bluuebunny/crossref_metadata_2025_split huggingface dataset:
THE MINERALOGY OF A RHÖNITE‐BEARING CALCIUM ALUMINUM RICH INCLUSION IN THE ALLENDE METEORITE
Authors: Fuchs Louis H. 
Published: June 2012

The meteoritic occurrence of rhönite‐bearing calcium and aluminum rich inclusions (CAIs) appears to be restricted to the Allende meteorite. These inclusions, however, are very scarce as only four are known to the author. This report is primarily concerned with the descriptive mineralogy of one of these rare CAIs.The inclusion contains, in decreasing order of abundance, melilite (Ak15‐Ak55), spinel, fassaite, rhönite, perovskite, grossular, Ni‐Fe metal, hibonite, troilite, a vanadium‐iron oxide (20–27 wt % V2O3), molybdenite, geikielo‐ilmenite and magnetite. Although this mineral assemblage formed under nonequilibrium conditions from a molten drop, the coarse grained texture does not indicate rapid crystallization. Local chemistries evidently played a significant role in the formation of metastable phase relationships and symplectites, thereby confounding the construction of a plausible thermal history for this CAI.
A METHOD FOR THE IDENTIFICATION AND ELIMINATION OF CONTAMINATION DURING CARBON ISOTOPIC ANALYSES OF EXTRATERRESTRIAL SAMPLES
Authors: Swart P.K. , Grady M.M. , Pillinger C.T. 
Published: June 2012

The study of carbon abundance and isotopic composition in extraterrestrial samples is fraught with problems related to contamination in the terrestrial environment and during sample handling. A stepped combustion method is described which demonstrates that progress can be made towards resolving the indigenous species from contamination which for the most part burns at low temperature (< 425 ± 25 ±C). The proposed method is not applicable to samples which have indigenous phases burning at low temperatures e.g. the C1 and C2 carbonaceous chondrites. A number of examples where its application is possible are given.Even meteorites collected immediately after their fall, such as Allende, contain a proportion of extraneous carbon which has deleterious effects on any bulk estimate of isotopic composition. “Falls” which have spent a considerable time in museum collections and “finds” (other than Antarctic samples) can be considered as grossly contaminated. Bulk isotope and carbon abundance measurements in the literature for most samples having less than 1 wt% C are thus of questionable value. Antarctic samples have much less contamination of an organic nature but all seem to contain a weathering component which can be easily recognised and hence disregarded in estimates of bulk composition. Stepped combustion, applied to an Apollo 11 lunar soil which has not been specially stored and which now contains, due to contamination, nearly twice as much carbon as when originally collected, can still afford data closely resembling those obtained from the sample when it was first returned to Earth.
PHOSPHATE‐SULFIDE ASSEMBLAGES AND Al/Ca RATIOS IN TYPE‐3 CHONDRITES
Authors: Rubin Alan E. , Grossman Jeffrey N. 
Published: June 2012

Phosphate‐sulfide assemblages are common constituents in type‐3 carbonaceous and ordinary chondrites. CV3 chondrites contain assemblages of pentlandite‐merrillite and troilite‐merrillite as well as isolated grains of Ca‐pyroxene; CO3 chondrites contain troilite‐merrillite (± schreibersite) as well as isolated grains of plagioclase; and, H‐L‐LL3 chondrites contain troilite‐merrillite (± metal), troilite‐chlorapatite, and metal‐chlorapatite. The phosphate‐bearing assemblages probably formed in the following manner: (1) metal grains with significant P formed in the nebula at high temperatures; (2) Schreibersite exsolved and crystallized at metal grain boundaries during cooling; (3) some metal grains were sulfurized at lower temperatures by H2S; (4) the metal‐schreibersite and sulfide‐schreibersite assemblages accreted rims of finegrained silicates; and, (5) the Schreibersite reacted with Ca, O and Cl from these silicates to form merrillite and chlorapatite.The reported bulk compositions of chondritic constituents have ***CI‐normalized Al/Ca ratios >1, whereas whole‐rock ratios are unfractionated. Even though the phosphate‐bearing assemblages and isolated grains of Ca‐bearing silicates are ubiquitous in type‐3 chondrites, they are insufficiently abundant to lower the Al/Ca ratios of the major chondritic components to those of the whole‐rocks. It seems probable that some of the analytical data are incorrect; bulk compositions determined by microprobe may yield erroneously high Al/Ca ratios if samples are analyzed with a broader electron beam than used for analyzing the standards. We recommend analyzing standards and samples with the same size beam.
RARE EARTH AND OTHER ELEMENTS IN COMPONENTS OF THE ABEE ENSTATITE CHONDRITE
Authors: Frazier Robert M. , Boynton William V. 
Published: June 2012

Abee clast samples, a matrix sample, a dark inclusion, magnetic and nonmagnetic samples, and bulk samples were analyzed by neutron activation analysis (NAA). The REE were determined by radiochemical NAA. Na, K, Sc, Cr, Mn, Fe, Co, Ni, Zn, As, Se, Sm, Ir, Au were determined by instrumental NAA.High abundances of As, Ir, and Au in the magnetic separate and the correlation of their abundances with the metal content of the clasts indicate that As, Ir, and Au chiefly occur in the metal. Correlations for Zn and Sc indicate that they chiefly occur in niningerite, but a significant amount of Sc may also occur in oldhamite. The dark inclusions do not follow the As and Zn correlations, suggesting that the dark inclusions and clasts are not equilibrated with each other. Correlation of the REE and oldhamite abundances for both the clasts and dark inclusions indicates that the REE chiefly occur in oldhamite.In view of the INAA and mineralogical evidence for non‐equilibration among the clasts and a dark inclusion (Sears et al., 1981), the similar REE patterns for clasts (3,3) and dark inclusion (5,1), and the similar mineral composition of oldhamite in clast (3,3) and dark inclusion (5,1), suggest that the oldhamite in the clasts and dark inclusions is of a common origin, which Sears et al. (1981) showed could be formed by condensation.A Tb anomaly of a factor of 2 was found in sample (2,9 and 9,2), and a La anomaly of a factor of 2 was found in clast (3,3). The only other REE anomaly in Abee, a factor of 3.5 for Yb, was found by Nakamura and Masuda(1973). In view of the evidence for equilibration among the clasts, this anomaly must have been introduced shortly before the brecciation process and indicates that no significant reheating has occurred. This concurs with the findings of Sugiura and Strangeway (1981) and Bogard et al. (1982).
Shock‐implanted noble gases II: Additional experimental studies and recognition in naturally shocked terrestrial materials
Authors: Bogard Donald , Hörz Friedrich , Johnson Pratt 
Published: June 2012

Abstract— Several experimentally and naturally shocked silicate samples were analyzed for noble gas contents to further characterize the phenomenon by which ambient gases can be strongly implanted into silicates by shock and to evaluate the possible importance of this process in capturing planetary atmospheres in naturally shocked samples. Gas implantation efficiency is apparently mineral independent, as mono‐mineralic powders of oligoclase, labradorite, and diopside and a powdered basalt shocked to 20 GPa show similar efficiencies. The retentivity of shock‐implanted gas during stepwise heating in the laboratory is defined in terms of two parameters: activation energy for diffusion as determined from Arrhenius plots, and the extraction temperature at which 50% of the gas is released, both of which correlate with shock pressure. These gas diffusion parameters are essentially identical for radiogenic 40Ar and shock‐implanted 40Ar in oligoclase and labradorite shocked to 20 GPa, suggesting that the two 40Ar components occupy analogous lattice sites. Our experiments indicate that gas implantation occurs through an increasing production of microcracks/defects in the lattice with increasing shock pressure. The ease of diffusive loss of implanted gas is controlled by the degree of annealing of these microcracks/defects.Identification of a shock‐implanted component requires relatively large concentrations of implanted gas which is strongly retained (i.e., moderate activation energy) in order to separate implanted gas from surface adsorbed gases. Literature data on shocked terrestrial samples indicate only weak evidence for shock‐implanted gases, with an upper limit for 40Ar of ∼ 10−6 cm3STP/g. New analyses of shocked samples from the Wabar Crater indicate the presence of shock‐implanted Ar, having concentrations (∼ 10−4 cm3STP/g) and activation energies for diffusive loss which are essentially that expected from experimental studies. Lack of sufficient target porosity or the presence of ground water may explain the sparse evidence for shock‐implanted gas at other terrestrial craters. Although Wabar Crater may represent an unusually favorable environment on Earth for shock‐implanting gases, surfaces of other planetary bodies, such as Mars, may frequently provide such environments. Analyses of returned samples from old Martian terraines may document temporal changes in earlier atmospheric composition.
The flux of meteorites on the Earth's surface
Authors: Halliday Ian , Blackwell Alan T. , Griffin Arthur A. 
Published: June 2012

Abstract— We derive values for the number and size distributions of meteorites landing on Earth from a study of photographic observations of bright fireballs with the Canadian camera network. The observations cover 11 years from 1974 to 1985. This analysis is an extension of a previous study and represents a 30% increase in the data base.The cumulative plot of numbers vs estimated mass of the largest fragment for each event shows a change in slope near 0.6 kg due to a deficiency of small meteorites surviving from the group of slow fireballs. The change can be explained by a mass dependence of the fraction of the incoming object that survives as the largest fragment. For larger falls, the main mass appears to represent a decreasing fraction of the total mass of the surviving meteorites and estimates of these effects are used to derive the final distribution of both main masses and total masses of meteoritic events. For total masses greater than 1 kg the population index is 1.82, close to previous estimates. About 9 events per year drop at least 1 kg of meteorites in an area of a million square km and the same area receives an annual influx of 54 kg from meteorite events with total masses between 0.01 and 100 kg.There is sufficient confidence in these results that they may be used for comparison of the present flux of meteorites with values inferred for other times, in particular the long accumulation times of the Antarctic meteorite collections.
Problems of origin of asteroids and comets
Authors: Safronov V. S. 
Published: June 2012

Abstract— Various hypotheses of the origin of asteroids and comets are briefly discussed. Interaction of planetesimals in the asteroid zone (AZ) with the gas, their perturbations by proto‐Jupiter, and sweeping them out by more massive Jupiter zone bodies when they penetrated the AZ are considered. If the gas was turbulent, it could prevent a settling of dust particles to the equatorial plane of the disk and formation of dust condensations due to gravitational instability. Then particles grew by sticking upon collision. Gas moved radially due to turbulent viscosity and its dissipation. Small particles moved more‐or‐less together with the gas. As a result of gas drag, larger particles and bodies moved relative to the gas in the direction of increasing gas pressure. Gas would remove much of the solid material from the AZ if most bodies larger than a few km disintegrated by collisions into fragments smaller than a few tens of meters. Most of these fragments would then move into the Martian zone, and the small mass of Mars would have no explanation.Resonant perturbations of asteroids by Jupiter are discussed. In the model of a small mass disk they could scan through the asteroid belt due to changes in Jupiter's distance from the Sun that occurred when this planet accreted the gas and ejected the bodies from the solar system. Such a scanning considerably accelerated the removal of asteroids from the AZ. Massive Jupiter zone bodies with large orbital eccentricities that crossed the AZ were probably efficient at sweeping out bodies. Larger bodies increased the random velocities of the remaining asteroids at close encounters to the present values ∼ 5 km/s.Restrictions on the runaway growth of giant planets, on the relative velocities of bodies and the disk surface density that follow from the consideration of the origin of the asteroid belt and the cometary cloud are considered.
Deformation and thermal histories of chondrules in the Chainpur (LL3.4) chondrite
Authors: Ruzicka Alex 
Published: June 2012

Abstract— Transmission‐electron‐microscopy (TEM) and optical data suggest that chondrules in the Chainpur (LL3.4) chondrite experienced varied thermal and deformation histories prior to the final agglomeration of the meteorite. Chainpur may be regarded as an agglomerate or breccia that experienced little deformation or heating during and after the final accumulation and compaction of its constituents.One chondrule in Chainpur was impact‐shocked to high pressures (∼ 20–50 GPa), almost certainly prior to final agglomeration, either while it was an independent entity in space or while it was in the regolith of a parent body. However, most (>85%) of the chondrules in Chainpur were evidently not significantly shock‐metamorphosed subsequent to their formation. The dearth of shock effects implies that most chondrules in Chainpur did not form by shock melting, although some chondrules may have formed by this process.Dusty‐metal‐bearing olivine grains, which are widely interpreted to have escaped melting during chondrule formation, contain moderate densities of dislocations (∼ 108 cm−2). The dislocations in these grains were introduced before or during the last episode of melting in at least one chondrule. This observation can be explained if olivine was impact‐deformed before or during chondrule formation, or if olivine was strained by reduction or thermally‐induced processes during chondrule formation.Low‐Ca pyroxene grains in chondrules are often strained. In most cases this strain probably arose as a by‐product of polytype transformations (protoenstatite → clinoenstatite/orthoenstatite and clinoenstatite → orthoenstatite) that occurred during the igneous crystallization and static annealing of chondrules.Droplet chondrules with glassy mesostases were minimally annealed, consistent with an origin as relatively rapidly cooled objects in an unconfined, cold environment. Some irregular chondrules and at least one droplet chondrule were thermally metamorphosed prior to final agglomeration, either as a result of moderately slow cooling (∼ 100 °C/hr) from melt temperatures (during autometamorphism) or as a result of reheating episodes. Two of the most annealed chondrules contain relatively abundant plagioclase feldspar, and one of these has a uniform olivine composition appropriate to that of an LL4 chondrite.
Alkali (Rb/K) abundances in Allende barred‐olivine chondrules: Implications for the melting conditions of chondrules
Authors: Matsuda Hitoshi , Nakamura Noboru , Noda Shinji 
Published: June 2012

Abstract— In order to study abundances of alkali metals in chondrules, 25 petrographically characterized chondrules, including 18 barred olivine (BO) chondrules from the Allende (CV3) meteorite, were analyzed for alkalis (K and Rb) and alkaline earths (Sr, Ba, Ca and Mg) by mass spectrometric isotope dilution.Most BO chondrules with higher alkalis (>CI level) have nearly CI‐chondritic Rb/K ratios, while those with lower alkalis clearly show higher Rb/K ratios than the CI‐chondritic. In general, BO chondrules with higher Rb/K exhibit more depletion of alkalis relative to Ca. The mean olivine Fa for individual chondrules positively correlates with bulk alkali concentrations in BO type but not in porphyritic type chondrules. These observations suggest that some BO chondrules formed from more reducing assemblages of precursor minerals, which experienced more intensive vaporization losses of alkalis, accompanied by Rb/K fractionation, during the chondrule‐formation melting.
Isotopic signatures of black tektites from the K‐T boundary on Haiti: Implications for the age and type of source material
Authors: Premo W. R. , Izett G. A. 
Published: June 2012

Abstract— U‐Th‐Pb, Rb‐Sr, and Sm‐Nd isotopic signatures of corroded, but unaltered, black glassy tektites from Cretaceous‐Tertiary (K‐T) boundary rock on Haiti are not consistent with their derivation from an impact on MOR‐derived oceanic crust or continental regions involving middle Proterozoic or older crustal material. Two single‐grain and two batches of these tektites yielded present‐day ∍Nd = −3.0 to −3.4, ∍Sr = +55 to 56, 206Pb/204Pb = 18.97; 207Pb/204Pb = 15.74; 208Pb/204Pb = 38.91 values, and Pb, Rb, Sr, Sm, and Nd concentrations of ∼6, ∼45, ∼535, ∼4.7, and ∼22 ppm, respectively. Initial ∍Nd and ∍Sr values for the tektites are different from time‐integrated Nd‐Sr isotopic signatures for almost all oceanic crustal types. Age‐corrected Pb isotopic values are similar to those for pelagic sediments with distinctly higher 207Pb/204Pb values compared to MORB. However, these results do not exclude the possibility of an oceanic impact site, if the tektites were derived from fine‐grained sediments that typically overlie such regions, although other mineralogic and chemical evidence from K‐T boundary debris suggests otherwise. Moreover, the Nd average crustal residence age of ∼ 1080 Ma (TDM) for the black tektites eliminates impact sites on continental crustal regions involving middle Proterozoic or older rocks, or sedimentary rocks largely derived from them.Previously reported major and trace element data from the black tektites suggest that the source material was possibly sedimentary with a composition similar to average shale or graywacke. If this is the case, then the Nd isotopic data suggest that the source rocks were not older than Silurian (TCHUR = 400 Ma) in age, and were composed largely of young (< 1080 Ma) crustal material. Of the suspected K‐T boundary impact sites, both the Manson (Iowa) and Chicxulub (Yucatan) structures occur in suitable lithologies to yield the Haitian black tektites, although neither structure has as yet proven to be the tektite source.
A survey of Rb‐Sr systematics of eucrites
Authors: Smoliar M. I. 
Published: June 2012

Abstract— This paper surveys the experimental material on the Rb‐Sr systematics of eucrites accumulated in the literature for the last two decades. Among the meteorites studied, those representatives have been chosen for which the absence of disturbances of isotope systems seems reliably fixed (according to evidence from Rb‐Sr, Sm‐Nd, and U‐Pb isotope data). These data have been subjected to joint statistical treatment. For ordinary eucrites the age has been estimated as T = 4.548 ± 0.058 Ga, and initial Sr isotope ratio, Iord = 0.698925 ± 14. For cumulate eucrites, the initial Sr isotope ratio has been estimated as Icum = 0.698872 ± 14. The substantial difference in initial Sr isotope composition suggests that those two eucrite groups originate from distinct parent bodies.Joint treatment of the available Rb‐Sr data for angrites has been carried out for comparison, and the angrites initial Sr ratio has been estimated. On the basis of the estimates obtained formation intervals for the corresponding parent bodies have been calculated.
Pecora Escarpment 91002: A member of the new Rumuruti (R) chondrite group
Authors: Rubin Alan E. , Kallemeyn Gregory W. 
Published: June 2012

Abstract— Pecora Escarpment (PCA)91002 is a light/dark‐structured chondrite breccia related to Carlisle Lakes and Rumuruti; the meteorite contains ∼10–20 vol% equilibrated (type −5 and −6) clasts within a clastic groundmass, much of which was metamorphosed to type‐3.8 levels. The olivine compositional distribution forms a tight cluster that peaks at Fa38–40; by contrast, low‐Ca pyroxene compositions are highly variable. Opaque phases identified in PCA91002 and its paired specimen, PCA91241, include pyrrhotite, pentlandite, pyrite, chromite, ilmenite, metallic Cu and magnetite. The majority of the rock is of shock stage S3‐S4; there are numerous sulfide‐rich shock veins and 50‐μm plagioclase melt pockets. Instrumental neutron activation analysis shows that, unlike Carlisle Lakes and ALH85151, PCA91002 exhibits no Ca enrichment or Au depletion; because PCA91002 is relatively unweathered, it seems probable that the Ca and Au fractionations in Carlisle Lakes and ALH85151 were caused by terrestrial alteration.The Rumuruti‐like (formerly Carlisle‐Lakes‐like) chondrites now include eight separate meteorites. Their geochemical and petrographic similarities suggest that they constitute a distinct chondrite group characterized by unfractionated refractory lithophile abundances (0.95 ± 0.05x CI), high bulk Δ17O, a low chondrule/groundmass modal abundance ratio, mean chondrule diameters in the 400 ± 100 μm range, abundant NiO‐bearing ferroan olivine, sodic plagioclase, titanian chromite, abundant pyrrhotite and pentlandite and negligible metallic Fe‐Ni. We propose that this group be called R chondrites after Rumuruti, the only fall.The abundant NiO‐bearing ferroan olivine grains, the occurrence of Cu‐bearing sulfide, and the paucity of metallic Fe‐Ni indicate that R chondrites are highly oxidized. It is unlikely that appreciable oxidation took place on the parent body because of the essential lack of plausible oxidizing agents (e.g., magnetite or hydrated silicates). Therefore, oxidation of R chondrite material must have occurred in the nebula. A few type‐I porphyritic olivine chondrules containing olivine grains with cores of Fa3–4 composition occur in PCA91002; these chondrules probably formed initially as metallic‐Fe‐Ni‐bearing objects at high nebular temperatures. As temperatures decreased and more metallic Fe was oxidized, these chondrules accreted small amounts of oxidized material and were remelted. The ferroan compositions of the >5‐μm olivine grains in the R chondrites reflect equilibration with fine‐grained FeO‐rich matrix material during parent body metatnorphism.