Technical note: apatite and zircon (U-Th)/He analysis using quadrupole and magnetic sector mass spectrometry

Abstract. Apatite and zircon (U-Th)/He thermochronological data are obtained through a combination of crystal selection, He content measurement by extraction from crystal and analysis using noble gas mass spectrometry, and measurement of U, Th and Sm contents by dissolution and solution analysis using inductively coupled plasma mass spectrometry (ICP-MS). In this contribution, we detail the complete protocols developed for over more than a decade that allow apatite and zircon (U-Th)/He data to be obtained with precision. More specifically, we show that the He content can be determined with a high precision using a calibration of the He sensibility based on the Durango apatite and its use also appears crucial to check for He, U-Th-Sm analytical problems. The Durango apatite used as a standard is therefore a suitable mineral to perform precise He calibration and yield (U-Th)/He ages of 31.1 ± 1.4 Ma with an analytical error of less than 5 %. The (U-Th)/He ages for the FCT zircon standard yields a dispersion of about 9 %, with mean age of 27.0 ± 2.6 Ma comparable to other laboratories. For the long-term quality control of the (U-Th)/He data, attention has been paid to evaluate the drift of He sensibility, blanks through time and those of (U-Th)/He ages and Th/U ratios (with Sm/Th when possible), all associated with the use of Durango apatite and Fish Canyon Tuff zircon as standards.