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)

TLDR

Age determinations using the 235U and 238U decay series to 207Pb and 206Pb in zircon are widely used to decipher geological processes, and the recently developed LA‑MC‑ICP‑MS method overcomes laborious sample preparation to provide isotopic ratios and age data with ten‑micron spatial resolution. The study presents the analytical set‑up and data‑reduction workflow used at the University of Brasília’s Geochronology Laboratory and evaluates the method’s precision and accuracy by cross‑analysing three international zircon standards. The authors employ laser ablation multi‑collector ICP‑MS to analyze zircon samples, applying the technique to both international standards and two natural zircons previously dated by other methods, thereby enabling high‑resolution isotopic measurements. The method achieves 1.9–3.7 % precision and 0.6–3.8 % accuracy for U isotopic ratios in standards, produces age results that agree within error limits with previous analyses, and demonstrates rapid, precise, and accurate U‑Pb isotopic analyses on the micron scale.

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.