Precise measurement of stable (δ 88/86Sr) and radiogenic (87Sr/86Sr) strontium isotope ratios in geological standard reference materials using MC-ICP-MS

Abstract

A new method for high-precision measurements of stable ( δ 88/86Sr) and radiogenic (87Sr/86Sr) Sr isotope ratios was developed on the MC-ICP-MS using a standard-sample-standard bracketing protocol. By carefully monitoring and controlling the accumulation of the on-peak backgrounds (i.e. the carryover memory) on the instrument, our method can provide an external precision of around ±0.010‰ to ±0.014‰ for δ 88/86Sr measurements. Such precision is comparable to that achieved by double-spike TIMS method (DS-TIMS), and 2-3 times better than those of the previously reported results by MC-ICP-MS without spiking. The results of the standard seawater IAPSO are also identical to those by DS-TIMS. However, our method is more advantageous, efficient and convenient to use for routine Sr isotopic analysis than the DS-TIMS method as there is no need to prepare and calibrate the 84Sr-87Sr double spike. Using this method, we measured the δ 88/86Sr values in a series of international geological rock standards, which show large variability, with the lowest value (-0.20‰) registered in JG-2, a terrestrial granite, and the highest value (0.539‰) in UB-N, a serpentinite. This may provide an additional means for isotopic characterization of geological processes, adding a new dimension over the traditional use of radiogenic Sr isotopic ratio 87Sr/86Sr.