Isotopic Compositions Of Rare Gases Research Articles

Chemical and isotopic characteristics of the Didwana‐Rajod (H5) chondrite

Abstract— The mineralogical and chemical characteristics of the Didwana‐Rajod chondrite are described. The mean mineral composition is found to be olivine (Fo83.2) and pyroxene (En83.5Wo0.7Fs15.8), and feldspar is mainly oligoclase. Oxygen isotopic analysis shows δ18O = +3.8%0 and δ17O = +2.59%0. The nitrogen content of Didwana‐Rajod is ∼2 ppm with δ15N ≅ 3.4%0. Based on microscopic, chemical, isotopic and electron probe microanalysis, the meteorite is classified as an H5 chondrite. Cosmogenic tracks, radionuclides and the isotopic composition of rare gases were also measured in this meteorite. The track density in olivines varies in a narrow range with an average value of (6.5 ± 0.5) × 105/cm2 for four spot samples taken at the four corners of the stone. The cosmic‐ray exposure age based on neon and argon is 9.8 Ma. 22Na/26Al ≅ 0.94 is lower than the solar‐cycle average value of ∼1.5 and is consistent with irradiation of the meteoroid to lower galactic cosmic‐ray fluxes as expected at the solar maximum. The track density, rare gas isotopic ratios, 60Co activity and other radionuclide data are consistent with a preatmospheric radius of ∼15 cm, corresponding to a mass of ∼50 kg. The cosmogenic properties are consistent with a simple exposure history in interplanetary space.

Solar and galactic cosmic‐ray records of the Fermo (H) chondrite regolith breccia

Abstract— ‐We demonstrate the presence of solar flare as well as neutron capture effects in the isotopic composition of rare gases in the Fermo regolith breccia acquired on its parent body based on the measurements of tracks, rare gases and radionuclides. The track density along a 3.2 cm long core decreases by a factor of about 6 and by more than a factor of 13 within the meteorite, indicating small (2–9 cm) and asymmetrical ablation. Rare gases show a large trapped component; the isotopic ratios, particularly 20Ne/22Ne ≃ 11 and 20Ne/36Ar = 10 are indicative of a solar component. The galactic cosmic‐ray exposure age is determined to be 8.8 Ma. Activities of a dozen radionuclides ranging in half‐life from 16 day 48V to 0.73 Ma 26Al are consistent with their expected production rates. Track, rare gas and radionuclide data show that the meteoroid was a small body (≤ 120 kg) and had a simple, one‐stage exposure history to cosmic rays in the interplanetary space. However, 82Kr and 128Xe show an excess due to neutron irradiation on the parent body of the meteorite. The presence of solar gases and the neutron capture effects indicate several stages of irradiation on the parent asteroid. The chemical composition of Fermo confirms that it belongs to the H group of ordinary chondrites with lithic clasts having varying compositions. δ15N is found to be 8.3 ± 1.2%0, close to the typical values observed in H chondrites.

Time-dependent models of U–Th–He and K–Ar evolution and the layering of mantle convection

High-resolution seismic tomography indicates that lithospheric plates penetrate the 670 km discontinuity. A common view holds that the isotopic composition of He and Ar is incompatible with geophysical evidence because (a) the inventory of terrestrial radiogenic argon requires that the lower mantle is largely undegassed, and (b) recycling of outgassed lithospheric material should destroy the primitive isotopic composition of helium and argon in the lower mantle. It is first shown, however, that the `argon problem' results from the assumption that the terrestrial K/U ratio is ≥1.2×10 4, a value that cannot be taken for granted. In addition, earlier geochemical models of steady-state He and Ar cycles are shown to be inadequate, because common concepts of residence time do not apply to multiple reservoir systems. A time-dependent model with four reservoirs (lower and upper mantle, crust, atmosphere) is evaluated in which elemental fractionation upon transfer and variable growth rate for crustal growth, atmosphere extraction, and lithosphere production are taken into account. Vigorous lithosphere production early in the history of the Earth enhances the degassing of the upper mantle in agreement with the very radiogenic character of 4He/ 3He and 40Ar/ 36Ar ratio in mid-ocean ridge basalts. Recycling of oceanic lithosphere stripped of its rare gases, but also of its U, Th, and K, into the lower mantle preserves the primitive isotopic signature of the rare-gas inventory of this reservoir. Accumulation in the lower mantle of ancient plates essentially barren of U, Th, and K, but nevertheless showing isotopic evidence of plate-tectonic processes in the less incompatible elements (Nd, Hf, Sr), can account for the isotopic composition of rare gases in hot-spot basalts. The outgassed character of the upper mantle was probably acquired during the early Archaean when mantle activity was intense and is still being enhanced by the fluxing of K, U, and Th from the descending lithospheric plates. The view of the mantle offered by seismic tomography is therefore not incompatible with the isotopic evidence provided by terrestrial rare gases.

Reconciling Mantle Rare Gas Geochemistry with Tomographic Evidence of Whole Mantle Convection

High-resolution seismic tomography clearly shows that lithospheric plates penetrate the 670 km discontinuity (e.g. van der Hilst et al, 1997). A common view defended by geochemists (Turekian, 1959; Allbgre et al. , 1996) holds that the isotopic composition of He and Ar is incompatible with geophysical evidence because (a) the inventory of terrestrial radiogenic argon requi-res that the lower mantle is largely undegassed and (b) recycling of outgassed lithospheric material should destroy the primitive isotopic composition of helium and argon in the lower mantle. It is first shown, however, that the 'argon problem' is largely a 'potassium problem' that results from the assumption that the terrestrial K/U ratio is 12,500 (Jochum et al., 1983). In view of the large variability of this ratio amongst telluric objects (from 3,000 in the Moon to 60,000 in chondrites), of the volatile character of alkalis demonstrated by the Sr isotope geochemistry of chondrites and terrestrial material, and of the uncertainties on the K/U fractionation during melting, such a value value cannot be taken for granted. In addition, earlier geochemical models of steady-state He and Ar cycles (e.g. O'Nions and Tolstikhin, 1996) are shown to be inadequate. The common concept of residence time is inappropriate for multiple reservoir systems. An unwanted artifact resulting from the assumption that an isotopically heterogeneous system is time-invariant, is actually a strong reduction of mass fluxes between reservoirs, with, in particular, a small exchange rate between the lower mantle and the upper mantle. The view that tomography reflects a present-day snapshot while the Earth's mantle was essentially layered until recently, requires some unsubstantiated serendipity. A regime of intermittent layering (avalanches) is geochemically indistinguishable from whole mantle convection and, as such, does not solve the dilemma. A time-dependent model with four reservoirs (lower and upper mantle, crust, atmo-sphere) is evaluated, in which elemental fractionation upon transfer and variable rate of crustal growth, atmosphere extraction, and lithosphere production are taken into account. Vigorous lithosphere production early in the history of the Earth enhances the degassing of the upper mantle in agreement with the very radiogenic character of 4He/3He and 4~ ratio in MORB. The preferential reinjection into the lower mantle of lithospheric material that lost its component rich in lithophile elements to the continental crust has been underestimated by previous modelers. Recycling of oceanic lithosphere stripped of its rare gases, but more importantly of its U, Th, and K, into the lower mantle preserves the primitive isotopic signature of the rare-gas inventory of this reservoir. Accumulation in the lower mantle of ancient plates essentially barren of U, Th, and K, but nevertheless showing isotopic evidence of platetectonic processes in the less incompatible elements (Nd, Hf, St), can account for the isotopic composition of rare gases in hot-spot basalts. A prominent effect of plate tectonics and whole mantle convection is to clutter and dilute the lower mantle with old barren lithospheric debris. The geochemistry of the lower mantle is therefore expected to show characters that somehow conflict with conventional views. Although the incompatible element content of the lower mantle has been diluted with respect to the bulk silicate Earth (BSE), the fractionation of elements, and in particular parentdaughter pairs such as Sm/Nd and Rb/Sr, is still intermediate between that of BSE and that of the upper mantle. For such a model to be acceptable, melting should reverse the relative level of enrichment of melts relative to that of their mantle source. OIB should be produced by very small degrees of melting of relatively barren lower mantle material and MORB by large degrees of melting of a relatively fertile upper mantle, regardless of the apparent 'depletion' or 'enrichment' indicated by the isotopic systems and trace element patterns. The outgassed character of the upper mantle was probably acquired during the early Archaean when mantle activity was intense and is still being enhanced by the fluxing of K, U, and Th from the descending lithospheric plates. The view of the mantle offered by seismic

Lunar highland meteorites and the composition of the lunar crust

Major, minor, and trace element data obtained by neutron activation techniques and by spark source mass spectrometry (SSMS) on two lunar meteorites MAC88104 and MAC88105 are reported. Both MAC samples were also analysed for their contents and isotopic compositions of rare gases. Additional SSMS-data were obtained on four lunar highland meteorites previously found in Antarctica: ALHA81005, Y791197, Y82192, and Y86032. MAC88104 and MAC88105 are very similar in chemistry, suggesting that they are pieces of a single fall event. The bulk chemical composition of MAC88104/5 is not very different from the other lunar highland meteorites: highly aluminous with relatively low contents of REE and siderophile element concentrations slightly above 1% of a CI-chondritic level. However, mafic element concentrations (Mg, Cr, Mn, etc.) are slightly lower in MAC88104/5 than in the other lunar highland meteorites. The contents of solar rare gases in the two MAC samples are low, indicating only a small regolith contribution in agreement with rare petrographically identifiable regolith components. The MAC samples and also Y82192 and Y86032 are classified as fragmental breccias with negligible regolith components, in contrast to ALHA81005 and Y791197 which are regolith breccias with high solar wind derived rare gas contents. There is no correlation among lunar meteorites between peak shock pressures and solar gas contents, indicating that peak shock pressures of up to 25 GPa do not lead to gas loss. A low 26Al activity ( Vogt et al., 1990) and high contents of cosmogenic rare gases in MAC88104/5 suggest a long exposure (400,000 years) in the lunar sub-surface. K-Ar ages are in excess of 3.9 by. Lunar highland meteorites and compositionally similar granulitic rocks from the Apollo 16 and 17 landing sites contain about 1% of a CI-chondritic component, according to siderophile and volatile element contents, but independent of the amount of regolith components. Apparently, the major fraction of meteoritic elements in these rocks was not provided by micrometeorites impacting the regolith. The abundances of siderophile (e.g., Ir) and volatile elements may therefore reflect the last spike of accretion of the Moon after the formation of the anorthositic crust. Lunar meteorites of highland origin are chemically different from the bulk of the Apollo 16 highland samples in having higher Fe Mg ratios and lower contents and less fractionated patterns of incompatible and siderophile elements. Since lunar highland meteorites are associated with at least three but probably four different fall events, and since they are not derived from chemically exotic front-side terranes, they may represent a better sampling of the average chemical composition of the lunar crust than previous estimates based on returned lunar samples and remote sensing data. A comparison between an average highland composition derived by Taylor (1982) and an estimate based on lunar highland meteorites shows that the Taylor composition contains higher concentrations and more fractionated incompatible elements mainly because of a substantial amount of KREEP (a trace element rich, highly fractionated component from the front side of the Moon not present at the sites from which the lunar meteorites come).

Isotopic compositions of rare gases in the Matsushiro earthquake fault region.

Elemental and isotopic compositions of rare gases in gas samples from “Helium spots” and CO2-spring in the fault zone formed by the Matsushiro earthquake swarms, reported by WAKITA et al. (1978), have been analyzed mass spectrometrically. Isotopic compositions of rare gases were atmospheric except for 3He/4He and 40Ar/36Ar ratios. 3He/4He ratios were (9.12 ± 0.58) × 10-6 and (9.10±0.69) × 10-6, and 40Ar/34Ar ratios were 308 ± 9 and 304 ± 8 for “Helium spots” and CO2-spring, respectively. Rare gas elemental abundance patterns for both samples were similar to a pattern produced by the dissolution equilibrium of atmospheric rare gases in water. The origin of rare gases is discussed. The 3He/4He ratio found in the fault zone suggests that the systematic observation of 3He/4He ratio may be useful for the detection of precursory phenomena of an earthquake.

Abundances and isotopic compositions of rare gases in granites.

The contents and the isotopic compositions have been measured mass-spectrometrically for neon, argon, krypton and xenon released from four specimens of granites in stepwise-heating experiments. These rocks contained appreciable amounts of 40Ar from the decay of 40K and 131-136Xe from the 238U spontaneous fission. The contents of neon, argon (excluding 40Ar), krypton and xenon in these granites were considerably lower than the estimated rare gas contents of rocks reported by BROWN (1949), except for the Red Rock granite, which contained an abnormally large amount of xenon. The Ne/Xe, Ar/Xe and Kr/Xe ratios in the Red Rock granite were abnormal in that they resembled the relative abundances in meteorites. Moreover, a large fraction of the total xenon released from the Red Rock granite appeared to be a mixture of AVCC xenon and the atmospheric xenon. The Red Rock granite is from the Sudbury structure, Ontario, Canada, which is believed to have been formed by the impact of an asteroid in an event that formed a crater 30 miles in diameter 1, 720 million years ago.

Isotopic compositions of rare gases in the carbonaceous chondrites Mokoia and Allende

The isotopic compositions have been measured mass spectrometrically for neon, argon, krypton and xenon released from the carbonaceous chondrites Mokoia and Allende in stepwise heating experiments. The isotopic compositions of rare gases released from the meteorites at different temperatures varied quite considerably. The observed minimum and maximum values were, for example, as follows: Ne 20 Ne 22 = 2.10 (Allende 800°C) and 12.95 (Mokoia 400°C); Ar 36 Ar 38 = 4.20 (Allende 1600°C) and 6.05 (Allende 800°C); Kr 84 Kr 86 = 3.140 (Allende 1000°C) and 3.306 (Mokoia 600°C); Xe 134 Xe 136 = 1.019 (Allende 800°C) and 1.192 (Allende 1400°C). A marked enrichment of Xe 129 due to the decay of extinct nuclide I 126 was observed in both meteorites. The variations of the isotopic ratios are partly caused by the presence of cosmic-ray spallation and neutron-capture products. In addition, however, a marked trend of mass-dependent variation of the isotopic ratios was observed in this work. The rare gas isotopes released from the meteorites appear to be systematically mass-fractionated relative to the relative abundances of the average carbonaceous chondrite (AVCC). It seems that this phenomenon can be best explained as due to the fact that there exist reservoirs of two isotopically distinct gases in the meteorites and mixtures of these gases are being released at each temperature fraction. The two different gases being considered here are most likely the so-called solar and planetary rare gases, whose isotopic compositions are quite different from each other.

Isotopic Composition of Rare Gases in Lunar Samples

Data from total melt and step-by-step heating experiments on the Apollo 11 lunar samples suggest a close affinity between lunar and meteoritic rare gases. Trapped neon-20/neon-22 ratios range from 11.5 to approximately 15, resembling those for the gas-rich meteorites. Trapped krypton and xenon in the lunar fines and in the carbonaceous chondrites are similar except for an interesting underabundance of the heavy isotopes in both lunar gases which suggests that the fission component found in carbonaceous chondrites is depleted in lunar material. Spallation gases are in most cases quite close to meteoritic spallation gases in isotopic composition.

642. Study by X-ray diffractions of thin layers of lanthanum fluoride: A Bourg and D Talbot, C R Acad Sc (Paris), 259 ( 25), Dec 1964, 4608–4611

High-resolution seismic tomography indicates that lithospheric plates penetrate the 670 km discontinuity. A common view holds that the isotopic composition of He and Ar is incompatible with geophysical evidence because (a) the inventory of terrestrial radiogenic argon requires that the lower mantle is largely undegassed, and (b) recycling of outgassed lithospheric material should destroy the primitive isotopic composition of helium and argon in the lower mantle. It is first shown, however, that the `argon problem' results from the assumption that the terrestrial K/U ratio is ≥1.2×104, a value that cannot be taken for granted. In addition, earlier geochemical models of steady-state He and Ar cycles are shown to be inadequate, because common concepts of residence time do not apply to multiple reservoir systems. A time-dependent model with four reservoirs (lower and upper mantle, crust, atmosphere) is evaluated in which elemental fractionation upon transfer and variable growth rate for crustal growth, atmosphere extraction, and lithosphere production are taken into account. Vigorous lithosphere production early in the history of the Earth enhances the degassing of the upper mantle in agreement with the very radiogenic character of 4He/3He and 40Ar/36Ar ratio in mid-ocean ridge basalts. Recycling of oceanic lithosphere stripped of its rare gases, but also of its U, Th, and K, into the lower mantle preserves the primitive isotopic signature of the rare-gas inventory of this reservoir. Accumulation in the lower mantle of ancient plates essentially barren of U, Th, and K, but nevertheless showing isotopic evidence of plate-tectonic processes in the less incompatible elements (Nd, Hf, Sr), can account for the isotopic composition of rare gases in hot-spot basalts. The outgassed character of the upper mantle was probably acquired during the early Archaean when mantle activity was intense and is still being enhanced by the fluxing of K, U, and Th from the descending lithospheric plates. The view of the mantle offered by seismic tomography is therefore not incompatible with the isotopic evidence provided by terrestrial rare gases.

634. Focussing mechanism of ion beam devices and application to machining: S Matsui, Bull Electrotech Lab, 28 ( 6), June 1964, 23–31 Japanese)

High-resolution seismic tomography indicates that lithospheric plates penetrate the 670 km discontinuity. A common view holds that the isotopic composition of He and Ar is incompatible with geophysical evidence because (a) the inventory of terrestrial radiogenic argon requires that the lower mantle is largely undegassed, and (b) recycling of outgassed lithospheric material should destroy the primitive isotopic composition of helium and argon in the lower mantle. It is first shown, however, that the `argon problem' results from the assumption that the terrestrial K/U ratio is ≥1.2×104, a value that cannot be taken for granted. In addition, earlier geochemical models of steady-state He and Ar cycles are shown to be inadequate, because common concepts of residence time do not apply to multiple reservoir systems. A time-dependent model with four reservoirs (lower and upper mantle, crust, atmosphere) is evaluated in which elemental fractionation upon transfer and variable growth rate for crustal growth, atmosphere extraction, and lithosphere production are taken into account. Vigorous lithosphere production early in the history of the Earth enhances the degassing of the upper mantle in agreement with the very radiogenic character of 4He/3He and 40Ar/36Ar ratio in mid-ocean ridge basalts. Recycling of oceanic lithosphere stripped of its rare gases, but also of its U, Th, and K, into the lower mantle preserves the primitive isotopic signature of the rare-gas inventory of this reservoir. Accumulation in the lower mantle of ancient plates essentially barren of U, Th, and K, but nevertheless showing isotopic evidence of plate-tectonic processes in the less incompatible elements (Nd, Hf, Sr), can account for the isotopic composition of rare gases in hot-spot basalts. The outgassed character of the upper mantle was probably acquired during the early Archaean when mantle activity was intense and is still being enhanced by the fluxing of K, U, and Th from the descending lithospheric plates. The view of the mantle offered by seismic tomography is therefore not incompatible with the isotopic evidence provided by terrestrial rare gases.