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| Titel |
First-principles calibration of 40Ar/39Ar mineral standards and complete extraction of 40Ar* from sanidine |
| VerfasserIn |
Leah Morgan, Klaudia Kuiper, Darren Mark, Onno Postma, Igor Villa, Jan Wijbrans |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250055099
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| Zusammenfassung |
| 40Ar/39Ar geochronology relies on comparing argon isotopic data for unknowns to those
for knowns. Mineral standards used as neutron fluence monitors must be dated by
the K-Ar method (or at least referenced to a mineral of known K-Ar age). The
commonly used age of 28.02 ± 0.28 Ma for the Fish Canyon sanidine (FCs) (Renne
et al., 1998) is based upon measurements of radiogenic 40Ar in GA1550 biotite
(McDougall and Roksandic, 1974), but underlying full data were not published (these
measurements were never intended for use as an international standard), so uncertainties
are difficult to assess. Recent developments by Kuiper et al. (2008) and Renne et
al. (2010) are limited by their reliance on the accuracy of other systems. Modern
technology should allow for more precise and accurate calibration of primary K-Ar and
40Ar/39Ar standards. From the ideal gas law, the number of moles of 40Ar in a
system can be calculated from measurements of pressure, volume, and temperature.
Thus we have designed and are proceeding to build a pipette system to introduce
well-determined amounts of 40Ar into noble gas extraction lines and mass spectrometers.
This system relies on components with calibrations traceable to international standards,
including a diaphragm pressure gauge (MKS Instruments), thermocouples, and a “slug”
of an accurately determined volume to be inserted into the reservoir for volume
determinations of the reservoir and pipette. The system will be renewable, with a
lifetime of ca. 1 month for gas in the reservoir, and portable, to permit interlaboratory
calibrations. The quantitative extraction of 40Ar* from the mineral standard is of
highest importance; for sanidine standards this is complicated by high melt viscosity
during heating. Experiments adding basaltic “zero age glass” (ZAG) to decrease melt
viscosity are underway. This has previously been explored by McDowell (1983) with a
resistance furnace, but has not been quantitatively addressed with laser heating. The
sensitivity of each participating mass spectrometer will be calibrated by the bracketing
standards approach, alternating measurements of pipette gas and mineral standards.
This will convert relative abundances into absolute molar quantities and allow for
quantification of interlaboratory systematic bias. Uncertainty propagation indicates
uncertainties of the molar quantity of 40Ar in mineral standards will be < 0.25% (2Ïă), a
considerable improvement of one component of the uncertainties involved in 40Ar/39Ar
geochronology.
The research leading to these results has received funding from the European
Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement n °
[215458]. |
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