Optimizing MC-ICP-MS with SEM protocols for determination of U and Th isotope ratios and 230Th ages in carbonates

Abstract

We have optimized techniques to measure uranium and thorium isotopic compositions using secondary electron multiplier (SEM) and peak-jumping protocols on a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS, Thermo Scientific Neptune Plus). This instrument is equipped with an advanced interface pump, X- skimmer cone, Jet sample cone, and desolvating nebulizer system (CETAC Aridus II) that enables overall ioni-zation and transmission efficiency up to 3% for both U and Th. Approximately 50 fg234U or 230Th can routinely yield a precision of 1–2‰ (2σ) for abundance and isotopic measurements. Non-linear behavior was not observed in the MasCom SEM with a retarding potential quadrupole in our MC-ICP-MS system when the beam intensity was limited to < ∼1 × 10 6 cps. We performed a series of tests to examine the influence of tuning parameters, peak shift, and operating voltage of the SEM on isotopic measurements. Results for U/Th standards are consistent with published values and demonstrate the reliability of our protocols. Lastly, the precision and accuracy of 230Th ages based on U and Th isotope ratios is presented for a variety of carbonates, including both modern and secular-equilibrium samples. For corals as young as ∼10-yr old, we have significantly reduced the procedural chemistry blanks so that only 40 mg is required to yield an analytical precision of ± 1.4 yr (2σ). All 230Th ages from modern and fossil corals and speleothems are in agreement with band counting ages.