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| Titel |
Combined apatite fission track and U-Pb dating by LA-ICPMS |
| VerfasserIn |
D. M. Chew, R. A. Donelick |
| 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 |
250059273
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| Zusammenfassung |
| Apatite is a common accessory mineral in igneous, metamorphic and clastic sedimentary
rocks. It is a nearly ubiquitous accessory phase in igneous rocks, is common in metamorphic
rocks of pelitic, carbonate, basaltic, and ultramafic composition and is virtually ubiquitous
in clastic sedimentary rocks. In contrast to the polycyclic behavior of the stable
heavy mineral zircon, apatite is unstable in acidic groundwaters and has limited
mechanical stability in sedimentary transport systems. Apatite has many potential
applications in provenance studies, particularly as it likely represents first-cycle
detritus.
Fission track and U-Pb dating are very powerful techniques in apatite provenance studies.
They yield complementary information, with the apatite fission-track system yielding
low-temperature exhumation ages and the U-Pb system yielding high-temperature cooling
ages which constrain the timing of apatite crystallization. This study focuses on integrating
apatite fission track and U-Pb dating by the LA-ICPMS method. Our approach is
intentionally broad in scope, and is applicable to any quadrupole or rapid-scanning
magnetic-sector LA-ICPMS system.
Calculating uranium concentrations in fission-track dating by LA-ICPMS increases the
speed of analysis and sample throughput compared to the conventional external detector
method and avoids the need for neutron irradiation (Hasebe et al., 2004). LA-ICPMS-based
uranium measurements in apatite are measured relative to an internal concentration standard
(typically 43Ca). Ca in apatite is not always stochiometric as minor cations (Mn2+, Sr2+,
Ba2+ and Fe2+) and REE can substitute with Ca2+. These substitutions must be quantified
by multi-elemental LA-ICPMS analyses. Such data are also useful for discriminating
between different apatite populations in sedimentary or volcaniclastic rocks based on their
trace-element chemistry.
Low U, Th and radiogenic Pb concentrations, elevated common Pb / radiogenic
Pb ratios and U-Pb elemental fractionation are challenges in apatite U-Pb dating
by LA-ICPMS. Isochron-based approaches to common Pb correction require a
significant spread in common Pb / radiogenic Pb ratios. This is not usually possible on
individual detrital apatite grains and hence the 204Pb-, 207Pb- and 208Pb-correction
methods are preferred. Uranium concentration measurements by ICPMS employ
large peak jumps (the internal standard is a Ca isotope) which require a quadrupole
or a rapid-scanning magnetic-sector LA-ICPMS system. These single-collector
instruments require a prohibitively long dwell time on the low intensity 204Pb peak
to measure it accurately and hence the 207Pb- and 208Pb-correction methods are
preferred.
Uranium-concentration measurements in fission-track dating require well-constrained
ablation depths during analysis and hence spot analyses are preferred to rastering.
Laser-induced U-Pb fractionation is corrected for by sample-standard bracketing using a
variety of apatite standards (Durango, Emerald Lake, Fish Canyon Tuff, Kovdor,
Otter Lake and McClure Mountain syenite). Of these, Emerald Lake (Chew et al.,
2011) and McClure Mountain syenite apatite are recommended as primary standards
with Durango apatite making a suitable secondary standard. Offline data-reduction
uses custom-written software for ICPMS data processing (the UPbICP package
of Ray Donelick) or the freeware IOLITE data-reduction package of Paton et al.
(2010).
References:
Chew et al. (2011) Chemical Geology 280, 200–216.
Hasebe et al. (2004) Chemical Geology 207, 135-145.
Paton et al. (2010) G3, 11 Q0AA06. |
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