High spatial resolution analysis of Pb and U isotopes for geochronology by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS)

Bernhard Bühn,Márcio M Pimentel,Massimo Matteini,Elton L Dantas

doi:10.1590/s0001-37652009000100011

Abstract

Age determinations using the 235U and 238U radioactive decay series to the daughter isotopes 207Pb and 206Pb, respectively, using the mineral zircon (ZrSiO4), are widely used to decipher geological processes. A new method developed in the last couple of years, the laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS), overcomes previous laborious sample preparation, and yields isotopic ratios and age data with a high spatial resolution of ten of microns. The present study describes the analytical set-up and data reduction process as presently applied at the Laboratory for Geochronology of the University of Brasília. It explores the precision and accuracy of the method by cross-analysing three international zircon standards. We arrive at a precision of 1.9 to 3.7% (2σ SD) and an accuracy of 0.6 to 3.8% (2σ SD) for and U isotopic ratios of the standards. We also apply the method to two natural zircon samples, which have previously been dated by other analytical methods. A comparison of the results show a good conformity of the age data,being whitin the error limits. The data demonstrate the great analytical potential of the method for rapid, precise and accurate U-Pb isotopic analyses on the micron scale.

Highlights

Absolute age determinations in the geosciences serve for a variety of applications including geotectonic studies, sedimentation ages and sediment provenance, as well as dating of igneous and metamorphic rocks
The technique is referred to as “laser ablation inductively coupled mass spectrometry” (LA-inductively coupled plasma (ICP)-MS). It avoids the laborious sample cleaning, dissolution and element separation in an ultraclean laboratory environment required by the traditional isotope dilution thermal ionization mass spectrometry (ID-TIMS)
Conventional Pb-U analyses for absolute age determination of geological materials commonly use the technique of ID-TIMS (Isotope Dilution Thermal Ionization Mass Spectrometry)

Summary

Introduction

Absolute age determinations in the geosciences serve for a variety of applications including geotectonic studies, sedimentation ages and sediment provenance, as well as dating of igneous and metamorphic rocks. Mineral standards for isotopic analyses by LA-ICP-MS or ion microprobe should meet these two requirements, i.e. they should be concordant, and void of common lead. Conventional Pb-U analyses for absolute age determination of geological materials commonly use the technique of ID-TIMS (Isotope Dilution Thermal Ionization Mass Spectrometry).

Results

Conclusion