the CO chondrites have not experienced temperatures above the order/disorder temperature for feldspar (500–600°C) and they cooled more slowly than comparable (i.e. type <3.5) type 3 ordinary chondrites. Colony and ALHA 77307 have atypical TL properties, including very low TL sensitivity, suggesting that phosphors other than feldspar are important. They have apparently experienced less metamorphism than the others, and may have also been aqueously altered.     

The New Meteorite Fall of Yanzhuang—A Severely Shocked H6 Chondrite with Black Molten Materials

At 21:45 hr (Beijing time) on Oct. 31, 1990 the Yanzhuang meteorite hit the ground at the Yanzhuang village, Wenyuan County, Guangdong Province. Several fragments, totalling 3.5 kg, were recovered during the field survey. This meteorite is a rare one of its kind due to its heavily shocked features and thick veins made up of black molten materials. Olivine and low-calcium pyroxene are compositionally homogeneous with Fa=18.59, Fs=16.35 and Wo=1.29. The chemical composition (total Fe=28.0%) and recrystallized texture of the chondritic mass show that the Yanzhuang is an H6 chondrite. The black molten materials occur in the form of blocks (up to 2×3×4 cm in size) and veins (0.1–15 mm in width), and contain a lot of rounded and elliptic FeNi-FeS blobs (up to 6–10 mm in length). The metal in these blobs exhibits distinct dendritic structure characteristic of rapid cooling. Unmolten and molten samples are very similar in chemical composition, fitting well with the average H-chondrites. Partial melting and FeNi/FeS-silicate separation have not been observed in the molten materials of the Yanzhuang. This project was financially supported by the Science Foundation of Guangdong Province.     

Impact features of enstatite-rich meteorites

Enstatite-rich meteorites include EH and EL chondrites, rare ungrouped enstatite chondrites, aubrites, a few metal-rich meteorites (possibly derived from the mantle of the aubrite parent body), various impact-melt breccias and impact-melt rocks, and a few samples that may be partial-melt residues ultimately derived from enstatite chondrites. Members of these sets of rocks exhibit a wide range of impact features including mineral-lattice deformation, whole-rock brecciation, petrofabrics, opaque veins, rare high-pressure phases, silicate darkening, silicate-rich melt veins and melt pockets, shock-produced diamonds, euhedral enstatite grains, nucleation of enstatite on relict grains and chondrules, low MnO in enstatite, high Mn in troilite and oldhamite, grains of keilite, abundant silica, euhedral graphite, euhedral sinoite, F-rich amphibole and mica, and impact-melt globules and spherules. No single meteorite possesses all of these features, although many possess several. Impacts can also cause bulk REE fractionations due to melting and loss of oldhamite (CaS) – the main REE carrier in enstatite meteorites. The Shallowater aubrite can be modeled as an impact-melt rock derived from a large cratering event on a porous enstatite chondritic asteroid; it may have been shock melted at depth, slowly cooled and then excavated and quenched. Mount Egerton may share a broadly similar shock and thermal history; it could be from the same parent body as Shallowater. Many aubrites contain large pyroxene grains that exhibit weak mosaic extinction, consistent with shock-stage S4; in contrast, small olivine grains in some of these same aubrites have sharp or undulose extinction, consistent with shock stage S1 to S2. Because elemental diffusion is much faster in olivine than pyroxene, it seems likely that these aubrites experienced mild post-shock annealing, perhaps due to relatively shallow burial after an energetic impact event. There are correlations among EH and EL chondrites between petrologic type and the degree of shock, consistent with the hypothesis that collisional heating is mainly responsible for enstatite-chondrite thermal metamorphism. Nevertheless, the apparent shock stages of EL6 and EH6 chondrites tend to be lower than EL3-5 and EH3-5 chondrites, suggesting that the type-6 enstatite chondrites (many of which possess impact-produced features) were shocked and annealed. The relatively young Ar–Ar ages of enstatite chondrites record heating events that occurred long after any ²⁶Al that may have been present initially had decayed away. Impacts remain the only plausible heat source at these late dates. Some enstatite meteorites accreted to other celestial bodies: Hadley Rille (EH) was partly melted when it struck the Moon; Galim (b), also an EH chondrite, was shocked and partly oxidized when it accreted to the LL parent asteroid. EH, EL and aubrite-like clasts also occur in the polymict breccias Kaidun (a carbonaceous chondrite) and Almahata Sitta (an anomalous ureilite). The EH and EL clasts in Kaidun appear unshocked; some clasts in Almahata Sitta may have been extensively shocked on their parent bodies prior to being incorporated into the Almahata Sitta host.     

Thermoluminescence of meteorites and their terrestrial ages

A new technique is described in which the natural thermoluminescence (TL) in chondritic meteorites is normalized to the TL sensitivity of each meteorite. The relationship between the normalized TL level and a meteorite's terrestrial age is examined. The average normalized TL level measured in 45 ordinary chondrites of known terrestrial age decreases with increasing terrestrial age. There is a factor of ten variation in the TL levels of fresh falls which is primarily due to differences in orbital temperatures (Melcher, 1981). Nevertheless, it is possible to distinguish between recent falls and meteorites greater than a few hundred years old if they have been stored at the same temperature. The decay characteristics of the TL are studied by means of initial rise experiments and isothermal decay experiments at elevated temperatures. The TL levels of 11 Antarctic meteorites are compared with the results of ¹⁴C, ²⁶Al and ³⁶Cl studies of the terrestrial ages of these objects. There is a rough correlation between the low temperature TL levels and the radionuclide activities. Since the TL method is quick and requires as little as 10mg of material, it is most valuable as a rapid screening process to select samples appropriate for study by other techniques.     

Studies of an artificially shock-loaded H group chondrite

Five samples of the naturally unshocked Kernouve (H6) meteorite were artificially shock-loaded to pressures of 70, 165, 270, and 390 kbar and the silicates and metal examined optically, by scanning and transmission electron microscopy and by thermoluminescence (TL). Olivine deformation is closely comparable to that in naturally shocked meteorites, producing dislocations with Burgers vector [001]. At pressures of ?165 kbar, these are formed in well-defined slip planes. At 270 kbar, olivine develops optical mosaicism, has high dislocation densities throughout and is also highly fractured. Recovery, due to heating is minimal. In orthopyroxene, the deformation mechanism changes, from the clino-inversion to unit-dislocation slip, between 70 and 165 kbar. In diopside, (001) and (100) twinning was produced. Plagioclase is inferred to have been progressively converted to maskelynite, but some is still present in 270 kbar sample.The microhardness of the kamacite in the samples increases with shock pressure. The α → ? transformation pressure in the kamacite is 30–40 kbar higher than observed for iron meteorites. Annealed kamacite displays incipient polycrystallinity and α-martensite and taenite sometimes contains slip lines. Troilite acquired cracks, undulose extinction, twins, polycrystallinity and finally melted as the shock pressure increased.At pressures over 200 kbar there was a systematic decrease in the natural TL and the TL sensitivity. Detailed considerations of changes in the natural $\text{TL}\text{TL}$ sensitivity ratio for various regions of the TL glow curve suggest that two processes were effective during shock; thermal drainage of electron traps and a reduction in the effective trap density. It is suggested that the latter process associated with the vitrification of feldspar, the TL phosphor.An additional sample was subjected to a shock pulse which was “spiked” instead of square. Very distinctive changes were apparent; thermal effects are conspicuous and with widespread annealing (~600–800°C) of metal and sulfide. Glassy, opaque veins were produced which are analogous to the black veins in shock-lithified gas-rich meteorites. Anomalous low-temperature TL was induced, suggesting that a new or modified phase or mineral has become the dominant TL phosphor.     

Chemical studies of L chondrites—III. Mobile trace elements and 40Ar39Ar ages

We report data for trace elements Ag, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Se, Te, Tl and Zn determined by radiochemical neutron activation analysis in L4–6 chondrites with undisturbed ⁴⁰Ar release patterns or with patterns showing some disturbance in the 4.4–4.6 Gyr plateau indicating shock-induced loss. Mean concentrations are lower, many significantly so, in 16 chondrites with disturbed patterns than in 4 with undisturbed ones, consistent with shock-induced mobilization. Similar trends were noted earlier in L4–6 chondrites having mineralogically observable shock indicators: mean concentrations are lower in strongly shocked (i.e. > 22 GPa) than in mildly shocked (<22 GPa) samples. From trace element contents, L4–6 chondrites with undisturbed ⁴⁰Ar release patterns are mildly shocked but chondrites with disturbed patterns are more strongly heated, on average, than those of shock facies d-f (i.e. 22 to > 57 GPa). Pooling these populations, significantly lower mean concentrations of nearly all trace elements in 26 strongly shocked L4–6 chondrites than in 14 mildly shocked ones indicate loss in shock-formed FeS-Fe eutectic and/or by vaporization during cooling of shock-heated collisional debris. Two-element correlations and the pattern of them, i.e. correlation profiles, are also consistent with this picture. Trace elements can act as thermometers for collisional episodes in L4–6 chondrites but not for earlier thermal fractionations, unless compensation can be made for late shock heating.     

Certain peculiarities in the distribution of mercury in meteorites

This paper presents chemical analyses of mercury in 123 specimens of 58 meteorites. A statistical comparison is made of the mercury content in meteorites (using all available data) as a function of their composition and texture.The average mercury abundance in stony meteorites is estimated as 6.6 ppm. Stony and iron meteorites differ significantly in mercury content. Among stones, chondrites and achondrites show significant differences in the mercury content. In ordinary cbondrites, mineralogy and texture do not seem to have a significant influence on the distribution of mercury, judging from the available number of analyses. Carbonaceous chondrites, in which we found up to 500 ppm mercury, stand out among all other varieties of chondrites. Iron meteorites fall at the other extreme, having the lowest mercury concentrations (generally 0-0x ppm). In specimens of several meteorite classes, troilite is a good mercury concentrator, having a consistently higher mercury content than the meteorite as a whole. Nonetheless, troilite generally accounts for less than one-half the total mercury content of the meteorite.