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Rajan, R. S.

Publications and source records attributed to Rajan, R. S..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Meteorite regolithic breccias

In addition to endogenic processes such as heating and aqueous activity, meteorite parent bodies were subjected also to exogenic processing brought about by the impact of the other solar-system objects. Such impacts can produce a variety of effects, ranging from shock metamorphism of individual mineral grains to production of breccias; i.e., rocks consisting of mixtures of disparate lithic units. The present paper reviews recent studies of such breccias, which have generated significant information about the accretional growth of parent bodies, as well as their evolution, composition, stratigraphy, and geological processing.

Bunch, T. E.↗
NASA NTRS1986

Nuclear tracks, Sm isotopes and neutron capture effects in the Elephant Morraine shergottite

Nuclear track studies, uranium concentration measurements and Sm-isotope studies have been performed on both lithologies A and B of the Elephant Morraine shergottite, EETA 79001. Track studies show that EETA 79001 was a rather small object in space with a preatmospheric radius of 12 + or - 2 cm, corresponding to a preatmospheric mass of 28 + or - 13 kg. Phosphates have U concentrations ranging from 0.3 to 1.3 ppm. There are occasional phosphates with excess fission tracks, possibly produced from neutron-induced fission of U and Th, during the regolith exposure in the shergottite parent body (SPB). Sm-isotope studies, while not showing any clear-cut excess in Sm-150, make it possible to derive meaningful upper limits to thermal neutron fluences of 2 to 3 x 10 to the 15th n/sq cm, during a possible regolith irradiation. These limits are consistent with the track data and also make it possible to derive an upper limit to the neutron exposure age of EETA 79001 of 55 Myr in the SPB regolith.

Rajan, R. S.↗
NASA NTRS1986

Gallium-bearing sphalerite in a metal-sulfide nodule of the Qingzhen (EH3) chondrite

The composition and possible history of the Qingshen (EH3) chondrite is presented. The chondrite contains a population of spheroidal metal-sulfide nodules, which display textural evidence of reheating and melting. Evidence of metal sulfuration is also present, suggesting replacement of metal by sulfide during melting. This process has led to the nucleation of perryite along metal-sulfide interfaces. The Ga-bearing sphalerite that was found may have formed by injection of molten sulfide droplets into the metal followed by subsolidus diffusion of Ga from the metal into the sulfide. The latter may occur because of Ga supersaturation in the metal during progressive sulfuration and its decreased affinity for the metal phase during cooling below the taenite-kamacite transition point.

Rambaldi, E. R.↗
NASA NTRS1986

Roedderite in the Qingzhen (EH3) chondrite

The rare mineral roedderite, (Na1.09 K0.89 Ca0.02)2.00 (Mg4.71 Fe0.27)4.98 (Si11.80 Al0.09)11.89 O30 has been found in accessory amounts in the highly unequilibrated enstatite chondrite, Qingzhen. It occurs in association with minor amounts of albite and SiO2 as inclusions within the metal or sulfide phases of metal-sulfide assemblages. The roedderite crystals are connected through oxide and silicate veins to the surrounding matrix. The presence of glass coated vesicles on the surface of the assemblages strongly suggests that roedderite originated in the presence of a fluid phase, presumably during post-accretional planetary processes.

Rambaldi, E. R.↗
NASA NTRS1986

Solar wind helium, neon and argon released by oxidation of metal grains from the Weston chondrite

A set of experiments were carried out to test the feasibility of determining unfractionated elemental and isotopic ratios for the noble gases in the presumably ancient solar wind present in the gas rich meteorites. The problems of diffusive loss was avoided by analyzing metal rather than the usual silicates. In order to avoid chemical, and even harsh physical, treatment of the sample, which might have affected the surfaces of metal grains, a means of analyzing the metal in the presence of residual silicate not removed by gentle crushing and magnetic separation was devised. Preliminary results given were obtained by taking advantage of the differing properties of metal and silicates with regard to diffusion. The results suggests that, with some modifications in the choice of pyrolysis and combustion temperatures and in the amount of O2 used, it should be possible, by oxidizing the surfaces of metal grains from gas rich meteorites, to obtain data on solar wind that has not been fractionated by diffusive loss.

Becker, R. H.↗
NASA NTRS1985

Chondrules in the Qingzhen type-3 enstatite chondrite Possible precursor components and comparison to ordinary chondrite chondrules

The mineral composition of chondrules from a fragment of Qingzhen (EH3) fall was analyzed by neutron activation method. Unlike the ordinary chondrite (OC) chondrules (Gooding and Keil, 1981), the Qingzhen radial pyroxene (RP) and porphyritic pyroxene (PP) chondrules have similar bulk compositions. Porphyritic olivine-pyroxene (POP) chondrules are richer than PP and RP chondrules in refractory lithophiles and siderophiles. Elements in each of the following sets intercorrelate significantly: (1) Fe-Co-Ni-Ir-Au, probably derived from a metal component; (2) Ca-Eu-Se, which suggests an oldhamite-rich precursor; (3) Al-Sc-Hf, occurring in high concentrations in POP chondrules, this set suggesting the existence of a refractory lithophile-rich and olivine-rich component; (4) Na REE; and (5) Cl-Br. Sets (2) and (4) were not precursors of OC. The interelement ratios of refractory lithophiles such as Ca, Al, Ti, Sc, and REE are similar to CI ratios, suggesting that they originated in the earliest phases as silicates, which were sulfurized before chondrule formation.

Grossman, J. N.↗
NASA NTRS1984

Occurrence of oxidized components in Qingzhen enstatite chondrite

It is shown that the most unequilibrated enstatite chondrite, Qingzhen, contains a population of enstatite grains which appear to have formed under more oxidizing nebular conditions than the bulk of the meteorite, which is highly reduced. These grains are black in transmitted light because of the presence of micron-sized inclusions of Ni-poor, Cr-rich metal, and occur either isolated within the matrix or in chondrule interiors. The textural occurrence of these grains argues against their having been introduced during collision of Qingzhen with an oxidized meteorite. Most likely, they originated in the same general nebular neighborhood of the reduced bulk Qingzhen material and were subsequently transported into the reducing environment either before or during the process of chondrule formation. The discovery of this once-oxidized material in Qingzhen posed significant constraints on existing models of formation of reduced matter in the solar system.

Rambaldi, E. R.↗
NASA NTRS1984

Lunar sample analysis

Results are presented from an extensive series of new high resolution scanning electron microscope studies of the very primative group of meteorites known as unequilibrated chondrites. These include quantitative analyses of micrometer sized phases and interpretation in terms of relevant phase equilibria. Several new meteorite minerals including high chromium metal, have been discovered.

Housley, R. M.↗
NASA NTRS1983

Evidence for relict grains in chondrules of Qingzhen, an E3 type enstatite chondrite

Petrographic and chemical studies of the Qingzhen chondrite strongly suggest that it is the most highly unequilibrated (type 3) enstatite chondrite recognized so far. Qingzhen contains abundant, well-defined chondrules, some of which were incompletely molten during the chondrule formation process. The relict olivine grains within these chondrules contain dusty inclusions of almost pure metallic Fe, which appear to be the in situ reduction product of the fayalitic component of the olivine. The reduction process presumably took place at the time of chondrule formation and the chondrule precursor material must have been more oxidized than average enstatilite chondrite material. It is believed that this oxidized material may have formed at the enstatite chondrite formation location in the solar nebula, provided fluctuations in the degree of oxidation of the nebular gas existed at such locations. Reheating of this material under more reducing conditions would lead to the observed reduction of the olivine. Igneous olivines within chondrules always contain detectable amounts of CaO, while relict olivines are essentially CaO-free. This seems to suggest thatg the relict olivines did not originate during a previous igenous process of chondrule formation and might represent condensation products from the early solar nebula.

Rambaldi, E. R.↗
NASA NTRS1982

Fission track studies of xenolithic chondrites - Implications regarding brecciation and metamorphism

Fission tracks in phosphates from one gas-poor chondrite and three gas-rich ones are studied to determine their thermal history and brecciation time scales. Model fission track ages are calculated for given track densities due to Pu-244 and U-238; track densities and uranium measurements for whitlockite are shown, and possible sources of the tracks are mentioned. Details of track density and uranium measurements are discussed for each meteorite separately. Whitlockites from all the meteorites give model fission track ages of 4.4 Gyr assuming a Pu/U ratio at 4.55 Gyr of 0.045. The final brecciation event definitely did not reset the track clock in phosphates of one meteor and probably not in another two. It is concluded that the observed fission track ages date the end of metamorphic cooling in the meteorite parent bodies and support the planetesimal model for the formation of xenolithic chondrites.

Kothari, B. K.↗
NASA NTRS1981

Metallic minerals, thermal histories and parent bodies of some xenolithic, ordinary chondrite meteorites

The metal grains were investigated in the hosts and lithic fragments of various petrologic types in four xenolithic chondrites using reflected-light microscopy and electron-probe analysis. The kamacite, taenite, tetrataenite, and troilite in Weston and Fayetteville have many textures; on a Wood plot of the central Ni content vs dimensions, the taenite content shows scatter if metal grains had cooled at rates of 10-1000 and 1-100 K/Myr through 700 K. In contrast, metallic minerals in Bhola and Mezo-Madaras have uniform textures and plot coherently, indicating cooling rates of 0.1 and 1 K/Myr, respectively, in the 700-600 K range. It is concluded that the host and xenoliths in these chondrites were cooled slowly after compaction; their clasts underwent peak metamorphic temperatures and slow cooling through 700 K in different environments.

Scott, E. R. D.↗
NASA NTRS1980

On the role of a supernova in the formation of the solar system

Four possible scenarios relating the proto-solar cloud to the 'last-minute' supernova presumed responsible for the isotopic anomalies in Allende and other meteorites are considered. The probability that a chance supernova occurred close enough to an already-collapsing proto-solar cloud to inject sufficient matter is extremely small, even if the sun formed in a region of enhanced supernova activity such as Orion OB1. The ambient level of Al-26 inside a molecular cloud in Orion is also apparently too low to account for the meteorite data, unless the supernova ejecta accumulates at the edges of the cloud and star formation occurs there preferentially. Two modes of supernova-induced star formation are discussed. In one, the supernova shock collapses a preexisting cloud; in the other, stars form within the snowplow shell of the supernova. Canis Major R1 and Monoceros R1 are possible present-day examples of such star formation regions.

Herbst, W.↗
NASA NTRS1975

Rare gases in etched 10084 ilmenite - A search for trapped solar-flare rare gases

Results are presented of rare-gas analyses of 10084 ilmenite samples which have been etched to depths of about 1 micrometer in an attempt to remove the surface layer of trapped solar-wind gases, thereby allowing detection of the more deeply implanted solar-flare ions in the remaining trapped gases. A tendency found for the Ne-20/Ne-22 ratios in the etched ilmenite to reflect the solar-wind ratio measured in the solar-wind composition experiment foils may represent a cancellation of mass-fractionation effects. The Ne-20/Ne-22 ratio in the saturated layer is lowered with respect to the solar wind by preferential diffusive loss, but is raised again in the gas diffusing inward by the same preferential mobilization of Ne-20 relative to Ne-22. Attention is also given to the significance of the findings in the case of argon, krypton, and xenon.

Leich, D. A.↗
NASA NTRS1974

On the irradiation history and origin of gas-rich meteorites

Transmission electron microscope study of the track density gradients and irradiation geometries of track-rich grains and chondrules in sections of Fayetteville and Kapoeta, and in sections of lunar breccias and grains from lunar soil. A substantial fraction (50 to 90%) of the meteoritic track-rich grains and chondrules show evidence of having been irradiated anisotropically in their different faces, as would be expected for irradiation on the surface of a parent body. The observations thus support the hypothesis that the irradiation of these grains and chondrules took place on the regoliths of asteroidal-sized bodies. Measurements of steepest track density gradients indicate that, while there are finite differences between spectra exhibited by individual gas-rich meteorites, the average solar flare spectral shapes have been similar over the last 4 b.y. or so.

Rajan, R. S.↗
NASA NTRS1974

Gas-rich meteorites - Possible evidence for origin on a regolith

The asymmetry of irradiation features of grains in the Kapoeta and Fayetteville meteorites suggests irradiation on a regolith before meteorite formation. Chondrules and broken grains require approximately 10,000 years of irradiation time between formation or fracturing and compaction into the meteorite. Shock erasure of tracks from irradiated Kapoeta feldspars requires a severe shock event during or after meteorite formation.

Macdougal, D.↗
NASA NTRS1973

Micrometeorite craters discovered on chondrule-like objects from Kapoeta meteorite

Craters attributable to hypervelocity impacts of micrometeorites have been discovered on rare chondrule-like objects from the gas-rich meteorite Kapoeta. These chondrule-like objects, probably generated by impacts themselves, provide further evidence for the regolith origin of Kapoeta. The micrometeorite flux at the time of formation of the meteorites was probably an order of magnitude higher than the present flux, but the solar luminosity could not have been higher than 1.7 times its present value.

Brownlee, D. E.↗