High-precision lithium isotopic analysis using the Nu Sapphire MC-ICP-MS

Abstract

Here, we present a high-precision analytical method for lithium (Li) isotopic analysis using standard-sample bracketing on a Nu Sapphire collision cell multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS). A modified chemical purification procedure and optimized instrumental setup allow high-precision determination of Li isotopic ratios. A precision of ±0.13 to 0.23‰ (2SD) has been achieved based on repeat measurements of Li standard IRMM-016 and geological reference materials, BCR-2 and JG-2, over an eight-month period. We also evaluate factors that may lead to instrumental mass fractionations, e.g., Li concentration mismatch between the sample and standard, and the potential effect of the Na matrix. The obtained δ⁷Li values of eight international reference materials (IRMM-016, BHVO-2, BCR-2, RGM-1, AGV-2, G-2, GSP-2, and JG-2) are consistent with published data within uncertainty (±2SD). In addition, we report seventeen Chinese geological reference materials from NRCG with the lowest Li content at about 2 μg g⁻¹. Among them, the Li isotopic compositions of GSR-1, GSR-6, and GSR-12 are in agreement with values published in the literature, and δ⁷Li values of the other fourteen NRCG samples are reported for the first time, including andesite GSR-2: 8.27 ± 0.14‰, basalt GSR-3: 5.72 ± 0.18‰, sandstone GSR-4: −3.19 ± 0.31‰, syenite GSR-7: 1.91 ± 0.24‰, trachyte GSR-8: 4.77 ± 0.21‰, diorite GSR-9: 3.47 ± 0.21‰, gabbro GSR-10: 6.36 ± 0.35‰, rhyolite GSR-11: 2.50 ± 0.15‰, calcite GSR-13: 7.17 ± 0.38‰, granite gneiss GSR-14: 4.49 ± 0.15‰, hornblendite GSR-15: 3.45 ± 0.19‰, diabase GSR-16: 3.28 ± 0.24‰, kimberlite GSR-17: −0.19 ± 0.21‰, and pegmatite GSR-18: 1.35 ± 0.10‰.