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

Abstract

Silicon isotope serves as a powerful geochemical tracer 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, remain to be systematically investigated and resolved. This study optimized the in-situ Si isotope method using nanosecond laser ablation (ns-LA) connected to a 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 ablating 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., 91500, 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.