Pyroxene-garnet transformation in coorara meteorite

Majorite is a new garnet in a veinlet of the Coorara meteorite. Its chemical composition is compatible with derivation mostly from original pyroxene, not from olivine as originally reported. Silicon is partly in sixfold coordination. Ringwoodite, a spinel of olivine composition, occurs as purple grains set in a matrix of fine-grained garnet. The similar mineralogy and texture of the Coorara and Tenham meteorites suggest a common parent body.     

First occurrence of the garnet-ilmenite transition in silicates

Pyrope garnet (Mg(3)Al(2)Si(3)O(12)) has been found to transform to an ilmenite-type phase at a loading pressure between 240 and 250 kilobars and at about 1000 degrees to 1400 degrees C in a diamond-anvil press coupled with laser heating. The lattice parameters for the ilmenite-type phase of (Mg(.75) Al(.25))(Si(.75) Al(.25))O(3) are a(0) = 4.755 +/- 0.002 and c(0) = 13.360 +/- 0.005 angstroms. The zero-pressure volume change associated with the garnet-ilmenite transition is calculated to be -7.1 percent. This result verifies the prediction that pyrope garnet would transform to the ilmenite structure at high pressure first suggested in 1962 by Clark et al. and Ringwood.     

Chondritic meteorite fragments associated with the Permian-Triassic boundary in Antarctica

Multiple chondritic meteorite fragments have been found in two sedimentary rock samples from an end-Permian bed at Graphite Peak in Antarctica. The Ni/Fe, Co/Ni, and P/Fe ratios in metal grains; the Fe/Mg and Mn/Fe ratios in olivine and pyroxene; and the chemistry of Fe-, Ni-, P-, and S-bearing oxide in the meteorite fragments are typical of CM-type chondritic meteorites. In one sample, the meteoritic fragments are accompanied by more abundant discrete metal grains, which are also found in an end-Permian bed at Meishan, southern China. We discuss the implications of this finding for a suggested global impact event at the Permian-Triassic boundary.     

Crystallization of some lunar mafic magmas and generation of rhyolitic liquid

During crystallization of lunar crystalline rocks 10022 and 10024, augite changes in composition almost continuously from titanaugite (Ca(36)Mg(47) Fe, (17) with TiO(2) 3 percent) to a very iron-rich variety (Ca(9)Mg(5)Fe(86)), pigeonite changes from Ca(9)Mg(66)Fe(25) to Ca(1O)Mg(51)Fe(39), and olivine changes (in 10022) from Mg(71)Fe(29) to Mg(41)Fe(59), whereas plagioclase stays as bytownite. These compositional variations, as well as the textural relations, may be explained by rapid crystallization of undercooled magmas. The residual liquids found as mesostasis are rhyolitic, which suggests that fractional crystallization of some lunar mafic magmas can generate rhyolitic magmas. Melting experiments were made on crystalline rocks to determine liquidus temperatures and crystallizing phases.     

The Effect of H2O on the 410-Kilometer Seismic Discontinuity

The 410-kilometer seismic discontinuity is generally considered to be caused by a phase transformation of the main constituent of the upper mantle, olivine, alpha-(Mg,Fe)(2)SiO(4), to beta-(Mg,Fe)(2)SiO(4). Recent data show that H(2)O dissolves in olivine and other nominally anhydrous mantle minerals and that the partitioning of H(2)O between olivine and beta-(Mg,Fe)(2)SiO(4) is about 1:10. Such behavior strongly affects the region over which the alpha to beta phase transformation occurs and hence the seismic discontinuity that results. The observed width of the discontinuity constrains the maximum H(2)O content of upper mantle olivine to about 200 parts per million by weight.     

Catalysis of the olivine to spinel transformation by high clinoenstatite

Although enstatite is a major constituent of the Earth's upper mantle and subducting lithosphere, most kinetic studies of olivine phase transformations have typically involved single-phase polycrystalline aggregates. Transmission electron microscopy investigations of olivine to spinel and modified spinel (beta phase) reactions in the (Mg, Fe)(2)SiO(4)-(Mg,Fe)SiO(3) system show that transformation of olivine in the stability field of spinel plus phase begins with coherent nucleation of spinel on high-clinoenstatite grains. These observations demonstrate that high clinoenstatite can catalyze the transformation by enhancing nucleation kinetics and therefore imply that secondary phases can influence reaction kinetics during high-pressure mineral transformations.     

Melting of (Mg,Fe)2SiO4 at the Core-Mantle Boundary of the Earth

The lower mantle of the Earth is believed to be largely composed of (Mg,Fe)O (magnesiowustite) and (Mg,Fe)SiO3 (perovskite). Radiative temperatures of single-crystal olivine [(Mg0.9,Fe0.1)2SiO4] decreased abruptly from 7040 +/- 315 to 4300 +/- 270 kelvin upon shock compression above 80 gigapascals. The data indicate that an upper bound to the solidus of the magnesiowustite and perovskite assemblage at 4300 +/- 270 kelvin is 130 +/- 3 gigapascals. These conditions correspond to those for partial melting at the base of the mantle, as has been suggested occurs within the ultralow-velocity zone beneath the central Pacific.     

Amphibole: first occurrence in a meteorite

This is the first report of an amphibole mineral found in any meteorite. The amphibole richterite (soda tremolite), Na(2)Ca(Mg, Fe)(5)Si(8)O(22)(OH, F)(2), occurs as a primary (preterrestrial) mineral enclosed within graphite nodules in the iron meteorite from Wichita County, Texas.     

Elemental compositions of comet 81P/Wild 2 samples collected by Stardust

We measured the elemental compositions of material from 23 particles in aerogel and from residue in seven craters in aluminum foil that was collected during passage of the Stardust spacecraft through the coma of comet 81P/Wild 2. These particles are chemically heterogeneous at the largest size scale analyzed ( approximately 180 ng). The mean elemental composition of this Wild 2 material is consistent with the CI meteorite composition, which is thought to represent the bulk composition of the solar system, for the elements Mg, Si, Mn, Fe, and Ni to 35%, and for Ca and Ti to 60%. The elements Cu, Zn, and Ga appear enriched in this Wild 2 material, which suggests that the CI meteorites may not represent the solar system composition for these moderately volatile minor elements.     

Precision Lattice-Parameter Determination of (Mg,Fe)SiO3 Tetragonal Garnets

The tetragonal garnet (Mg,Fe)SiO(3) is a high-pressure phase of pyroxene that is thought to be a major constituent of the earth's upper mantle. Its crystal structure is similar to that of cubic garnet, but it is slightly distorted to tetragonal symmetry so that its x-ray powder diffraction pattern shows a very small line splitting. A suite of tetragonal garnets with different compositions in the MgSiO(3)-rich portion of the MgSiO(3)-FeSiO(3) system was synthesized at about 20 gigapascals and 2000 degrees C. The lattice parameters a and c of quenched samples were determined by whole-powder-pattern decomposition analysis of Fe Kalpha x-ray powder diffraction data, which has the capacity to resolve to a high degree heavily overlapping reflections. It was found that the lattice parameters can be obtained from the following equations; a (in angstroms) = 11.516 + 0.088x and c (in angstroms) = 11.428 + 0.157x, where x, teh mole fraction of FeSiO(3), is 0.0 相似文献   

Exsolution in ferromagnesian olivine of the divnoe meteorite

Ferromagnesian olivine, one of the most common minerals in the solar system, has been widely regarded as a continuous solid solution, although several thermodynamic analyses have suggested the possibility of a miscibility gap at low temperatures. Natural ferromagnesian olivine from the Divnoe meteorite contains compositionally different exsolution lamellae, providing direct evidence for the existence of a miscibility gap in iron-magnesium olivine solid solutions.     

Crystallization studies of lunar igneous rocks: crystal structure of synthetic armalcolite

Crystals of armalcolite, Mg(0.5)Fe(0.5)Ti(2)O(5), up to several millimeters in length have been grown from a glass initially having the composition of lunar rock 10017. A single-crystal x-ray study has confirmed that the crystals are isomorphous with pseudobrookite and has shown that the cations are strongly ordered, with the Ti(4+) ions occupying the 8f sites and the Fe(2+) and Mg(2+) ions randomly distributed over the 4c sites. An examination of karrooite, MgTi(2)O(5), has revealed a similar distribution of Mg(2+) and Ti(4+) ions. A reexamination of earlier x-ray and M?ssbauer data for pseudobrookite, Fe(2)TiO(5), has shown that it is more consistent with this type of ordering than with the inverse structure that has been generally assumed.     

Shock-produced olivine glass: first observation

Transmission electron microscope (TEM) observations of an experimentally shock-deformed single crystal of natural peridot, (Mg(0.88)Fe(0.12))(2)SiO(4), recovered from peak pressures of about 56 x 10(9) pascals revealed the presence of amorphous zones located within crystalline regions with a high density of tangled dislocations. This is the first reported observation of olivine glass. The shocked sample exhibits a wide variation in the degree of shock deformation on a small scale, and the glass appears to be intimately associated with the highest density of dislocations. This study suggests that olivine glass may be formed as a result of shock at pressures above about 50 to 55 x 10(9) pascals and that further TEM observations of naturally shocked olivines may demonstrate the presence of glass.     

Crystal site location of iron and trace elements in a magnesium-iron olivine by a new crystallographic technique

A new crysallographic technique has been developed, which has been applied to the problem of locating the cations in a natural olivine crystal with the composition (Mg(0.90)Fe(0.10)Ni(0.004)Mn(0.002))(2)SiO(4). The method uses the variation of characteristic x-ray emission with the direction of an exciting electron beam in an analytical transmission electron microscope. It may be applied to nanometer-sized areas and to concentrations as low as 0.1 atomic percent, is capable of distinguishing neighbors in the periodic table, and does not require external standards. The iron atoms in this crystal are evenly distributed between the two available crystal sites M1 and M2 (49.6 +/- 1 percent on M1), whereas the trace elements nickel and manganese occupy the M1 and M2 positions, respectively (97 +/- 5 percent nickel on M1 and 1 +/- 5 percent manganese on M1).     

Iron-titanium oxides and olivine from 10020 and 10071

A new mineral (approximately Fe(0.5)Mg(0.5)Ti(2)O(5)) related to the pseudobrookite series has been discovered in section 10071,28. Electron-probe analyses for this mineral, a coexisting ilmenite, and a chromian ulv?spinel-ilmenite assemblage in section 10020,40 indicate crystallization under highly reducing conditions. Analytical and optical absorption studies of the olivine in 10020 show it to contain unusually high Cr (1400 parts per million) probably as Cr(2+).     

Allende meteorite: a high-voltage electron petrographic study

Electron-transparent sections of the Allende meteorite, a carbonaceous chondrite, have been prepared by ion-thinning and examined by high-voltage (800-kilovolt) transmission electron microscopy. The matrix crystals, mainly olivine, range in size from approximately 5 to approximately 0.01 micrometers; carbon is present as intergranular films of poorly crystalline graphite. The chondrules exhibit extensive radiation damage, a feature lacking in the matrix. In addition, both chondrules and matrix are undeformed and contain negative crystals; submicroscopic exsolution lamellae are present in pyroxenes. Comparison of the substructure in the Allende meteorite with that in the Parnallee meteorite and in lunar and selected terrestrial rocks leads to the conclusion that chondrule irradiation preceded cold accretion during formation of the solar system and that the meteorite has since been undisturbed.     

Seismic evidence for olivine phase changes at the 410- and 660-kilometer discontinuities

The view that the seismic discontinuities bounding the mantle transition zone at 410- and 660-kilometer depths are caused by isochemical phase transformations of the olivine structure is debated. Combining converted-wave measurements in East Asia and Australia with seismic velocities from regional tomography studies, we observe a correlation of the thickness of, and wavespeed variations within, the transition zone that is consistent with olivine structural transformations. Moreover, the seismologically inferred Clapeyron slopes are in agreement with the mineralogical Clapeyron slopes of the (Mg,Fe)2SiO4 spinel and postspinel transformations.     

Direct observation of dissociated dislocations in garnet

Dislocation core structures in garnet [grossularite (Ca(2.9)Fe(II)(0.1))(Al(1.9)Fe(III)(0.1)Si(3.0)O(12)] have been examined with near atomic resolution transmission electron microscopy. Dissociated dislocations have been observed as parallel a/4<111> partial dislocations that are separated by stacking faults. The partial dislocations have narrow cores ( approximately 3 burgers vectors), and the stacking fault zone between the narrow partial dislocations is apparently a low-energy configuration that results from the occupancy of previously unfilled dodecahedral and tetrahedral sites. Previous studies of garnet dislocations suggested that dissociation involves departures from garnet stoichiometry (that is, trace amounts of impurities), but evidence of detectable amounts of impurities has not been found even in the highest resolution images. These results have implications for mantle mineral rheology and transformations as well as for ceramics of material science interest.     

Composition of the Earth

New estimates of solar composition, compared to earlier measurements, are enriched in Fe and Ca relative to Mg, Al, and Si. The Fe/Si and Ca/Al atomic ratios are 30 to 40 percent higher than chondritic values. These changes necessitate a revision in the cosmic abundances and in the composition of the nebula from which the planets accreted (which have been based on chondritic values). These new values imply that the mantle could contain about 15 weight percent FeO and more CaMgSi(2)O(6) than has been supposed. Geophysical data are consistent with a dense, FeO-rich lower mantle and a CaMgSi(2)O(6) (diopside)-rich transition region. FeO contents of 13 to 18 weight percent appear to be typical of the mantles of bodies in the inner solar system. The oldest komatiites (high-temperature MgO-rich magmas) have a similar chemistry to the derived mantle. These results favor a chemically zoned mantle.     

Early irradiation of matter in the solar system: magnesium (proton, neutron) scheme

The occurrence of positive and negative (26)Mg anomalies in inclusions of the Allende meteorite is explained in terms of proton bombardment of a gas of solar composition. A significant fraction of (26)Mg in the irradiated gas is stored temporarily in the form of radioactive (26)Al by the reaction (26)Mg(p,n) (26)Al. Proton fluxes of 10(17) to 10(19) protons per square centimeter per year at l million electron volts are inferred. Aluminum-rich materials condensing from the gas phase have positive (26)Mg anomalies, whereas magnesium-rich materials have negative (26)Mg anomalies. The proton flux required to account for the observed magnesium anomalies is used to investigate possible isotopic anomalies in the elements from oxygen to argon. Detectable isotopic anomalies are predicted only for neon. The anomalous neon is virtually pure (22)Ne from (22)Na decay. The predicted amount of anomalous (22)Ne is about 10(-8) cubic centimeter (at standard temperature and pressure) per milligram of sodium.