Precise measurement of 40K isotopic anomalies in small samples using a TIMS with a 1013 ohm amplifier

Abstract

The paradox of volatile depletion in terrestrial planets is critical to the early evolution of planets and the emergence of life. However, the depletion mechanism of volatiles, such as K, remains vague. Potassium isotopic anomalies, controlled only by the variable mixing of isotopically distinct primordial disk reservoirs, could be a promising tool to indicate the original distribution of volatiles. Conventional methods used a large ion beam (3 nA for ³⁹K) and a 10¹⁰ Ω amplifier to measure the ³⁹K to enlarge the signal of ⁴⁰K (³⁹K/⁴⁰K ≈ 8 × 10⁴), which required large sample sizes. Instead, we present a method using a two-step ion-exchange purification procedure and a Triton plus Thermal Ionization Mass Spectrometer (TIMS) equipped with a 10¹³ Ω amplifier to collect the signal of the ⁴⁰K (∼400 mV on the 10¹³ Ω amplifier) when the signal of ³⁹K is approximately 0.35 nA (∼35 V on the 10¹¹ Ω amplifier). A new method for gain and baseline calibrations for the 10¹³ Ω amplifier was also developed to correct its time and temperature drift. With all these improvements, we achieved a long-term reproducibility (2σ) of 0.40 for ε⁴⁰K in Alfa-K with a sample size of ∼50 ng. Seven USGS geological standards and three carbonaceous chondrites were measured. No resolvable K isotopic anomalies were found in terrestrial samples, but different degrees of enrichment in ⁴⁰K were observed in chondrites, indicating the heterogeneous distribution of ⁴⁰K among the Solar System materials.