Although deep carbon recycling plays an important role in the atmospheric CO2 budget and climate changes through geological time, the precise mechanisms remain poorly understood. Since recycled sedimentary carbonate through plate subduction is the main light-δ26Mg reservoir within deep-Earth, Mg isotope variation in mantle-derived melts provides a novel perspective when investigating deep carbon cycling. Here, we show that the Late Cretaceous and Cenozoic continental basalts from 13 regions covering the whole of eastern China have low δ26Mg isotopic compositions, while the Early Cretaceous basalts from the same area and the island arc basalts from circum-Pacific subduction zones have mantle-like or heavy Mg isotopic characteristics. Thus, a large-scale mantle low δ26Mg anomaly in eastern China has been delineated, suggesting the contribution of sedimentary carbonates recycled into the upper mantle, but limited into the lower mantle. This large-scale spatial and temporal variation of Mg isotopes in the mantle places severe constraints on deep carbon recycling via oceanic subduction.

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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.

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