Heinrich Baur

He, Ne, and Ar from the solar wind and solar energetic particles in lunar ilmenites and pyroxenes

He, Ne, and Ar in ilmenite and pyroxene separates from two lunar regolith samples exposed to the solar corpuscular radiation at widely different epochs were analyzed by the closed system stepped etching technique. This method avoids element or isotope fractionation due to diffusion during the experiment and thus depth profiles of element and isotopic composition of implanted solar noble gases can be determined. Ilmenites from a relatively recently irradiated soil release in the first steps He and Ne with isotopic compositions identical to modern solar wind values. We conclude that ilmenite retains isotopically unfractionated He, Ne, and Ar from the solar wind (SW) in the outermost grain layers. The isotopic composition of SW noble gases has remained constant over the last 100 Ma but there is some evidence for a slight change over a Ga timescale. The ratio 36Ar/38Ar in the solar wind is 2–3% larger than that in the terrestrial atmosphere. We give further evidence for the identification of a solar energetic particle component (SEP), which was implanted with energies exceeding those in the solar wind. All ilmenites and pyroxenes contain SEP-Ne with 20Ne/22Ne = 11.2±0.2, in agreement with SEP-Ne found earlier in lunar plagioclase. (21Ne/22Ne)SEP is consistent with SEP-Ne being derived from SW-Ne by mass dependent fractionation. In addition, the new data reveal SEP-He and SEP-Ar, both isotopically heavier than the SW components. The isotopic fractionation factors required to relate SW and SEP gases are approximately equal to the square of the mass ratios. Element ratios in the SEP dominated steps are similar (He: Ar) or identical (Ne: Ar) to present-day solar wind values, indicating little or no element fractionation in the SEP reservoir.

Cosmogenic noble gas studies in the oldest landscape on earth: surface exposure ages of the Dry Valleys, Antarctica

Extraordinarily high surface exposure ages have been determined for Sirius Group tillites of Mt. Fleming and Mt. Feather as well as at localities in the Inner Dry Valleys using cosmogenic helium and neon. Ages of 10 Ma at Mt. Fleming, 5.3 Ma at Mt. Feather and 6.5 Ma at Insel Mountain are among the highest nominal exposure ages published so far. These values are minimal ages as they are based on the assumption of zero erosion and uplift. The Mt. Feather sample independently confirms the pre-Pliocene age of the Sirius Group sediments in the Dry Valleys as previously determined at Mt. Fleming. The Insel Mountain samples provide evidence for a landscape formation of the Inner Dry Valleys not later than Late Miocene time. Assuming conservatively low values of 2.5 cm Ma−1 for erosion rate and 50 m Ma−1 for uplift rate we infer that the Sirius Group tillites at Mt. Fleming were deposited earlier than 20 Ma ago. This indicates that the overriding of the Dry Valleys block of the Transantarctic Mountains by the East Antarctic Ice Sheet occurred not later than the Early Miocene. Maximum long-term erosion rates in the Inner Dry Valleys must be <15 cm Ma−1 down to altitudes <1000 m. Since such low erosion rates require permanently cold and hyperarid conditions, the response of Antarctica to the Pliocene warm climatic episode must have been small. Cosmogenic nuclide data from both the Inner Dry Valleys and the Sirius Group sediment localities support the hypothesis of a stable East Antarctic Ice Sheet since at least Late Miocene time, implying that the climate of Antarctica was decoupled from that of lower southern latitudes. We present also new elemental 21Ne production rates of P21(Mg) = 196 atoms g−1 yr−1 and P21(Al) = 55 atoms g−1 yr−1 at sea level and high geomagnetic latitude. These figures are consistent with a 3He production rate of P3 = 110 atoms g−1 yr−1, similar to previously published values. This consistency provides evidence that pyroxene is retentive for both helium and neon over at least 10 Ma. Cosmogenic Ne in quartz and pyroxene has a (22Ne/21Ne)cos ratio of 1.266 ± 0.040 and 1.159 ± 0.040, respectively.

The limited influence of glaciations in Tibet on global climate over the past 170 000 yr

Extensive ice cover on the Tibetan Plateau would significantly influence Earth’s climate in general and the Asian monsoon system in particular, but extent and timing of Quaternary glaciations in Tibet remain highly controversial. We dated erratics on top of moraines in the climatic key areas of Central and East Tibet using cosmogenic 10Be, 26Al, and 21Ne. Consistent exposure ages obtained by various nuclides indicate a continuous period of exposure since the deposition of the samples. Our data imply that glacial advances were restricted to a few 10 km during the last 170 kyr in Central Tibet and during the peak of the last glaciation (∼24–13 kyr ago) in Eastern Tibet. Advances of Tibetan glaciers were much less prominent than elsewhere in the northern hemisphere most likely due to very arid conditions and high sublimation rates. A proposed ice-dome covering the entire Plateau can be excluded. Thus, albedo increase of Tibet most likely did trigger neither northern hemisphere ice ages nor paleomonsoon changes during the last two glacial cycles. The glacial advance during Marine Isotope Stage 2 in East Tibet and the absence of significant glacial events during the Holocene suggest a relation of snowline lowering in East Tibet to North Atlantic cooling events rather than to periods of high precipitation by an intensified monsoon.

A model for the production of cosmogenic nuclides in chondrites

Abstract A model is presented which permits the calculation of production rates of cosmic-ray-produced light noble gases He, Ne and Ar as well as 10Be, 26Al and 53Mn in chondrites of variable size and shape The production of a given nuclide is described by a semi-empirical equation used by Signer and Nier (1960) to describe the production of the light noble gases in the iron meteorite Grant. For a given nuclide, this equation contains two free parameters that are fitted to the depth profiles measured in the Knyahinya chondrite. Model predictions are tested by comparison with literature data for depth profiles in other meteorites (ALH78084, St. Severin, Keyes). The agreement between model predictions and experimental data is found to be 5% for the concentrations of 10Be, 21Ne, 22Ne, 38Ar and 53Mn, respectively and to be better than 1% for the 22 Ne 21 Ne ratios. The model predicts that 3-nuclide correlations that use the same reference nuclide are linear. The correlation between P( 10 Be) P( 21 Ne) and 22 Ne 21 Ne ratios is experimentally verified over a wide range of irradiation conditions. With this relation, shielding and size corrected exposure ages can be derived from the measurements of 10Be and of the Ne isotopes in a single sample. The calculated exposure ages are believed to have a precision of 5%.

Noble gases from solar energetic particles revealed by closed system stepwise etching of lunar soil minerals

He, Ne, and Ar abundances and isotopic ratios in plagioclase and pyroxene separates from lunar soils were determined using a closed system stepwise etching technique. This method of noble gas release allows one to separate solar wind (SW) noble gases from those implanted as solar energetic particles (SEP). SEP-Ne with 20Ne22Ne = 11.3 ± 0.3 is present in all samples studied. The abundances of SEP-Ne are 2–4 orders of magnitude too high to be explained exclusively as implanted solar flare gas. The major part of SEP-Ne possibly originates from solar “suprathermal ions” with energies < 0.1 MeV/amu. The isotopic composition of Ne in these lower energy SEP is, however, probably identical to that of real flare Ne. The suggestion that SEP-Ne might have the same isotopic composition as planetary Ne and thus possibly represent an unfractionated sample of solar Ne is not tenable. SW-Ne retained in plagioclase and pyroxene is less fractionated than has been deduced by total fusion analyses. Ne-B is a mixture of SW-Ne and SEP-Ne rather than fractionated SW-Ne. In contrast to SEP-Ne, SEP-Ar has probably a very similar composition as SW-Ar.

Characterisation of Q-gases and other noble gas components in the Murchison meteorite

Abstract Noble gases in several HF/HCl resistant residues of the CM2 chondrite Murchison were measured by closed-system stepped etching, in order to study the planetary gases in their major carrier “Q”—an ill-defined minor phase, perhaps merely a set of adsorption sites. Neon, Ar, Kr, Xe, and probably also He in “Q” of Murchison have the same isotopic and nearly the same elemental abundances as their counterparts in Allende (CV3). The isotopic composition of Ne-Q is consistent with mass-dependent fractionation of either solar wind Ne or Ne from solar energetic particles. Unlike Allende, Murchison during HNO 3 attack releases, besides Q-gases, large amounts of two other Ne-components, Ne-E and Ne-A3, a third subcomponent of Ne-A. This work confirms that Q-gases of well-defined composition were an important noble gas component in the early solar system and are now found in various classes of meteorites, such as carbonaceous chondrites, ureilites, and ordinary chondrites. Ne-Q may have played a role in the formation of noble gas reservoirs in terrestrial planets.

The oldest ice on Earth in Beacon Valley, Antarctica: new evidence from surface exposure dating

Abstract Beacon Valley, Antarctica, contains unique remnants of glacier ice underneath a till layer covering the valley floor. To constrain the age and evolution of this important indicator of Antarctic paleoclimate, we analyzed two dolerite erratics from the till surface and one from within the ice for cosmogenic helium and neon. A conservative minimum exposure age of the older surface sample is 2.3 Ma, but taking into account erosion, the true exposure age of this boulder is likely to be considerably higher. The buried sample contains more than 20 times less cosmogenic noble gases than the old surface sample, although its current shielding would imply only a three times lower production rate. This indicates that the ice level has slowly been lowered by sublimation at the rate of a few m/Ma. The high exposure age of the surface sample as well as the very low sublimation rate of the relict ice both support the conclusion that the remnant ice in Beacon Valley was deposited many million years ago [Sugden et al., Nature 376 (1995) 412–414] and has never been thinner than at present. In addition, we found that cosmogenic helium and neon are released quantitatively from pyroxene at temperatures of 1000°C, respectively.

Solar Wind Neon from Genesis: Implications for the Lunar Noble Gas Record

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 NASAs 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.

Fast delivery of meteorites to Earth after a major asteroid collision

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 ∼480 million year old sediments. The transfer times deduced from the noble gases are as short as ∼105 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.

Dating of Sirius Group tillites in the Antarctic Dry Valleys with cosmogenic3He and21Ne

We measured in situ produced cosmogenic3He and21Ne in boulder and bedrock surface samples in and near Sirius Group tillites from two localities in Southern Victoria Land, Antarctica. Pyroxene quantitatively retains cosmogenic3He 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 ≈ 50% higher than the stated value. The concentrations of cosmogenic Ne in our samples constrain uplift rates of the Transantarctic Mountains to < 170 m/Ma. These data contradict the hypothesis of a collapse of the East Antarctic ice sheet in the Pliocene coupled with high uplift rates and support the model of a stable ice sheet since the middle Miocene.