Precise determination of δ88Sr in rocks, minerals, and waters by double-spike TIMS: a powerful tool in the study of geological, hydrological and biological processes

We present strontium isotopic (Sr-88/Sr-86 and Sr-87/Sr-86) results obtained by Sr-87-Sr-84 double spike thermal ionization mass-spectrometry (DS-TIMS) for several standards as well as natural water samples and mineral samples of abiogenic and biogenic origin. The detailed data reduction algorithm and a user-friendly Sr-specific stand-alone computer program used for the spike calibration and the data reduction are also presented. Accuracy and precision of our delta Sr-88 measurements, calculated as permit (parts per thousand) deviations from the NIST SRM-987 standard, were evaluated by analyzing the NASS-6 seawater standard, which yielded delta Sr-88 = 0.378 +/- 0.009 parts per thousand. The first DS-TIMS data for the NIST SRM-607 potassium feldspar standard and for several US Geological Survey carbonate, phosphate, and silicate standards (EN-1, MAPS-4, MAPS-5, G-3, BCR-2, and BHVO-2) are also reported. Data obtained during this work for Sr-bearing solids and natural waters show a range of delta Sr-88 values of about 2.4 parts per thousand, the widest observed so far in terrestrial materials. This range is easily resolvable analytically because the demonstrated external error (+/- SD, standard deviation) for measured delta Sr-88 values is typically <= 0.02 parts per thousand. It is shown that the true Sr-87/Sr-86 value obtained by the DS-TIMS or any other external normalization method combines radiogenic and mass-dependent mass-fractionation effects, which cannot be separated. Therefore, the true Sr-87/Sr-86 and the delta Sr-87 parameter derived from it are not useful isotope tracers. Data presented in this paper for a wide range of naturally occurring sample types demonstrate the potential of the delta Sr-88 isotope tracer in combination with the traditional radiogenic Sr-87/Sr-86 tracer for studying a variety of biological, hydrological, and geological processes.