Sluggish lithium isotopic response of continental intraplate basalts to recycled sedimentary carbonate

Abstract

Recognizing the signal of recycled carbonate in the deep mantle and associated basalts is crucial for depicting deep carbon cycling. It has long been recognized that marine sedimentary carbonates have much heavier lithium isotope compositions (δ7Li) than the depleted mantle. However, Li isotope systematics are rarely employed to trace recycled carbonate. To investigate basalt Li isotopic response to recycled carbonate, we carried out whole-rock geochemical and Li isotopic studies on a series of the Cenozoic continental intraplate basalts from eastern China that sample a typical carbonated mantle. The basalts have been reported to have light magnesium (Mg) isotope compositions, but their δ7Li values vary in a limited range (0.9–5.7‰) around the depleted mantle (~3.8 ± 1.4‰). The decoupled LiMg isotopic records are closely related to recycled Mg-rich carbonates (dolomite) that have lighter Mg isotope compositions than the depleted mantle but show mantle-like δ7Li values. After modeling the Li isotopic effects of carbonate sediment, silicate sediment and altered oceanic crust (AOC) recycling, we find that even calcium-rich carbonate recycling is inadequate to shift basalt Li isotope composition. Collectively, Li isotopic responses of intraplate basalts to all types of recycled sedimentary carbonates are more sluggish than expected. Our modeling also suggests that recycling of AOC and silicate sediments is likely responsible for the basalts with δ7Li values higher and lower than the depleted mantle, respectively. In addition, seawater δ7Li values probably exert a first-order role in the Li isotopic response. Low δ7Li values of paleo-seawater (< 20‰) for most of the Earth's history have generated AOC with δ7Li values indistinguishable from the depleted mantle, which goes against the emergence of high δ7Li values for intraplate basalts. Although some limitations exist in Li isotopic applications, we propose that a combination of Li isotopic data and whole-rock element ratios is effective in tracing recycled crustal materials hidden in the intraplate basalts.