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| Titel |
A simple method for in-situ zircon U-Th-He dating |
| VerfasserIn |
P. Vermeesch, A. Carter |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250063652
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| Zusammenfassung |
| In situ U-Th-He dating of zircon by laser ablation offers significant advantages over the
current practice of whole grain degassing and dissolution:
It dramatically increases sample throughput. Measuring the U and Th content
of zircon by isotope dilution requires dissolution in hydrofluoric acid at high
temperature and pressure using a Parr bomb for up to 48 hours. In contrast,
measuring the U and Th content by LA-ICP MS, SIMS, or EMPA can be done
in a matter of minutes.
The process of in-situ measurements of U and Th content of grains yields
U-Th-Pb ages as a by-product. Thus, in-situ dated zircon crystals are
double-dated by default, opening up exciting new research opportunities in
detrital geochronology.
We here propose a simple four step procedure to measure in-situ U-Th-He ages without
the need to know any absolute U, Th, or He concentrations:
Polish and mount two sets of grains in indium: a standard of known U-Th-He
age, and the sample of interest, whose age is unknown.
Ablate the grains and measure the raw helium signal (in A, V, or Hz) of the
sample along with helium measurements of the age standard.
Measure the U and Th signals of the standard and the sample by LA-ICP-MS (in
Hz).
Obtain the U-Th-He age of the sample by scaling its U, Th, and He signals to
those of the standard.
In practice, the standard and the unknown are combined on a pairwise basis. This is done
by calculating the normalised U, Th, and He concentrations of the standard from its known
age and measured Th/U ratio, and then using this normalised composition as a benchmark
against which to compare the U, Th, and He signals of the unknown. The Th/U ratio of the
standard can be determined from its measured 208Pb/206Pb ratio, removing the need to use
NIST glass as a reference material:
232 208 λ238t
23T8h-= 206Pb(eλ–t–1)
U Pb(e 232 - 1)
(1)
It is important to note that the Th/U ratio only needs to be determined for the standard. In
other words, the use of Equation 1 does not require U-Th-Pb concordance of the
sample.
The pairwise dating makes one assumption, namely that the ablation pit volume is the
same for the U-Th-He age standard and the sample. Violation of this assumption may
result in systematic errors. It is straightforward to correct these errors by defining a
‘scaling factor’ for the ablation pit depth measured, for example, by interferometric
microscopy. However, performing depth measurements adds another analytical step and
partly defeats the purpose of the pairwise dating method. As a potential solution
to this problem, we propose an LA-ICP-MS-based ‘drill rate proxy’ to be used
instead of the interferometric microscope. The idea behind the drill rate proxy is
that the beam intensity of a stoichiometric nuclide, such as 29Si should increase
proportionally with the rate of laser ablation. If this is correct, then the average
ratio (fSi) of the time-resolved 29Si spectrum of the unknown sample over that of
the standard should equal the ratio of the ablation pit depths (fD). It then suffices
to divide the normalised helium signal by fSi to account for the differential drill
rates.
We have applied the pairwise dating method on three shards of Sri Lanka zircon,
obtaining ages that are in good agreement with previously published conventional U-Th-He
ages. |
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