High‐precision magnesium isotope analysis of geological and environmental reference materials by multiple‐collector inductively coupled plasma mass spectrometry

Abstract

Rationale High‐precision magnesium (Mg) isotopic analysis for geological and environmental reference materials is a prerequisite to ensure data quality before using Mg isotopes to trace geochemical and environmental processes. However, the Mg‐isotopic ratios of many commonly used reference materials, especially sediments, have rarely been reported. Furthermore, published values for some commonly used reference materials exhibit a significant inconsistency across laboratories and thus need more data comparison. Methods We developed different Mg purification schemes for silicate rocks, high‐Ca carbonates and carbonatites, and high‐Mn samples because of their significantly different matrices. We then used synthetic solutions to evaluate potential effects on measurement using multiple‐collector inductively coupled plasma mass spectrometry (MC‐ICP‐MS). The accuracy and precision of our procedures were assessed by measurement on both synthetic solutions and well‐studied geostandards. Results The three different schemes for routine, high‐Ca, and high‐Mn samples can remove matrices efficiently with nearly 100% Mg yield. However, the presence of acid molarity and concentration mismatch, matrix elements, and fluctuations in room temperature can significantly affect the precision and accuracy of Mg isotope analysis, and must be avoided. The Mg isotopic ratios of reference materials obtained in this study are identical to the previously published values within ±0.06‰, verifying that our procedures are robust. Conclusions This study presented a thorough set of tests for high precision and accuracy of Mg isotope measurements using MC‐ICP‐MS, which demonstrate reproducibility and accuracy better than 0.05‰ for δ25Mg values and 0.06‰ for δ26Mg values. We reported high‐quality Mg isotopic data for 16 geological and environmental reference materials to aid the inter‐laboratory calibration of Mg isotope measurements in the future.

Purification of Mg from extremely low-Mg felsic rocks for isotopic ratio determination by MC-ICP-MS

“…As demonstrated in the previous studies, 21,22,25,26,33 the factors affecting the precision and accuracy of Mg isotopic analyses include mismatch of acid molarity and Mg concentration between samples and bracketing standards, as well as matrix effects. These effects have been evaluated detailedly by Gao et al…”

“…where d 25 Mg and d 26 Mg values of GSB Mg relative to the DSM-3 are À1.044 AE 0.024& and À2.032 AE 0.038& (2SD, N ¼ 225). 25 During each measurement, the standard-sample sequences were usually repeated four times. Mg isotopic data are reported as the average values of the four repeat analyses.…”

“…Mg isotopes were determined in a low-mass resolution mode by standard-sample bracketing. 24 Mg, 25 Mg and 26 Mg were collected statically by Faraday cups L3, C and H3. Each analysis consisted of a 3 s idle time and 30 cycles of 4.19 s integration time.…”

“…Magnesium is one of the most abundant elements and is widely distributed in the silicate earth, hydrosphere and biosphere. [1][2][3] It has three stable isotopes, 24 Mg, 25 Mg and 26 Mg, with relative abundances of 78.99%, 10.00% and 11.01%, respectively. The mass difference of $8% between 24 Mg and 26 Mg makes Mg isotopes a potential tool to trace geological processes, such as magma differentiation, [4][5][6][7] chemical/thermal diffusion, 8 crustal weathering, 9,10 global geochemical cycling, [11][12][13] and biomineralization.…”

“…Although these purication schemes obtained satisfactory Mg recovery, these previously established procedures have only been applied to geological reference materials with MgO > 0.16 wt% (Table 1). [21][22][23][24][25][26][27][28][29] As shown in Fig. 1b, samples with MgO < 0.15 wt% reported in the literature show enormous Mg isotopic variation (larger than 1.5&).…”