21Ne Production Research Articles

Cosmogenic neon in recent lavas from the western United States

The absolute cosmogenic production of 21Ne (21Nec) and 3He (3Hec) in olivine (Fo81) is measured for the well‐dated Tabernacle Hill flow. Values scaled to sea level and high latitude ( > 60°) are 45 atoms/g/yr for 21Nec and 109 atoms/g/yr for 3Hec for the past 17,800 years.Production rates for 21Nec are about 50% higher than the theoretical values given by Lal [1991]. A ratio of 21Nec/3Hec of 0.41 ± 0.05 is measured in olivines that have a range of 25 in cosmogenic exposure; there is no evidence of 3Hec loss in samples older than 100,000 years.Volcanic plagioclase is shown to be a suitable mineral for cosmogenic 21Nec dating of surfaces. 21Nec in co‐existing plagioclase and olivine phenocrysts have a constant ratio of 0.42 ± .04 in samples with more than 106 atoms of 21Nec. Using all of the 21Nec data and the 3Hec as a cosmic ray flux monitor, production of 21Nec in Ab36 plagioclase is calculated at 16.8 ± 1.7 atoms/g‐yr for the past 17,800 years. The production of 21Nec from 23Na is estimated at 160 ± 50 atoms/g/yr at 60° and sea level or equivalent to that on 24Mg.

On the production of cosmogenic nuclides in meteoroids by galactic protons

Abstract— A purely physical model is presented describing the depth‐ and size‐dependence of the production of cosmogenic nuclides in meteoroids with radii up to 85 cm and in planetary surfaces by galactic cosmic ray protons. The model is based on Monte Carlo calculations of the intra‐ and internuclear cascades, by which depth‐ and size‐dependent spectra of primary and secondary protons and of secondary neutrons are derived, and on experimental and theoretical thin‐target cross sections of the underlying nuclear reactions. Model calculations are presented for production rates of 53Mn, 26Al, 22Ne, and 21Ne in H‐ and L‐chondrites and of 53Mn and 26Al in lunar surface material and compared with experimental data. From the analysis of 53Mn and 26Al in the Apollo 15 lunar drill core and in the L‐chondrite Knyahinya GCR p‐spectra and integral particle fluxes at 1 A.U. and in the meteoroid orbits averaged over the last 10 Ma are derived. An analysis of experimental depth profiles in four H‐ and L‐chondrites demonstrates, that the new model is well capable of describing depth‐ and size‐dependences of production rates of cosmogenic nuclides. Moreover, it is possible to determine exposure ages for these meteorites on the basis of the theoretical 21Ne production rates. The model calculations further explain the depth‐ and size‐dependence of 22Ne/21Ne‐ratios and the dependences on these ratios of 21Ne, 26Al and 53Mn production rates. The future requirements for model calculations of cosmogenic nuclide production rates in extraterrestrial matter are outlined.

Cosmic-ray production rates of neon isotopes in meteorite minerals

The rates of production of21Ne and22Ne in spallation reactions, both due to solar as well as galactic cosmic rays, in some major meteoritic minerals, e.g. olivines, feldspars and pyroxenes, are calculated using their energy spectra and excitation functions. The production profiles of21Ne and22Ne due to galactic cosmic rays, and the22Ne/21Ne ratio depend upon the size of the meteoroid. The22Ne/21Ne ratio is very sensitive to the abundance of sodium and consequently its depth profile is distinctly different in feldspars, the ratio increasing with depth rather than decreasing as in pyroxenes and olivines. In the near-surface regions, up to a depth of 2cm, production due to solar flare protons dominates, giving rise to a steep gradient in isotopic production as well as in the22Ne/21Ne ratio. Composite production profiles are given and compared with measurements in some meteorites.

Investigations of cosmic-ray-produced nuclides in iron meteorites, 6. The Signer-Nier model and the history of the cosmic radiation

About thirty years ago, a simple model was proposed by Martin and by Ebert and Wänke to describe the interaction of cosmic rays with meteoritic iron. The model was later used by Signer and Nier to calculate the spatial distribution of cosmogenic noble gas nuclides in spherical meteoroids. A rigorous mathematical evaluation of the model—known as the Signer-Nier model—is presented. The model is broadly compatible with data on noble gas nuclides and on 41K/ 40K exposure ages: (1)|In the framework of the model, the abundance ratio of any two of the cosmogenic nuclides 3He, 4He, 21Ne, and 38Ar is correlated with the ratio of any other pair of these nuclides. These ratios depend only on the effective irradiation hardness in the sample. Analysis of these nuclides in a single sample does not permit determination of radiation dosage, of size of the meteoroid, and of location of the sample within the meteoroid. The model-derived correlations agree with the corresponding experimental correlations except for samples from relatively large depths. (2)|The model makes it possible to derive a P 21- 4He 21Ne diagram predicting how 21Ne production rate P 21 and 4He/ 21Ne ratio depend on meteoroid size and sample depth. Possible combinations of values for P 21 and 4He/ 21Ne are restricted, they define the “allowed field” of the diagram. Each location in this field is associated with a distinct combination of values for meteoroid size and sample depth. The diagram is a tool to determine model-dependent values for sizes and depths. Values of P 21 and 4He/ 21Ne from previous noble gas analyses and from 41K/ 40K age determinations fall into or close to the allowed field. The model represents, therefore, a reasonable description of the cosmic ray interaction with iron meteoroids. Analysis of age-, size-, and depth-distributions indicates that space erosion was probably effective with a rate of the order of 1 cm per 100 Ma. An investigation of the compatibility of noble gas, potassium, and radionuclide data reveals the following: (1) the traditional age scale of the 41K/ 40K method is correct; (2) the intensity of the cosmic radiation was essentially constant in the course of the last 1–2 Ga; (3) the intensity exceeded its long-term average by 50% during the last few million years.

Half‐life of 26Al

The half‐life of 26Al has been redetermined because of suggestions of an error in the accepted value based on its use in calculating 21Ne production rates from cosmic rays in meteorites. Two solutions of 26Al were analyzed for the specific radioactivity and mass spectrometric determination of the 26Al concentration. The half‐life obtained for 26Al was 7.05×105 years ±3.4%. This is identical to the accepted value of 7.16×105 years and indicates that problems with the 21Ne production rate is not due to an erroneous half‐life.

The 21Ne production rate in stony meteorites estimated from 10Be and other radionuclides

The 10Be contents of 28 stony meteorites with known 21Ne contents range from 0.97 to 23 dpm/kg and give an average 21Ne production rate ( P 21) of (0.28 ± 0.02) × 10 8 cm 3 STP/g-Myr for shielding conditions corresponding to 22Ne 21Ne = 1.114 in an H-chondrite. Our P 21( 10Be) agrees with others' P 21 based on 22Na, 81Kr and 53Mn but not on 26A1. Temporal variations in the cosmic ray flux do not explain the disagreement satisfactorily; major errors in the radionuclide half-lives are not indicated. The discrepancy seems rooted in the data selection and the difficulties of making accurate corrections for shielding, chemical composition and other sources of variability.

Cosmic-ray constancy and cosmogenic production rates in short-lived chondrites

Five chondrites with short cosmic ray exposure ages (< 3Myr) have been analyzed for 53Mn ( t 1.2 = 3.7Myr), 26Al( t 1.2 = 0.72Myr) and the stable isotopes of He, Ar, and in particular, Ne. In addition, rare gases were determined for five other short-lived chondrites from the same aliquots which had previously been used for 26Al and 53Mn determinations by the Cologne Laboratory. Improved curves of growth were constructed for 26Al and 53Mn to deduce the respective saturation values 26Al 0 and 53Mn 0 and to obtain 21Ne production rates ( 21 P). 26Al and 53Mn saturation values deduced from short-lived chondrites alone agree within 20%, with those deduced from all chondrites. Values of 21 p based on 53Mn and 26Al are 0.30 ± 0.03 and 0.46 ± 0.05, respectively, in H chondrites. Possible causes for the difference include half-life errors, data selection and a recent (≲2.5Myr) increase in the cosmic ray intensity.

Investigations on cosmic-ray-produced nuclides in iron meteorites, 3. Exposure ages, meteoroid sizes and sample depths determined by mass spectrometric analyses of potassium and rare gases

Three physical quantities define the essentials of the cosmic ray exposure history of a sample of an iron meteorite: (1) the cosmic ray exposure age T, (2) the pre-atmospheric “size” S of the irradiated body, and (3) the location, i.e. the “depth” D, of the samples within the body. To establish these quantities for a given sample three independent quantities must be determined experimentally. In the present work T is ascertained by the 41K/ 40K method and the 4He and 21Ne concentrations ( C 4 and C 21) are measured by the isotope dilution method. Signer and Nier's evaluation of the rare gas distribution in the meteorite Grant and the measured exposure age for this meteorite provide the relationships allowing to ascertain for any meteorite the quantities S and D from the 21Ne production rate ( P 21 = C 21/ T) and the 4He/ 21Ne ratio. Earlier measurements have provided data on the isotopic composition of potassium in 74 different iron meteorites. New rare gas measurements are reported for some 40 samples. Results on the age, size and depth are obtained for almost 60 samples. These data suggest that Signer and Nier's model is well suited for describing not only the rare gas distribution in a single selected meteorite (Grant) but also the exposure histories of the great majority of all irons. For a few samples, however, secondary breakups of the meteoroid and a two- or multiple-stage irradiation must be invoked. Further measurements are proposed for testing and, possibly, refining the still somewhat uncertain relationships between the abundances of cosmogenic nuclides and the quantities T, S, and D in very large meteorites. Histograms are presented showing the age distributions for irons of different chemical groups and of different size ranges. The feasibility and the relative merits of other methods for the determination of T, S, and D are discussed.

The reaction Mg(n,α)Ne at 14.1 and 14.7 MeV: cross sections and implications for meteorites

Mass spectrometric analyses of neutron-irradiated targets of natural magnesium yield cross sections of59 ± 14,160 ± 8, and11.0 ± 3.3mb for 20Ne, 21Ne, and 22Ne, respectively, at 14.1 MeV and of94 ± 8,152 ± 12, and13.0 ± 2.0mb at 14.7 MeV. With the incorporation of these cross sections, calculations modeling cosmic-ray interactions in stony meteoroids of radii 20 and 26 cm predict that between the surface and center the 22Ne/ 21Ne ratio falls more than 10% while the 21Ne production rate rises about 30%. The reaction 24Mg(n,α) 21Ne predominantly controls these trends: the 22Ne/ 21Ne ratio due to magnesium decreases over 15% while that due to silicon remains constant with increasing depth. The calculations are compared with published neon measurements for the Keyes and St. Séverin meteorites.

Isotopic anomalies of noble gases in meteorites and their origins: 2. Separated minerals from Allende

Eleven mineral fractions from the Allende C3 chondrite were analyzed for He, Ne, Ar, Kr, and Xe by mass spectrometry. As was shown previously (Lewis et al., 1975), most of the heavy noble gases, of unexceptional isotopic composition, are contained in an ill-defined HNO3-soIuble mineral fraction (‘Q’) comprising 0.04% of the meteorite. The remaining noble gases, of decidedly nonsolar isotopic composition, are contained in chromite (0.2%) and amorphous carbon (0.2%). They have virtually identical concentrations and isotopic compositions in the two minerals: 20Ne/22Ne = 8.7 and 36Ar/38Ar = 4.82. Xenon is enriched up to twofold in both heavy and light isotopes, while Kr is enriched only in heavy isotopes. The enrichment of the heavy isotopes is presumably due to fission, but the fission spectra derived depend critically on the assumed composition of the trapped component. The enrichment patterns of the light xenon isotopes differ slightly for chromite and carbon. Partial destruction of the minerals by chemical treatments gave no evidence for other gas-rich phases or other gas components and only orginal evidence for heterogeneities within the known gas-rich phases. Spinel has ≪0.1 the gas content of other phases. The chromite-carbon fractions contain 30–60% more spallogenic 21Ne than does the bulk meteorite, although they are deficient in target elements for the production of 21Ne. A possible explanation is recoil of 21Ne from adjacent Mg-rich phases. All of the available isotopic and chemical data suggest that chromite, carbon, and Q, and hence their associated gas components, are of local rather than extra-solar system origin. The excess heavy Kr and Xe isotopes may have been produced by fission of a superheavy element, while the remaining isotopic anomalies may be due to mass fractionation during trapping. The virtual identity of the noble gas components of chromite and carbon suggests that both minerals formed simultaneously, perhaps by a reaction such as Fe + 2Cr + 4CO → FeCr2O4 + 4C.

Cosmogenic neon produced from sodium in meteoritic minerals

Cosmogenic neon in sodium-rich oligoclase feldspar from the ordinary chondrites St. Severin and Guaren˜a is characterized by an unusually high 22Ne/ 21Ne = 1.50 ± 0.02. This high ratio is due to the cosmogenic 22Ne/ 21Ne production ratio in sodium which is 2.9 ± 0.3, two to three times the production ratio in any other target element. The relative production rate of 21Ne per gram sodium is one quarter the production rate per gram magnesium. The striking enrichment of 22Ne relative to 21Ne in sodium arises from enhanced indirect production from 23Na via 22Na. The unusual composition of cosmogenic neon in sodium and sodium-rich minerals explains the high 22Ne/ 21Ne ratios observed in inclusions of the Allende carbonaceous chondrite, and observed during low-temperature extraction of neon from ordinary chondrites. The isotopic composition of cosmogenic neon released during the stepwise heating of a trapped gas-rich meteorite containing sodium-rich phases can be expected to vary, and use of a constant cosmogenic neon composition to derive the composition of the trapped gas may not be justified. Preferential loss of this 22Ne-enriched cosmogenic neon from meteoritic feldspar can result in a 2–3% drop in the measured cosmogenic 22Ne/ 21Ne ratio in a bulk meteorite sample. This apparent change in composition can lead to overestimation of the minimum pre-atmospheric mass of the meteorite by a factor of two.

Noble gas concentrations and cosmic ray exposure ages of eight recently fallen chondrites

Abundances and isotopic compositions of He, Ne, Ar, and Xe have been measured in eight recently fallen chondrites. Ratios of concentrations of cosmic ray-produced 3He, 21Ne, 22Ne and 38Ar indicate that all eight samples experienced less than average cosmic ray shielding. 3He and 21Ne exposure ages were calculated using shielding corrected chondritic production rates and the measured 22Ne 21Ne . Exposure ages calculated from 22Na 22Ne and 26Al 21Ne ratios and constant relative production rates show a bias between the two ages due to variations in 22Na 26Al . Arguments are presented that this bias is due to irradiation hardness differences, and therefore the use of constant values for both the 22Na 22Ne and 26Al 21Ne production ratios is not permitted. Dwaleni, Swaziland, was found to be an unusual gas-rich chondrite with high concentrations of solar-derived He and Ne and planetary-type Xe.

He, Ne and Ar in chondritic NiFe as irradiation hardness sensors

Noble gas analyses of the Ni-Fe of 9 L, 5 H and 2 LL chondrites quantitatively support previous suggestions of radiogenic 4He recoil and 3He deficits. Furthermore, noble gases in the Ni-Fe show evidence for in situ produced radiogenic 4He and in some cases for recoil loss of 38Ar and gain of 21Ne. The ratio of spallogenic 21Ne and 38Ar in the metal phase is found to correlate strongly with 3He/ 21Ne and 22Ne/ 21Ne in bulk samples of these chondrites. This is proof of the dependence of these ratios on the irradiation hardness experienced by the meteoroid in space. ‘Hardness indices’ n = 1.9–2.2 are found, indicating that on the average the stone meteoroids from which the samples came were smaller in mass than iron meteoroids. The spallogenic 21Ne/ 38Ar ratio in metallic Ni-Fe can be used with the semi-empirical production model deduced from the Grant iron meteorite to calibrate spallogenic 3He/ 21Ne and 4Ne/ 21Ne in bulk samples of L, LL and H chondrites for meteoroid size and sample location allowing the estimation of minimal meteoroid masses. 3He and 21Ne production rates calculated from previously determined 36Ar/ 38Ar exposure ages for four L chondrites indicate that they are probably not single-valued functions of the 3He/ 21Ne ratio. The ratio of 3He in bulk samples to 38Ar in metal samples of the same meteorite is constant (= 20 ± 3) whereas the ratio of 21Ne in the bulk to 38Ar in the metal varies by as much as a factor of two in correlation with 3He/ 21Ne.