Accurate Hf isotope determinations of complex zircons using the “laser ablation split stream” method

Abstract

[1] The ''laser ablation split stream'' (LASS) technique is a powerful tool for mineral-scale isotope analyses and in particular, for concurrent determination of age and Hf isotope composition of zircon. Because LASS utilizes two independent mass spectrometers, a large range of masses can be measured during a single ablation, and thus, the same sample volume can be analyzed for multiple geochemical systems. This paper describes a simple analytical setup using a laser ablation system coupled to a singlecollector (for U-Pb age determination) and a multicollector (for Hf isotope analyses) inductively coupled plasma mass spectrometer (MC-ICPMS). The ability of the LASS for concurrent Hf 1 age technique to extract meaningful Hf isotope compositions in isotopically zoned zircon is demonstrated using zircons from two Proterozoic gneisses from northern Idaho, USA. These samples illustrate the potential problems associated with inadvertently sampling multiple age and Hf components in zircons, as well as the potential of LASS to recover meaningful Hf isotope compositions. We suggest that such inadvertent sampling of differing age and Hf components can be a significant cause of excess scatter in Hf isotope analyses and demonstrate that the LASS approach offers a robust solution to these issues. The veracity of the approach is demonstrated by accurate analyses of 10 reference zircons with well-characterized age and Hf isotopic composition, using laser spot diameters of 30 and 40 mm. In order to expand the database of high-precision Lu-Hf isotope analyses of reference zircons, we present 27 new isotope dilution-MC-ICPMS Lu-Hf isotope measurements of five U-Pb zircon standards: FC1, Temora, R33, QGNG, and 91500.

Transfer of Metasupracrustal Rocks to Midcrustal Depths in the North Cascades Continental Magmatic Arc, Skagit Gneiss Complex, Washington

“…For the Hf‐isotope analyses, a ~40 μm laser spot was placed over the previous U‐Pb ablation pit. Detailed methodology and description of results for both the U‐Pb and Hf‐isotope analyses can be found in Text S1 and Tables S1 and S2 in the supporting information (Bouvier et al, ; Fisher et al, ; Fisher, Vervoort, & Dufrane, ; Fisher, Vervoort, & Hanchar, , Gehrels, ; Gehrels et al, ; Jackson et al, ; Paton et al, ; Slama et al, ; Söderlund et al, ; Wiedenbeck et al, ; Woodhead & Hergt, ). A summary of the U‐Pb and Hf‐isotope results is presented below, in Table and in Figures .…”

The earliest low and high δ18O caldera-forming eruptions of the Yellowstone plume: implications for the 30–40 Ma Oregon calderas and speculations on plume-triggered delaminations

“…δ 18 O compositions of mounted and imaged zircon crystals were further refined in their δ 18 O values by targeting cores and rims in situ using the Cameca 1280 ion microprobe at the University of Alberta (±0.16 2σ). The Hf isotopic composition of zircon was then determined for some of these same spots at Washington State University's Radiogenic Isotope and Geochronology Lab (±0.8-2.0 εHf) (Fisher et al, 2014). Individual zircon cores and rims were then analyzed for 238 U-206 Pb ages using the CAMECA ims 1270 ion microprobe at UCLA.…”

Hafnium Isotope Characteristics of Palaeoarchaean Zircon OG1/OGC from the Owens Gully Diorite, Pilbara Craton, Western Australia

“…, Fisher et al . ), underlines the requirement for a zircon reference material with simple, well‐constrained U‐Pb age systematics (i.e., without the complexities of radiogenic lead loss, age inheritance or significant common lead content) and an accurately known Hf isotope composition. It is essential that reference zircons for laser ablation Hf isotope analysis have moderately high, and variable, REE content so that the veracity of the large corrections for the isobaric interference of 176 Lu and 176 Yb on 176 Hf can be properly assessed (e.g., Fisher et al .…”