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| Titel |
Ar-Ar dating of biotite - grain size dependent ages and possible interpretations |
| VerfasserIn |
Blanka Sperner, Jörg A. Pfänder, Alexandra Scherer, Lothar Ratschbacher |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250043780
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| Zusammenfassung |
| Ar-Ar dating of biotite is a well-established method in geochronology and can be used to date
rocks as young as several 100 ka. We tested the grain size dependence of Ar-Ar ages of
biotite for samples from eastern Tibet. The size of the studied minerals ranged
from 80 to 2500 μm; up to four different size fractions were measured for one
and the same rock sample. Sample weight was between 1 and 7 mg. Dating was
performed by step-heating (in most cases with 20-25 steps) using a CO2 thermal
laser. Gas purification was achieved by two SAES AP10N getter pumps, and Ar
isotope compositions were measured using the ARGUS multicollector gas mass
spectrometer at Freiberg (see abstract of Pfänder et al., same session). Weighted plateau
ages reveal differences in age between different grain size fractions in the order
of a few percent and show a negative correlation between grain size and age: the
larger the grains, the younger the apparent age. Due to diffusion theory and the
concept of closure temperature an opposite trend would be expected. Low cooling
rates (as we revealed for these rock samples) could explain the range of ages we
measured, but still the trend should be opposite (larger grain size resulting in older
ages). A possible explanation for the measured trend could be the loss of 39Ar
during irradiation (recoil effect) which mainly affects small grains. Alternatively,
optically invisible alteration (e.g. chloritization) may be responsible for the age range
observed.
Despite of the high precision of the Ar-Ar method, age variations in the range of several
percent (i.e. multiple times the measurement error) should not be overinterpreted. They might
be the effect of sample treatment, e.g. by choosing a specific grain size fraction, or
may result from secondary phase transitions and related argon loss. Competing
trends in the correlation between age and grain size (recoil effect with older smaller
grains vs. closure temperature with older larger grains) complicate the interpretation. |
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