Determination of silicon isotopic composition of zircon reference materials by ns-LA-MC-ICP-MS

Abstract

Silicon isotopes serve as powerful geochemical tracers for elucidating crustal recycling processes and plate tectonic dynamics. Although significant progress has been made in in situ Si isotope analysis, the influence of laser parameters on the accuracy and precision of in situ Si isotope measurements, as well as the matrix effects during the analytical process, remains to be systematically investigated and resolved. This study optimized the in situ Si isotope method using nanosecond laser ablation (ns-LA) coupled with multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Our results of comprehensive tests indicated that a laser repetition rate of 4–14 Hz, an energy density exceeding 1.5 J cm⁻², and a spot size of 33–90 µm are ideal conditions to obtain adequate signal intensity for the zircon reference material MTUR1. The external reproducibility of δ³⁰Si obtained by long-term single spot and line scan ablation of MTUR1 zircon was 0.26‰ and 0.11‰ (2SD), respectively. Significant matrix effects were consistently observed during non-matrix-matched Si isotope analysis. Although the introduction of water before the ablation cell and Z-axis position adjustment partially suppressed matrix effects, considerable analytical bias remained. We applied this optimized method to examine natural and synthetic zircon reference materials (e.g., 91 500, Plešovice, SA01, GJ-1, Tanz, Penglai, Ban-1, Qinghu, GHR1, and Jilin). In addition to MUN-zircon, which yielded a δ³⁰Si value of −2.10 ± 0.64‰ (2SD), the other zircon reference materials gave relatively homogeneous Si isotopic compositions, with δ³⁰Si values ranging from −0.71 ± 0.21‰ to −0.29 ± 0.13‰ (2SD). Our data reveal significant intra- and inter-grain variations in Si isotopic composition within the Qinghu, LKZ-1, Jilin, Temora-2, GHR1, and MUN-zircon zircon reference materials, indicating that Si isotopic data from these materials hold promise for tracing magmatic differentiation.