Tightly coupled Ca-Zn-Sr isotope co-variations in basalts caused by recycled calcium carbonate in the mantle source

The deep carbon cycle requires tracers to understand carbon sequestration and mobility in the deep Earth. Recently, the use of metal stable isotopes has emerged as a promising and innovative approach to address element cycles, such as the use of magnesium and zinc isotopes to trace the magnesium carbonate cycle. However, despite the prevalence of calcium carbonate as the dominant sedimentary carbonate during the Phanerozoic era, its tracking through the use of metal stable isotopes has proven challenging. Here we show that deep subduction of Phanerozoic calcium carbonates can significantly decrease the δ^44/40Ca value, while simultaneously increasing the δ⁶⁶Zn value and ⁸⁷Sr/⁸⁶Sr ratio of the mantle. The Neogene Dalihu basalts in Inner Mongolia have high ⁸⁷Sr/⁸⁶Sr ratios, ranging from 0.7058 to 0.7063, which are positively correlated with Sr/Nd ratio, indicating the presence of sedimentary carbonate within their mantle source. The Dalihu basalts exhibit significantly lower δ^44/40Ca values (0.51 to 0.70‰) but higher δ⁶⁶Zn values (0.35 to 0.45 ‰) than the mantle. These low δ^44/40Ca values cannot be attributed to partial melting, even when considering the jadeite effect. Our calculations indicate that adding 5–10% recycled calcium carbonate into the mantle source explains the high ⁸⁷Sr/⁸⁶Sr ratio and δ⁶⁶Zn values but low δ^44/40Ca values of the Dalihu basalt. Therefore, recycled calcium carbonate directly influences correlated Ca-Zn-Sr isotope variation, thereby providing compelling evidence for its role in the deep carbon cycle. Our findings suggest that combining metal stable isotopes with radiogenic Sr isotope is a powerful tracer of the deep carbon cycle.