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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Chemical fractionations in group IIIAB iron meteorites - Origin by dendritic crystallization of an asteroidal core

Results of an investigation of the crystallization history of the asteroidal core that produced nearly 200 iron meteorites of group IIIAB are presented. The accuracy of the published distribution of coefficients between solid and liquid metal for seven elements and their dependence on the concentrations of S and P are critically assessed to develop a nonideal fractional crystallization model that reproduces all of the major features of the overall chemical trends in group IIIAB. It is suggested that the apparent distribution coefficient of S was was much higher than its equilibrium value (not greater than 0.01) and that it increased during crystallization. In the present preferred model the apparent distribution coefficient for S increases from 0.6 to 0.8, while the S concentration of the liquid increases from 6 to 13 wt pct. It is inferred that light S-rich liquid accumulates preferentially at the top of the core and in structural traps formed by the advancing solid.

Haack, Henning↗
NASA NTRS1988

Trace element partitioning between taenite and kamacite - Relationship to the cooling rates of iron meteorites

Instrumental neutron activation analysis (INAA) was used to determine Ni, Co, Cu, Ga, As, Au, W, Re and Ir in taenite lamellae isolated by acid dissolution from eight iron meteorites from groups IA, IIIAB and IVA. Taenite is enriched in Ni, Cu, Ga, As, Au, W, Re and Ir relative to kamacite, whereas taenite is depleted in Co. Taenite/kamacite partition ratios in slowly cooled IAB meteorites are farther from unity than those in rapidly cooled IVA meteorites. Taenite/kamacite partition ratios for Cu, Ir, Au and Co may be sensitive cooling rate indicators.

Rasmussen, Kaare L.↗
NASA NTRS1984

Cosmic ray exposure ages of iron meteorites, complex irradiation and the constancy of cosmic ray flux in the past

While previous calculations of potassium ages assumed a constant cosmic ray flux and a single stage (no change in size) exposure of iron meteorites, present calculations relaxed these constancy assumptions and the results reveal multistage irradiations for some 25% of the meteorites studied, implying multiple breakup in space. The distribution of exposure ages suggests several major collisions (based on chemical composition and structure), although the calibration of age scales is not yet complete. It is concluded that shielding-corrected (corrections which depend on size and position of sample) production rates are consistent for the age bracket of 300 to 900 years. These production rates differ in a systematic way from those calculated for present day fluxes of cosmic rays (such as obtained for the last few million years).

Marti, K.↗
NASA NTRS1979

74001 drive tube - Siderophile elements match II B iron meteorite pattern

Eight siderophile elements, seven volatile elements, and U were determined in three samples from 74001 by radiochemical neutron activation analysis. The siderophile pattern closely corresponds to the addition of approximately 0.1% of II B iron meteorites, and the enrichment of nonsiderophile volatile elements seems to be derived from lunar sources with variation generally following Se content. In addition, in 74001 (predominantly black glass), moderately volatile elements (Ag, Zn) are more abundant than In, Cd, and particularly Bi, while in 74220 (mainly orange glass), Ag, Zn, In, and Cd are almost equally abundant, but Bi is as low as in 74001.

Morgan, J. W.↗
NASA NTRS2002

A IAB-Complex Iron Meteorite Containing Low-Ca Clinopyroxene: Northwest Africa 468 and its Relationship to Iodranites and Formation by Impact Melting

Northwest Africa 468 (NWA 468) is a new ungrouped, silicate-rich member of the IAB complex of nonmagmatic iron meteorites. The silicates contain relatively coarse (approximately 300 micron-size) grains of low-Ca clinopyroxene with polysynthetic twinning and inclined extinction. Low-Ca clinopyroxene is indicative of quenching from high temperatures (either from protoenstatite in a few seconds or high-temperature clinoenstatite in a few hours). It seems likely that NWA 468 formed by impact melting followed by rapid cooling to less than or equal to 660 C. After the loss of a metal-sulfide melt from the silicates, sulfide was reintroduced, either from impact-mobilized FeS or as an S2 vapor that combined with metallic Fe to produce FeS. The O-isotopic composition (delta O-17 = -1.39 %) indicates that the precursor material of NWA 468 was a metal-rich (e.g., CR) carbonaceous chondrite. Lodranites are similar in bulk chemical and O-isotopic composition to the silicates in NWA 468; the MAC 88177 lodranite (which also contains low-Ca clinopyroxene) is close in bulk chemical composition. Both NWA 468 and MAC 88177 have relatively low abundances of REE (rare earth elements) and plagiophile elements. Siderophiles in the metal-rich areas of NWA 468 are similar to those in the MAC 88177 whole rock; both samples contain low Ir and relatively high Fe, Cu and Se. Most unweathered lodranites contain approximately 20 - 38 wt. % metallic Fe-Ni. These rocks may have formed in an analogous manner to NWA 468 (i.e., by impact melting of metal-rich carbonaceous-chondrite precursors) but with less separation of metal-rich melts from silicates.

Rubin, Alan E.↗
NASA NTRS1987

Nucleogenic noble gas components in the Cape York iron meteorite

The paper reports data on neutron capture products of the secondary cosmic ray component, the inferred proton and neutron fluences, and the identification of double beta decay of Se-82 in heavily shielded samples of the Cape York iron meteorite. One purpose of this study is to develop a new chronometer for cosmic ray exposure, based on the nuclides I-129 (16 My half-life) and Xe-129 from low energy cosmic ray reactions on Te. The abundance ratio of these two nuclides permits the determination of an (effective) exposure age of 93 + or - 16 My, which represents the first exposure age datum of Cape York. The very small concentrations of spallogenic Ar-38 = 6.5 x 10 to the -10th cu cm STP/g in the metal and troilite (per g Fe) document the heavily shielded locations of the sample. An excess of Xe-129 in the troilite is shown to be entirely due to the decay of cosmic-ray-produced I-129. On the other hand, an inclusion in the troilite reveals the presence of Xe-129 from extinct I-129 and documents its about 4.5 Gy formation age. Mono-isotopic excess of Kr-82 is identified as due to beta-beta-decay of Se-82 with an inferred half-life of 1.0 x 10 to the 20th y. This represents the first beta-beta-decay product observed in a meteorite.

Murty, S. V. S.↗
NASA NTRS1984

The low-energy secondary cosmic ray flux: Detectors in iron meteorites

The cosmic ray-produced nuclides in the interiors of very large bodies are mainly due to secondary particle reactions. The records of low-energy products, specifically the neutron capture products in meteorites, may provide new information regarding multi-stage irradiation and parent body break-ups. Noble gas nuclides, in general, are the best candidates for these investigations. The troilite nodules of iron meteorites contain very low amounts of trapped gas and, because of their elemental abundance systematics of suitable target elements for low energy production of noble gas nuclides, they are ideal candidates for this purposes. Results of a study of noble gases and nitrogen in a troilite nodule of the Cape York iron are presented.

Murty, S. V. S.↗
NASA NTRS2001

A Henry's Law Test for Experimental Partitioning Studies of Iron Meteorites

Low-level doped solid metal/liquid metal experiments analyzed by laser ablation ICP-MS allow Henry's Law to be tested. The results indicate Henry's Law is obeyed and the experimental partition coefficients can be applied to iron meteorites. Additional information is contained in the original extended abstract.

Chabot, N. L.↗
NASA NTRS1990

Compositions of large metal nodules in mesosiderites - Links to iron meteorite group IIIAB and the origin of mesosiderite subgroups

The compositions of large metal nodules in 12 mesosiderites, ranging from 3 mm to 4 cm in minimum dimensions and from 10 to 200 mg/g in silicate content, were analyzed by neutron activation; also analyzed were two additional samples (Pennyweight Point and Murchison Downs) which were originally designated iron meteorites but were later classified as Mes nodules. It was found that mesosiderites subgroups classified on the basis of metal composition are virtually the same as those based on silicate composition. The general covariation of the silicate and the metal compositional characteristics suggests that these features were established in the same process or in linked processes. A mesosiderite formation model is proposed.

Hassanzadeh, Jamshid↗
NASA NTRS1977

Pallasites - Metal composition, classification and relationships with iron meteorites

A comparative study was conducted of the metal composition of 34 pallasites in order to shed further light on the origin of these meteorites. Concentrations of Au, As, Co, Ga, Ge, Ir, Ni, and W in pallasitic metal were determined. Most pallasites are found to have similar compositions indicating a close genetic relationship, and are designated as main group. The Eagle Station Trio is unrelated to the main group as indicated by higher Ni, Ge, and Ir and lower As, Au, and Ga contents in the metal, and olivine richer in Fe and Sc and poorer in Mg and Mn. The trio of Springwater, Rawlinna and Phillips County have metal compositions appropriate to high-Ni main group members, but their fayalite contents suggest they are not closely related to it. Pavlodar and Glorieta Mountain appear to be unique pallasites, and Brenham an anomalous main group member. Krasnoyarsk is classified as a main group member. Main group pallasites have metal compositions which overlap those of IIIAB iron meteorites on a Ga-Ge plot, and they have similar Au, As, Cr, Ir, Ni and W contents to high-Ni IIIAB irons.

Scott, E. R. D.↗
NASA NTRS1988

Low temperature phase transformations in the metallic phases of iron and stony-iron meteorites

The nickel content and the structure of kamacite and decomposed taenite (clear taenite 1, CT-1; the cloudy zone, CZ; and clear taenite 2, CT-2) in the metallic phases of meteorites were determined using X-ray microanalysis techniques in the AEM. The kamacite near the CT-1 interface was found to contain about 4 wt pct Ni. The CT-1 structure contains 51.4-45.6 wt pct Ni; it is ordered FeNi with the L1(0) superstructure. The CZ structure consists of two phases: a globular phase (ordered FeNi containing 50.9 wt pct Ni) and a surrounding honeycomb martensitic phase containing 11.7 wt pct Ni. The CT-2 was found in all of the iron meteorite groups studied and in the pallasites, but not in the mesosiderites. Based on the preliminary evidence, this region is believed to be ordered Fe3Ni. Possible mechanisms for the decomposition of taenite are discussed.

Reuter, K. B.↗
NASA NTRS1989

Atomic environments in iron meteorites using EXAFS

Extended x ray absorption fine structure (EXAFS) is observed as a modulation on the high energy side of an x ray absorption edge. It occurs when the photo-ejected electron wave is scattered by neighboring atoms in a solid, and interference occurs between the outgoing and scattered waves. The result is that the absorption spectrum carries a signature that is characteristic of the identity and disposition of scattering atoms around the absorbing atom. Therefore, it may be shown that the Fourier transform of the normalized EXAFS can provide detailed information about the immediate environment of specific atoms in a solid and is ideally suited to the study of cosmic dusts. A study of cosmic dust was initiated using EXAFS and other techniques. The simplest type of cosmic material, namely iron meteorites, was investigated.

Cressey, G.↗
NASA NTRS1984

Extreme incompatibility of Pb during the crystallization of magmatic iron meteorites

The partitioning behavior of Pb in the three-phase system metal-troilite-sulfide liquid was investigated. Microscopic examination and electron microprobe analysis of a polished section showed three phases--Fe metal, troilite and an Fe-S-Pb-O metallic liquid. The Pb concentration of the metallic liquid was 1.4 wt.%; the Pb contents of both the troilite and the Fe metal were below the detection limit of the electron probe. To better define the Pb concentrations of the solid phases, the same sample was analyzed for Pb and Fe using the M.I.T., Harvard and Brown ion microprobe. Even at this level of precision, some conclusions and speculations are possible: (1) The D(Pb) (troilite/metallic liquid) is low (.01 to .001) and, contrary to expectation, troilite, like metal, excludes Pb very effectively during the crystallization of metallic liquids; (2) Pb in iron meteorites is conceivably concentrated in trace phases which are less susceptible to contamination than metal and troilite; and (3) taken at face value D(Pb)(metal/troilite) is 0.1, but given the Pb and Fe ion yields from simple endmember phases (Fe, FeS, Pb, PbS), this D is more likely to be 0.05 to 0.03.

Jones, J. H.↗
NASA NTRS1978

Fractional crystallization of iron meteorites, an experimental study

Measurements of the distribution coefficients of various elements (Ni, Co, P, Pt, Ir, Ge, Cr, and Au) between solid and liquid phases were made for FeNi alloys. A high thermal gradient crystal growing furnace was employed in order to promote plane front solidification, rapid mixing in the liquid and a minimum of diffusion in the solid. Eight measurements of the nickel distribution coefficient gave an average value of 0.91 plus or minus 0.03 and this value was independent of the Ni concentration in the range 5.5 to 8.75 wt. % Ni. Equilibrium distribution coefficients were also calculated by correcting for the diffusion boundary layer present in the liquid. These values are within 20% of the measured distribution coefficient values. Except for Co and Ni, all the measured distribution coefficient values are closer to 1.0 than those calculated by Scott (1972) for the IIIAB irons. Using the experimental values and the calculated equilibrium values for ternary additions to FeNi melts, it appears that fractional crystallization processes will not explain the observed chemical variations in the major iron meteorite groups. Alternative explanations are suggested which also recognize the fact that cooling rates vary widely in several chemical groups.

Goldstein, J. I.↗
NASA NTRS2018

Arcjet Ablation of Stony and Iron Meteorites

A test campaign was conducted placing meteorites in the 60 MW plasma Arcjet Interaction Heating Facility at NASA Ames Research Center, with the aim to achieve flight-relevant conditions for asteroid impacts in Earth's atmosphere and to provide insight into how meteoritic materials respond to extreme entry heating environments. The test conditions at heat flux of 4000 W/m2 and 140 kPa stagnation pressure are comparable to those experienced by a 30-meter diameter asteroid moving at 20 km/s velocity at 65 km altitude in the Earth's atmosphere. Test objects were a stony type H5 ordinary chondrite (Tamdakht) and an iron type IAB-MG meteorite (Campo Del Cielo), and included the terrestrial analogs Dense Flood Basalt and Fused Silica. All samples were exposed for only a few seconds in the plasma stream. Significant melt flow and vaporization was observed for both the stony and iron meteorites during exposure. Mass loss from spallation of fragments was also observed. Vapor emitted atomic lines from alkali metals and iron, but did not emit the expected MgO molecular band emissions. The meteoritic melts flowed more rapidly, indicating lower viscosity, than those of Fused Silica. The surface recession was mapped. The effective heat of ablation derived from this showed that ablation under these conditions occurred in the melt-dominated regime. Ablation parameters have an effect on ground damage estimates. A bias in ablation parameters towards the melt-dominated regime would imply that impacting asteroids survive to lower altitude, and therefore could possibly have airbursts with a larger ground damage footprint.

Asteroid↗
NASA NTRS2018

Arcjet Ablation of Stony and Iron Meteorites

A test campaign was conducted placing meteorites in the 60 MW plasma Arcjet Interaction Heating Facility at NASA Ames Research Center, with the aim to achieve flight-relevant conditions for asteroid impacts in Earth's atmosphere and to provide insight into how meteoritic materials respond to extreme entry heating environments. The test conditions at heat flux of 4000 W/m2 and 140 kPa stagnation pressure are comparable to those experienced by a 30-meter diameter asteroid moving at 20 km/s velocity at 65 km altitude in the Earth's atmosphere. Test objects were a stony type H5 ordinary chondrite (Tamdakht) and an iron type IAB-MG meteorite (Campo Del Cielo), and included the terrestrial analogs Dense Flood Basalt and Fused Silica. All samples were exposed for only a few seconds in the plasma stream. Significant melt flow and vaporization was observed for both the stony and iron meteorites during exposure. Mass loss from spallation of fragments was also observed. Vapor emitted atomic lines from alkali metals and iron, but did not emit the expected MgO molecular band emissions. The meteoritic melts flowed more rapidly, indicating lower viscosity, than those of Fused Silica. The surface recession was mapped. The effective heat of ablation derived from this showed that ablation under these conditions occurred in the melt-dominated regime. Ablation parameters have an effect on ground damage estimates. A bias in ablation parameters towards the melt-dominated regime would imply that impacting asteroids survive to lower altitude, and therefore could possibly have airbursts with a larger ground damage footprint.

Arcjet↗