Some results

Primitive meteorites contain nanodiamonds (average size 2.6 nm) of which at least some fraction is presolar, i.e. older than the Solar System. They contain noble gases that most likely were introduced by ion implantation. We have simulated this process by implanting into artificial detonation nanodiamonds noble gases (energy approx. 1 keV). In stepwise heating the implanted noble gases show the same double peak structure as a function of release temperature as seen in the analysis of the natural nanodiamonds. The low temperature component has the isotopic compositon of the reservoir from which the noble gases were taken, while the more tightly bound component is fractionated favoring the heavy isotopes.

Applied to the case of the natural nanodiamonds, our results indicate:
a) The isotopically anomalous HL component is presdent in krypton and xenon only.
b) Isotopically anomalous krypton not only shows an enhancement of the heaviest isotopes (Kr-H), but also - as is the case
    for xenon - also of the lightest isotopes (Kr-L).
c) Apparent anomalies in He, Ne, and Ar can be explained by mass-dependent fractionation. Exceptions are ³He and ²¹Ne
   that show contributions due to cosmic ray effects.
d) In case of ³He the dominant cosmic ray contribution may be trapped cosmic rays. This may offer a possibility for dating
    the presolar nanodiamonds.

In another experiment we have investigated the recoil loss during radioactive decay of trace radionuclides within nanodiamonds. In order to do so we implanted radioactive ²²Na, determined the decay rate by measuring their radioactivity and measured the abundance of the decay product ²²Ne. The results agree with theoretical expectations within error.
 

Transmission electron microscopy image of a nanodiamond
(F. Banhart)

Collaboration with Karpov Institute for Physical Chemistry (Moscow) und University of Hawaii.

"Noble gases in presolar diamonds III: Implications of ion implantation experiments with synthetic nanodiamonds". G.R. Huss, A.P. Koscheev 
and U. Ott  (2008). Meteoritics and Planetary Science 43, 1811-1826.

"Trapping of cosmic ray helium by interstellar diamond".  U. Ott and G.R. Huss. (2008). Meteoritics and Planetary Science 43, A125.

"Investigating recoil loss from ²²Na decay within nanograins". E. Marosits and U. Ott (2006). Meteoritics and Planetary Science 41, A113.

The age of presolar silicon carbide

Possibly the only way to determine an age for presolar grains in meteorites is via a cosmic ray exposure age, which records the time interval between their formation and their incorporation into the Solar system. However, due to the finite grain size, there is always some recoil loss of the spallation products produced by the interaction with the cosmic rays.

Addressing the recoil problem in the past we have performed simulation experiments to determine these recoil losses. We have applied our knowledge in this field in a collaboration with ETH Zürich (measurement of cosmogenic neon in single large "Jumbo" SiC grains, with sizes of some ten micrometers) and Washington University St. Louis (preparation of the grains). Resulting ages are few million years only in many cases, but reach up to about 800 Ma for one grain. Presolar ages determined similarly via cosmogenic lithium, however, are generally hundreds of millions of years. Tasks that remain are understanding the reason for the differences and extending the investigations to the smaller more typical silicon carbide grains.

Secondary electron microscopy image of a "Jumbo" silicon carbide grain (F. Gyngard)

"Spallation recoil and presolar age of interstellar grains in meteorites". U. Ott und F. Begemann (2000). Meteoritics and Planetary Science 35,
53-63.

"Spallation recoil II: Xenon evidence for young SiC grains". U. Ott, M. Altmaier, U. Herpers, J. Kuhnhenn, S. Merchel, R. Michel und R.K. Mohapatra (2005). Meteoritics and Planetary Science 40, 1635-1652.

"Interstellar exposure ages of large presolar SiC grains from the Murchison meteorite". F. Gyngard, S. Amari, E. Zinner und U. Ott (2009). Astrophysical Journal 694, 359-366.

"Interstellar residence times of presolar SiC dust grains from the Murchison carbonaceous meteorite". P.R. Heck, F. Gyngard, U. Ott, M.M.M. Meier, J.N. Ávila, S. Amari, E.K. Zinner, R.S. Lewis, H. Baur und R. Wieler (2009).  Astrophysical Journal 698, 1155-1164.

"Cosmic-ray exposure ages of large presolar SiC grains". F.Gyngard, S. Amari, E. Zinner und U. Ott (2009).  Publications of the Astronomical Society of Australia 26, 278-283.

"He and Ne ages of large presolar silicon carbide grains: Solving the recoil problem". U. Ott, P.R. Heck, F. Gyngard, R. Wieler, F. Wrobel, S. Amari  und E. Zinner E. (2009).  Publications of the Astronomical Society of Australia 26, 297-302.

Noble gases and nitrogen in Ocean Island Basalts (OIB)

We have determined abundances and isotopic compositions of noble gases and nitrogen in Ocean Island Basalts. The systematics of He, Ar and Xe isotopes is similar to that reported in the literature, however the results also demonstrate the importance of recycling and of air contamination. We suggest for the isotopic composition of nitrogen in the Earth's mantle a delta ¹⁵N value of about -25 ‰ (delta ¹⁵N  is the deviation of the ¹⁵N/¹⁴N ratio from the value for nitrogen in air).

"Noble gas and nitrogen isotopic components in Oceanic Island Basalts". R.K. Mohapatra, D. Harrison, U. Ott, J.D. Gilmour und M. Trieloff  (2009).  Chemical Geology 266, 29-37.

Noble-metal nuggets - the oldest surviving condensates of the Solar System?

In the process of isolating presolar silicon carbide grains from the Murchison meteorite we discovered in the acid-resistant residues noble metal nuggets with sizes in the sub-micrometer range. The size distribution is log-normal and the chemical composition and morphology point to a condensation origin. This follows in particular from the combination of structurally incompatible elements. The abundance ratios of the metals correspond to the predictions from equilibrium condensation calculations for the solar nebula in the temperature range between 1620 and 1450 K. The data also allow an estimate for the cooling rate of the solar nebula. In this range it cannot have exceeded 0.5 K per year.

Secondary electron microscopy image of one of the larger noble metal nuggets (T. Berg)

Collaboration with University of Mainz (Physics Institute) and Senckenberg Research Institute and Museum of Natural Science (Frankfurt).

"Direct evidence for condensation in the early solar system and implications for nebular cooling rates". T. Berg, J. Maul, G. Schönhense, E. Marosits, P. Hoppe, U. Ott und H. Palme (2009). Astrophysical Journal Letters 702, L172-L176.

"Die älteste Materie des Sonnensystems". Th. Berg, U. Ott und H. Palme (2010). Sterne und Weltraum 5/2010, 28-37.

Landscape evolution in Antarctica

The Earth’s atmosphere and magnetic field are potent shields against the flux of cosmic rays omnipresent in space. They are not perfect shields, however. Especially at high geographic latitude and high altitude, abundant energetic secondary particles are hitting the Earth’s surface. These cause detectable production of spallogenic radionuclides that can be used in geomorphological studies. Complementary information can be gained from measurements of noble gases, which are extremely rare in solid earth materials. Thus stable low-abundance noble gas isotopes can serve as tracers extending the accessible range of surface exposure ages. In particular they allow the determination of surface exposure ages and erosion rates.

Together with colleagues from the University of Cologne we have conducted corresponding investigations on samples from Queen Maud Land (East Antarctica). Radioactive ¹⁰Be and ²⁶Al were measured at ETH Zürich by accelerator mass spectrometry (AMS), while we determined the abundance of stable cosmogenic ²¹Ne. Especially remarkable are the results for the location "Petermann Ketten", where we found minimum surface exposure ages up to more than 8 million years and where erosion rates partially cannot have exceeded 5 cm per million year. Based on the argument that such small erosion rates can exist only under extremely cold and hyperarid conditions, a suspected prolonged period of warm climate in Queen Maud Land within the last 8 million years can be excluded.

Map of Queen Maud Land. Adapted from Antarctic Digital database, Scientific Committee on Antarctic Research (2009)

"Glaciation history of Queen Maud Land (Antarctica) reconstructed from in-situ produced cosmogenic ¹⁰Be, ²⁶Al and ²¹Ne".  M. Altmaier, U. Herpers, G. Delisle, S. Merchel und U. Ott (2010).  Polar Science 4, 42-61.