A most exciting event in the study of meteorites has been
the discovery that many primitive meteorites contain stardust – grains
of presolar origin, older than the Solar System itself. Formed as circumstellar
grains around dying stars, and having survived all subsequent events
in the interstellar medium and in the Solar System, they carry information
about the processes by which chemical elements are created in stars
(nucleosynthesis). In turn, from our concepts about nucleosynthesis
we can infer the stellar sources of the grains. Complementing previous
analyses of stardust diamonds by the Max Planck group, scientists from
the Karpov Institute of Physical Chemistry and from the Max Planck Institute
for Chemistry have studied the introduction of diagnostic trace elements
into terrestrial analog diamonds. The results are used to draw conclusions
about the early history of “stellar diamonds” (Nature, August
9, 2001).

Presolar grains known to be present in meteorites are thermally and
chemically extremely stable minerals such as diamond, graphite, silicon
carbide, corundum (aluminum oxide) and silicon nitride. Although discovered
first and by far the most abundant (ca. 1 per mill by weight in the
most primitive meteorites), the diamonds are the least understood, and
their very identification as being presolar is based on the isotopic
composition of trace elements they carry. Noble gases have played a
special role among these trace elements, and it is primarily the unusual
isotopic composition of xenon – ca. 100% enrichment of the lightest
and heaviest isotopes – which suggests that they came from supernova
explosions.

How, when, and where introduction of xenon and other trace elements
occurred may provide crucial information on formation and early history
of the diamond grains, and there are strong indirect arguments that
introduction was by implantation of ions. To test the viability of the
process, a simulation experiment was performed: a noble gas mixture
consisting of helium, argon, krypton and xenon ions with an energy of
700 electronvolts was implanted into a layer of terrestrial nanodiamonds
of similar size as the presolar nanodiamonds (extremely small,only a
few nanometer; s. figure), and after irradiation the release of the
implanted noble gases was studied. Surprisingly, release as a function
of temperature was bimodal, with one peak in the 200-700°C range
and another one above 1000°C. This situation – after a single implantation
– at first glance is similar to the case of the meteoritic nanodiamonds,
there is a complication, however. In the case of the “stellar”
diamonds differences in isotopic composition demand that at least two
different events must have been involved in the introduction of noble
gases: isotopically unremarkable noble gases are released primarily
at low temperature, gases of presumably supernova origin at higher temperature.

If indeed, ion implantation is the mechanism by which trace elements
were introduced into stardust diamonds and if, as the simulation study
suggests, ion implantation results in the gases being located in two
different sites within the diamonds of different thermal stability,
the following sequence of events seems required:
– formation of diamonds presumably by chemical vapor deposition;
– irradiation of the diamonds (or a subfraction of them) with supernova
trace elements;
– loss of the less retentively held fraction of implanted supernova
material;
– irradiation of the diamonds at some later time (or of a different
subfraction at an unspecified time) with trace elements of commonplace
isotopic composition, possibly in the interstellar medium or the early
Solar System;
– no more exposure to elevated temperature for any significant length
of time (e.g. no more than ca. 10,000 years at more than 100°C).

A second important information from the implantation study is that
the more retentively sited gases are isotopically fractionated relative
to the starting composition. How this may have affected the inferred
abundance and isotopic composition of the supernova implants into the
stardust diamonds and how important the resulting changes are for the
inferred nuclear processes remains to be worked out in detail.

   
  Presolar diamond grain observed by transmission
electron microscopy. High resolution image – obtained by F. Banhart
(then at Max Planck Institute for Metal Research, Stuttgart) – shows
the crystallographic[III] planes (distance 0.206 nanometer) of a
typical-sized grain.		  

 

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