Irradiation records in regolith materials. I: isotopic compositions of solar-wind neon and argon in single lunar mineral grains

We have applied a stepwise pyrolytic extraction technique to eleven individual lunar regolith grains to investigate the compositions of light noble gases embedded in grain surfaces by solar wind irradiation, with emphasis on the rather poorly known isotopic composition of solar-wind argon. Results are intriguing: average ²⁰Ne/²²Ne ratios observed in early pyrolytic releases from ilmenite grains separated from lunar soils 71501, 79035 and 10084 agree very well with both direct measures of the solar wind neon composition in the Apollo foils and with values obtained in first releases from acid-etched ilmenites by the Zürich laboratory, whereas these same pyrolytic and acid-etch fractions carry argon isotopic signatures that significantly disagree—average ³⁶Ar/³⁸Ar ratios near 5.8 for thermal extraction compared to 5.4–5.5 for chemical etching at Zürich. Consideration of the isotopic and elemental data from these grains in the context of first-order diffusive modeling calculations points to gas release at low temperatures, without significant isotopic or elemental fractionation, from isolated grain-surface reservoirs of solar wind composition. The physical nature of these reservoirs is presently unknown. In this interpretation the preferred solar wind ²⁰Ne/²²Ne and ²¹Ne/²²Ne ratios deduced from this study are respectively 13.81 ± 0.08 and 0.0333 ± 0.0003, both within error of the Zürich acid-etch values, and ³⁶Ar/³⁸Ar = 5.77 ± 0.08. It may be possible to reconcile the discrepancy between the acid-etch and pyrolytic estimates for the solar wind ³⁶Ar/³⁸Ar ratio in the context of arguments originally advanced by Benkert et al. (1993) to account for their He and Ne isotopic compositions. At the other, high-temperature end of the release profile from one of these grains there are clear isotopic indications of the presence of a Ne constituent with ²⁰Ne/²²Ne close to the 11.2 ratio found at Zürich and attributed by these workers to a deeply-sited component implanted by solar energetic particles.     

Noble gas composition of the solar wind as collected by the Genesis mission

We present the elemental and isotopic composition of noble gases in the bulk solar wind collected by the NASA Genesis sample return mission. He, Ne, and Ar were analyzed in diamond-like carbon on a silicon substrate (DOS) and ^84,86Kr and ^129,132Xe in silicon targets by UV laser ablation noble gas mass spectrometry. Solar wind noble gases are quantitatively retained in DOS and with exception of He also in Si as shown by a stepwise heating experiment on a flown DOS target and analyses on other bulk solar wind collector materials. Solar wind data presented here are absolutely calibrated and the error of the standard gas composition is included in stated uncertainties. The isotopic composition of the light noble gases in the bulk solar wind is as follows: ³He/⁴He: (4.64 ± 0.09) × 10⁻⁴, ²⁰Ne/²²Ne: 13.78 ± 0.03, ²¹Ne/²²Ne: 0.0329 ± 0.0001, ³⁶Ar/³⁸Ar 5.47 ± 0.01. The elemental composition is: ⁴He/²⁰Ne: 656 ± 5, and ²⁰Ne/³⁶Ar 42.1 ± 0.3. Genesis provided the first Kr and Xe data on the contemporary bulk solar wind. The preliminary isotope and elemental composition is: ⁸⁶Kr/⁸⁴Kr: 0.302 ± 0.003, ¹²⁹Xe/¹³²Xe: 1.05 ± 0.02, ³⁶Ar/⁸⁴Kr 2390 ± 150, and ⁸⁴Kr/¹³²Xe 9.5 ± 1.0. The ³He/⁴He and the ⁴He/²⁰Ne ratios in the Genesis DOS target are the highest solar wind values measured in exposed natural and artificial targets. The isotopic composition of the other noble gases and the Kr/Xe ratio obtained in this work agree with data from lunar samples containing “young” (∼100 Ma) solar wind, indicating that solar wind composition has not changed within at least the last 100 Ma. Genesis could provide in many cases more precise data on solar wind composition than any previous experiment. Because of the controlled exposure conditions, Genesis data are also less prone to unrecognized systematic errors than, e.g., lunar sample analyses. The solar wind is the most authentic sample of the solar composition of noble gases, however, the derivation of solar noble gas abundances and isotopic composition using solar wind data requires a better understanding of fractionation processes acting upon solar wind formation.     

Solar wind helium, neon, and argon isotopic and elemental composition: Data from the metallic glass flown on NASA’s Genesis mission

Solar wind (SW) helium, neon, and argon trapped in a bulk metallic glass (BMG) target flown on NASA’s Genesis mission were analyzed for their bulk composition and depth-dependent distribution. The bulk isotopic and elemental composition for all three elements is in good agreement with the mean values observed in the Apollo Solar Wind Composition (SWC) experiment. Conversely, the He fluence derived from the BMG is up to 30% lower than values reported from other Genesis bulk targets or in-situ measurements during the exposure period. SRIM implantation simulations using a uniform isotopic composition and the observed bulk velocity histogram during exposure reproduces the Ne and Ar isotopic variations with depth within the BMG in a way which is generally consistent with observations. The similarity of the BMG release patterns with the depth-dependent distributions of trapped solar He, Ne, and Ar found in lunar and asteroidal regolith samples shows that also the solar noble gas record of extraterrestrial samples can be explained by mass separation of implanted SW ions with depth. Consequently, we conclude that a second solar noble gas component in lunar samples, referred to as the “SEP” component, is not needed. On the other hand, a small fraction of the total solar gas in the BMG released from shallow depths is markedly enriched in the light isotopes relative to predictions from implantation simulations with a uniform isotopic composition. Contributions from a neutral solar or interstellar component are too small to explain this shallow sited gas. We tentatively attribute this superficially implanted gas to low-speed, current-sheet related SW, which was fractionated in the corona due to inefficient Coulomb drag. This fractionation process could also explain relatively high Ne/Ar elemental ratios in the same initial gas fraction.     

Subduction of solar-type noble gases from extraterrestrial dust: constraints from high-pressure low-temperature metamorphic deep-sea sediments

Solar-type helium (He) and neon (Ne) in the Earths mantle were suggested to be the result of solar-wind loaded extraterrestrial dust that accumulated in deep-sea sediments and was subducted into the Earths mantle. To obtain additional constraints on this hypothesis, we analysed He, Ne and argon (Ar) in high pressure–low temperature metamorphic rocks representing equivalents of former pelagic clays and cherts from Andros (Cyclades, Greece) and Laytonville (California, USA). While the metasediments contain significant amounts of ⁴He, ²¹Ne and ⁴⁰Ar due to U, Th and K decay, no solar-type primordial noble gases were observed. Most of these were obviously lost during metamorphism preceding 30 km subduction depth. We also analysed magnetic fines from two Pacific ODP drillcore samples, which contain solar-type He and Ne dominated by solar energetic particles (SEP). The existing noble gas isotope data of deep-sea floor magnetic fines and interplanetary dust particles demonstrate that a considerable fraction of the extraterrestrial dust reaching the Earth has lost solar wind (SW) ions implanted at low energies, leading to a preferential occurrence of deeply implanted SEP He and Ne, fractionated He/Ne ratios and measurable traces of spallogenic isotopes. This effect is most probably caused by larger particles, as these suffer more severe atmospheric entry heating and surface ablation. Only sufficiently fine-grained dust may retain the original unfractionated solar composition that is characteristic for the Earths mantle He and Ne. Hence, in addition to the problem of metamorphic loss of solar noble gases during subduction, the isotopic and elemental fractionation during atmospheric entry heating is a further restriction for possible subduction hypotheses.     

Do lunar and meteoritic archives record temporal variations in the composition of solar wind noble gases and nitrogen? A reassessment in the light of Genesis data

Since about half a century samples from the lunar and asteroidal regoliths been used to derive information about elemental and isotopic composition and other properties of the present and past solar wind, predominantly for the noble gases and nitrogen. Secular changes of several important compositional parameters in the solar wind were proposed, as was a likely secular decrease of the solar wind flux. In 2004 NASA’s Genesis mission returned samples which had been exposed to the solar wind for almost 2.5 years. Their analyses resulted in an unprecendented accuracy for the isotopic and elemental composition of several elements in the solar wind, including noble gases, O and N. The Genesis data therefore also allow to re-evaluate the lunar and meteorite data, which is done here. In particular, claims for long-term changes of solar wind composition are reviewed.Outermost grain layers from relatively recently irradiated lunar regolith samples conserve the true isotopic ratios of implanted solar wind species. This conclusion had been made before Genesis based on the agreement of He and Ne isotopic data measured in the aluminum foils exposed to the solar wind on the Moon during the Apollo missions with data obtained in the first gas release fractions of stepwise in-vacuo etch experiments. Genesis data allowed to strengthen this conclusion and to extend it to all five noble gases. Minor variations in the isotopic compositions of implanted solar noble gases between relatively recently irradiated samples (<100 Ma) and samples irradiated billions of years ago are very likely the result of isotopic fractionation processes that happened after trapping of the gases rather than indicative of true secular changes in the solar wind composition. This is particularly important for the ³He/⁴He ratio, whose constancy over billions of years indicates that hardly any ³He produced as transient product of the pp-chains has been mixed from the solar interior into its outer convective zone. The He isotopic composition measured in the present-day solar wind therefore is identical to the (D + ³He)/⁴He ratio at the start of the suns’s main sequence phase and hence can be used to determine the protosolar D/H ratio.Genesis settled the long-standing controversy on the isotopic composition of nitrogen in lunar regolith samples. The ¹⁵N/¹⁴N ratio in the solar wind as measured by Genesis is lower than in any lunar sample. This proves that nitrogen in regolith samples is dominated by non-solar sources. A postulated secular increase of ¹⁵N/¹⁴N by some 30% over the past few Ga is not tenable any longer. Genesis also provided accurate data on the isotopic composition of oxygen in the solar wind, invaluable for cosmochemisty. These data superseded but essentially confirmed one value – and disproved a second one – derived from lunar regolith samples shortly prior to Genesis.Genesis also confirmed prior conclusions that lunar regolith samples essentially conserve the true elemental ratios of the heavy noble gases in the solar wind (Ar/Kr, Kr/Xe). Several secular changes of elemental abundances of noble gases in the solar wind had been proposed based on lunar and meteoritic data. I argue here that lunar data – in concert with Genesis – provide convincing evidence only for a long-term decrease of the Kr/Xe ratio by almost a factor of two over the past several Ga. It appears that the enhancement of abundances of elements with a low first ionisation potential in the solar wind (FIP effect) changed with time.Finally, Genesis allows a somewhat improved comparison of the present-day flux of solar wind Kr and Xe with the total amount of heavy solar wind noble gases in the lunar regolith. It remains unclear whether the past solar wind flux has been several times higher on average than it is today.     

Re-examination of the formation ages of the Apollo 16 regolith breccias

The lunar regolith is exposed to irradiation from the solar wind and to bombardment by asteroids, comets and inter-planetary dust. Fragments of projectiles in the lunar regolith can potentially provide a direct measure of the sources of exogenous material being delivered to the Moon. Constraining the temporal flux of their delivery helps to address key questions about the bombardment history of the inner Solar System.Here, we use a revised antiquity calibration (after Eugster et al., 2001) that utilises the ratio of trapped ⁴⁰Ar/³⁶Ar (‘parentless’ ⁴⁰Ar derived from radioactive decay of ⁴⁰K, against solar wind derived ³⁶Ar) to semi-quantitatively calculate the timing of the assembly of the Apollo 16 regolith breccias. We use the trapped ⁴⁰Ar/³⁶Ar ratios reported by McKay et al. (1986). Our model indicates that the Apollo 16 ancient regolith breccia population was formed between ∼3.8 and 3.4 Ga, consistent with regoliths developed and assembled after the Imbrium basin-forming event at ∼3.85 Ga, and during a time of declining basin-forming impacts. The material contained within the ancient samples potentially provides evidence of impactors delivered to the Moon in the Late-Imbrian epoch. We also find that a young regolith population was assembled, probably by local impacts in the Apollo 16 area, in the Eratosthenian period between ∼2.5 and 2.2 Ga, providing insights to the sources of post-basin bombardment. The ‘soil-like’ regolith breccia population, and the majority of local Apollo 16 soils, were likely closed in the last 2 Ga and, therefore, potentially provide an archive of projectile types in the Eratosthenian and Copernican periods.     

Noble Gases in the Lunar Regolith

The most fundamental character of lunar soil is its high concentrations of solar-wind-implanted elements, and the concentrations and behavior of the noble gases He, Ne, Ar, and Xe, which provide unique and extensive information about a broad range of fundamental problems. In this paper, the authors studied the forming mechanism of lunar regolith, and proposed that most of the noble gases in lunar regolith come from the solar wind. Meteoroid bombardment controls the maturity of lunar soil, with the degree of maturation decreasing with grain size; the concentrations of the noble gases would be of slight variation with the depth of lunar soil but tend to decrease with grain size. In addition, the concentrations of noble gases in lunar soil also show a close relationship with its mineral and chemical compositions. The utilization prospects of the noble gas ^3He in lunar regolith will be further discussed.     

History and origin of aubrites

The cosmic ray exposure (CRE) ages of aubrites are among the longest of stone meteorites. New aubrites have been recovered in Antarctica, and these meteorites permit a substantial extension of the database on CRE ages, compositional characteristics, and regolith histories. We report He, Ne, and Ar isotopic abundances of nine aubrites and discuss the compositional data, the CRE ages, and regolith histories of this class of achondrites. A Ne three-isotope correlation reveals a solar-type ratio of ²⁰Ne/²²Ne = 12.1, which is distinct from the present solar wind composition and lower than most ratios observed on the lunar surface. For some aubrites, the cosmic ray-produced noble gas abundances include components produced on the surface of the parent object. The Kr isotopic systematics reveal significant neutron-capture-produced excesses in four aubrites, which is consistent with Sm and Gd isotopic anomalies previously documented in some aubrites. The nominal CRE ages confirm a non-uniform distribution of exposure times, but the evidence for a CRE age cluster appears doubtful. Six meteorites are regolith breccias with solar-type noble gases, and the observed neutron effects indicate a regolith history. ALH aubrites, which were recovered from the same location and are considered to represent a multiple fall, yield differing nominal CRE ages and, if paired, document distinct precompaction histories.     

Argon, krypton, and xenon in the bulk solar wind as collected by the Genesis mission

We present bulk solar wind isotopic and elemental ratios for Ar, Kr, and Xe averaged from up to 14 individual analyses on silicon targets exposed to the solar wind for ∼2.3 years during NASA’s Genesis mission. All averages are given with 1σ standard errors of the means and include the uncertainties of our absolute calibrations. The isotopic ratios ⁸⁶Kr/⁸⁴Kr and ¹²⁹Xe/¹³²Xe are 0.303 ± 0.001 and 1.06 ± 0.01, respectively. The elemental ratios ³⁶Ar/⁸⁴Kr and ⁸⁴Kr/¹³²Xe are 2390 ± 120 and 9.9 ± 0.3, respectively. Average fluxes of ⁸⁴Kr and ¹³²Xe in the bulk solar wind in atoms/(cm² s) are 0.166 ± 0.009 and 0.017 ± 0.001, respectively. The flux uncertainties also include a 2% uncertainty for the determination of the extracted areas. The bulk solar wind ³⁶Ar/³⁸Ar ratio of 5.50 ± 0.01 and the ³⁶Ar flux of 397 ± 11 atoms/(cm² s) determined from silicon targets agree well with the ³⁶Ar/³⁸Ar ratio and the ³⁶Ar flux determined earlier on a different type of target by Heber et al. (2009). A comparison of the solar wind noble gas/oxygen abundance ratios with those in the solar photosphere revealed a slight enrichment of Xe and, within uncertainties a roughly uniform depletion of Kr-He in the solar wind, possibly related to the first ionization potentials of the studied elements. Thus, the solar wind elemental abundances He-Kr display within uncertainties roughly photospheric compositions relative to each other. A comparison of the Genesis data with solar wind heavy noble gas data deduced from lunar regolith samples irradiated with solar wind at different times in the past reveals uniform ³⁶Ar/⁸⁴Kr ratios over the last 1-2 Ga but an increase of the ⁸⁴Kr/¹³²Xe ratio of about a factor of 2 during the same time span. The reason for this change in the solar wind composition remains unknown.     

Argon 40-argon 39 chronology of lithic clasts from the Kapoeta howardite

⁴⁰Ar-³⁹Ar age spectra have been measured on plagioclase separates from three basaltic clasts (A, B, C), a pyroxene separate from clast B, and a total sample of a fourth basaltic clast (ρ) from the Kapoeta achondritic meteorite. The Ar data show that three of the four clasts crystallized ≥4.5 AE ago. Xe measurements indicate all four formed within a 0.1 AE period (Huneke, et al., 1977, Lunar Science VIII, pp. 484–486). Three clasts have suffered various degrees of ⁴⁰Ar loss since that time. The times of ⁴⁰Ar degassing do not cluster about a single time analogous to the lunar cataclysm. The survival of ≥4.5 AE ages contrasts with the general absence of ages ≥4.0 AE on the moon.The Ar retention age of clast B of ≥4.57 AE is atypically older than the Rb-Sr age of 3.6 AE (Papanastassiouet al., 1974, Lunar Science V, p. 583). The 3.5 AE Ar age of clast A is distinctly younger than the Rb-Sr age of 3.9 AE (Papanastassiou et al., 1974). The K-Ar and Rb-Sr systems are clearly not equivalent dating techniques in these instances.The combined evidence of Ar, Xe and Rb-Sr studies suggests the period of volcanism on the Kapoeta parent planet was restricted to the first ~0.2 AE of solar system history. The subsequent thermal metamorphic histories recorded in each of the four clasts after formation are distinctly different. The clasts must have existed as independent fragments at least as recently as 3.5 AE ago. The cosmic ray exposure ages of all the four clasts are similar (~ 3 Myr), and are not significantly different from that of the bulk meteorite. The clasts spent essentially all of the time prior to the formation of Kapoeta at depths greater than a few meters.     

Two oxygen isotopic components with extra-selenial origins observed among lunar metallic grains - In search for the solar wind component

Oxygen isotopic analyses were performed in the surface layers of lunar metallic grains from lunar regolith samples 71501 and 79035, presumably exposed at the Moon surface at different times. We were able to reproduce the two extreme O components previously found [Hashizume K. and Chaussidon M. (2005) A non-terrestrial ¹⁶O-rich isotopic composition for the protosolar nebula. Nature434, 619-622; Ireland T. R., Holden P., Norman M. D. and Clarke J. (2006) Isotopic enhancements of ¹⁷O and ¹⁸O from solar wind particles in the lunar regolith. Nature440, 776-778], with a range observed of −12 ± 5 < Δ¹⁷O < +33 ± 3‰ (1σ). The relatively minor ¹⁶O-rich component corresponding to an end-member Δ¹⁷O value lower than −20‰ is likely the solar component. This comes from the fact that its concentration roughly agrees with the maximum solar wind abundance expected among the grains from the two samples. At variance the ¹⁶O-poor component is 5-10 times more abundant and thus likely non-solar. The δ¹⁸O range found for the ¹⁶O-poor component may reflect various processes such as isotope exchange reaction during oxidation of metallic iron and/or isotope fractionation by evaporation/condensation at the surface of the Moon or during implantation at depth in the lunar metallic grains. The present study suggests that planetary solid materials in bulk are systematically depleted in ¹⁶O relative to the solar isotopic composition, suggesting the existence of non-mass-dependent isotopic fractionations associated to the formation of solids in the accretion disk.     

Evidence from Ar/Ar ages of lunar impact glasses for an increase in the impact rate ∼800 Ma ago

Geochemical and ⁴⁰Ar/³⁹Ar data on nine impact glasses from the Apollo 14, 16, and 17 landing sites indicate at least seven distinct impact events with ages ∼800 Ma. Rock fragments analyzed by Barra et al. [Barra F., Swindle T. D., Korotev R. L., Jolliff B. L., Zeigler R. A., and Olsen E. (2006) ⁴⁰Ar-³⁹Ar dating of Apollo 12 regolith: implications for the age of Copernicus and the source of nonmare materials, Geochim. Cosmochim. Acta,70, 6016-6031] from the Apollo 12 landing site and some Apollo 12 spherules reported by Levine et al. [Levine J., Becker T. A., Muller R. A., Renne P. R. (2005) ⁴⁰Ar/³⁹Ar dating of Apollo 12 impact spherules, Geophys. Res. Let., 32, L15201, doi: 10.1029/2005GL022874.] show ∼800 Ma ages, close to the accepted age of the Copernicus event, 800 ± 15 Ma [Bogard D. D., Garrison D. H., Shih C. Y., and Nyquist L. E. (1994) ³⁹Ar-⁴⁰Ar dating of two lunar granites: The age of Copernicus, Geochim. Cosmochim. Acta, 58, 3093-3100]. These Apollo 12 samples are thought to have been affected by material from the Copernicus event since there is a Copernicus ray going through the Apollo 12 landing site. When all of these data are viewed collectively, including an Apollo 16 glass bomb [Borchardt R., Stöffler D., Spettel B., Palme H. and Wänke H. (1986) Composition, structure, and age of the Apollo 16 subregolith basement as deduced from the chemistry of post-Imbrium melt bombs. In Proceedings, 17th Lunar and Planetary Science Conference, pp. E43-E54], and in the context of diverse compositional range and sample location, there is a suggestion that there may have been a transient increase in the global lunar impact flux at ∼800 Ma. Therefore, the Copernicus impact event could have been one of many. If correct, there should be evidence for this increased impact flux around 800 Ma ago in the age statistics of terrestrial impact samples.     

Regolith history of the aubritic meteorite parent body revealed by neutron capture effects on Sm and Gd isotopes

Enstatite achondrites (aubrites) when compared to other stone meteorites have unusually long cosmic-ray exposure (CRE) ages. We report here the ¹⁵⁰Sm/¹⁴⁹Sm and ¹⁵⁸Gd/¹⁵⁷Gd ratios in six different structural phases, i.e., light and dark (shocked) grains and in matrix materials of Pesyanoe, in three different fragments from Pena Blanca Spring, and in one from Norton County, Shallowater, and Khor Temiki, to investigate the regolith history on the aubrite parent body. The results from phases components of Pesyanoe confirm earlier reported evidence for regolith irradiation of several aubrites. The inferred neutron fluences for six Pesyanoe separates vary between (2.13 and 2.82) × 10¹⁶ n cm⁻². The fluences also significantly exceed those expected from cosmic-ray irradiation during transit to Earth and approach those observed in the lunar regolith. These observations confirm that the brecciated Pesyanoe meteorite, which contains solar wind (SW) gases only in dark phases, was processed in a regolith and that structural phases were differentially irradiated before compaction. On the other hand, in some aubrites (Mt. Egerton, Shallowater, Pena Blanca Spring, Norton County) neutron capture effects may entirely be due to space irradiation.     

Rare gas isotopes and parent trace elements in ultrabasic-alkaline-carbonatite complexes, Kola Peninsula: identification of lower mantle plume component

During the Devonian magmatism (370 Ma ago) ∼20 ultrabasic-alkaline-carbonatite complexes (UACC) were formed in the Kola Peninsula (north-east of the Baltic Shield). In order to understand mantle and crust sources and processes having set these complexes, rare gases were studied in ∼300 rocks and mineral separates from 9 UACC, and concentrations of parent Li, K, U, and Th were measured in ∼70 samples. ⁴He/³He ratios in He released by fusion vary from pure radiogenic values ∼10⁸ down to 6 × 10⁴. The cosmogenic and extraterrestrial sources as well as the radiogenic production are unable to account for the extremely high abundances of ³He, up to 4 × 10⁻⁹ cc/g, indicating a mantle-derived fluid in the Kola rocks. In some samples helium extracted by crushing shows quite low ⁴He/³He = 3 × 10⁴, well below the mean ratio in mid ocean ridge basalts (MORB), (8.9 ± 1.0) × 10⁴, indicating the contribution of ³He-rich plume component. Magnetites are principal carriers of this component. Trapped ³He is extracted from these minerals at high temperatures 1100°C to 1600°C which may correspond to decrepitation or annealing primary fluid inclusions, whereas radiogenic ⁴He is manly released at a temperature range of 500°C to 1200°C, probably corresponding to activation of ⁴He sites degraded by U, Th decay.Similar ⁴He/³He ratios were observed in Oligocene flood basalts from the Ethiopian plume. According to a paleo-plate-tectonic reconstruction, 450 Ma ago the Baltica (including the Kola Peninsula) continent drifted not far from the present-day site of that plume. It appears that both magmatic provinces could relate to one and the same deep-seated mantle source.The neon isotopic compositions confirm the occurrence of a plume component since, within a conventional ²⁰Ne/²²Ne versus ²¹Ne/²²Ne diagram, the regression line for Kola samples is indistinguishable from those typical of plumes, such as Loihi (Hawaii). ²⁰Ne/²²Ne ratios (up to 12.1) correlate well with ⁴⁰Ar/³⁶Ar ones, allowing to infer a source ⁴⁰Ar/³⁶Ar ratio of about 4000 for the mantle end-member, which is 10 times lower than that of the MORB source end-member. In (³He/²²Ne)_PRIM versus (⁴He/²¹Ne)_RAD plot the Kola samples are within array established for plume and MORB samples; almost constant production ratio of (⁴He/²¹Ne)_RAD ≅ 2 × 10⁷ is translated via this array into (³He/²²Ne)_PRIM ∼ 10. The latter value approaches the solar ratio implying the non-fractionated solar-like rare gas pattern in a plume source.The Kola UACC show systematic variations in the respective contributions of in situ-produced radiogenic isotopes and mantle-derived isotopes. Since these complexes were essentially plutonic, we propose that the depth of emplacement exerted a primary control on the retention of both trapped and radiogenic species, which is consistent with geological observations. The available data allow to infer the following sequence of processes for the emplacement and evolution of Kola Devonian UACC: 1) Ascent of the plume from the lower mantle to the subcontinental lithosphere; the plume triggered mantle metasomatism not later than ∼700 to 400 Ma ago. 2) Metasomatism of the lithosphere (beneath the central part of the Kola Peninsula), including enrichment in volatile (e.g., He, Ne) and in incompatible (e.g., U, Th) elements. 3) Multistage intrusions of parental melts, their degassing, and crystallisation differentiation ∼370 Ma ago. 4) Postcrystallisation migration of fluids, including loss of radiogenic and of trapped helium. Based on model compositions of the principle terrestrial reservoirs we estimate the contributions (by mass) of the plume material, the upper mantle material, and the atmosphere (air-saturated groundwater), into the source of parent melt at ∼2%, 97.95%, and ∼0.05%, respectively.     

稀有气体同位素对岩浆侵入方向的制约:以夏日哈木镍铜硫化物矿床为例

我国东昆仑造山带新发现的夏日哈木镍铜硫化物矿床是造山带环境产出全球镍资源最大的岩浆镍铜矿床。含矿岩体不同类型岩石中橄榄石和辉石的He、Ne和Ar同位素组成表明:~3He/~4He(0.39~0.03Ra)和~(40)Ar/~(36)Ar比值(292.0~316.9)较低,~(20)Ne/~(22)Ne和~(21)Ne/~(22)Ne沿放射性成因Ne及大陆地壳线分布,表明岩浆起源演化过程中有大陆地壳组分和大气饱和流体在橄榄石结晶前加入。He和Ar同位素混合模型计算表明岩浆中有7.8%再循环洋壳组分和87.7%大气饱和流体的加入,再循环洋壳可能带入了大气及地壳组分。~3He/~4He和~(40)Ar/~(36)Ar比值的自西向东系统性降低,以及微量元素、成矿元素和稀有气体同位素的空间协同变化特征表明地壳物质的逐步加入,即岩浆可能自西向东方向侵入,侵位过程中地壳流体的加入促使硫饱和及硫化物的熔离成矿。     

Improving noble gas based paleoclimate reconstruction and groundwater dating using Ne/Ne ratios

The interpretation of noble gas concentrations in groundwater with respect to recharge temperature and fractionated excess gas leads to different results on paleo-climatic conditions and on residence times depending on the choice of the gas partitioning model. Two fractionation models for the gas excess are in use, one assuming partial re-equilibration of groundwater supersaturated by excess air (PR-model, Stute et al., 1995), the other assuming closed-system equilibration of groundwater with entrapped air (CE-model, Aeschbach-Hertig et al., 2000). In the example of the Continental Terminal aquifers in Niger, PR- and CE- model are both consistent with the data on elemental noble gas concentrations (Ne, Ar, Kr, and Xe). Only by including the isotope ratio ²⁰Ne/²²Ne it can be demonstrated that the PR-model has to be rejected and the CE-model should be applied to the data. In dating applications ³He of atmospheric origin (³He_atm) required to calculate ³H-³He water ages is commonly estimated from the Ne excess presuming that gas excess is unfractionated air (UA-model). Including in addition to the Ne concentration the ²⁰Ne/²²Ne ratio and the concentration of Ar enables a rigorous distinction between PR-, CE- and UA-model and a reliable determination of ³He_atm and of ³H-³He water ages.     

Combined U-Th/He and Ar/Ar geochronology of post-shield lavas from the Mauna Kea and Kohala volcanoes, Hawaii

Late Quaternary, post-shield lavas from the Mauna Kea and Kohala volcanoes on the Big Island of Hawaii have been dated using the ⁴⁰Ar/³⁹Ar and U-Th/He methods. The objective of the study is to compare the recently demonstrated U-Th/He age method, which uses basaltic olivine phenocrysts, with ⁴⁰Ar/³⁹Ar ages measured on groundmass from the same samples. As a corollary, the age data also increase the precision of the chronology of volcanism on the Big Island. For the U-Th/He ages, U, Th and He concentrations and isotopes were measured to account for U-series disequilibrium and initial He. Single analyses U-Th/He ages for Hamakua lavas from Mauna Kea are 87 ± 40 to 119 ± 23 ka (2σ uncertainties), which are in general equal to or younger than ⁴⁰Ar/³⁹Ar ages. Basalt from the Polulu sequence on Kohala gives a U-Th/He age of 354 ± 54 ka and a ⁴⁰Ar/³⁹Ar age of 450 ± 40 ka. All of the U-Th/He ages, and all but one spurious ⁴⁰Ar/³⁹Ar ages conform to the previously proposed stratigraphy and published ¹⁴C and K-Ar ages. The ages also compare favorably to U-Th whole rock-olivine ages calculated from ²³⁸U-²³⁰Th disequilibria. The U-Th/He and ⁴⁰Ar/³⁹Ar results agree best where there is a relatively large amount of radiogenic ⁴⁰Ar (>10%), and where the ⁴⁰Ar/³⁶Ar intercept calculated from the Ar isochron diagram is close to the atmospheric value. In two cases, it is not clear why U-Th/He and ⁴⁰Ar/³⁹Ar ages do not agree within uncertainty. U-Th/He and ⁴⁰Ar/³⁹Ar results diverge the most on a low-K transitional tholeiitic basalt with abundant olivine. For the most alkalic basalts with negligible olivine phenocrysts, U-Th/He ages were unattainable while ⁴⁰Ar/³⁹Ar results provide good precision even on ages as low as 19 ± 4 ka. Hence, the strengths and weaknesses of the U-Th/He and ⁴⁰Ar/³⁹Ar methods are complimentary for basalts with ages of order 100-500 ka.     

Sources, degassing, and contamination of CO2, H2O, He, Ne, and Ar in basaltic glasses from Kolbeinsey Ridge, North Atlantic

New volatile data (CO₂, H₂O, He, Ne, and Ar) are presented for 24 submarine basaltic glasses from the Kolbeinsey Ridge, Tjörnes Fracture Zone and Mohns Ridge, North Atlantic. Low CO₂ and He contents indicate that magmas were strongly outgassed with the extent of degassing increasing toward the south, as expected from shallower ridge depths. Ne and Ar are significantly more abundant in the southernmost glasses than predicted for degassed melt. The strong atmospheric isotopic signal associated with this excess Ne and Ar suggests syn- or posteruptive contamination by air. Degassing, by itself, cannot generate the large variations in δ¹³C values of dissolved CO₂ or coupled CO₂-Ar variations. This suggests that δ¹³C values were also affected by some other processes, most probably melt-crust interaction. Modelling indicates that degassing had a negligible influence on water owing to its higher solubility in basaltic melt than the other volatiles. Low H₂O contents in the glasses reflect melting of a mantle source that is not water-rich relative to the source of N-MORB.Before eruption, Kolbeinsey Ridge melts contained ∼400 ppm CO₂ with δ¹³C of −6‰, 0.1 to 0.35 wt.% H₂O, ³He/⁴He ∼11 R_A, and CO₂/³He of ∼2 × 10⁹. We model restored volatile characteristics and find homogeneous compositions in the source of Kolbeinsey Ridge magmas. Relative to the MORB-source, He and Ne are mildly fractionated while the ⁴⁰Ar/³⁶Ar may be low. The ³He/⁴He ratios in Tjörnes Fracture Zone glasses are slightly higher (13.6 R_A) than on Kolbeinsey Ridge, suggesting a greater contribution of Icelandic mantle from the south, but the lack of ³He/⁴He variation along the Kolbeinsey Ridge is inconsistent with active dispersal of Icelandic mantle beyond the Tjörnes Fracture Zone.     

Ar-Ar dating of plagioclase grain size separates from silicate inclusions in IAB iron meteorites and implications for the thermochronological evolution of the IAB parent body

In order to better constrain the thermochronological evolution of the IAB parent body we performed a ⁴⁰Ar/³⁹Ar age study on individual silicate inclusions of the IAB irons Caddo County, Campo del Cielo, Landes, and Ocotillo. In contrast to earlier studies, several plagioclase separates of different grain sizes and quality grades were extracted from each inclusion to reduce the complexity of the age spectra and study the influence of these parameters on the Ar-Ar ages. In nearly all inclusions we found significantly different Ar-Ar ages among the separates (Caddo County: 4.472 ± 0.02-4.562 ± 0.02 Ga; Campo del Cielo 2: 4.362 ± 0.04-4.442 ± 0.03 Ga; Landes 2: 4.412 ± 0.05-4.522 ± 0.04 Ga; Ocotillo: 4.382 ± 0.04-4.462 ± 0.03 Ga). These ages were calculated using the new ⁴⁰K decay constant published by [Mundil R., Renne P. R., Min K. and Ludwig K. R. (2006) Resolvable miscalibration of the ⁴⁰Ar/³⁹Ar geochronometer. Eos Trans. AGU 87, Fall Meet. Suppl., Abstract V21A-0543]. The ages did not systematically correlate with the respective grain size of the separate as expected, i.e., smaller grains did not necessarily show younger ages due to later closure to Ar diffusion or easier re-opening of the system in the course of a reheating event compared to larger grains. Based on the large range of Ar-Ar ages we suggest that the individual inclusions are composed of silicate grains from different locations within the IAB parent body. While some grains remained in a hot (deep) environment that allowed Ar diffusion over an extended time period—in some cases combined with grain coarsening—, others cooled significantly earlier (near surface) through the K/Ar blocking temperature. These different grains where brought together during an impact followed by mixing and reassembly of the debris as proposed by Benedix et al. [Benedix G. K., McCoy T. J., Keil K. and Love S. G. (2000) A petrologic study of the IAB iron meteorites: constraints on the formation of the IAB-Winonaite parent body. Meteorit. Planet. Sci.35, 1127-1141]. Due to rapid cooling after the impact some of the age differences among the grains could be preserved. Based mainly on our Caddo County Ar-Ar age information, the IAB parent body was destroyed by impact and reassembled between ∼4.5 and 4.47 Ga. However, IAB silicate Ar-Ar ages should depend much more on the pre- and post-impact cooling rate and burial depth than on the time of the actual impact. This is supported by a compilation of our and literature IAB and winonaite Ar-Ar ages ranging smoothly from the time of accretion of the chondritic IAB parent body down to the time of its final cooling through the K-Ar blocking temperature after impact and reassembly, instead of showing a peak in Ar-Ar ages at the time of the destructive impact.     

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.