Ordinary (mesostasis) and not‐so‐ordinary (symplectites) late‐stage assemblages in howardites     

Andrea PATZER  Harry Y McSWEEN JR. 《Meteoritics & planetary science》2012,47(9):1475-1490

Abstract– Two categories of symplectites have been observed in howardites: three‐phase, composed of vermicular intergrowths of ferroan augite, fayalitic olivine, and silica, and two‐phase, composed of vermicular intergrowths of orthopyroxene and troilite. Three‐phase symplectites have been previously shown to represent the breakdown products of metastable pyroxene. In howardites, they appear to be genetically related to gabbroic eucrites. In some cases and under yet‐to‐be specified conditions, ferroan clinopyroxene in gabbroic eucrites may undergo only localized decomposition resulting in oriented exsolution‐like features. Breakdown phases in those cases are fayalitic olivine, silica, and—depending on the MgO content of the system—orthopyroxene. As opposed to three‐phase symplectites, two‐phase symplectites are most likely of diogenitic origin. They probably formed via impact‐induced localized melting of diogenitic orthopyroxene in the presence of troilite (grain boundary melting). Three‐phase symplectites in howardites occasionally contain accessory amounts of ilmenite, troilite, and/or kamacite and are exclusively associated with medium‐grained FeO‐rich pyroxene, silica, and plagioclase. All minerals involved are late‐stage crystallites or mesostasis phases. In general, highly evolved eucritic lithologies constitute only a minor fraction of howardites. However, considering that three‐phase symplectites are generated in a low‐pressure, i.e., near‐surface, environment, FeO‐ and CaO‐rich eucritic rocks may be exposed locally on Vesta’s surface. This, in turn, is highly relevant to the ongoing DAWN mission.  相似文献   

The Meteoritical Bulletin,No. 76, 1994 January: The U.S. Antarctic Meteorite Collection*     

Jeffrey N. Grossman 《Meteoritics & planetary science》1994,29(1):100-143

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Chemical condensation sequences in supernova ejecta     

James M. Lattimer  Lawrence Grossman 《Earth, Moon, and Planets》1978,19(2):169-184

The mineralogical composition of grains produced in supernova ejecta is explored via chemical equilibrium condensation computations. These calculations are carried out for chemical compositions characteristic of each of several supernova zones, taking into account the pressure decrease due to adiabatic expansion and condensation. The distributions of the major elements among the various gaseous species and solid phases are graphically displayed. These computations reveal that many of the major condensates from supernova ejecta are also stable against evaporation in a gas of solar composition at high temperatures. This is especially true for minerals containing the elements O, Mg, Al, Si, Ca, Fe and Ti. Grains which form in supernova ejecta are less likely to become homogenized with solar nebular gas than SN gas and are thus potential sources of exotic isotopic compositions in the early solar system. The calculated elemental distributions of supernova condensates are applied to problems concerning isotopic anomalies and large mass-dependent isotopic fractionations discovered in the meteorite Allende. The order in which the major elements become totally condensed is found to be nearly independent of the supernova zone considered, being the same as that for a solar gas. The consequence of this may be that some of the observed depletions of heavy elements in the interstellar gas are due to supernova-produced dust.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.  相似文献   

High fO2 During Sillimanite Zone Metamorphism of Part of the Barrovian Type Locality, Glen Clova, Scotland     

AGUE  JAY J.; BAXTER  ETHAN F.; ECKERT  JAMES O.  JR 《Journal of Petrology》2001,42(7):1301-1320

The redox state of sillimanite zone (650–700°C, 5–6kbar) metasediments of the Barrovian type area, Scotland, wasinvestigated using estimates of metamorphic oxygen fugacity(f_O2), sulfur fugacity (f_S2), and fluid chemistry based on newdeterminations of mineral and rock compositions from 33 samples.A total of 94% of the samples lack graphite, contain both ilmenite–hematitesolid solutions (RHOMOX) and magnetite, and had metamorphicf_O2 about 2 log₁₀ units above the quartz–fayalite–magnetite(QFM) buffer. The regional variation in metamorphic f_O2 forthese rocks was minimal, about ±0·3 log₁₀ units,reflecting either a protolith that was homogeneous with respectto redox state, or an initially variable protolith whose redoxstate was homogenized by metamorphic fluid–rock interaction.RHOMOX inclusions in garnet porphyroblasts that become richerin ilmenite from the interior to the edge of the host porphyroblastsuggest that at least some syn-metamorphic reduction of rockoccurred. Significant variations in bulk-rock oxidation ratio(OR) that are probably inherited from sedimentary protolithsare found from one layer to the next; OR ranges mostly between  相似文献   

Reflectivities of uniform and broken layered clouds   总被引：2，自引：0，他引：2  

JAMES A. COAKLEY  JR. 《Tellus. Series B, Chemical and physical meteorology》1991,43(5):420-433

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Pontlyfni: a differentiated meteorite related to the group IAB irons     

Andrew M. Davis  R. Ganapathy  Lawrence Grossman 《Earth and Planetary Science Letters》1977,35(1):19-24

The abundances of 23 major and trace elements in the Pontlyfni meteorite have been measured by instrumental neutron activation analysis. The compositions of the metal and silicate fractions suggest a genetic relationship between Pontlyfni and the group IAB irons.  相似文献   

Trace elements in the Allende meteorite—III. Coarse-grained inclusions revisited     

Lawrence Grossman  R Ganapathy 《Geochimica et cosmochimica acta》1977,41(11):1647-1664

New RNAA determinations of Ba, Sr, Zr, U, Re, Pd, Ag, Zn and Se and INAA measurements of Lu are added to published data for 21 other elements in the same suite of ten samples. On the average, 21 refractory elements are not significantly fractionated from one another. The mean of their enrichment factors relative to Cl chondrites is 17.5 ± 0.4, indicating that the high-temperature condensate inclusions represent 5.7 wt% of the total condensable matter. Os, Ir, Ru, Re and most of the W condensed in one or more refractory siderophile element alloys along with small fractions of the Pd, Co, Au and Ag. The bulk of the Eu and Sr condensed in solid solution in melilite. Sc, Zr, Hf, Ta, U and the remaining REE condensed in a phase whose abundance in the inclusions is negatively correlated with that of melilite, either diopside or one or more minor or trace phases, including perovskite. Ba condensed in a different phase, separately from all these elements. In individual inclusions, fractionations are common between elements which were carried in by different condensate phases. Smaller fractionations are also observed for elements which condensed together. These may be due to variable proportions of them in a common condensate phase in response to different nebular equilibration temperatures or to multiple condensate phases containing different proportions of these elements. Available evidence indicates that some trace elements no longer reside in the phases which carried them into the inclusions, indicating a post-accretion thermal event which redistributed some of them. From the minimal variation of the Zr/Hf ratio in the inclusions, the solar system ratio is estimated to be 29.6 ± 1.8. From the mean U content of the inclusions and estimates of the bulk terrestrial and lunar U abundances, the Earth and Moon are estimated to contain 21% and 22–30% high-temperature condensates, respectively.  相似文献   

Condensation and fractionation of rare earths in the solar nebula     

Andrew M Davis  Lawrence Grossman 《Geochimica et cosmochimica acta》1979,43(10):1611-1632

Using the most recent thermodynamic data, we calculated the condensation behavior of REE and investigated several models to explain ‘group II’ REE patterns in Allende inclusions. All models involve removal of large fractions of the more refractory heavy REE in an early condensate, probably perovskite, followed by condensation of the remainder at lower temperature. Boynton (1975 Geochim. Cosmochim. Acta39, 569–584), found that the pattern of one such inclusion could not be fit by that of the gas remaining after ideal solution of REE in perovskite and, assuming the presence of only one REE component, calculated relative activity coefficients for REE in perovskite that would be needed to produce a match. In attempting to fit 20 group II patterns with this type of model, we found that these activity coefficients could not be used for most inclusions and that the relationship between ionic radius and required activity coefficients had to change rapidly and irregularly over a narrow range of perovskite removal temperature. Because this feature and the high degree of nonideality needed are most unreasonable, we propose a different model in which two REE components control the patterns: (1) the gas remaining after removal of perovskite in which REE dissolve in ideal solution; (2) a material uniformly enriched in all REE. Two-component models in which solid solution of REE in perovskite is slightly non-ideal and activity coefficients vary negligibly over a narrow temperature range cannot be ruled out. By varying perovskite removal temperatures and the relative proportions of the two components, all 20 REE patterns can be satisfactorily explained.By using a thermodynamically reasonable model, we conclude that perovskite removal occurred over a very narrow temperature range, that multiple refractory element-bearing components are present, indicating a complex history for these inclusions, and that the undeniable gas-solid fractionations that produced the REE patterns may have taken place under somewhat more reducing conditions than those of a normal solar gas.  相似文献   

The Antarctic achondrite ALHA 76005: a polymict eucrite     

Lawrence Grossman  Edward Olsen  Andrew M. Davis  Tsuyoshi Tanaka  Glenn J. MacPherson 《Geochimica et cosmochimica acta》1981,45(8):1267-1279

ALHA 76005 is a basaltic achondrite containing few. if any, orthopyroxenes. Its bulk major and trace element composition is like that of a non-cumulate eucrite, and unlike that of a howardite. It contains a variety of igneous clasts which differ in their textures, pyroxene/plagioclase ratios and pyroxene and plagioclase compositions. One clast, No. 4, was found to have the REE pattern of a cumulate eucrite and an oxygen isotopic composition different from that of the bulk meteorite. Both the chemical and oxygen isotopic composition of clast No. 4 suggest that it was derived from a source different from its host. These observations lead to the conclusion that ALHA 76005 is a polymict eucrite.  相似文献   

Amoeboid olivine aggregates in the Allende meteorite     

Lawrence Grossman  Ian M Steele 《Geochimica et cosmochimica acta》1976,40(2):149-155

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.  相似文献