Metal-troilite-magnetite assemblage in shock veins of Sixiangkou meteorite

The first natural occurrence of metal-troilite-magnetite assemblage composed of Fe-Ni metal and magnetite dendrites and a groundmass of troilite was identified in the shock veins of the Sixiangkou L6 chondrite, which contain abundant high-pressure minerals. This assemblage suggests a liquid miscibility among metal, FeS, and iron oxide, and subsequently quenching under pressure. Components of magnetite could be, in origin, related to chromite that was embedded and dissolved in an Fe-Ni-S liquid. Cr₂O₃ dissociated from chromite was mainly incorporated into garnet and magnesiowüstite in the fine-grained matrix of shock veins, in which chromium behaves as a lithophile element at the P-T conditions experienced by the shock veins. The occurrence of metal-troilite-magnetite assemblage suggests that the shock veins were still under pressure at temperatures from 900 to 950°C during solidification of Fe-Ni-S or Fe-Ni-S-O liquid, hence indicating a long duration of high pressure in the shock veins.     

Natural CaTi2O4-structured FeCr2O4 polymorph in the Suizhou meteorite and its significance in mantle mineralogy

The first natural occurrence of a high-pressure polymorph of chromite has been discovered in the shock-metamorphosed Suizhou meteorite. The composition of this high-pressure polymorph is identical to that of the precursor chromite. The Raman spectrum of this polymorph is distinct from that of chromite. Synchrotron X-ray diffraction analysis revealed that this polymorph has an orthorhombic CaTi₂O₄- type structure. The cell parameters are: a = 9.462(6) Å, b = 9.562(9) Å, c = 2.916(1) Å, V = 263.8(4) ų (Z=4), space group = Bbmm, and the density = 5.63 g/cm³ (the numbers in parentheses are standard deviations on the last significant digits). This polymorph is 11.5% denser than chromite. The P-T conditions for the phase transformation from chromite to the CaTi₂O₄-structured polymorph are estimated at 20-23 GPa and 1800 to 2000 °C, respectively. This dense CaTi₂O₄-structured FeCr₂O₄ phase could be a host phase for Cr, Al, Fe, Mg and Mn and other metallic elements in the deep Earth.     

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.     

Compression studies of gibbsite and its high-pressure polymorph

Various X-ray diffraction methods have been applied to study the compression behavior of gibbsite, Al(OH)₃, in diamond cells at room temperature. A phase transformation was found to take place above 3 GPa where gibbsite started to convert to its high-pressure polymorph. The high-pressure (HP) phase is quenchable and coexists with gibbsite at the ambient conditions after being unloaded. This HP phase was identified as nordstrandite based on the diffraction patterns obtained at room pressure by angle dispersive and energy dispersive methods. On the basis of this structural interpretation, the bulk modulus of the two polymorphs, i.e., gibbsite and nordstrandite, could be determined as 85 ± 5 and 70 ± 5 GPa, respectively, by fitting a Birch-Murnaghan equation to the compression data, assuming their K_o ^′ as 4. Molar volume cross-over occurs at 2 GPa, above which the molar volume of nordstrandite is smaller than that of gibbsite. The differences in the molar volume and structure between the two polymorphs are not significant, which accounts for the irreversibility of the phase transition. In gibbsite, the axial compressibility behaves as c/c _o > a/a _o > b/b _o. This is due to the fact that the dioctahedral sheets along the c-axis are held by the relatively weak hydrogen bonding, which results in the greater compressibility along this direction. In nord- strandite, the axial compressibility is b/b _o > c/c _o > a/a _o, which can also be interpreted as resulting from the the existence of hydrogen bonds along the b-axis. Received: 28 September 1998 / Revised, accepted: 22 December 1998     

A new high-pressure and high-temperature polymorph of FeS

A new polymorph of FeS has been observed at pressures above 30 GPa at 1,300 K by in situ synchrotron X-ray diffraction measurements in a laser-heated diamond anvil cell. It is stable up to, at least, 170 GPa at 1,300 K. The new phase (here called FeS VI) has an orthorhombic unit cell with lattice parameters a = 4.8322 (17) Å, b = 3.0321 (6) Å, and c = 5.0209 (8) Å at 85 GPa and 300 K. Its topological framework is based on the NiAs-type structure as is the case for the other reported polymorphs (FeS I-V). The unit cell of FeS VI is, however, more distorted (contracted) along the [010] direction of the original NiAs-type cell. For example, the c/b axial ratio is ～1.66 at 85 GPa and 300 K, which is considerably smaller than that of orthorhombic FeS II (～1.72) and NiAs-type hexagonal FeS V (=√3 ≈ 1.73). The phase boundary between FeS IV and VI is expected to be located around 30 GPa at 1,300 K. The phase transition is accompanied by gradual and continuous changes in volume and axial ratios and may be second order. At room temperature, FeS VI becomes stable over FeS III at pressures above 36 GPa. It is, therefore, suggested that the phase boundary of FeS III–VI and/or FeS IV–VI has negative pressure dependence.     

Transmission electron microscopic study of the fine-grained vein matrix in the Suizhou L6 meteorite

The mineralogy of shock vein matrix in the Suizhou meteorite has been investigated by optical and transmission electron microscopy. It was revealed that the vein matrix is composed of majorite-pyrope garnet, magnesiowüstite, and ringwoodite, with FeNi–FeS intergrowths. The observation and character of ring-like selected electron diffraction (SAED) patterns indicate that the idiomorphic garnet crystals in the vein matrix have different orientations. The polycrystalline nature of magnesiowüstite is also confirmed by a ring-like SAED pattern. Both garnet and magnesiowüstite crystals showed sharp diffraction spots, signifying the good crystallinity of these two minerals. The SAED pattern of cryptocrystalline ringwoodite shows only diffuse concentric diffraction rings. FeNi metal and troilite (FeS), which were molten during the shock event, occur in the matrix as fine eutectic FeNi–FeS intergrowths filling the interstices between garnet and magnesiowüstite grains. Based on the phase diagram of the Allende chondrite and the results of this TEM study, it is inferred that majorite-pyrope garnet first crystallized from the Suizhou chondritic melt at 22–26 GPa, followed by crystallization of magnesiowüstite at 20–24 GPa, and then ringwoodite at 18–20 GPa. The eutectic intergrowths of FeNi-metal and troilite are proposed to have crystallized during meteorite cooling and solidified at the last stage of vein formation.     

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。     

A modeling of the structure and favorable H-docking sites and defects for the high-pressure silica polymorph stishovite

Employing first-principles methods, the docking sites for H were determined and H, Al, and vacancy defects were modeled with an infinite periodic array of super unit cells each consisting of 27 contiguous symmetry nonequivalent unit cells of the crystal structure of stishovite. A geometry optimization of the super-cell structure reproduces the observed bulk structure within the experimental error when P1 translational symmetry was assumed and an array of infinite extent was generated. A mapping of the valence electrons for the structure displays mushroom-shaped isosurfaces on the O atom, one on each side of the plane of the OSi₃ triangle in the nonbonded region. An H atom, placed in a cell near the center of the super cell, was found to dock upon geometry optimization at a distance of 1.69 Å from the O atom with the OH vector oriented nearly perpendicular to the plane of the triangle such that the OH vector makes a angle of 91° with respect to [001]. However, an optimization of a super cell with an Al atom replacing Si and an H atom placed nearby in a centrally located cell resulted in an OH distance of 1.02 Å with the OH vector oriented perpendicular to [001] as observed in infrared studies. The geometry-optimized position of the H atom was found to be in close agreement with that (0.44, 0.12, 0.0) determined in an earlier study of the theoretical electron density distribution. The docking of the H atom at this site was found to be 330 kJ mol^–1 more stable than a docking of the atom just off the shared OO edge of the octahedra as determined for rutile. A geometry optimization of a super cell with a missing Si generated a vacant octahedra that is 20% larger than that of the SiO₆ octahedra. The valence electron density distribution displayed by the two-coordinate O atoms that coordinate the vacant octahedral site is very similar to those displayed by the bent SiOSi angles in coesite. The internal distortions induced by the defect were found to diminish rather rapidly with distance, with the structure annealing to that observed in the bulk crystal to within about three coordination spheres.     

Constraints from high-pressure veins in eclogites on the composition of hydrous fluids in subduction zones

Hydrous high-pressure veins formed during dehydration of eclogites in two paleo-subduction zones (Trescolmen locality in the Adula nappe, central Alps and Münchberg Gneiss Massif, Variscan fold belt, Germany) constrain the major and trace element composition of solutes in fluids liberated during dehydration of eclogites. Similar initial isotopic compositions of veins and host eclogites at the time of metamorphism indicate that the fluids were derived predominantly from the host rocks. Quartz, kyanite, paragonite, phengite, zoisite and omphacite are the dominant minerals in the veins. The major element compositions of the veins are in agreement with experimental evidence indicating that the composition of solutes in such fluids is dominated by SiO₂ and Al₂O₃. Relative to N-MORB, the veins show enrichments of Cs, Rb, Ba, Pb, and K, comparable or slightly lower abundances of Sr, U, and Th, and very low abundances of Nd, Sm, Zr, Nb, Ti and Y. The differential fractionation of highly incompatible elements such as K, U and Th in the veins, as well as the presence of hydrous minerals in the eclogites rule out partial melting as a cause for vein formation. These results confirm previous suggestions that fluids derived from subducted basalt may have low abundances of high field strength elements, rare earth elements and Y. Variable vein-eclogite enrichment factors of incompatible alkalis and to a lesser extent Pb appear to reflect mineralogical controls (phengite, epidote-group minerals) on partitioning of these elements during dehydration of eclogite in subduction zones. However, abundance variations of incompatible elements in minerals from eclogites suggest that the composition of fluids released from eclogites at temperatures <700°C may not reflect true equilibrium partitioning during dehydration. Simple models for the trace elements U and Th indicate the relative importance of the basaltic and sedimentary portions of subducted oceanic crust in producing the characteristic chemical signatures of these elements in convergent plate margin volcanism.     

Time-resolved measurement of high-pressure phase transition of fluorite under shock loading

In situ time-resolved measurements of shock wave profiles for anisotropic fluorite crystals with two different crystal orientations were carried out up to a pressure of 34 GPa that is above the transition pressure for the fluorite to cotunnite phase. They indicate that the Hugoniot elastic limit varies with the crystal orientation and final pressure and that high-pressure phase transition from fluorite to a cotunnite-type structure occurs at 13 GPa in 10–20 ns for CaF₂ [100]-oriented crystals and at 17 GPa in more than 50 ns for CaF₂ [111]-oriented crystals, respectively. These results are in disagreement with those from static compression. The phase transition at static pressures has been known to be very sluggish, but the present results indicate a large sensitivity of kinetics to the relationship between crystallographic orientation and shock direction, supporting a martensitic mechanism for the fluorite to cotunnite phase transition that is enhanced by the effect of shock-induced shear. It is also helpful to explain the observation that the in situ emission spectra for shocked Eu-doped fluorite became weak and had no shift above ~15 GPa.     

Thermoluminescence and the shock and reheating history of meteorites—II: Annealing studies of the Kernouve meteorite

Samples of the unshocked, equilibrated chondrite, Kernouve (H6), have been annealed for 1–100 hours at 500–1200°C, their thermoluminescence sensitivity measured and Na, K, Mn, Ca and Sc determined by instrumental neutron activation analysis. The TL sensitivity decreased with temperature until by 1000°C it had fallen by 40%. The process responsible has an activation energy of ~8 kcal/mole and probably involves diffusion. Samples annealed 1000–1200°C had TL sensitivities 10^?2 times the unannealed values, most of the decrease occurring ~1100°C. This process has an activation energy of ~100 kcal/mole and is probably related to the melting of the TL phosphor, feldspar, with some decomposition and loss of Cs, Na and K. Meteorites whose petrography indicates healing > 1100°C by natural shock heating events (shock facies d-f). have TL sensitivities similar to samples annealed > 1100°C. Our own and literature compositional data indicate that TL is more stable to annealing than Ag, In, Tl, Bi, Zn and Te and less stable than Na, K, Mn, Ca, Se and Co, while the TL decrease resembles very closely the pattern of Cs loss on annealing.     

Thermochemistry of the new silica polymorph moganite

Samples of microcrystalline silica varieties containing variable amounts of the new silica polymorph moganite (up to R~82 wt.%) have been studied by a combination of high temperature solution calorimetry using lead borate (2 PbO · B₂O₃) solvent and transposed temperature drop calorimetry near 977 K, in order to investigate the thermochemical stability of this new silica mineral. The enthalpy of solution at 977 K and the heat content (H₉₇₇ — H₂₉₈) of “pure” moganite phase were estimated to be -7.16 ± 0.35 kJ/mol and 43.62 ± 0.50 kJ/mol, respectively. The standard molar enthalpy of formation is-907.3 ± 1.2 kJ/mol. Thus, calorimetry strongly supports results of previous X-ray and Raman spectroscopic studies that moganite is a distinct silica polymorph. Its thermochemical instability relative to quartz at 298 K of 3.4 ± 0.7 kJ/mol is marginally higher than those of cristobalite and tridymite. Structurally, this instability may be related to the presence of distorted 4-membered rings of SiO₄ tetrahedra, which are not found in the quartz structure. The metastability relative to quartz may also explain the apparent scarcity of moganite in altered rocks and in rocks that are older than 130 my.     

Amino acids in the Murchison meteorite

Continued investigation of the Murchison meteorite by gas chromatography combined with mass spectrometry has led to the characterization of at least 17 amino acids in addition to the 18 identified in earlier work. The total population consists of a wide variety of linear and cyclic difunctional and polyfunctional amino acids, of which the linear difunctional amino acids show a general decrease in concentration as the number of carbon atoms in the amino acid molecule increases. These results are consistent with the hypothesis that the amino acids are present as the result of an extraterrestrial, abiotic synthesis.     

Refractory inclusions in the Murchison meteorite

Mineralogical and petrographic studies of a wide variety of refractory objects from the Murchison C2 chondrite have revealed for the first time melilite-rich and feldspathoid-bearing inclusions in this meteorite, but none of these is identical to any inclusion yet found in Allende. Blue spinel-hibonite spherules have textures indicating that they were once molten, and thus their SiO₂-poor bulk composition requires that they were exposed to higher temperatures (>1550°C) than those deduced so far from any Allende inclusion. Melilite-rich inclusions are similar to Allende compact Type A's, but are more Al-, Ti-rich. One inclusion (MUCH-1) consists of a delicate radial aggregate of hibonite crystals surrounded by alteration products, and probably originated by direct condensation of hibonite from the solar nebular vapor. The sinuous, nodular and layered structures of another group of inclusions, spinel-pyroxene aggregates, suggest that these also originated by direct condensation from the solar nebular gas. Each type of inclusion is characterized by a different suite of alteration products and/or rim layers from all the other types, indicating modification of the inclusions in a wide range of different physico-chemical environments after their primary crystallization. All of these inclusions contain some iron-free rim phases. These could not have formed by reaction of the inclusions with fluids in the Murchison parent body because the latter would presumably have been very rich in oxidized iron. Other rim phases and alteration products could have formed at relatively low temperatures in the parent body, but some inclusions were not in the locations in which they were discovered when this took place. Some of these inclusions are too fragile to have been transported from one region to another in the parent body, indicating that low temperature alteration of these may have occurred in the solar nebula.     

Nitrogen and water bubbles, oxygen isotopes, shock effects: Deciphering the history of the Bencubbin meteorite breccia

We have observed vesicles filled with heavy nitrogen gas and water vapor in three settings in the Bencubbin CB chondrite: in the mesostasis of the silicate clasts, in the mesostasis of the chondrules of an ordinary chondrite (OC) xenolith, and in grains we refer to as bubble grains, and interpret as remelted OC chondrule mesostasis. In our view, these bubbles are a local phenomenon and formed as a consequence of the impact of the OC fragment onto the Bencubbin parent body. The heavy nitrogen in the bubbles came from one or several of its carrier phases in Bencubbin, and the water came from hydrous silicates. As formulated by Meibom et al. (Meibom A., Righter K., Chabot N., Dehn G., Antignano A., McCoy T. J., Krot A. N., Zolensky M. E., Petaev M. I. and Keil K. (2005) Shock melts in QUE 94411, Hammadah al Hamra 237, and Bencubbin: remains of the missing matrix? Meteorit. Planet. Sci.40, 1377-1391) these hydrous phases, similar to the hydrated clasts now found in CH and CB_b chondrites, were probably common in the Bencubbin parent body at that time. They were later almost totally destroyed by a large scale shock event, and contributed to form the impact melt that now fills space in between the large clasts of Bencubbin. Our observations indirectly confirm this hypothesis. From our composition measurements, we infer that the silicate part of the impact melt was made in roughly equal proportions of melted phyllosilicates and melted anhydrous silicates. The oxygen isotopic composition of the impact melt is much heavier than that of the silicate clasts, probably reflecting the composition of the water at the origin of the phyllosilicates. The O isotope measurements of the OC inclusion chondrules present some features that seem to be common in OCs: a composition of the chondrule crystals slightly lighter than that of whole chondrules, and one olivine crystal with a very light composition.     

Stereoisomers of isovaline in the Murchison meteorite

Isovaline is present in the Murchison meteorite as a racemic mixture (about equal concentrations of the R and S enantiomers). Since isovaline does not have a hydrogen atom on its asymmetric α-carbon atom, the racemic mixture could not have formed by commonly accepted mechanisms of racemization. Thus, isovaline in the meteorite most probably was synthesized as a racemic mixture and is not the result of the racemization of either the R or S enantiomer. Other chiral amino acids in the meteorite are present as racemic mixtures, and were probably synthesized in a similar manner by abiotic, extraterrestrial processes.     

Noble gases in the unique Kaidun meteorite

Two examined fragments of the Kaidun meteorite principally differ in the concentrations of isotopes of noble gases and are very heterogeneous in terms of the isotopic composition of the gases. Because these fragments belong to two basically different types of meteoritic material (EL and CR chondrites), these characteristics of noble gases could be caused by differences in the cosmochemical histories of the fragments before their incorporation into the parent asteroid. As follows from the escape kinetics of all gases, atoms of trapped and cosmogenic noble gases are contained mostly in the structures of two carrier minerals in the samples. The concentrations and proportions of the concentrations of various primary noble gases in the examined fragments of Kaidun are obviously unusual compared to data on most currently known EL and CR meteorites. In contrast to EL and CR meteorites, which contain the primary component of mostly solar provenance, the elemental ratios and isotopic composition of Ne and He in the fragments of Kaidun correspond to those typical of the primary components of A and Q planetary gases. This testifies to the unique conditions under which the bulk of the noble gases were trapped from the early protoplanetary nebula. The apparent cosmic-ray age of both of the Kaidun fragments calculated based on cosmogenic isotopes from ³He to ¹²⁶Xe varies from 0.027 to 246 Ma as a result of the escape of much cosmogenic isotopes at relatively low temperatures. The extrapolated cosmic-ray age of the Kaidun meteorite, calculated from the concentrations of cosmogenic isotopes of noble gases, is as old as a few billion years, which suggests that the material of the Kaidun meteorite could be irradiated for billions of years when residing in an unusual parent body.     

Purines and triazines in the Murchison meteorite

Two samples of the Murchison C2 chondrite were examined for organic nitrogen compounds, using mass spectrometry as well as paper and thin-layer chromatography. Under mild extraction conditions (water or formic acid) only aliphatic amines and C₂-C₆ alkylpyridines were seen; the latter may be contaminants. Under drastic extraction conditions (hot, 3–6 M HCl or CF₃COOH), a variety of basic nitrogen compounds appeared, in the following amounts (ppm): adenine (15), guanine (5), melamine (20), cyanuric acid (20–30), guanylurea (30–45), urea (25), etc. Apparently these compounds are present mainly in macromolecular material, and are released only upon acid hydrolysis.These findings support our earlier identifications of these compounds in the Orgueil meteorite. They also suggest that the recent failure by Folsomeet al. (Nature232, 108–109, 1971; Geochim. Cosmochim. Acta37, 455–465, 1973) to find purines or triazines in carbonaceous chondrites was due to inadequate extraction conditions: water and formic acid, rather than HCl. Conversely, we were unable to detect the principal compound class reported by Folsome et al.: 4-hydroxypyrimidines.     

Amoeboid olivine aggregates in the Allende meteorite

Greyish-brown, irregularly-shaped aggregates composed predominantly of olivine make up ~2% of the Allende meteorite by volume. Many of the aggregates are constructed of subspherical lumps of micron-sized crystals of olivine, pyroxene, nepheline and sodalite surrounded by coarsergrained olivine. Rarely, anorthite, spinel and perovskite are also present. The olivine ranges in composition from Fo64 to Fo99. Pyroxenes range from aluminous diopside to hedenbergite to very Al-rich and Ti-Al-rich varieties. The nepheline contains 1.6–2.4% K₂O and 1.6–5.2% CaO but the sodalite is significantly poorer in these elements. The spinel contains 2.1–13.4% FeO. Textural information and oxygen isotopic data suggest that the aggregates are composed of primary, solid condensates from the solar nebula. The perovskite. spinel and Ti-Al-rich pyroxenes are the remains of high-temperature condensates but the olivine compositions and the presence of feldspathoids indicate that some of the grains continued to react with the solar nebular vapor in the temperature range 500–900°K.