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ABSTRACT: Concentrations of stable and radioactive nuclides produced by cosmic ray particles in meteorites allow us to track the long
term average of the primary flux of galactic cosmic rays (GCR). During the past ∼10Ma, the average GCR flux remained constant
over timescales of hundreds of thousands to millions of years, and, if corrected for known variations in solar modulation,
also during the past several years to hundreds of years. Because the cosmic ray concentrations in meteorites represent integral
signals, it is difficult to assess the limits of uncertainty of this statement, but they are larger than the often quoted
analytical and model uncertainties of some 30%. Time series of concentrations of the radionuclide 10Be in terrestrial samples strengthen the conclusions drawn from meteorite studies, indicating that the GCR intensity on a
∼0.5 million year scale has remained constant within some ±10% during the past ∼10 million years. The very long-lived radioactive
nuclide 40K allows to assess the GCR flux over about the past one billion years. The flux over the past few million years has been the
same as the longer-term average in the past 0.5–1 billion years within a factor of ∼1.5. However, newer data do not confirm
a long-held belief that the flux in the past few million years has been higher by some 30–50% than the very long term average.
Neither does our analysis confirm a hypothesis that the iron meteorite data indicate a ∼150 million year periodicity in the
cosmic ray flux, possibly related to variations in the long-term terrestrial climate.
KeywordsGalactic cosmic ray intensity–Cosmogenic nuclides–Meteorites–Beryllium-10 in sediments–Exposure ages
Space Science Reviews 04/2012; · 3.61 Impact Factor
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Pieter Vermeesch,
Greg Balco,
Pierre-Henri Blard,
Tibor J Dunai,
Florian Kober,
Samuel Niedermann,
David L Shuster,
Stefan Strasky,
Finlay M Stuart, Rainer Wieler,
Laurent Zimmermann
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ABSTRACT: a b s t r a c t We performed an interlaboratory comparison study with the aim to determine the accuracy of cosmo-genic 21 Ne measurements in quartz. CREU-1 is a natural quartz standard prepared from amalgamated vein clasts which were crushed, thoroughly mixed, and sieved into 125e250 mm and 250e500 mm size fractions. 50 aliquots of CREU-1 were analyzed by five laboratories employing six different noble gas mass spectrometers. The released gas contained a mixture of 16e30% atmospheric and 70e84% non-atmospheric (predominantly cosmogenic) 21 Ne, defining a linear array on the 22 Ne/ 20 Ne-21 Ne/ 20 Ne three isotope diagram with a slope of 1.108 AE 0.014. The internal reproducibility of the measurements is in good agreement with the formal analytical precision for all participating labs. The external repro-ducibility of the 21 Ne concentrations between labs, however, is significantly overdispersed with respect to the reported analytical precision. We report an average reference concentration for CREU-1 of 348 AE 10 Â 10 6 at [ 21 Ne]/g[SiO 2 ], and suggest that the 7.1% (2s) overdispersion of our measurements may be representative of the current accuracy of cosmogenic 21 Ne in quartz. CREU-1 was tied to CRONUS-A, which is a second reference material prepared from a sample of Antarctic sandstone. We propose a reference value of 320 AE 11 Â 10 6 at/g for CRONUS-A. The CREU-1 and CRONUS-A intercalibration materials may be used to improve the consistency of cosmogenic 21 Ne to the level of the analytical precision. Ó 2012 Elsevier B.V. All rights reserved.
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ABSTRACT: Abstract– We studied the mineralogy, petrology, and bulk, trace element, oxygen, and noble gas isotopic compositions of a composite clast approximately 20 mm in diameter discovered in the Larkman Nunatak (LAR) 04316 aubrite regolith breccia. The clast consists of two lithologies: One is a quench-textured intergrowth of troilite with spottily zoned metallic Fe,Ni which forms a dendritic or cellular structure. The approximately 30 μm spacings between the Fe,Ni arms yield an estimated cooling rate of this lithology of approximately 25–30 °C s−1. The other is a quench-textured enstatite-forsterite-diopside-glass vitrophyre lithology. The composition of the clast suggests that it formed at an exceptionally high degree of partial melting, perhaps approaching complete melting, and that the melts from which the composite clast crystallized were quenched from a temperature of approximately 1380–1400 °C at a rate of approximately 25–30 °C s−1. The association of the two lithologies in a composite clast allows, for the first time, an estimation of the cooling rate of a silicate vitrophyre in an aubrite of approximately 25–30 °C s−1. While we cannot completely rule out an impact origin of the clast, we present what we consider is very strong evidence that this composite clast is one of the elusive pyroclasts produced during pyroclastic volcanism on the aubrite parent body (Wilson and Keil 1991). We further suggest that this clast was not ejected into space but retained on the aubrite parent body by virtue of the relatively large size of the clast of approximately 20 mm. Our modeling, taking into account the size of the clast, suggests that the aubrite parent body must have been between approximately 40 and 100 km in diameter, and that the melt from which the clast crystallized must have contained an estimated maximum range of allowed volatile mass fractions between approximately 500 and approximately 4500 ppm.
Meteoritics & Planetary Science. 10/2011; 46(11):1719 - 1741.
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ABSTRACT: Abstract– We report concentrations and isotopic compositions of He, Ne, and Ar measured with high spatial resolution along a radial traverse of a silicon carbide (SiC) quadrant of the Genesis mission concentrator target. The Ne isotopic composition maps instrumental fractionation as a function of radial position in the target: the maximum observed isotopic fractionation is approximately 33‰ per mass unit between the center and periphery. The Ne fluence is enhanced by a factor of 43 at the target center and decreases to 5.5 times at the periphery relative to the bulk solar wind fluence. Neon isotopic profiles measured along all four arms of the “gold cross” mount which held the quadrants in the concentrator target demonstrate that the concentrator target was symmetrically irradiated during operation as designed. We used implantation experiments of Ne into SiC and gold to quantify backscatter loss and isotopic fractionation and compared measurements with numerical simulations from the code “stopping and range of ions in matter.” The 20Ne fluence curve as a function of radial distance on the target may be used to construct concentration factors relative to bulk solar wind for accurate corrections for solar wind fluences of other light elements to be measured in the concentrator target. The Ne isotopic composition as a function of the radial distance in the SiC quadrant provides a correction for the instrumental mass-dependent isotopic fractionation by the concentrator and can be used to correct measured solar wind oxygen and nitrogen isotopic compositions to obtain bulk solar wind isotopic compositions.
Meteoritics & Planetary Science. 03/2011; 46(4):493 - 512.
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ABSTRACT: Abstract— We present Ne data from plagioclase separates from the solar noble-gas-rich meteorite Kapoeta, obtained mainly by in vacuo etching. samples rich in solar gases contain an excess of cosmogenic ne compared to solar-gas-poor samples, testifying to an exposure to cosmic rays in the parent body regolith. The 21Ne/22Ne ratio of the excess component is slightly lower than that of the Ne acquired during the meteoroid flight. Model calculations indicate that the observed isotopic composition of the excess Ne can be produced by galactic cosmic rays at a reasonable mean shielding of around a hundred to a few hundred grams per square centimeter. No substantial contribution from Ne produced by solar cosmic rays is needed to explain the data. We therefore conclude that they do not offer evidence for a substantially enhanced flux of solar energetic particles early in solar history, contrary to other claims. This conclusion is in agreement with solar flare track data.
Meteoritics & Planetary Science. 02/2010; 35(2):251 - 257.
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ABSTRACT: Abstract— Chondrules are generally believed to have lost most or all of their trapped noble gases during their formation. We tested this assumption by measuring He, Ne, and Ar in chondrules of the carbonaceous chondrites Allende (CV3), Leoville (CV3), Renazzo (CR2), and the ordinary chondrites Semarkona (LL3.0), Bishunpur (LL3.1), and Krymka (LL3.1). Additionally, metalsulfide-rich chondrule coatings were measured that probably formed from chondrule metal. Low primordial 20Ne concentrations are present in some chondrules, while even most of them contain small amounts of primordial 36Ar. Our preferred interpretation is that-in contrast to CAIs-the heating of the chondrule precursor during chondrule formation was not intense enough to expel primordial noble gases quantitatively. Those chondrules containing both primordial 20Ne and 36Ar show low presolar-diamond-like 36Ar/20Ne ratios. In contrast, the metal-sulfide-rich coatings generally show higher gas concentrations and Q-like 36Ar/20Ne ratios. We propose that during metalsilicate fractionation in the course of chondrule formation, the Ar-carrying phase Q became enriched in the metal-sulfide-rich chondrule coatings. In the silicate chondrule interior, only the most stable Ne-carrying presolar diamonds survived the melting event leading to the low observed 36Ar/20Ne ratios. The chondrules studied here do not show evidence for substantial amounts of fractionated solar-type noble gases from a strong solar wind irradiation of the chondrule precursor material as postulated by others for the chondrules of an enstatite chondrite.
Meteoritics & Planetary Science. 01/2010; 39(1):117 - 135.
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ABSTRACT: Abstract— Calcium-aluminum-rich inclusions (CAIs) were among the first solids in the solar system and were, similar to chondrules, created at very high temperatures. While in chondrules, trapped noble gases have recently been detected, the presence of trapped gases in CAIs is unclear but could have important implications for CAI formation and for early solar system evolution in general. To reassess this question, He, Ne, and Ar isotopes were measured in small, carefully separated and, thus, uncontaminated samples of CAIs from the CV3 chondrites Allende, Axtell, and Efremovka.The 20Ne/22Ne ratios of all CAIs studied here are <0.9, indicating the absence of trapped Ne as, e.g., Ne-HL, Ne-Q, or solar wind Ne. The 21Ne/22Ne ratios range from 0.86 to 0.72, with fine-grained, more altered CAIs usually showing lower values than coarse-grained, less altered CAIs. This is attributed to variable amounts of cosmogenic Ne produced from Na-rich alteration phases rather than to the presence of Ne-G or Ne-R (essentially pure 22Ne) in the samples. Our interpretation is supported by model calculations of the isotopic composition of cosmogenic Ne in minerals common in CAIs. The 36Ar/38Ar ratios are between 0.7 and 4.8, with fine-grained CAIs within one meteorite showing higher ratios than the coarse-grained ones. This agrees with higher concentrations of cosmogenic 36Ar produced by neutron capture on 35Cl with subsequent β−-decay in finer-grained, more altered, and thus, more Cl-rich CAIs than in coarser-grained, less altered ones.Although our data do not strictly contradict the presence of small amounts of Ne-G, Ne-R, or trapped Ar in the CAIs, our noble gas signatures are most simply explained by cosmogenic production, mainly from Na-, Ca-, and Cl-rich minerals.
Meteoritics & Planetary Science. 01/2010; 39(5):767 - 778.
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ABSTRACT: Keywords: cosmogenic nuclides attenuation length neon helium quartz All currently used scaling models for Terrestrial Cosmogenic Nuclide (TCN) production rates are based on neutron monitor surveys. Therefore, an assumption underlying all TCN studies is that production rates are directly proportional to secondary cosmic ray intensities for all cosmogenic nuclides. To test this crucial assumption, we measured cosmogenic 3 He and 21 Ne in artificial quartz targets after one year of exposure at mountain altitudes in the Swiss Alps. The targets were inconel steel tubes containing 1 kg of artificial quartz sand (250–500 µm), degassed for one week at 700 °C in vacuum prior to exposure. From August 2006 until August 2007, ten of these targets were exposed at five locations in Switzerland and Italy: Zürich (556 m), Davos (1560 m), Säntis (2502 m), Jungfraujoch (3571 m), and Monte Rosa (4554 m). Additionally, a sixth set of two blank targets was kept in storage and effectively shielded from cosmic ray exposure. Cosmogenic noble gases were measured at room temperature and at 700 °C. Up to 9% of the cosmogenic 3 He was measured in the cold step, indicating that 3 He diffuses out of quartz at room temperature on short time scales. The remaining 3 He and all 21 Ne were released at 700 °C, as shown by a repeat measurement at 800 °C for the Monte Rosa target, which yielded no additional cosmogenic helium and neon. As expected, the Monte Rosa target contained the highest cosmogenic nuclide content, with 1.56 ± 0.07 × 10 6 atoms of excess 3 He and 4.5 ± 1.2 × 10 5 atoms of excess 21 Ne (all errors are 2σ). The raw measurements were corrected for non-atmospheric blanks, shielding (roof + container wall), tritiogenic helium and solar modulation (normalised to the average neutron flux over the past five solar cycles). The 3 He/ 21 Ne production rate ratio of 6.8 ± 0.9 indicates that cosmogenic 3 He production by the container walls is negligible. The main goal of the artificial target experiment was to determine the production rate attenuation length. Because all our targets had an identical design and were exposed under identical conditions, all systematic errors cancel out in the calculation of an attenuation length. Our best estimates for the 3 He and 21 Ne attenuation lengths are 134.8 ± 5.9 g/cm 2 and 135 ± 25 g/cm 2 , respectively, agreeing very well with currently used scaling models. We conclude that TCN production rates are indeed proportional to neutron monitor count rates, and that 3 He and 21 Ne production rates follow the same altitudinal scaling relationships as the cosmogenic radionuclides. Finally, the measurements were scaled to sea level and high latitude using the empirical attenuation length, yielding weighted mean production rates of 107.6 ± 6.6 at/g/yr for 3 He and 15.4 ± 2.1 at/g/yr for 21 Ne. Despite the significant uncertainties associated with the corrections for shielding, solar modulation and especially the 3 He/ 3 H branching ratio, these estimates are in good agreement with production rates derived from long-term exposure experiments at natural calibration sites and physics-based simulations.
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ABSTRACT: Abstract— We present noble gas analyses of sediment-dispersed extraterrestrial chromite grains recovered from ˜470 Myr old sediments from two quarries (Hällekis and Thorsberg) and of relict chromites in a coeval fossil meteorite from the Gullhögen quarry, all located in southern Sweden. Both the sediment-dispersed grains and the meteorite Gullhögen 001 were generated in the L-chondrite parent body breakup about 470 Myr ago, which was also the event responsible for the abundant fossil meteorites previously found in the Thorsberg quarry. Trapped solar noble gases in the sediment-dispersed chromite grains have partly been retained during ˜470 Myr of terrestrial residence and despite harsh chemical treatment in the laboratory. This shows that chromite is highly retentive for solar noble gases. The solar noble gases imply that a sizeable fraction of the sediment-dispersed chromite grains are micrometeorites or fragments thereof rather than remnants of larger meteorites. The grains in the oldest sediment beds were rapidly delivered to Earth likely by direct injection into an orbital resonance in the inner asteroid belt, whereas grains in younger sediments arrived by orbital decay due to Poynting-Robertson (P-R) drag. The fossil meteorite Gullhögen 001 has a low cosmic-ray exposure age of ˜0.9 Myr, based on new He and Ne production rates in chromite determined experimentally. This age is comparable to the ages of the fossil meteorites from Thorsberg, providing additional evidence for very rapid transfer times of material after the L-chondrite parent body breakup.
Meteoritics & Planetary Science. 02/2008; 43(3):517 - 528.
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ABSTRACT: Lunar soils have been thought to contain two solar noble gas components with distinct isotopic composition. One has been identified as implanted solar wind, the other as higher-energy solar particles. The latter was puzzling because its relative amounts were much too large compared with present-day fluxes, suggesting periodic, very high solar activity in the past. Here we show that the depth-dependent isotopic composition of neon in a metallic glass exposed on NASA's Genesis mission agrees with the expected depth profile for solar wind neon with uniform isotopic composition. Our results strongly indicate that no extra high-energy component is required and that the solar neon isotope composition of lunar samples can be explained as implantation-fractionated solar wind.
Science 12/2006; 314(5802):1133-5. · 31.20 Impact Factor
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ABSTRACT: Abstract— We report noble gas data for the second chassignite, Northwest Africa (NWA) 2737, which was recently found in the Moroccan desert. The cosmic ray exposure (CRE) age based on cosmogenic 3He, 21Ne, and 38Ar around 10–11 Ma is comparable to the CRE ages of Chassigny and the nakhlites and indicates ejection of meteorites belonging to these two families during a discrete event, or a suite of discrete events having occurred in a restricted interval of time. In contrast, U-Th/He and K/Ar ages <0.5 Ga are in the range of radiometric ages of shergottites, despite a Sm-Nd signature comparable to that of Chassigny and the nakhlites (Misawa et al. 2005). Overall, the noble gas signature of NWA 2737 resembles that of shergottites rather than that of Chassigny and the nakhlites: NWA 2737 does not contain, in detectable amount, the solar-like xenon found in Chassigny and thought to characterize the Martian mantle nor apparently fission xenon from 244Pu, which is abundant in Chassigny and some of the nakhlites. In contrast, NWA 2737 contains Martian atmospheric noble gases trapped in amounts comparable to those found in shergottite impact glasses. The loss of Martian mantle noble gases, together with the trapping of Martian atmospheric gases, could have occurred during assimilation of Martian surface components, or more likely during shock metamorphism, which is recorded in the petrology of this meteorite.
Meteoritics & Planetary Science. 04/2006; 41(5):739 - 748.
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ABSTRACT: Very large collisions in the asteroid belt could lead temporarily to a substantial increase in the rate of impacts of meteorites on Earth. Orbital simulations predict that fragments from such events may arrive considerably faster than the typical transit times of meteorites falling today, because in some large impacts part of the debris is transferred directly into a resonant orbit with Jupiter. Such an efficient meteorite delivery track, however, has not been verified. Here we report high-sensitivity measurements of noble gases produced by cosmic rays in chromite grains from a unique suite of fossil meteorites preserved in approximately 480 million year old sediments. The transfer times deduced from the noble gases are as short as approximately 10(5) years, and they increase with stratigraphic height in agreement with the estimated duration of sedimentation. These data provide powerful evidence that this unusual meteorite occurrence was the result of a long-lasting rain of meteorites following the destruction of an asteroid, and show that at least one strong resonance in the main asteroid belt can deliver material into the inner Solar System within the short timescales suggested by dynamical models.
Nature 08/2004; 430(6997):323-5. · 36.28 Impact Factor
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ABSTRACT: Abstract— The HF/HCI-resistant residues of the chondrites CM2 Cold Bokkeveld, CV3 (ox.) Grosnaja, CO3.4 Lancé, CO3.7 Isna, LL3.4 Chainpur, and H3.7 Dimmitt have been measured by closed-system stepped etching (CSSE) in order to better characterise the noble gases in “phase Q”, a major carrier of primordial noble gases. All isotopic ratios in phase Q of the different meteorites are quite uniform, except for (20Ne/22Ne)Q. As already suggested by precise earlier measurements (Schelhaas et al., 1990; Wieler et al., 1991, 1992), (20Ne/22Ne)Q is the least uniform isotopic ratio of the Q noble gases. The data cluster ∼10.1 for Cold Bokkeveld and Lancé and 10.7 for Chainpur, Grosnaja, and Dimmitt, respectively. No correlation of (20Ne/22Ne)Q with the classification or the alteration history of the meteorites has been found. The Ar, Kr, and Xe isotopic ratios for all six samples are identical within their uncertainties and similar to earlier Q determinations as well as to Ar-Xe in ureilites. Thus, an unknown process probably accounts for the alteration of the originally incorporated Ne-Q. The noble gas elemental compositions provide evidence that Q consists of at least two carbonaceous carrier phases “Q1” and “Q2” with slightly distinct chemical properties. Ratios (Ar/Xe)Q and (Kr/Xe)Q reflect both thermal metamorphism and aqueous alteration. These parent-body processes have led to larger depletions of Ar and Kr relative to Xe. In contrast, meteorites that suffered severe aqueous alteration, such as the CM chondrites, do not show depletions of He and Ne relative to Ar but rather the highest (He/Ar)Q and (Ne/Ar)Q ratios. This suggests that Q1 is less susceptible to aqueous alteration than Q2. Both subphases may well have incorporated noble gases from the same reservoir, as indicated by the nearly constant, though very large, depletion of the lighter noble gases relative to solar abundances. However, the elemental ratios show that Q1 and Q2 must have acquired (or lost) noble gases in slightly different element proportions. Cold Bokkeveld suggests that Q1 may be related to presolar graphite. Phases Q1 and Q2 might be related to the subphases that have been suggested by Gros and Anders (1977). The distribution of the 20Ne/22Ne ratios cannot be attributed to the carriers Q1 and Q2. The residues of Chainpur and Cold Bokkeveld contain significant amounts of Ne-E(L), and the data confirm the suggestion of Huss (1997) that the 22Ne-E(L) content, and thus the presolar graphite abundances, are correlated with the metamorphic history of the meteorites.
Meteoritics & Planetary Science. 08/2000; 35(5):949 - 973.
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ABSTRACT: Abstract— A large hand sample and numerous polished thin sections, made from the hand sample, of the Kapoeta howardite and its many diverse lithic clasts were studied in detail by optical microscopy and electron microprobe techniques in an attempt to understand the surface processes that operated on the howardite-eucrite-diogenite (HED) parent body (most likely the asteroid 4 Vesta). Four unique, unusually large clasts, designated A (mafic breccia), B (granoblastic eucrite), D (howardite) and H (melt-coated breccia), were selected for detailed study (modal analysis, mineral microprobe analysis, and noble gas measurements). Petrographic studies reveal that Kapoeta consists of a fine-grained matrix made mostly of minute pyroxene and plagioclase fragments, into which are embedded numerous different lithic and mineral clasts of highly variable sizes. The lithic clasts include pyroxene-plagioclase (eucrite), orthopyroxenite (diogenite), howardite, impact-melt, metal-sulfide-rich, and carbonaceous chondrite clasts. The howardite clasts include examples of lithic clasts that constitute breccias-within-breccias, suggesting that at least two regolith generations are represented in the Kapoeta sample we studied. The clast assemblage suggests that repeated shock lithification was an important process during regolith evolution. Noble gas analyses of clast samples fall into two populations: (a) solar-gas-rich clasts H (rim only) and D and (b) clasts A and B, which are essentially free of solar gases. The concentrations of solar noble gases in the two matrix samples differ by a factor of ∼40. It appears that clast D is a true regolith breccia within the Kapoeta howardite (breccia-within-breccia), while clast H is a regolith breccia that has been significantly impact reworked. Our data indicate that the Kapoeta howardite is an extraordinarily heterogeneous rock in modal mineral and lithic clast abundances, grain size distributions, solar-wind noble gas concentrations and cosmic-ray exposure ages. These results illustrate the repetitive nature of impact comminution and lithification in the regolith of the HED parent body.
Meteoritics & Planetary Science. 06/1998; 33(4):835 - 851.
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ABSTRACT: We measured in situ produced cosmogenic 3He and 21Ne in boulder and bedrock surface samples in and near Sirius Group tillites from two localities in Southern Victoria Land, Antarctica. Pyroxene quantitatively retains cosmogenic 3He for millions of years. The deposit at Mt. Fleming has a minimum exposure age of ~} 6.5 Ma, while ~{ 6 Ma is a probable minimum age for the Table Mountain tillite. These lower limits (not taking into account erosion of the sampled surfaces) are based on the samples with the highest concentrations of cosmogenic noble gases and currently accepted production rates. Since the plagioclase-bearing Mt. Fleming samples almost certainly lost part of their cosmogenic Ne, the minimum exposure age at this location is presumably even $[$ap$]$ 50% higher than the stated value. The concentrations of cosmogenic Ne in our samples constrain uplift rates of the Transantarctic Mountains to ice sheet in the Pliocene coupled with high uplift rates and support the model of a stable ice sheet since the middle Miocene.
Earth and Planetary Science Letters 01/1997; 147(1-4):37-54. · 4.18 Impact Factor
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ABSTRACT: We etched lunar soil samples in several steps in a vacuum-tight device
and analyzed the evolved solar noble gases on line in a mass
spectrometer. This technique avoids diffusive noble gas fractionation
during analysis and therefore provides reliable element abundance ratios
as a function of depth in the grains. The ratios He/Ar and Ne/Ar
strongly increase in the course of the etching and reach present-day
solar wind ratios toward the end of the runs. This shows that the solar
wind component, residing in the top few hundred angstroms, lost much of
its He and Ne, whereas even these mobile light noble gases are retained
nearly unfractionated in the solar energetic particle (SEP) component at
larger depths. In contrast to the light gases, the Kr/Xe ratio is
constant throughout all runs, and the same is essentially true also for
Ar/Kr. This strongly suggests that the relative abundances of the three
heavy noble gases in the incoming solar corpuscular radiation are
conserved in lunar samples. The Kr/Xe ratio in samples irradiated in the
past ˜100 Myr is about a factor of 2.5 lower than the most
probable value in the Sun. The same ratio was another factor of 2 lower
1-3 Gyr ago. Kr/Ar in the solar corpuscular radiation is also slightly
fractionated. Xe is overabundant relative to Ar by about the same
factor as are elements with a first ionization potential (FIP) of less
than ˜10 eV relative to high-FIP elements. This seems astonishing,
since Xe has a FIP above 10 eV. However, Geiss, Gloeckler, & von
Steiger showed recently that the Xe overabundance deduced here is
expected if actually the first ionization time rather than the FIP
governs the fractionation in the solar wind source region.
The Astrophysical Journal 10/1995; 453:987. · 6.02 Impact Factor
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ABSTRACT: We present Kr and Xe isotope data obtained by closed system stepped
etching of ilmenite separates from two lunar samples exposed to the
solar corpuscular radiation at different epochs. Helium, neon, and argon
in the same samples were reported to consist of two components:
isotopically unfractionated solar wind (SW) released in the first steps,
and an isotopically heavier component (SEP) released later and, thus,
sited at larger depth. The same release characteristic is now observed
for the heavy noble gases. We also conclude that solar Kr and Xe consist
of two isotopically different components, implanted with different
energies. The SW-Kr in a recently irradiated soil has a composition very
close to atmospheric Kr, which agrees with other newly reported data
from stepped etch- and combustion runs. No clear evidence for temporally
variable SW-Kr or SW-Xe spectra was found. 'Surface correlated' Kr and
Xe components 'SUCOR' and 'BEOC 12001' are a mixture of SW and SEP. The
isotopic fractionation factors relating SW and SEP are close to the
square of the mass ratios for all five noble gases. We infer that the
measured Kr/Xe ratio in ilmenite is essentially identical to this ratio
in the solar corpuscular radiation.
Meteoritics 08/1994; 29:570-580.
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ABSTRACT: Results are presented from an analysis, in nearly pure form, of noble gases from the 'phase-Q' in an HF/HCl residue of the Allende C3V meteorite, using the closed-system stepped etching technique developed by Wieler et al. (1986) and Benkert et al. (1988) to extract noble gases from the residue. The results yield precise values of element and isotope abundances of all five noble gases in phase-Q, which is the major carrier of the planetary gases in carbonaceous chondrites. It was found that Ne-Q and Xe-Q in Allende are very similar to trapped gases in ureilites and in oxidizable carriers in several classes of ordinary chandrites, indicating that Q-gases are present in the formation locations of all these meteorites.
07/1991;
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ABSTRACT: The Olton ordinary chondrites (two stones found in 1948) are H group, solar wind-bearing regolith breccias. The proximity of the recovery site to the Dimmitt strewnfield, and the similarity in texture, composition, noble gas contents and isotopic ratios to Dimmitt indicate that Olton is paired with Dimmitt.
01/1991;
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ABSTRACT: U-rich minerals were investigated for the products of radioactive decay, fission and specific nuclear reactions induced thereby. Geologically undisturbed uranium minerals with concordant U-Pb ages were used to determine the product of the 238U spontaneous fission decay constant λsf and the yield iYsf of a specific Xe or Kr isotope, the isotopic ratios of spontaneous fission xenon and krypton and the spontaneous fission ratio. Values of (5.7 ±0.4) · 10−18/a and 6.1 ± 0.1 were obtained for λsf · 136Ysf and , respectively. With our result for λsf · 136Ysf and literature data on 238U spontaneous fission yields, we propose that the proper value for λsf is around 8.6 · 10−17/a rather than the 6.4 · 10−17/a as mainly proposed by fission track investigators.Isotopic ratios of neon and argon differ significantly from atmospheric composition. No evidence for nucleogenic neon and argon from strongly asymmetric fission or from (n,α) or (n, γ) reactions was found. These gases are produced by the nuclear reactions 18O(α,n)21Ne,19F(α,n)22Ne,35Cl(α,p)38Ar and 35Cl(n,γ)36Ar as already proposed in 1954 by Wetherill. For these reactions the production ratios P(21)norm, P(22)norm and P(38)norm of 21Nenuc, 22Nenuc and 38Arnuc relative to the α-dose and the target element concentrations in the minerals were deduced. The problem of diffusional 4He losses was circumvented by using fissiogenic Xe as a measure of the α-dose. The contributions of 22Nenuc and 38Arnuc to the Earth's atmosphere are negligible, whereas up to 2.4% of 21Ne in the atmosphere may be nucleogenic.
Geochimica et Cosmochimica Acta.
