Cosmic Ray Exposure Ages of Large Presolar SiC grains derived from 6Li excesses

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Cosmic Ray Exposure Ages of Large Presolar SiC
grains derived from 6Li excesses
Frank Gyngard, Sachiko Amari, Janaina Avila, Uli
Ott, and Ernst Zinner Laboratory for Space
Sciences Washington University St. Louis, MO
Australian National University,
Canberra Max-Planck Institut für Chemie, Mainz
The Origins of the Elements Heavier than Fe,
September 27, 2008
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Silicon carbide
Corundum
Almost all analyses of presolar SiC have been
made on grains 5µm or smaller. Average size of
SiC grains lt0.5µm
Graphite
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1988-1990 at the University of Chicago two
detailed physical and chemical separations of
Murchison were made, resulting in samples of
presolar SiC grains
K-series
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L-series
The L-series fractions LS and LU contain
unusually large SiC grain
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Some of the grains
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Virag et al. (1993) analyzed 40 LSLU grains for
their C, N, Al-Mg, Si isotopic compositions,
trace element contents, and optical properties.
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Virag et al. (1993) noticed clustering of the
LSLU grains in their C and Si isotopic ratios.
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We decided to revisit the LSLU grains. Ill
concentrate on determination of cosmic-ray
exposure ages from 6Li excesses.
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• Tang and Anders (1988) and Lewis et al. (1994)
  used 21Ne excesses in SiC to determine cosmic-ray
  exposure ages.
• These results were invalidated by realistic
  determination of recoil losses (Ott and Begemann,
  2000).
• Ott et al. (2005) set limits on CR exposure ages
  from cosmogenic 126Xe.
• The large size of LSLU grains and their low Li
  concentrations opens the possibility of using
  cosmogenic Li for age determination.

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We measured B and Li isotopic compositions in
LSLU grains and found excesses in 10B and 6Li.
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The only reasonable explanation for 6Li is that
it is of cosmogenic origin.
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6Li is completely destroyed in stellar
environments. Under special circumstances, 7Li
can be produced by the Cameron-Fowler (1971)
mechanism 3He(a,g)7Be(e,n)7Li. Any stellar Li
left in the grains must be mostly
7Li. 6Li/7Lisolar 0.08 6Li/7LiGCR 0.5 We
assumed a mixture of solar and GCR-produced Li, a
spallation production ratio of Li from C, and an
average flux of Galactic cosmic rays.
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We used the calculation for retention of 6Li by
Greiner et al. (1975) to obtain GCR exposure ages.
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The GCR exposure ages from 6Li are compared with
theoretical estimates of lifetimes of
insterstellar grains and of previous estimates
from noble gases.
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21Ne
New 21Ne results on LSLU are added (next talk).
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The LSLU grains analyzed have a limited range in
Si isotopic ratios.
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Clustering is more apparent if C isotopic ratios
are also considered. Can have grains with
similar isotopic ratios have different ages and
thus come from different stars?
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Ti isotopic ratios of LSLU grains (Ireland et
al., 1991) are similar to those of mainstream
grains from the K-series. Do LSLU grains show
any s-process signatures?
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Al contents and Al/Mg ratios in LSLU grains are
generally lower than in smaller grains. Only one
grain shows a clear signature of 26Al.
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There is no correlation, indicating no GCE
effects for 25Mg.
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Most LSLU grains have only upper limits for
26Al/27Al.
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Unlike smaller grains from the K-series, most
LSLU grains dont show s-process signatures in
their Ba isotopic ratios. (Janaina Avila)
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One LSLU grain has a definite 151Eu
excess. See poster by Avila et al.
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Several LSLU grains show s-process signatures in
their Gd isotopic compositions. See poster by
Avila et al.
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• CONCLUSIONS
• Large SiC grains from the Murchison LSLU
  separate are unique in several ways.
• From 6Li excesses we could derive Galactic cosmic
  ray exposure ages for several grains.
• Most LSLU grains seem to lack evidence for 26Al
  and some lack s-process signatures. Did the
  parent stars of these grains experience much TDU?