Rainer Wieler

Late formation and prolonged differentiation of the Moon inferred from W isotopes in lunar metals.

The Moon is thought to have formed from debris ejected by a giant impact with the early ‘proto’-Earth and, as a result of the high energies involved, the Moon would have melted to form a magma ocean. The timescales for formation and solidification of the Moon can be quantified by using 182Hf–182W and 146Sm–142Nd chronometry, but these methods have yielded contradicting results. In earlier studies, 182W anomalies in lunar rocks were attributed to decay of 182Hf within the lunar mantle and were used to infer that the Moon solidified within the first ∼60 million years of the Solar System. However, the dominant 182W component in most lunar rocks reflects cosmogenic production mainly by neutron capture of 181Ta during cosmic-ray exposure of the lunar surface, compromising a reliable interpretation in terms of 182Hf–182W chronometry. Here we present tungsten isotope data for lunar metals that do not contain any measurable Ta-derived 182W. All metals have identical 182W/184W ratios, indicating that the lunar magma ocean did not crystallize within the first ∼60 Myr of the Solar System, which is no longer inconsistent with Sm–Nd chronometry. Our new data reveal that the lunar and terrestrial mantles have identical 182W/184W. This, in conjunction with 147Sm–143Nd ages for the oldest lunar rocks, constrains the age of the Moon and Earth to  Myr after formation of the Solar System. The identical 182W/184W ratios of the lunar and terrestrial mantles require either that the Moon is derived mainly from terrestrial material or that tungsten isotopes in the Moon and Earth’s mantle equilibrated in the aftermath of the giant impact, as has been proposed to account for identical oxygen isotope compositions of the Earth and Moon.

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.

A petrologic, chemical, and isotopic study of Monument Draw and comparison with other acapulcoites: Evidence for formation by incipient partial melting

Abstract We have conducted petrologic, chemical, and isotopic studies of acapulcoites (Acapulco, Monument Draw, Yamato 74063, ALH A77081, ALH A81261, ALH A81315, ALH 78230, ALH A81187 and ALH 84190) in an attempt to constrain their genesis. Acapulcoites have distinctly different oxygen isotopic compositions than silicate inclusions in IAB and IIICD irons, winonaites and ureilites and, thus, formed on a different parent body. Oxygen isotopic compositions, which are slightly heterogeneous within the group, overlap with lodranites, indicating a likely origin on a common parent body. These groups can be distinguished on the basis of mafic silicate grain size. All acapulcoites have mafic silicate compositions intermediate between E and H chondrites, roughly chondritic mineralogies, achondritic, equigranular textures, micrometer to centimeter sized veins of Fe,NiFeS which cross-cut silicate phases, rapid metallographic cooling rates at ∼600−400°C (103–105°C/Myr) and trapped noble gas abundances comparable to type 3–4 ordinary chondrites. They exhibit variable mafic silicate zoning, abundance of Fe,NiFeS veins, REE abundances and patterns and, possibly, cosmic ray exposure ages (∼5–7 Ma). Momument Draw and Yamato 74063 retain rare relict chondrules. Phosphates are associated with Fe,NiFeS veins or form separate veins in Monument Draw and Acapulco. Heating and cooling of acapulcoites occurred very early in the history of the Solar System, as evidenced by the 39Ar40Ar ages of ∼4.51 Ga. These ages appear distinctly younger than the likely formation time for Acapulco of 4.557 Ga, but are older than analogous 39Ar40Ar ages for most chondrites. Acapulcoites formed from a precursor chondrite which differs from known chondrites in mineral and oxygen isotopic compositions. Heating to ∼950–1000°C resulted in melting at the Fe,NiFeS cotectic, but silicates did not melt. Silicate textures resulted from extensive solid-state recrystallization. Heating was by noncollisional sources (e.g., 26Al, electromagnetic induction). Despite uncertainties owing to a lack of data, acapulcoites may have experienced a three-stage thermal history of slow cooling at high temperature, rapid cooling at intermediate temperatures, and slow cooling at low temperatures, possibly resulting from breakup and gravitational reassembly of the parent body. The complex thermal history is also reflected in disequilibrium REE abundances. One or at most two impact events (∼7 Ma and possibly ∼5 Ma ago) are consistent with the cosmic ray exposure ages of all four acapulcoites for which cosmogenic noble gas data exist.

A petrologic and isotopic study of lodranites: Evidence for early formation as partial melt residues from heterogeneous precursors

Abstract We have conducted petrologic, chemical, and isotopic studies of lodranites in an attempt to constrain their genesis. Lodran, Gibson, Y-791491, Y-791493, Y-74357, Y-8002, Y-75274, MAC 88177, LEW 88280, EET 84302, FRO 90011, and QUE 93148 are classified as lodranites. Lodranites and acapulcoites are indistinguishable on the basis of oxygen isotopic compositions but are distinct in average grain sizes of their mafic silicates, with lodranites being significantly coarser-grained. Lodranites exhibit a diverse range of petrologic and mineralogic features: they range widely in mafic silicate compositions (Fa3–13), plagioclase (0–11.4 vol%), Fe,Ni metal (0.5–20 vol%), and troilite (0.2–5.3 vol%) contents; and shock levels (S1–S4) . They appear to have experienced high peak temperatures and rapid cooling in the temperature range recorded by metallographic cooling rates (i.e., 700-350°C). The only dated lodranite, Gibson, cooled to Ar closure temperatures at 4.49 ± 0.01 Ga. Lodranites formed from chemically and isotopically heterogeneous precursors in which the mineral and oxygen isotopic compositions were correlated. Heating of their parent body to temperatures between ∼ 1050–1200°C resulted in formation of Fe,NiFeS and basaltic partial melts. Depletions of troilite and/ or plagioclase in most lodranites testify to the removal of some of these partial melts, although melt migration was complex. Lodranites appear to have experienced a complex cooling history of slow cooling at high temperatures, followed by rapid cooling at intermediate temperatures, possibly related to breakup of the parent body. Lodranites were liberated from their parent body during 1–3 impact events, with most having cosmic ray exposure ages of 5.5–7 Ma. The acapulcoites are samples from the same parent body but were heated to lower temperatures and, thus, experienced lower degrees of partial melting.

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.

Correction of in situ cosmogenic nuclide production rates for geomagnetic field intensity variations during the past 800,000 years

We present integrated relative production rates for cosmogenic nuclides in rock surfaces, which take into account reported variations of the geomagnetic field intensity during the past 800,000 yr. The calculations are based on the model simulating cosmic ray particle interactions with the Earth’s atmosphere given by Masarik and Beer [“Simulation of particle fluxes and cosmogenic nuclide production in the Earth’s atmosphere,” J. Geophys. Res. 104(D10), 12099–12111, 1999]. Corrections are nearly independent on altitude between sea level and at least 5000 m. The correction factors are essentially identical for all stable and radioactive cosmogenic nuclides with half-lives longer than a few hundred thousand years. At the equator, integrated production rates for exposure ages between ∼40,000 to 800,000 yr are 10 to 12% higher than the present-day values, whereas at latitudes >40°, geomagnetic field intensity variations have hardly influenced in situ cosmogenic nuclide production. Correction factors for in situ 14C production rates differ from those of longer-lived nuclides. They are always smaller than ∼2% because the magnetic field intensity remained rather constant during the past ∼10 kyr, when the major fraction of the 14C extant today was produced.

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.