δ26Mg Values of Thirty-One Geological Reference Materials Analysed by the Critical Mixture Double Spike Technique

Stable Mg delta values (the relative deviation from a certain reference material, e.g., DSM-3 here and expressed as δ²⁶Mg_DSM-3) were routinely measured by the sample-standard bracketing (SSB) method on a multi-collector ICP-MS, as only three isotopes (i.e., ²⁴Mg, ²⁵Mg and ²⁶Mg) naturally exist. Due to potential inaccuracy in correcting mass bias during measurements, considerable measurement bias has been reported among laboratories. Recently, a critical mixture double spike (CMDS) technique has been developed and demonstrated to accurately correct mass bias of measurement results with precision of ± 0.03‰ for δ²⁶Mg. Here, we measured thirty-one geological reference materials including igneous, metamorphic and sedimentary rocks, sediments and minerals using the CMDS technique, with the purpose of better characterising their δ²⁶Mg_DSM-3 values. Aligning with the data previously reported, uncorrected bias, on average measured by ∆²⁶Mg_SSB-CMDS (i.e., δ²⁶Mg_SSB - δ²⁶Mg_CMDS) as -0.071 ± 0.092‰ (2s, n = 42), has been reaffirmed for the traditional SSB method. Such uncorrected bias positively correlates with sample Mg/(Si+Al+Ca), and thus may result from the accumulative effect of residual matrix elements. The new data set herein can aid future inter-laboratory comparison and data quality control.