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Timescales and settings for alteration of chondritic meteorites

Description: 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.
Date: November 16, 2005
Creator: Krot, A. N.; Hutcheon, I. D.; Brearley, A. J.; Pravdivtseva, O. V.; Petaev, M. I. & Hohenberg, C. M.
Partner: UNT Libraries Government Documents Department

Oxygen Isotopes in Chondritic Interplanetary Dust: Parent-Bodies and Nebular Oxygen Reservoirs

Description: Planetary objects have preserved various amounts of oxygen issued from isotopically different oxygen reservoirs reflecting their origin and physico-chemical history. An {sup 16}O-rich component is preserved in refractory inclusions (CAIs) whereas meteorites matrices are enriched in an {sup 16}O-poor component. The origin of these components is still unclear. The most recent models are based on isotope selective photodissociation of CO in a {sup 16}O-rich nebula/presolr cloud resulting in a {sup 16}O-poor gas in the outer part of the nebula. However because most meteorite components are thought to be formed in the inner 3AU of the solar nebula, the precise isotopic composition of outer solar system components is yet unknown. In that respect, the oxygen isotopic composition of cometary dust is a key to understand the origin of the solar system. The Stardust mission will bring back to the Earth dust samples from comet Wild2, a short period comet from the Jupiter family. A precise determination of the oxygen isotope composition of Wild2 dust grains is essential to decipher the oxygen reservoirs of the outer solar system. However, Stardust samples may be extremely fragmented upon impact in the collector. In addition, interplanetary dust particles (IDPs) collected in the stratosphere are likely to contain comet samples. Therefore, they started to investigate the oxygen isotopic composition of a suite of chondritic interplanetary dust particles that includes IDPs of potential cometary origin using a refined procedure to increase the lateral resolution for the analysis of Stardust grains or IDP subcomponents down to {approx} 3 {micro}m. High precision data for 4 IDPs were previously reported, here they have measured 6 additional IDPs.
Date: February 14, 2006
Creator: Aleon, J; McKeegan, K D & Leshin, L
Partner: UNT Libraries Government Documents Department

On the origin of porphyritic chondrules

Description: A computer program for the complex equilibria in a cooling nebular gas was used to explore a possible origin of porphyritic chondrules, the major class of chondrules in chondritic meteorites. It uses a method of accurately calculating the thermodynamic properties of molten multicomponent aluminosilicates, which deduces the silicate condensates vs temperature and pressure of a nebular gas. This program is coupled with a chemical equilibrium algorithm for systems with at least 1000 chemical species; it has a data base of over 5000 solid, liquid, and gaseous species. Results are metastable subcooled liquid aluminoscilicates with compositions resembling types IA and II porphyritic chondrules at two different temperatures at any pressure between 10{sup {minus}2} and 1 (or possibly 10{sup {minus}3} to 5) atm. The different types of chondrules (types I, II, III) could have been produced from the same gas and do not need a different gas for each apparent oxidation state; thus, the difficulty of current models for making porphyritic chondrules by reheating different solids to just below their liquidus temperatures in different locations is not necessary. Initiation of a stage of crystallization just below liquidus is part of the natural crystallization (recalescence) process from metastable subcooled liquidus and does not require an improbably heating mechanism. 2 tabs.
Date: May 1, 1994
Creator: Blander, M.; Unger, L.; Pelton, A. & Ericksson, G.
Partner: UNT Libraries Government Documents Department

Primordial Compositions of Refractory Inclusions

Description: Bulk chemical and oxygen, magnesium and silicon isotopic compositions were measured for each of 17 Types A and B refractory inclusions from CV3 chondrites. After bulk chemical compositions were corrected for non-representative sampling in the laboratory, the Mg and Si isotopic compositions of each inclusion were used to calculate its original chemical composition assuming that the heavy-isotope enrichments of these elements are due to Rayleigh fractionation that accompanied their evaporation from CMAS liquids. The resulting pre-evaporation chemical compositions are consistent with those predicted by equilibrium thermodynamic calculations for high-temperature nebular condensates but only if different inclusions condensed from nebular regions that ranged in total pressure from 10{sup -6} to 10{sup -1} bar, regardless of whether they formed in a system of solar composition or in one enriched in OC dust relative to gas by a factor of ten relative to solar composition. This is similar to the range of total pressures predicted by dynamic models of the solar nebula for regions whose temperatures are in the range of silicate condensation temperatures. Alternatively, if departure from equilibrium condensation and/or non-representative sampling of condensates in the nebula occurred, the inferred range of total pressure could be smaller. Simple kinetic modeling of evaporation successfully reproduces observed chemical compositions of most inclusions from their inferred pre-evaporation compositions, suggesting that closed-system isotopic exchange processes did not have a significant effect on their isotopic compositions. Comparison of pre-evaporation compositions with observed ones indicates that 80% of the enrichment in refractory CaO + Al{sub 2}O{sub 3} relative to more volatile MgO + SiO{sub 2} is due to initial condensation and 20% due to subsequent evaporation for both Type A and Type B inclusions.
Date: February 20, 2008
Creator: Grossman, L.; Simon, S. B.; Rai, V. K.; Thiemens, M. H.; Hutcheon, I. D.; Williams, R. W. et al.
Partner: UNT Libraries Government Documents Department

A possible origin of EL6 chondrites from a high temperature-high pressure solar gas

Description: Condensates from a gas of ``solar`` composition were calculated to investigate the origins of EL6 chondrites using a free energy minimization program with a data base for the thermodynamic properties of multicomponent molten silicates as well as for other liquids solids, solid solutions and gaseous species. Because of high volatility of silicon and silica, the high silicon content of metal (2.6 mole %) can only be produced at pressures 10{sup {minus}2} atm at temperatures above 1475 K. At 100--500 atm, a liquid silicate phase crystallizes at a temperature where the silicon content of the metal, ferrosilite content of the enstatite and albite concentration in the plagioclase are close to measured values. In pyrometallurgy, liquid silicates are catalysts for reactions in which Si-O-Si bridging bonds are broken or formed. Thus, one attractive mode for freezing in the compositions of these three phases is disappearance of fluxing liquid. If the plagioclase can continue to react with the nebula without a liquid phase, lower pressures of 10{sup {minus}1} to 1 atm might be possible. Even if the nebula is more reducing than a solar gas, the measured properties of EL6 chondrites might be reconciled with only slightly lower pressures (less than 3X lower). The temperatures would be about the same as indicated in our calculations since the product of the silicon content of the metal and the square of the ferrosilite content of the enstatite constitute a cosmothermometer for the mineral assemblage in EL6 chondrites.
Date: May 1, 1994
Creator: Blander, M.; Unger, L.; Pelton, A. & Eriksson, G.
Partner: UNT Libraries Government Documents Department

Comet 81P/Wild 2 under a microscope

Description: The Stardust spacecraft collected thousands of particles from comet 81P/Wild 2 and returned them to Earth for laboratory study. The preliminary examination of these samples shows that the nonvolatile portion of the comet is an unequilibrated assortment of materials that have both presolar and solar system origin. The comet contains an abundance of silicate grains that are much larger than predictions of interstellar grain models, and many of these are high-temperature minerals that appear to have formed in the inner regions of the solar nebula. Their presence in a comet proves that the formation of the solar system included mixing on the grandest scales. Stardust was the first mission to return solid samples from a specific astronomical body other than the Moon. The mission, part of the NASA Discovery program, retrieved samples from a comet that is believed to have formed at the outer fringe of the solar nebula, just beyond the most distant planet. The samples, isolated from the planetary region of the solar system for billions of years, provide new insight into the formation of the solar system. The samples provide unprecedented opportunities both to corroborate astronomical (remote sensing) and sample analysis information (ground truth) on a known primitive solar system body and to compare preserved building blocks from the edge of the planetary system with sample-derived and astronomical data for asteroids, small bodies that formed more than an order of magnitude closer to the Sun. The asteroids, parents of most meteorites, formed by accretion of solids in warmer, denser, more collisionally evolved inner regions of the solar nebula where violent nebular events were capable of flash-melting millimeter-sized rocks, whereas comets formed in the coldest, least dense region. The samples collected by Stardust are the first primitive materials from a known body, and as such they provide contextual ...
Date: October 12, 2006
Creator: Brownlee, D; Tsou, P.; Aleon, J.; Alexander, C.; Araki, T.; Bajt, S. et al.
Partner: UNT Libraries Government Documents Department

Light Element Isotopic Compositions of Cometary Matter Returned by the STARDUST Mission

Description: Hydrogen, carbon, nitrogen, and oxygen isotopic compositions are heterogeneous among comet 81P/Wild2 particle fragments, however extreme isotopic anomalies are rare, indicating that the comet is not a pristine aggregate of presolar materials. Non-terrestrial nitrogen and neon isotope ratios suggest that indigenous organic matter and highly volatile materials were successfully collected. Except for a single circumstellar stardust grain, silicate and oxide minerals have oxygen isotopic compositions consistent with solar system origin. One refractory grain is {sup 16}O-enriched like refractory inclusions in meteorites, suggesting formation in the hot inner solar nebula and large-scale radial transport prior to comet accretion in the outer solar system.
Date: October 10, 2006
Creator: McKeegan, K. D.; Aleon, J.; Bradley, J.; Brownlee, D.; Busemann, H.; Butterworth, A. et al.
Partner: UNT Libraries Government Documents Department

Mineralogy and Petrology of Comet Wild 2 Nucleus Samples

Description: The bulk of the Wild 2 samples appear to be weakly-constructed mixtures of nanometerscale grains with occasional much larger (>1{micro}m) ferromagnesian silicates, Fe-Ni sulfides, Fe-Ni metal and accessory phases. The very wide range of olivine and low-Ca pyroxene compositions in Wild 2 require a wide range of formation conditions, probably reflecting different formation locations in the protoplanetary disk. The restricted compositional ranges of Fe-Ni sulfides, the wide range for silicates, and absence of hydrous phases indicate that Wild 2 experienced little or no aqueous alteration. Less abundant Wild 2 materials include a refractory particle, whose presence appears to require large-scale radial transport in the early protoplanetary disk. The nature of cometary solids is of fundamental importance to our understanding of the early solar nebula and protoplanetary history. Until now we have had to study comets from afar using spectroscopy, or settle for analyses of interplanetary dust particles (IDPs) of uncertain provenance. We report here mineralogical and petrographic analyses of particles derived directly from Comet Wild 2. All of the Wild 2 particles we have thus far examined have been modified in various ways by the capture process. All particles that may have been loose aggregates, ''traveling sand piles'', disaggregated into individual components with the larger, denser components penetrating more deeply into the aerogel. Individual grains experienced a wide range of heating effects that range from excellent preservation to melting (Fig. 1); such behavior was expected (1, 2 ,3). What is remarkable is the extreme variability of these modifications and the fact that severely modified and unmodified materials can be found within a micrometer of each other, requiring tremendous local temperature gradients. Fortunately, we have an internal gauge of impact collection heating. Fe-Ni sulfides are ubiquitous in the Wild 2 samples, are very sensitive indicators of heating, and accurate chemical analyses ...
Date: October 11, 2006
Creator: Zolensky, M. E.; Zega, T. J.; Yano, H.; Wirick, S.; Westphal, A. J.; Weisberg, M. K. et al.
Partner: UNT Libraries Government Documents Department

Chemical Engineering Division physical inorganic chemistry. Annual report, July 1975--June 1976

Description: The thermal conductivity of acetonitrile vapor has been measured at temperatures between 338 and 387/sup 0/K and pressures between 100 and 1050 torr. The pressure dependence of the thermal conductivities indicates the presence of dimeric vapor species with an association constant given by RT ln K/sub 2/ (atm/sup -1/) = 5002 - 21.15 T. Molecular orbital studies of hydrogen-bonded methyl alcohol vapor complexes indicate that, the cyclic tetramer is especially stable. Preliminary calculations on trifluoroethanol indicate that its dimer is more stable than that of methanol. Molecular dynamics calculations have been utilized to calculate the second-order coefficient of conformal ionic solution (CIS) theory. The results indicate that the repulsive part of the pair potential most commonly used for calculations on molten salts is inaccurate and that calculation of the CIS coefficient provides a sensitive test of pair potentials. Laser-Raman spectra of molten solutions of YCl/sub 3/ in CsCl, KCl and LiCl were obtained and compared with spectra of solids of known structure. The spectra at low concentrations of YCl/sub 3/ (X/sub YCl/sub 3// is less than 0.25) indicate an octahedrally coordinated YCl/sub 6//sup 3 -/ species. Raman and infrared spectra of claudetite and arsenolite were measured and compared with the spectra of vitreous and liquid As/sub 2/O/sub 3/, as well as with the As/sub 4/O/sub 6/ vapor molecules.The spectrum of claudetite has some resemblance to that of vitreous and liquid As/sub 2/O/sub 3/. Raman spectra of arsenous oxide indicate that the dimeric As/sub 2/O/sub 3/ species is present. The Raman spectra of aluminum chloride vapor (AlCl/sub 3/ and Al/sub 2/Cl/sub 6/ vapor molecules), provided nine new bands not observed before and permitted a more complete assignment and analysis of the structure of the species to be made. The Raman spectra of In(I), In(II) and In(III) chlorides indicate the presence of ...
Date: January 1, 1976
Creator: Blander, M.; Curtiss, L. A.; Hsu, C. C.; Kucera, G. H.; Marr, J. J.; Papatheodorou, G. N. et al.
Partner: UNT Libraries Government Documents Department