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At least 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Determination of the half-life of Ca-41 from measurements of Antarctic meteorites

Accelerator mass spectrometry is utilized to determine the half-life of Ca-41 from the decrease of its concentration with terrestrial age in five Antarctic meteorites and a recent fall. The meteorites were selected on the basis of their Cl-36 concentrations, which showed a span of terrestrial ages of about 600 ka, and on the basis of other cosmogenic nuclide concentrations which indicated that the meteorites had small preatmospheric sizes, and sufficiently long irradiation times in space that the concentrations of Ca-41 and Cl-36 were in secular equilibrium prior to the meteorites' fall to earth. The half-life of Ca-41 is determined at 103 + or - 7 ka. Topics discussed include the effects of undersaturation (short exposure time in space), shielding (the samples are from the interior of a large meteorite), and weathering on the cosmogenic nuclide concentrations in meteorites.

Klein, Jeffrey↗
NASA NTRS1991

New evidence for a dramatic rise in atmosphere oxygen ca. 1,900 m.y. ago

Several lines of geologic evidence have pointed to a significant increase in P sub O2 about 2,000 m.y. ago, but the magnitude of P sub O2 before and after that time has been quite uncertain. The data that we have recently accumulated suggest that P sub O2 was approximately less than 2 x 10 (exp -3) atm more than 2,000 m.y. ago, and approximately greater than 0.03 atm more recently than ca. 1,900 m.y. ago. These estimates are based on the behavior of iron in Precambrian weathering horizons. More than ca. 2,000 m.y. ago, Fe(+2), released during the weathering of basalts was not oxidized to Fe(+3), and was removed in ground water from the upper layers of soil horizons. More recently than ca. 1,850 m.y. ago, Fe(+2) was oxidized to Fe(+3) and precipitated as iron oxides and hydroxides in such soil horizons and in the weathering products of a carbonate facies banded iron formation in Griqualand West, South Africa. The O2 content of the atmosphere must have increased dramatically about 1,900 m.y. ago to explain these observations. The reasons for the increase are still obscure, but are probably related to changes in the biologic productivity of the oceans. Eukaryotes appear to have developed shortly after the increase in P sub O2, perhaps in response to the subsequent increase in the supply of nitrate from the atmosphere to the oceans.

Holland, H. D.↗
NASA NTRS1991

Effects of Mg II and Ca II ionization on ab-initio solar chromosphere models

Acoustically heated solar chromosphere models are computed considering radiation damping by (non-LTE) emission from H(-) and by Mg II and Ca II emission lines. The radiative transfer equations for the Mg II k and Ca II K emission lines are solved using the core-saturation method with complete redistribution. The Mg II k and Ca II K cooling rates are compared with the VAL model C. Several substantial improvements over the work of Ulmschneider et al. (1987) are included. It is found that the rapid temperature rises caused by the ionization of Mg II are not formed in the middle chromosphere, but occur at larger atmospheric heights. These models represent the temperature structure of the 'real' solar chromosphere much better. This result is a major precondition for the study of ab-initio models for solar flux tubes based on MHD wave propagation and also for ab-initio models for the solar transition layer.

Rammacher, W.↗
NASA NTRS1993

Ca-,Al-rich inclusions in the unique chondrite ALH85085 - Petrology, chemistry, and isotopic compositions

A comprehensive study is performed for the Ca-,Al-rich inclusions (CAIs) in the unique chondrite ALH85085. The ALH85085 inclusions are smaller (5-80 microns) and more refractory than their counterparts in carbonaceous chondrites. The study includes 42 inclusions for petrography and mineralogy, 15 for bulk major and minor element chemical composition, six for Mg-Al isotopic systematics, 10 for Ca isotopes, nine for Ti isotopes, and six for trace element abundances. In addition, oxygen-isotopic compositions were determined in minerals from a single inclusion. No correlation is found between mineralogy, major element chemistry, and trace element abundances. It is further shown that the high-temperature geochemical behavior of ultrarefractory trace elements is decoupled from that of the major elements Ca and Ti (Ti is correlated with the relatively volatile elements Nb and Yb) implying that perovskite is of only minor importance as carrier of ultrarefractories.

Kimura, Makoto↗
NASA NTRS1993

Chronology of lunar granite 12033,576: Resetting of Rb-Sr and K-Ca isochrons

Lunar granite 12033,576 is a subsample of the 'large' (approximately 1 g) felsite 12033,507 which was identified from a collection of 4-10 mm particles from the 12033 soil sampled from the north rim of Head Crater in the eastern part of Oceanus Procellarum. Discordant ages of approximately 3.6, approximately 0.8, approximately 3.9, and approximately 2.2 Ga for this lunar granite were obtained, respectively, by the K-Ca, Ar-39/Ar-40, and U-Pb zircon methods in previous studies and by the Rb-Sr method is this study. Assuming the granite crystallized approximately 3.9 Ga ago (zircon age) and was shocked by meteoritic impacts at 0.8 Ga ago (Ar-39-Ar-40 age), the intermediate apparent ages by the Rb-Sr and K-Ca methods can be interpreted as reset by diffusion of the parent and daughter nuclides. The Rb-Sr age is less resistant to resetting than the K-Ca age, but more resistant that the Ar-39/Ar-40 age.

Shih, C.-Y.↗
NASA NTRS1995

Injection of freshly synthesized Ca-41 in the early solar nebula by an asymptotic giant branch star

We show that ejecta from the envelope of one asymptotic giant branch star of M is approximately 3 solar mass may account for many of the short-lived nuclei in the early solar system and also for the recent evidence of the presence of Ca-41 (bar-tau(sub 41) = 1.50 x 10(exp 5) yr) in early solar nebular condensates. This would require that the injection into the protosolar molecular cloud took place within a narrow time interval of (5-7) x 10(exp 5) yr before the formation of the solar system. If true, this places extremely tight constraints on the whole process of injection mixing and collapse. The timescales for both Ca-41 and Al-26 require that the placental medium be a dense molecular cloud (2 x 10(exp 3) -8 x 10(exp 3( H/cc). If the observed residual Ca-41 is instead produced by a proton bombardment mechanism within the early solar system, similar to what appears necessary to explain Mn-53, then the time interval is relaxed but would still be (1-2) x 10(exp 6) yr from consideration of Al-26.

Wasserburg, G. J.↗
NASA NTRS1995

Neutron-capture Cl-36, Ca-41, Ar-36, and Sm-150 in large chondrites: Evidence for high fluences of thermalized neutrons

We have measured significant concentrations of Cl-36, Ca-41, Ar-36 from decay of Cl-36, and Sm-150 produced from the capture of thermalized neutrons in the large Chico L6 chondrite. Activities of Cl-36 and Ca-41, corrected for a high-energy spallogenic component and a terrestrial age of approximately 50 ka, give average neutron-capture production rates of 208 atoms/min/g-Cl and 1525 atoms/min/kg-Ca, which correspond to thermal neutron (n) fluxes of 6.2 n/sq cm/s and 4.3 n/sq cm/s, respectively. If sustained for the approximately 65 Ma single-stage, cosmic ray exposure age of Chico, these values correspond to thermal neutron fluences of approximately 1.3 x 10(exp 16) and 0.8 x 10(exp 16) n/sq cm for Cl-36 and Ca-41, respectively. Stepwise temperature extraction of Ar in Chico impact melt shows Ar-36/Ar-38 ratios as large as approximately 9. The correlation of high Ar-36/Ar-38 with high Cl/Ca phases in neutron-irradiated Chico indicates that the excess Ar-36 above that expected from spallation is due to decay of neutron-produced Cl-36. Excess Ar-36 in Chico requires a thermal neutron fluence of 0.9-1.7 x 10(exp 16) n/sq cm. Decreases in Sm-149/Sm-152 due to neutron-capture by Sm-149 correlate with increases in Sm-150/Sm-152 for three samples of Chico, and one of the Torino H-chondrite. The 0.08% decrease in Sm-149 shown by Chico corresponds to a neutron fluence of 1.23 x 10(exp 16) n/sq cm. This fluence derived from Sm considers capture of epithermal neutrons and effects of chemical composition on the neutron energy distribution. Excess Ar-36 identified in the Arapahoe, Bruderheim, and Torino chondrites and the Shallowater aubrite suggest exposure to neutron fluences of approximately 0.2-0.2 x 10(exp 16) n/sq cm. Depletion of Sm-149 in Torino and the LEW86010 angrite suggest neutron fluences of 0.8 x 10(exp 16) n/sq cm and 0.25 x 10(exp 16) n/sq cm, respectively. Neutron fluences of approximately 10(exp 16) n/sq cm in Chico are almost as large as those previously observed for some lunar soils. Consideration of exposure ages suggests that the neutron flux in Chico may have been greater than that in many lunar soils.

Bogard, D. D.↗
NASA NTRS2000

Crystallization of Ca-Al-Rich Inclusions: Experimental Studies on the Effects of Repeated Heating Events

The observed textures and chemistry of Ca-Al-rich inclusions (CAIs) are presumed to be the culmination of a series of repeated heating and cooling events in the early history of the solar nebula. We have examined the effects of these heating/cooling cycles experimentally on a bulk composition representing an average Type B Ca-Al-rich inclusion composition. We have tested the effect of the nature of the starting material. Although the most recent and/or highest temperature event prior to incorporation into the parent body dominates the texture and chemistry of the CAI, prior events also affect the phase compositions and textures. We have determined that heating precursor grains to about 1275 C prior to the final melting event increases the likelihood of anorthite crystallization in subsequent higher temperature events and a prior high temperature even that produced dendritic melilite results in melilite that shows evidence of rapid crystallization in subsequent lower temperature events. Prior low temperature pre-crystallization events produce final ran products with pyroxene compositions similar to Type B Ca-Al-rich inclusions, and the glass (residual liquid) composition is more anorthitic than any other experiments to date. The addition of Pt powder to the starting material appears to enhance the ability of anorthite to nucleate from this composition.

Paque, Julie M.↗
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 NTRS2003

Trace Elements in High-Ca Pyroxene and Plagioclase in the Bilanga Diogenite: Implications for the Magmatic Evolution of Diogenites

High-Ca pyroxene and plagioclase are typically present as minor phases in diogenites. However, although the trace element content of diogenite orthopyroxene has been measured in a number of studies; almost no trace element data is available for the high-Ca pyroxene and plagioclase with which it routinely coexists in these meteorites. These data could provide insights into the nature and evolution of the melts from which diogenites crystallized in the HED parent body. In this study we have obtained initial measurements of several REEs in high-Ca pyroxene, plagioclase, and orthopyroxene in the Bilanga. Measurement of additional incompatible trace element concentrations in these phases is currently in progress.

Domanik, K. J.↗
NASA NTRS1991

Partial purification and characterization of a Ca(2+)-dependent protein kinase from pea nuclei

Almost all the Ca(2+)-dependent protein kinase activity in nuclei purified from etiolated pea (Pisum sativum, L.) plumules is present in a single enzyme that can be extracted from chromatin by 0.3 molar NaCl. This protein kinase can be further purified 80,000-fold by salt fractionation and high performance liquid chromatography, after which it has a high specific activity of about 100 picomoles per minute per microgram in the presence of Ca2+ and reaches half-maximal activation at about 3 x 10(-7) molar free Ca2+, without calmodulin. It is a monomer with a molecular weight near 90,000. It can efficiently use histone III-S, ribosomal S6 protein, and casein as artificial substrates, but it phosphorylates phosvitin only weakly. Its Ca(2+)-dependent kinase activity is half-maximally inhibited by 0.1 millimolar chlorpromazine, by 35 nanomolar K-252a and by 7 nanomolar staurosporine. It is insensitive to sphingosine, an inhibitor of protein kinase C, and to basic polypeptides that block other Ca(2+)-dependent protein kinases. It is not stimulated by exogenous phospholipids or fatty acids. In intact isolated pea nuclei it preferentially phosphorylates several chromatin-associated proteins, with the most phosphorylated protein band being near the same molecular weight (43,000) as a nuclear protein substrate whose phosphorylation has been reported to be stimulated by phytochrome in a calcium-dependent fashion.

NASA Discipline Number 40-50↗
NASA NTRS2005

Olivine and Ca-Phosphate in the Diogenites Manegaon and Roda

The textural relationships between the different primary minerals in igneous rocks provide one of the most fundamental pieces of evidence available for inferring the crystallization history of their parent magmas. Unfortunately, the high degree of brecciation that characterizes most diogenites, along with the low modal abundance and small grain sizes of minerals other than orthopyroxene, combine to make identifying and interpreting such textural relationships extremely difficult in this class of meteorites. A few descriptions of primary igneous contacts between orthopyroxene and chromite, troilite, and to a lesser extent, olivine in diogenites have been provided in the literature. In addition to these, in previous work, our research group has characterized several types of igneous contacts between Ca-pyroxene, plagioclase and orthopyroxene in the Bilanga diogenite. We have also described primary igneous inclusions of troilite + kamacite + chromite + Ca-pyroxene +/- Ca-phosphate in orthopyroxene (i.e. Metal/Troilite- Ball inclusions) in the diogenites Bilanga, Manegaon, Johnstown, Roda, Shalka, and Tatahouine. However, for the most part, detailed data on igneous textural relations between minerals other than orthopyroxene in diogenites are still sparse. Of the diogenite samples that we have examined,

Domanik, K. J.↗
NASA NTRS1996

Infrared Optical Properties of La(sub 0.7)Ca(sub 0.3)MnO(sub 3) Epitaxial Films

The reflectance of La(sub 0.7)Ca(sub 0.3)MnO(sub 3) epitaxial films on perovskite substrates with a range of lattice constants is studied in the frequency range of 50 cm(sup -1) - 5000cm(sup -1). The complex dielectric functions of the bare La(sub 0.7)Ca(sub 0.3)MnO(sub 3) films are obtained by modeling the measured reflectivity spectra of the two-layer La(sub 0.7)Ca(sub 0.3)MnO(sub 3)/substrate system with separaately measured dielectric functions of the bare substrate.

lattice distortion epitaxial films lattice polaron↗
NASA NTRS2011

K-Ca and Rb-Sr Dating of Lunar Granite 14321 Revisited

K-Ca and Rb-Sr age determinations were made for a bulk feldspar-rich portion of an Apollo rock fragment of the pristine lunar granite clast (14321,1062), an acid-leached split of the sample, and the leachate. K-Ca and Rb-Sr data were also obtained for a whole rock sample of Apollo ferroan anorthosite (FAN, 15415). The recent detection [1] of widespread intermediate composition plagioclase indicates that the generation of a diversity of evolved lunar magmas maybe more common and therefore more important to our understanding of crust formation than previously believed. Our new data strengthen the K-Ca and Rb-Sr internal isochrons of the well-studied Apollo sample 14321 [2], which along with a renewed effort to study evolved lunar magmas will provide an improved understanding of the petrogenetic history of evolved rocks on the Moon.

Simon, Justin I.↗
NASA NTRS2015

Early Solar System Alkali Fractionation Events Recorded by K-Ca Isotopes in the Yamato-74442 LL-Chondritic Breccia

Radiogenic ingrowth of Ca-40 due to decay of K-40 occurred early in the solar system history causing the Ca-40 abundance to vary within different early-former reservoirs. Marshall and DePaolo ] demonstrated that the K-40/Ca-40 decay system could be a useful radiogenic tracer for studies of terrestrial rocks. Shih et al. [3,4] determined 40K/40Ca ages of lunar granitic rock fragments and discussed the chemical characteristics of their source materials. Recently, Yokoyama et al. [5] showed the application of the K-40/Ca-40 chronometer for high K/Ca materials in ordinary chondrites (OCs). High-precision calcium isotopic data are needed to constrain mixing processes among early solar system materials and the time of planetesimal formation. To better constrain the solar system calcium isotopic compositions among astromaterials, we have determined the calcium isotopic compositions of OCs and an angrite. We further estimated a source K/Ca ratio for alkali-rich fragments in a chondritic breccia using the estimated solar system initial Ca-40/Ca-44.

Tatsunori, T.↗
NASA NTRS2019

Three CA-Related Covariance Issues and Their Solutions

This presentation addresses three issues that arise in the use of DoD-produced satellite state estimate covariances in the conjunction assessment process: the realism of the provided covariances, how to address correlated error between two satellites' covariance matrices, and how to proceed when a furnished covariance is non-positive-definite. To address the first, DoD has implemented a set of two "consider parameters" with which to alter/expand the covariance to account for atmospheric density forecast error and satellite frontal area uncertainty, the two largest sources of unmodeled position prediction error for LEO orbits; these values are governed by a satellite's orbital parameters and ballistic coefficient histories, as well as the current and predicted space weather situation. This approach substantially improves the realism of the covariance by accounting for expected prediction errors that are not part of the fit process that generates the covariance. The second issue of covariance correlation is a complicated one, but the main shared error source—global atmospheric density error that is common to both satellites can be characterized and, through sensitivity vectors, quantified at the conjunction's time of closest approach and removed from the joint covariance, which is used in the CA probability of collision (Pc) calculation. Finally, while according to the orbit determination theory a non-positive-definite (NPD) covariance is not possible, numerical truncation and covariance interpolation can conspire to produce NPD results, which render the covariance unusable for certain CA risk assessment calculations, such as Monte Carlo Pc determination. Three similar techniques are profiled and the simplest of them recommended as a reasonable remediation technique when NPD covariances are encountered by CA practitioners.

Dynamic Consider Parameter↗
DOE OSTI2020

Materials Data on Ca(ScS2)2 by Materials Project

Ca(ScS2)2 crystallizes in the orthorhombic Pnma space group. The structure is three-dimensional. Ca2+ is bonded in a 8-coordinate geometry to eight S2- atoms. There are a spread of Ca–S bond distances ranging from 2.92–3.22 Å. There are two inequivalent Sc3+ sites. In the first Sc3+ site, Sc3+ is bonded to six S2- atoms to form a mixture of edge and corner-sharing ScS6 octahedra. The corner-sharing octahedra tilt angles range from 48–61°. There are a spread of Sc–S bond distances ranging from 2.54–2.65 Å. In the second Sc3+ site, Sc3+ is bonded to six S2- atoms to form a mixture of edge and corner-sharing ScS6 octahedra. The corner-sharing octahedra tilt angles range from 48–61°. There are a spread of Sc–S bond distances ranging from 2.56–2.63 Å. There are four inequivalent S2- sites. In the first S2- site, S2- is bonded in a 5-coordinate geometry to two equivalent Ca2+ and three equivalent Sc3+ atoms. In the second S2- site, S2- is bonded in a 5-coordinate geometry to two equivalent Ca2+ and three Sc3+ atoms. In the third S2- site, S2- is bonded to two equivalent Ca2+ and three equivalent Sc3+ atoms to form a mixture of distorted edge and corner-sharing SCa2Sc3 square pyramids. In the fourth S2- site, S2- is bonded to two equivalent Ca2+ and three Sc3+ atoms to form a mixture of distorted edge and corner-sharing SCa2Sc3 trigonal bipyramids.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ca(BeN)2 by Materials Project

Ca(BeN)2 crystallizes in the tetragonal I4/mcm space group. The structure is three-dimensional. Ca2+ is bonded in a 8-coordinate geometry to eight equivalent N3- atoms. All Ca–N bond lengths are 2.69 Å. Be2+ is bonded in a trigonal planar geometry to three equivalent N3- atoms. There is one shorter (1.61 Å) and two longer (1.66 Å) Be–N bond length. N3- is bonded in a 7-coordinate geometry to four equivalent Ca2+ and three equivalent Be2+ atoms.

36 MATERIALS SCIENCE↗