Determination of Silicon Isotopic Compositions of Rock and Soil Reference Materials by MC-ICP-MS

With the analytical technique development, the precision of Si isotopes analysis increases rapidly.Silicon isotopes are widely used in geochemistry, cosmochemistry, environmental chemistry and so on, and can be used to trace the circulation of crust-mantle material, the source of subducting fluid, and constrain the origin and evolution of the moon and extraterrestrial materials.To compare the precision and accuracy of Si isotope analysis results in different laboratories, it is necessary to analyze Si isotopes of reference materials with published Si isotope data.As generally used USGS reference materials are currently unavailable, it is important to report Si isotopes of new reference materials.

In order to continuously conduct research in various fields with high-precision silicon isotope data, by providing a supply of new reference materials.Silicon isotopes of 30 GBW reference materials with different compositions, including 11 igneous rocks, 2 sedimentary rocks, 2 metamorphic rocks, 6 river/marine sediments and 9 soils, were analyzed.The SiO₂ content of these reference materials ranged from 32.69% to 90.36%, covering the variation range of most natural samples.

Alkali fusion method was used for sample digestion.Approximately 3-5mg of sample powder and 200mg of powdery NaOH were weighed in a 10mL silver crucible and heated.The Si purification was obtained using cation exchange resin AG50W-X12.6mol/L HNO₃ and ultrapure water were used to clean the resin before sample loading.Silicon isotopes were measured by multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS, Neptune Plus) at the laboratory in the University of Science and Technology of China (USTC), and the instrument mass bias was corrected by standard-sample-standard method, with a bracketing standard of NBS-28.The long-term reproducibility (over two years) of Si isotope analysis of one in-house standard (USTC-Si) and one international rock reference material (BHVO-2) were represented, with the δ³⁰Si values of USTC-Si and BHVO-2 of-0.07‰±0.06‰(n=117, 2SD) and-0.29‰±0.06‰(n=320, 2SD), respectively.During Si isotope analysis, two rock reference materials BHVO-2 and AGV-2 were also analyzed to ensure the precision and accuracy of the data.The Si isotopic compositions of BHVO-2 and AGV-2 were consistent with the reported data in the previous literature (Figure 3), demonstrating the reliability of this measurement.

Except one sediment and two soil samples, the δ³⁰Si values of most reference materials analyzed in this study range from-0.42‰ to-0.07‰, within the range of upper continental crust.The drainage sediment GBW07310 has the highest δ³⁰Si value (0.85‰±0.01‰), while the yellow-red soil GBW07405 and the latosol GBW07407 have the lowest δ³⁰Si values of-0.68‰±0.03‰ and-1.82‰±0.03‰, respectively.

The high-precision Si isotope data of 30 GBW reference materials helps replenish the database for Si isotope analysis.The Si isotope data of these standard materials show that the river sediment GBW07310 has a very high δ³⁰Si value of-1.82‰±0.03‰, indicating that it may be formed by dissolved silicon precipitation, which are enriched in heavy Si isotopes; while highly weathered yellow-red soil GBW07405 and the latosol GBW07407 have the lowest δ³⁰Si values of-0.68‰±0.03‰ and-1.82‰±0.03‰, respectively, indicating that the weathering and desiliconization process may lead to the loss of heavy Si isotopes.The δ³⁰Si values of most remaining reference materials analyzed in this study vary from-0.42‰ to-0.07‰, within the variation range of the upper continental crust.There is no obvious correlation between δ³⁰Si values and SiO₂ contents of the 11 igneous rock reference materials, revealing that their Si isotopes were not controlled by partial melting or mineral crystallization processes, and there may be other processes which would affect the Si isotopic composition of these standards.In the case that generally used USGS reference materials have been sold out, these high-precision Si isotope data of GBW reference materials will supplement basic data for Si isotope testing in different laboratories and lay solid isotope research in various fields.