# The data treatment dependent variability of U-Pb zircon ages obtained using mono-collector, sector field, laser ablation ICP-MS

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Accès restreint UNIL

Etat: Public

Version: Final published version

Accès restreint UNIL

Etat: Public

Version: Final published version

ID Serval

serval:BIB_D14F5F28E062

Type

**Article**: article d'un périodique ou d'un magazine.

Collection

Publications

Institution

Titre

The data treatment dependent variability of U-Pb zircon ages obtained using mono-collector, sector field, laser ablation ICP-MS

Périodique

Journal of Analytical Atomic Spectrometry

ISSN-L

0267-9477

Statut éditorial

Publié

Date de publication

2012

Peer-reviewed

Oui

Volume

27

Pages

663-676

Langue

anglais

Notes

ISI:000301496700011

Résumé

U-Pb dating of zircons by laser ablation inductively coupled plasma mass

spectrometry (LA-ICPMS) is a widely used analytical technique in Earth

Sciences. For U-Pb ages below 1 billion years (1 Ga), Pb-206/U-238 dates

are usually used, showing the least bias by external parameters such as

the presence of initial lead and its isotopic composition in the

analysed mineral. Precision and accuracy of the Pb/U ratio are thus of

highest importance in LA-ICPMS geochronology. We consider the evaluation

of the statistical distribution of the sweep intensities based on

goodness-of-fit tests in order to find a model probability distribution

fitting the data to apply an appropriate formulation for the standard

deviation. We then discuss three main methods to calculate the Pb/U

intensity ratio and its uncertainty in the LA-ICPMS: (1)

ratio-of-the-mean intensities method, (2) mean-of-the-intensity-ratios

method and (3) intercept method. These methods apply different functions

to the same raw intensity vs. time data to calculate the mean Pb/U

intensity ratio. Thus, the calculated intensity ratio and its

uncertainty depend on the method applied. We demonstrate that the

accuracy and, conditionally, the precision of the ratio-of-the-mean

intensities method are invariant to the intensity fluctuations and

averaging related to the dwell time selection and off-line data

transformation (averaging of several sweeps); we present a statistical

approach how to calculate the uncertainty of this method for transient

signals. We also show that the accuracy of methods (2) and (3) is

influenced by the intensity fluctuations and averaging, and the extent

of this influence can amount to tens of percentage points; we show that

the uncertainty of these methods also depends on how the signal is

averaged. Each of the above methods imposes requirements to the

instrumentation. The ratio-of-the-mean intensities method is

sufficiently accurate provided the laser induced fractionation between

the beginning and the end of the signal is kept low and linear. We show,

based on a comprehensive series of analyses with different ablation pit

sizes, energy densities and repetition rates for a 193 nm ns-ablation

system that such a fractionation behaviour requires using a low ablation

speed (low energy density and low repetition rate). Overall, we conclude

that the ratio-of-the-mean intensities method combined with low sampling

rates is the most mathematically accurate among the existing data

treatment methods for U-Pb zircon dating by sensitive sector field

ICPMS.

spectrometry (LA-ICPMS) is a widely used analytical technique in Earth

Sciences. For U-Pb ages below 1 billion years (1 Ga), Pb-206/U-238 dates

are usually used, showing the least bias by external parameters such as

the presence of initial lead and its isotopic composition in the

analysed mineral. Precision and accuracy of the Pb/U ratio are thus of

highest importance in LA-ICPMS geochronology. We consider the evaluation

of the statistical distribution of the sweep intensities based on

goodness-of-fit tests in order to find a model probability distribution

fitting the data to apply an appropriate formulation for the standard

deviation. We then discuss three main methods to calculate the Pb/U

intensity ratio and its uncertainty in the LA-ICPMS: (1)

ratio-of-the-mean intensities method, (2) mean-of-the-intensity-ratios

method and (3) intercept method. These methods apply different functions

to the same raw intensity vs. time data to calculate the mean Pb/U

intensity ratio. Thus, the calculated intensity ratio and its

uncertainty depend on the method applied. We demonstrate that the

accuracy and, conditionally, the precision of the ratio-of-the-mean

intensities method are invariant to the intensity fluctuations and

averaging related to the dwell time selection and off-line data

transformation (averaging of several sweeps); we present a statistical

approach how to calculate the uncertainty of this method for transient

signals. We also show that the accuracy of methods (2) and (3) is

influenced by the intensity fluctuations and averaging, and the extent

of this influence can amount to tens of percentage points; we show that

the uncertainty of these methods also depends on how the signal is

averaged. Each of the above methods imposes requirements to the

instrumentation. The ratio-of-the-mean intensities method is

sufficiently accurate provided the laser induced fractionation between

the beginning and the end of the signal is kept low and linear. We show,

based on a comprehensive series of analyses with different ablation pit

sizes, energy densities and repetition rates for a 193 nm ns-ablation

system that such a fractionation behaviour requires using a low ablation

speed (low energy density and low repetition rate). Overall, we conclude

that the ratio-of-the-mean intensities method combined with low sampling

rates is the most mathematically accurate among the existing data

treatment methods for U-Pb zircon dating by sensitive sector field

ICPMS.

Création de la notice

24/09/2012 20:35

Dernière modification de la notice

20/08/2019 16:51