42 research outputs found
Silica-bearing objects in the Dengli H3.8 and Gorlovka H3-4 chondrites
Silica-bearing objects are enigmatic components of the olivine-normative ordinary chondrites. Several papers have been devoted to the study of these objects in various chondrite types. While a relatively large body of information has been collected, the origin of these objects is still controversial. Here we report new data on silica-bearing objects in the unequilibrated H-chondrites Dengli and Gorlovka. The crystallization history of these objects could be explained on the basis of the phase diagram of the Q-Ol-Pl (Al2O3) system, but the origin of the silica-rich liquids remains unclear
Photometric Light Curves and Polarization of Close-in Extrasolar Giant Planets
The close-in extrasolar giant planets [CEGPs], \ltorder 0.05 AU from their
parent stars, may have a large component of optically reflected light. We
present theoretical optical photometric light curves and polarization curves
for the CEGP systems, from reflected planetary light. Different particle sizes
of three condensates are considered. In the most reflective case, the
variability is micromagnitudes, which will be easily detectable
by the upcoming satellite missions MOST, COROT, and MONS, and possibly from the
ground in the near future. The least reflective case is caused by small, highly
absorbing grains such as solid Fe, with variation of much less than one
micromagnitude. Polarization for all cases is lower than current detectability
limits. We also discuss the temperature-pressure profiles and resulting
emergent spectra of the CEGP atmospheres. We discuss the observational results
of Tau Boo b by Cameron et al. (1999) and Charbonneau et al. (1999) in context
of our model results. The predictions - the shape and magnitude of the light
curves and polarization curves - are highly dependent on the size and type of
condensates present in the planetary atmosphere.Comment: 33 pages, accepted by Ap
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Timescales and settings for alteration of chondritic meteorites
Most groups of chondritic meteorites experienced diverse styles of secondary alteration to various degrees that resulted in formation of hydrous and anhydrous minerals (e.g., phyllosilicates, magnetite, carbonates, ferrous olivine, hedenbergite, wollastonite, grossular, andradite, nepheline, sodalite, Fe,Ni-carbides, pentlandite, pyrrhotite, Ni-rich metal). Mineralogical, petrographic, and isotopic observations suggest that the alteration occurred in the presence of aqueous solutions under variable conditions (temperature, water/rock ratio, redox conditions, and fluid compositions) in an asteroidal setting, and, in many cases, was multistage. Although some alteration predated agglomeration of the final chondrite asteroidal bodies (i.e. was pre-accretionary), it seems highly unlikely that the alteration occurred in the solar nebula, nor in planetesimals of earlier generations. Short-lived isotope chronologies ({sup 26}Al-{sup 26}Mg, {sup 53}Mn-{sup 53}Cr, {sup 129}I-{sup 129}Xe) of the secondary minerals indicate that the alteration started within 1-2 Ma after formation of the Ca,Al-rich inclusions and lasted up to 15 Ma. These observations suggest that chondrite parent bodies must have accreted within the first 1-2 Ma after collapse of the protosolar molecular cloud and provide strong evidence for an early onset of aqueous activity on these bodies
The Formation of Wassonite: A New Titanium Monosulfide Mineral in the Yamato 691 Enstatite Chondrite
Wassonite, ideally stoichiometric TiS, is a titanium monosulfide not previously observed in nature, that was discovered within the Yamato 691 EH3 enstatite chondrite [1]. Because of the submicrometer size of the wassonite grains, it was not possible to determine conventional macroscopic properties. However, the chemical composition and crystal structure were well constrained by extensive quantitative energy dispersive x-ray analysis and electron diffraction using transmission electron microscopy (TEM). The crystal system for wassonite is rhombohedral (a = 3.42 plus or minus 0.07, c = 26.50 plus or minus 0.53 Angstroms) with space group: R(sup 3 raised bar) m (R9 type), cell volume: 268.4 plus or minus 0.53 Angstroms(sup 3), Z=9, density (calculated): 4.452 grams per cubic centimeter, empirical formula: (Ti(sub 0.93), Fe(sub 0.06), Cr(sub 0.01))S. In this study, we discuss possible formation mechanisms of wassonite and its associated minerals based on the petrology, mineralogy, crystallography, thermodynamic calculations, Al/Mg isotopic systematics and the O-isotopic composition of the wassonite-bearing BO chondrule
Thermal Processing of Silicate Dust in the Solar Nebula: Clues from Primitive Chondrite Matrices
The most abundant matrix minerals in chondritic meteorites, hydrated
phyllosilicates and ferrous olivine crystals, formed predominantly in asteroids
during fluid-assisted metamorphism. We infer that they formed from minerals
present in three less altered carbonaceous chondrites that have silicate
matrices composed largely of micrometer- and nanometer-sized grains of
crystalline forsterite, Mg2SiO4, and enstatite MgSiO3, and amorphous,
ferromagnesian silicate. Compositional and structural features of enstatite and
forsterite suggest that they formed as condensates that cooled below 1300 K at
\~1000 K/h. Most amorphous silicates are likely to be solar nebula condensates
also, as matrix, which is approximately solar in composition, is unlikely to be
a mixture of genetically unrelated materials with different compositions. Since
chondrules cooled at 10-1000 K/h, and matrix and chondrules are chemically
complementary, most matrix silicates probably formed close to chondrules in
transient heating events. Shock heating is favored as nebular shocks capable of
melting millimeter-sized aggregates vaporize dust. The crystalline and
amorphous silicates in the primitive chondrite matrices share many
characteristic features with silicates in chondritic interplanetary dust
particles suggesting that most of the crystalline silicates and possibly some
amorphous silicates in the interplanetary dust particles are also nebular
condensates. Except for small amounts of refractory oxides that formed with
Ca-Al-rich inclusions at the inner edge of the disk and presolar dust, most of
the crystalline silicate dust that accreted into chondritic asteroids and
long-period comets appears to have formed from shock heating at ~2-10 AU.
Forsterite crystals around young stars may have a similar origin.Comment: 16 page
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Differentiation of metal-rich meteoritics parent bodies: I. Measurements of PGEs, Re, Mo, W, and Au in meteoritic Fe-Ni metal
We describe an analytical technique for measurements of Fe, Ni, Co, Mo, Ru, Rh, W, Re, Os, Ir, Pt, and Au in bulk samples of iron meteorites. The technique involves EPMA (Fe, Ni, Co) and LA-ICP-MS analyses of individual phases of iron meteorites, followed by calculation of bulk compositions based on the abundances of these phases. We report, for the first time, a consistent set of concentrations of Mo, Ru, Rh, Pd, W, Re, Os, Ir, Pt, and Au in the iron meteorites Arispe, Bennett County, Grant, Cape of Good Hope, Cape York, Carbo, Chinga, Coahuila, Duchesne, Gibeon, Henbury, Mundrabilla, Negrillos, Odessa, Sikhote-Alin, and Toluca and the Divnoe primitive achondrite. The comparison of our LA-ICP-MS data for a number of iron meteorites with high-precision isotope dilution and INAA data demonstrates the good precision and accuracy of our technique. The narrow ranges of variations of Mo and Pd concentrations within individual groups of iron meteorites suggest that these elements can provide important insights into the evolution of parent bodies of iron meteorites. Under certain assumptions, the Mo concentrations can be used to estimate mass fractions of the metal-sulfide cores in the parent bodies of iron meteorites. It appears that a range of Pd variations within a group of iron meteorites can serve as a useful indicator of S content in the core of its parent body.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202
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Nebular history of amoeboid olivine aggregates
Minor element (Ca, Cr, and Mn) concentrations in amoeboid olivine aggregates (AOAs) from primitive chondrites were measured and compared with those predicted by equilibrium condensation in the solar nebula. CaO concentrations in forsterite are low, particularly in porous aggregates. A plausible explanation appears that an equilibrium Ca activity was not maintained during the olivine condensation. CaO and MnO in forsterite are negatively correlated, with CaO being higher in compact aggregates. This suggests that the compact aggregates formed either by a prolonged reheating of the porous aggregates or by condensation and aggregation of forsterite during a very slow cooling in the nebula.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202
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Silica-rich igneous rims around magnesian chondrules in CR carbonaceous chondrites: Evidence for condensation origin from fractionated nebular gas
The outer portions of many type I chondrules (Fa and Fs 800 K) ambient nebular temperatures and escaped remelting at lower ambient temperatures. We suggest that these rims formed either by gas-solid condensation of silica-normative materials onto chondrule surfaces and subsequent incomplete melting, or by direct SiO (gas) condensation into chondrule melts. In either case, the condensation occurred from a fractionated, nebular gas enriched in Si, Na, K, Mn, and Cr relative to Mg. The fractionation of these lithophile elements could be due to isolation (in the chondrules) of the higher temperature condensates from reaction with the nebular gas or to evaporation-recondensation of these elements during chondrule formation. These mechanisms and the observed increase in pyroxene/olivine ratio toward the peripheries of most type I chondrules in CR, CV, and ordinary chondrites may explain the origin of olivine-rich and pyroxene-rich chondrules in general.The Meteoritics & Planetary Science archives are made available by the Meteoritical Society and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202