In situ S isotopic analysis in apatite using laser ablation-multiple collector-inductively coupled plasma mass spectrometry

The S isotope composition (34S/32S) in apatite has been considered as a new powerful tool to trace S source reservoirs and to evaluate redox state and dynamic sulfur behavior in magmatic systems. This study presents the first in situ method to analyze S isotope compositions in apatite using laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). The sulfur isotope analysis of low-S apatite suffered the problem of high background interference, resulting in a low ratio of signal to noise (S/N). A detailed investigation found that ∼50% background of S isotopes originated from memory effect in the aerosol particles transformation system and can be removed through careful cleaning or replacement. A linear regression calculation method was proposed instead of the traditional data reduction method to improve the performance of correcting the S background signal. Femtosecond laser ablation system (fs-LA) was used to reduce the thermal effect and suppress the potential fractionation of S isotopes during the laser ablation process, significantly improving the stability of δ34SV-CDT values at different laser ablation parameters. Seven natural apatite samples with a large S concentration range were measured by the presented method. A good agreement of δ34SV-CDT values between fs-LA-MC-ICP-MS and IR-MS was obtained. The reproducibility of δ34SV-CDT obtained from fs-LA-MC-ICP-MS was similar to that in SIMS. These results confirm that the presented method can provide high-quality S isotopic compositions in natural apatites. Four apatite samples (Sly-1, Sly-2, MG, and OL-1) with various concentrations of SO3 (0.21% to 1.26%) show the homogeneous S isotope compositions based on the large amounts of repeated analyses using fs-LA-MC-ICP-MS. They are suggested as the high-quality external reference materials for in situ S isotope analysis for LA-MC-ICP-MS and SIMS.