Optimization of a Laser Ablation‐Single Collector‐Inductively Coupled Plasma‐Mass Spectrometer (Thermo Element 2) for Accurate, Precise, and Efficient Zircon U‐Th‐Pb Geochronology

Abstract

Many applications specific to detrital mineral U‐Th‐Pb geochronology in the Earth sciences necessitate large numbers of age observations to be made from samples and require accurate and precise isotope measurements across wide dynamic ranges in elemental concentrations and signal intensities. This implies that the laser system and mass spectrometer cannot be tuned between individual analyses as to optimize measurements based on the isotope composition and concentrations of samples and that intensity matching between the unknowns to be dated and the reference material(s) used for fractionation correction is impossible to ensure. We describe methodologies for optimization of laser ablation‐single collector‐inductively coupled plasma‐mass spectrometer for the accurate determination of initial‐Pb‐corrected (using measured 204Pb) U‐Th‐Pb zircon ages, taking full advantage of the high sensitivity provided by the Thermo Element 2 ICP‐MS instruments fitted with a high‐performance low ultimate vacuum Jet interface. “We describe an approach that corrects for nonlinearity of the detector—the primary obstacle avoided with sample‐specific tuning—as well as element‐ and mass‐dependent fractionation and instrumental drift by using a suite of three zircon reference materials with known isotopic ratios from isotope dilution‐thermal ionization mass spectrometry measurements but with differing U and Pb concentrations.” This approach allows for (experimentally) determining an instrumental fractionation versus ion beam intensity curve used for standard‐sample bracketing, thus taking into consideration an important instrumental variable that is commonly ignored in most applications of U‐Pb dating using laser ablation‐single collector‐inductively coupled plasma‐mass spectrometer. We show that these methodologies yield uncertainties and age offsets typically better than ±2.0% for individual measurements of small (e.g., 10‐μm depth × 20‐μm diameter) volumes of material.

Tectonics of the Eastern Kunlun Range: Cenozoic Reactivation of a Paleozoic‐Early Mesozoic Orogen

“…Zircon grains for U‐Pb dating were separated from 20 sandstones and 26 igneous rocks samples by traditional methods and mounted in epoxy with standard zircons (e.g., Gehrels et al, , ; Pullen et al, ). Individual zircon grains were picked under a binocular microscope.…”

The Ancestral Lhasa River: A Late Cretaceous trans-arc river that drained the proto–Tibetan Plateau

“…Sample information and all analytical data are reported in Tables DR1-DR3 in the Data Repository. Analysis and data reduction followed the methods of Gehrels and Pecha (2014) and Pullen et al (2018).…”

U-Pb and Hf Isotopic Analysis of Detrital Zircons From Paleozoic and Cretaceous Strata of Southern Vancouver Island, British Columbia

“…Zircon separation was performed at the Arizona LaserChron Center (www.laserchron.org) using methods described by Gehrels et al (2008), Gehrels and Pecha (2014), and Pullen et al (2018), including crushing/pulverizing, followed by separation with a Wifley table, Frantz magnetic separator, and heavy liquids (MI). Grains from each sample were poured in a 1" epoxy mount alongside fragments of U-Pb zircon standards (FC-1, SL2, and R33) and Hf zircon standards (Mud Tank, Temora-2, FC-1, 91500, Plesovice, R33, and SL2).…”