High-precision ID-TIMS zircon U-Pb geochronology using new 1013 Ohm resistors

Abstract:

Faraday cups equipped with high gain amplifiers provide a means to measure small ion beams in static mode without the limited linear range of ion counting systems. We tested the application of newly available 10¹³ Ohm resistors to ID-TIMS zircon U-Pb geochronology using a range of natural and synthetic reference materials. The TritonPlus-RPQ at the Institute of Geochemistry and Petrology, ETH Zurich, is equipped with five new 10¹³ Ohm resistors and one MasCom secondary electron multiplier, allowing to measure the ^{202-204-205-206-207-208}Pb masses in static mode. U is measured subsequently as U-oxide (^265-267-270UO₂) during a second step, also in static Faraday mode. The gain calibration of the 10¹³ Ohm resistors was performed using the procedure of Trinquier (2014), with ¹⁴⁴Nd-¹⁴⁶Nd being measured using 10¹¹ Ohm resistor and ^{142-143-145-148-150}Nd being measured using 10¹³ Ohm resitors (Trinquier, 2014; Koornneef et al., 2014). Standard deviations of the noise in all five new 10¹³ Ohm resistors are lower than 5.0 x 10^-6 over a 6 month period, with no shift occurring over this time interval.

This new detector set-up was tested by analyzing natural zircon standard materials and synthetic U/Pb solutions (www.earthime.org), ranging in age from ~2 Ma to ~600 Ma. All natural zircon standards were chemically abraded (Mattinson, 2005) and all samples were spiked with the ET2535 tracer solution. U-Pb dates obtained using the static measurement routine are compared to measurements employing dynamic peak jumping routines on the MasCom multiplier. This study illustrates the benefits and current limitations of using high gain amplifiers to measure small ion beams for zircon U-Pb geochronology compared to conventional dynamic ion counting techniques.

Mattinson, J.M. (2005) Chemical Geology 220:47-66;

Trinquier, A. (2014) Application Note 30281;

Koornneef, J. et al (2014) Analytica Chimica Acta 819:49-55.