An improved in situ technique for the analysis of the Os isotope ratio in sulfides using laser ablation-multiple ion counter inductively coupled plasma mass spectrometry

Abstract

A new technique is developed for in situ measurement of ¹⁸⁷Os/¹⁸⁸Os in sulfides using LA-MC-ICP-MS, with the improvement of the spatial resolution and quantification limits. In this study, an all-ion counter (all-IC) configuration is adopted for static concurrent measurement of ¹⁸⁷Os + ¹⁸⁷Re and ¹⁸⁸Os along with ¹⁸⁵Re. To cross-calibrate the detection efficiencies of the different ion counters, an Au standard solution is introduced into the instrument and its signal is measured in each ion counter in a dynamic peak jumping mode. Then, the signal intensities of IC1, IC2 and IC3 are all normalized to that of IC0. The sample-standard bracketing (SSB) method is adopted not only for the Re interference and Os mass fractionation correction but also for the monitoring of detection efficiency during continuous measurement. In detail, an in-house sulfide standard CR-1P (only spiking Re) is specifically employed for correcting interference ¹⁸⁷Re on ¹⁸⁷Os and monitoring detection efficiency variation in IC0 and IC1. The mass fractionation of Os and detection efficiency variation of IC1 and IC2 are calibrated against another in-house sulfide standard CO-1P (only spiking Os). Even though the ¹⁸⁸Os intensity decreases approximately 20 times, our improved method still provides accurate and precise results of the ¹⁸⁷Os/¹⁸⁸Os ratio, whereas published methods that employ Faraday detectors for the signal collection do not work. Furthermore, the threshold of reliable interference correction of ¹⁸⁷Re on ¹⁸⁷Os is estimated by analysing synthetic sulfides with different Re/Os ratios. The analytical results demonstrate that the relative deviations (RD) of the calculated ¹⁸⁷Os/¹⁸⁸Os ratio will be limited to within 0.8%, 1%, 3% and 5%, when ¹⁸⁷Re/¹⁸⁸Os is lower than 0.055, 0.09, 0.42 and 0.75 (i.e., Re/Os: 0.012, 0.019, 0.089 and 0.159), respectively. In general, this new method significantly improves the spatial resolution and quantification limits of in situ measurements, which is beneficial for analysing sulfides in the depleted subcontinental lithospheric mantle (SCLM).