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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS2014

Preliminary Report on U-Th-Pb Isotope Systematics of the Olivine-Phyric Shergottite Tissint

Geochemical studies of shergottites suggest that their parental magmas reflect mixtures between at least two distinct geochemical source reservoirs, producing correlations between radiogenic isotope compositions, and trace element abundances.. These correlations have been interpreted as indicating the presence of a reduced, incompatible-element- depleted reservoir and an oxidized, incompatible-element-rich reservoir. The former is clearly a depleted mantle source, but there has been a long debate regarding the origin of the enriched reservoir. Two contrasting models have been proposed regarding the location and mixing process of the two geochemical source reservoirs: (1) assimilation of oxidized crust by mantle derived, reduced magmas, or (2) mixing of two distinct mantle reservoirs during melting. The former clearly requires the ancient martian crust to be the enriched source (crustal assimilation), whereas the latter requires a long-lived enriched mantle domain that probably originated from residual melts formed during solidification of a magma ocean (heterogeneous mantle model). This study conducts Pb isotope and U-Th-Pb concentration analyses of the olivine-phyric shergottite Tissint because U-Th-Pb isotope systematics have been intensively used as a powerful radiogenic tracer to characterize old crust/sediment components in mantle- derived, terrestrial oceanic island basalts. The U-Th-Pb analyses are applied to sequential acid leaching fractions obtained from Tissint whole-rock powder in order to search for Pb isotopic source components in Tissint magma. Here we report preliminary results of the U-Th-Pb analyses of acid leachates and a residue, and propose the possibility that Tissint would have experienced minor assimilation of old martian crust.

Moriwaki, R.↗
NASA NTRS2020

Spatial distribution of the X-ray-emitting plasma of U Geminorum in quiescence and outburst

We present our analysis of the Suzaku data of U Geminorum (U Gem) from 2012 both in quiescence and outburst. Unlike SS Cygni (SS Cyg), the hard X-ray flux of U Gem is known to increase at times of optical outburst. A sophisticated spectral model and reliable distance estimate now reveal that this can be attributed to the fact that the mass accretion rate onto the white dwarf (WD) does not exceed the critical rate that causes the optically thin to thick transition of the boundary layer. From comparison of the X-ray and optical light curves, the X-ray outburst peak seems to be retarded by 2.1 ± 0.5 d, although there remains uncertainty in the X-ray peak identification, due to short data coverage. The larger delay than SS Cyg (0.9–1.4 d) also supports the lower accretion rate in U Gem. A fluorescent iron 6.4 keV emission line bears significant information about the geometry of the X-ray-emitting hot plasma and the accretion disk (AD) that reflects the hard X-ray emission. Our reflection simulation has shown that the optically thick AD is truncated at a distance of 1.20–1.25 times the white dwarf radius (RWD) in quiescence, and the accreting matter in the disk turns into the optically thin hard-X-ray-emitting plasma at this radius. In outburst, on the other hand, our spectral analysis favors the picture that the optically thick disk reaches the WD surface, although disk truncation can take place in the region of <1.012 RWD. From the profile of the 6.4 keV line, we have also discovered that the accreting matter is heated up close to the maximum temperature immediately after the matter enters the boundary layer at the disk truncation radius. This is consistent with the fact that the hard X-ray spectra of dwarf novae, in general, can be well represented with the cooling flow model.

Mai Takeo↗
NASA NTRS1972

U-Th-Pb and Rb-Sr measurements on some Apollo 14 lunar samples.

The concentrations of selected elements and elemental ratios in some Apollo 14 samples are presented in a table. Another table shows the isotopic composition of lead and strontium in some Apollo 14 samples. Graphs are provided with lead isotope ratio plots and a strontium evolution diagram. Rb-Sr ages are discussed together with isochron age methods of U-Th-Pb, a primary Pb-Pb isochron, a secondary Pb-Pb isochron, U-Pb concordia age relations, and U-Th-Pb concordia relations. Breccias and some soil samples as well as a basalt sample give evidence of significant enrichment of lead relative to uranium and thorium billions of years ago at the Fra Mauro site.

Tatsumoto, M.↗
NASA NTRS1974

U-Th-Pb and Rb-Sr systematics of Apollo 17 boulder 7 from the North Massif of the Taurus-Littrow valley

Portions of highland breccia boulder 7 collected during the Apollo 17 mission were studied using U-Th-Pb and Rb-Sr systematics. A Rb-Sr internal isochron age of 3.89 plus or minus 0.08 b.y. with an initial Sr-87/Sr-86 of 0.69926 plus or minus 0.00008 was obtained for clast 1 (77135,57) (a troctolitic microbreccia). A troctolitic portion of microbreccia clast 77215,37 yielded a U-Pb internal isochron of 3.8 plus or minus 0.2 b.y. and an initial Pb-206/Pb-207 of 0.69. These internal isochron ages are interpreted as reflecting metamorphic events, probably related to impacts, which reset Rb-Sr and U-Pb mineral systems of older rocks.

Nunes, P. D.↗
NASA NTRS1977

Early history of the moon: Implications of U-Th-Pb and Rb-Sr systematics

Anorthosite 60015 contains the lowest initial Sr-87/Sr-86 ratio yet reported for a lunar sample. The initial ratio is equal to that of the achondrite Angra dos Reis and slightly higher than the lowest measured Sr-87/Sr-86 ratio for an inclusion in the C3 carbonaceous chondrite Allende. The Pb-Pb ages of both Angra does Reis and Allende are 4.62 X 10 to the ninth power yr. Thus, the initial Sr/87/Sr-86 ratio found in lunar anorthosite 60015 strongly supports the hypothesis that the age of the Moon is about 4.65 b.y. The U-238/Pb-204 value estimated for the source of the excess lead in "orange soil" 74220 is approximately 35 and lower than the values estimated for the sources of KREEP (600-1000), high-K (300-600), and low-K (100-300) basalts. From these and other physical, chemical and petrographic results it was hypothesized that (1) the moon formed approximately 4.65 b.y. ago; (2) a global-scale gravitational differentiation occurred at the beginning of lunar history; and (3) the differentiation resulted in a radical chemical and mineralogical zoning in which the U-238/Pb-204 ratios increased toward the surface, with the exception of the low U-238/Pb-204 surficial anorthositic layer which "floated" at the beginning of the differentiation relative to the denser pyroxene-rich material.

Tatsumoto, M.↗
NASA NTRS1978

Origin and history of the adcumulate eucrite, Moama as inferred from REE abundances, Sm-Nd and U-Pb systematics

The abundances of Sm-Nd and U-Pb isotopes and of rare earth elements (REE) in Moama cumulate eucrite were investigated. The Sm-Nd data plotted on a Nd-143/Nd-144 vs Sm-147/Nd-144 evolution diagram define a line with a slope corresponding to an age of 4.58 plus or minus 0.05 billion years and an initial Nd-143/Nd-144 ratio of about 0.50684. Moama was found to contain the lowest overall trivalent REE abundances, the most light REE-depleted abundance pattern, and largest positive Eu anomaly of any eucrite yet studied. The REE data indicate that Moama could have been derived by about 1 to 5% fractional crystallization from a liquid with REE contents similar to Juvinas, or about 20 to 30% fractional crystallization from a Sioux County-like liquid. The data are consistent with the hypothesis that the Moama, Serra de Mage, and Moore county cumulate meteorites could have been derived from the same parent liquid. Results of the U-Th-Pb study indicate that the majority of the Pb in Moama is of terrestrial origin, and suggest that the meteorite has undergone a minor thermal event causing re-equilibration of the U-Pb system.

Hamet, J.↗
NASA NTRS1979

U-Th-Pb age of the Barwell chondrite - Anatomy of a 'discordant' meteorite

A Pb-Pb internal isochron for the Barwell L5-6 chondrite yields an age of 4.530 plus or minus 0.005 billion years, using the measured U-238/U-235 ratio of 135.24 plus or minus .17. If the terrestrial U isotope composition is used, an age of 4.559 billion years is obtained. The Pb isotopic composition is distinctly different from that of a terrestrial contaminant found in the fusion crust of the Barwell stone. When the U-Th-Pb data are plotted on the concordia diagram, the data define a line that intersects the concordia curve at approximately 4.53 and 0 billion years, and nearly all of the data plot above the concordia curve, regardless of the initial Pb correction. This discordancy and the Pb isotopic composition of the triolite are attributed to a recent reequilibration of Pb and not to terrestrial contamination.

Unruh, D. M.↗
NASA NTRS1980

Electron-impact excitation of the singlet states of N2. I - The Birge-Hopfield system /b 1 pi u - X 1 Sigma g +/

Results of a study of the electron-impact excitation of the b 1 pi u state of N2, one of the singlet states thought to be excited by precipitating electrons in the auroral zones, and of its predissociation and radiative relaxation through the emission of the Birge-Hopfield band system (b 1 pi u - X 1 Sigma g +) are presented. A collimated electron beam was passed through N2 gas producing a variety of atomic states through dissociative excitation, and the radiation resulting from relaxation of these states was observed by VUV and visible-IR monochromators. Absolute emission cross sections for 11 Birge-Hopfield bands are obtained for energies from threshold to 500 eV, and used to calculate the absolute transition probabilities for BH(1, v-prime) bands and the variation of the electric dipole moment with internuclear distance. With the exception of the v-prime equals 1, 5 and 6 vibrational levels, all b 1 pi u levels are found to predissociate with a specific predissociation branching ratio greater than 0.99, representing a major source of nitrogen atoms.

Zipf, E. C.↗
NASA NTRS1984

Observations of quasi-coherent soft X-ray oscillations in U Geminorum and SS Cygni

During the HEAO 1 satellite's detection of soft X-ray pulsations from U Geminorum and SS Cygni, both of these dwarf novae were on the declining phase of an optical outburst. The pulsations are quasi-coherent in character, with average pulse amplitudes of 15 percent for U Gem and 18 percent for SS Cyg. From pulse phase X-ray spectroscopic data, an upper limit of 50 percent may be set on any variation in temperature across U Gem's pulse. Attention is given to all data on the X-ray and optical pulsations thus far detected in these two novae, and the observational constraints on the various current models for the pulsations are discussed.

Cordova, F. A.↗
NASA NTRS1986

Formation ages and evolution of Shergotty and its parent planet from U-Th-Pb systematics

The isotopic composition of Pb from Shergotty, Zagami, and EETA 79001 meteorites was determined for different phases. Using phases with low U-238/Pb-204 ratio, the initial leads of these meteorites were defined. Samples from all three meteorites were shown to have distinct initial leads, and, thus to have evolved from different reservoirs over most of solar system history in a low U-238/Pb-204 environment. It follows that the parent planet of the shergottites has a high Pb-204 concentration relative to U and must also be high in other volatiles. The possibility of the Martian origin of the SNC-type meteorites is discussed.

Chen, J. H.↗
NASA NTRS1991

The Estherville mesosiderite - U-Pb, Rb-Sr, and Sm-Nd isotopic study of a polymict breccia

The U-Pb, Rb-Sr, and Sm-Nd isotopic systematics of the Estherville mesoderite were studied using analytical procedures and mass spectrometric techniques similar to those reported by Nakamura et al. (1976), Tatsumoto et al. (1987), and Premo et al. (1989) to analyze 21 separates of a 13-g Estherville clast, obtained either by handpicking or by using density and magnetic separation methods. The results on the Pb-Pb and U-Pb ages (about 4555 and about 4560 Ma, respectively) indicate that at least a part of the Estherville silicate fraction was formed early in the history of solar system. Younger Pb-Pb and U-Pb ages (about 4.43 Ga) were also obtained, confirming the heterogeneity of the Estherville mesoderite that is in agreement with the Wasson and Rubin (1985) suggestion of several generations of meteoritic impacts. The Sm-Nd and Rb-Sr ages were found to be close to 4.56 Ga.

Brouxel, M.↗
NASA NTRS1993

Observations of the white dwarf in the U Geminorum system with the Hopkins Ultraviolet Telescope

The Hopkins Ultraviolet Telescope was used to obtain an FUV spectrum of U Gem about 10 d after the end of an outburst. Most of the FUV emission from U Gem appears to rise from the white dwarf (WD) in the system. Various possibilities to explain the hot component in the present spectrum are explored. It is most likely due to radiation from the boundary-layer region of the WD surface rather than from the accretion disk which exists in U Gem in quiescence. If the standard picture of boundary-layer emission is correct, then the hot component is not due to ongoing accretion onto the WD because the temperature observed is too low. Delayed emission from a viscously heated, differentially rotating atmosphere is argued to be a more valid explanation.

Long, Knox S.↗
NASA NTRS1993

U-Pb isotopic ages and characteristics of ancient (greater than 4.0 Ga) lunar highland rocks

A review of the present (and certainly sparse) U-Pb isotopic database for ancient (greater than 4.0 Ga) lunar highland rocks indicates that both early-forming anorthosites and high-Mg suite rocks were formed from sources with a wide variety of U-238/Pb-204(mu) values as early as 4.44 Ga. The U-Pb data from Apollo 16 anorthosites 67075, 60025, and 62337 indicate source-mu values between approximately 55 and approximately 350; whereas values for high-Mg suite rocks are tentatively considered to be over 500, similar to KREEP sources. At present, I interpret the data to indicate that either early cumulates were formed slowly in a magma environment that allowed isotopic reequilibration or mixing over a long period of crystallization, or these cumulates formed from rapidly-evolving (both geochemically and isotopically) residual liquids. It is possible that an early bombardment period played a role in the mixing process.

Premo, W. R.↗
NASA NTRS1997

Temperature Dependence of the Collisional Removal of O2(A(sup 3)Sigma(sup +)(sub u), upsilon=9 ) with O2 and N2

The temperature dependence of the collisional removal of O2 molecules in the upsilon = 9 level of the A(sup 3)Sigma(sup +)(sub u) electronic state has been studied for the colliders O2 and N2, over the temperature range 150 to 300 K. In a cooled flow cell, the output of a pulsed dye laser excites the O2 to the upsilon = 9 level of the A(sup 3)Sigma(sup +)(sub u) state, and the output of a time-delayed second laser monitors the temporal evolution of this level via a resonance-enhanced ionization. We find the u thermally averaged removal cross section for O2 collisions is constant (approx. 10 A(sup 2)) between room temperature and 200 K, then increases rapidly with decreasing temperature, doubling by 150 K. In contrast, the N2 cross section at 225 K is approx. 8% smaller and gradually increases to a value at 150 K that is approx. 60% larger than the room temperature value. The difference between the temperature dependence of the O2 and N2 collision cross section implies that the removal by oxygen becomes more important at the lower temperatures found in the mesosphere, but removal by N2 still dominates.

Hwang, Eunsook S.↗
NASA NTRS2017

Complex Volcanism at Oppenheimer U Floor-Fractured Crater

Recent remote sensing studies have identified complex volcanism in the floor-fractured crater (FFC) Oppenheimer U, located in the northwest floor of Oppenheimer crater (35.2degS, 166.3degW, 208 km dia., Figure 1) within the "South Pole - Aitken basin" (SPA) region of the lunar far side. Up to 15 sites of pyroclastic volcanism have been identified in the floor of Oppenheimer crater. Studies of Moon Mineralogy Mapper data (M3, 0.4-3 microns, 86 bands, [5]) indicated that the pyroclastic deposits are comprised of mixtures of clinopyroxene and iron-rich glass, with the Oppenheimer U deposit showing variable composition within the FFC and having the most iron-rich volcanic glass thus far identified on the Moon. Here we examine the floor of Oppenheimer U in more detail and show evidence for possible multiple eruptive vents.

Gaddis, L. R.↗
NASA NTRS2019

Unravelling Complex Geologic Histories Using U–Pb and Trace Element Systematics of Titanite

Unravelling the spatio-temporal evolution of orogenic terranes requires a comprehensive understanding of the duration and extent of metamorphic events and hydrothermal alteration. Commonly used minerals such as zircon and monazite may not fully record geological histories in complex tectonic settings because their elemental constituents do not react under many metamorphic and metasomatic conditions. Here, we complement the current geochronological record of the Capricorn Orogen, Western Australia, with titanite U–Pb geochronology and geochemistry of felsic intrusive rocks to draw conclusions about the use of titanite in understanding the evolution of orogenic terranes. Because titanite usually incorporates common-Pb and may be variably reset by multiple metamorphic and hydrothermal events, a workflow is provided here for the systematic and robust interpretation of titanite U-Pb data. The addition of trace element data in titanite is particularly effective for differentiating whether a grain is igneous, recrystallized or metamorphic. We have developed several petrogenetic indices to differentiate these three types of titanite using Zr-in-titanite temperature, Th/U, Th/Pb, Al/(Al + Fe), light to heavy rare earth element ratio, and Eu anomalies. The addition of trace element geochemistry can also highlight anomalously radiogenic (sup 207)Pb/(sup 206)Pb reservoirs. Utilization of our workflow in the Capricorn Orogen reveals that titanite ages from the same samples as published zircon U–Pb data range from coeval to several hundreds of Myr of age difference between the two minerals. Titanite geochronology and trace element geochemistry indicates ~20 Myr of previously unrecognized prolonged cooling for the Capricorn Orogeny to ca. 1750 Ma. The spatial extent of the ca. 1210–1170 Ma part of the Mutherbukin Tectonic Event is also broadened significantly farther north and south than previously recognized. Incorporating titanite geochronology and trace geochemistry with more commonly used techniques (e.g., zircon and monazite petrochronology) extends our ability to resolve the complete history of large-scale orogenic terranes.

Hugo K. H. Olierook↗
NASA NTRS2021

Multiwavelength Observations of the RV Tauri Variable System U Monocerotis: Long-term Variability Phenomena That Can Be Explained by Binary Interactions with a Circumbinary Disk

We present an X-ray through submillimeter observations of the classical RV Tauri(RVb-type)variable U Mon, apost-asymptotic giant branch(AGB)binary with a circumbinary disk(CBD). Our SMA observations indicate aCBD diameter of550 au. Our XMM-Newton observations make U Mon thefirst RV Tauri variable detected inX-rays. The X-ray emission is characteristic of a hot plasma(∼10 MK), withLX=5×1030erg s−1, and weconsider its possible origin from U Mon, its companion, and/or binary system interactions. Combining DASCHand AAVSO data, we extend the time-series photometric baseline back to the late 1880s andfind evidence thatU Mon has secular changes that appear to recur on a timescale of∼60 yr, possibly caused by a feature in the CBD.From literature radial velocities wefind that the binary companion is a∼2MeA-type main-sequence star. Theorientation of the binary’s orbit lies along our line of sight(ω=95°), such that apastron corresponds to photometricRVb minima, consistent with the post-AGB star becoming obscured by the near side of the CBD. In addition, wefind the size of the inner-CBD hole(∼4.5–9au)to be comparable to the binary separation, implying that one orboth stars may interact with the CBD at apastron. The obscuration of the post-AGB star implicates the companionas the likely source of the enhanced Hαobserved at RVb minima and of the X-ray emission that may arise fromaccreted material.

Laura D Vega↗
NASA NTRSDate not supplied

Zircon (U-Th)/He Impact Crater Thermochronometry and the Effects of Shock Microstructures on Helium Diffusion Kinetics

Absolute age determination of impact cratering events remains difficult and often controversial; a challenge that has resulted in < 50% of known impact craters regarded as accurately and precisely dated. Besides conventional 40Ar/39Ar and U-Pb methods, zircon (U-Th)/He (ZHe) dating of impactites has been applied to large- to medium-sized impact structures. ZHe dates can be fully reset in minutes at 1000°C, which is commonly reached in central sections of the melt sheet, whereas resetting of ZHe at <300°C, which might be encountered near the crater margins or persist in post-impact hydrothermal systems, may take >103-6 years. There is a critical need to quantify the effects of shock-induced microstructures and impact metamorphism on helium diffusion kinetics in well-characterized, variably shocked zircon to further establish the reliability of (U-Th)/He for dating impacts. For this purpose, we investigated suevite and impact melt samples from two impact structures, the Chicxulub multi-ring basin and the Ries complex crater, which enables us to compare zircon helium diffusion kinetics from impact structures with differing sizes, ages, and hydrothermal system longevities. Shock microstructures were characterized by backscattered-electron (BSE) imaging prior to diffusion step-heating fractional release experiments using light-bulb furnace with prograde and retrograde incremental 10°C steps from 250°C to 600°C. Afterward, we characterize the diffusion domain sizes and their interconnectivity within the shocked zircon grains using electron backscatter diffraction (EBSD). We find that zircon with few shock microstructures exhibit no significant deviation from helium diffusion kinetics of undamaged zircon. In contrast, zircon grains with planar deformation features (PDFs) and granular textures classified by BSE and EBSD are characterized by a dramatic decrease in helium retentivity, similar to radiation damaged grains, due to a reduction in the effective domain size and the introduction of interconnected fast diffusion pathways created by shock microstructures. A subset of grains were dated by ZHe after the external morphology of the grains was determined by BSE imaging. The euhedral grains yielded a weighted mean age within the uncertainty of the accepted impact ages, whereas the grains with PDFs or granular textures exhibited younger ages. Thus, these diffusion experiments and ZHe dates suggest that the dramatic decrease in domain size likely renders shocked grains more susceptible to impact-induced hydrothermal resetting and subsequent overprinting. Hence, characterization of shock microstructures is critical for determining accurate impact ages using ZHe methods especially when applied to previously unconstrained craters. The thermochronometer also offers the opportunity to determine the magnitude and duration of post-impact hydrothermal circulation.

zircon↗