Accurate Determination of Silver Isotopic Composition in Silicate Rocks with Low Silver Abundance

Owing to the extremely low abundance of silver in the Earth's crust and mantle, it remains a major challenge to eliminate matrix impurities to obtain accurate silver isotopic compositions in silicate rocks. To determine precise and accurate silver isotopic data in silicates, we have modified the traditional pretreatment procedures, assessed isobaric interference and matrix effects, and analyzed silver isotopic compositions in silicates. By modification of the silicate digestion and ion-exchange procedures, efficient elution of silver was achieved. The doping experimental results indicated that the matrix effect induced by Ti and Cr could be satisfactorily corrected using the internal standard Pd isotope pair of Pd-108-Pd-106. The modified chemical chromatographic method effectively separates Ni from Ag in silicate samples, thereby minimizing the significant isobaric interference from Ni cations. As a result, the shifts in the delta Ag-109 value caused by cations can be corrected to less than 0.02 parts per thousand. There are considerable shifts down to -0.82 parts per thousand in delta Ag-109 from the accepted delta Ag-109 value when soluble metasilicate is present in the solution, which might explain the discrepancies in measured delta Ag-109 values for silicate materials. To accurately analyze silver isotopic compositions, especially of silicates with extremely low silver abundance, a silver standard doping method with an optimum doping proportion (sample-to-standard material ratio of 2:8) has been shown to produce an acceptable measurement uncertainty from 0.04 to 0.06 parts per thousand (2SD). The high-precision delta Ag-109 value determined in this study for ultramafic rocks from Balmuccia and the basalt reference material, BHVO-2, of -0.044 +/- 0.062 parts per thousand is consistent with that of -0.16 +/- 0.07 parts per thousand reported by previous studies. Our study paves the way for the more extensive use of silver isotopes in studies of terrestrial/extraterrestrial rocks, something that will be of help in constraining the sources of precious metals in polymetallic ore deposits as well as core formation and volatile-element depletion in the early solar system.