Recycling of carbon from the stagnant paleo-Pacific slab beneath Eastern China revealed by olivine geochemistry

Abstract

Plate subduction provides a potential link of carbon between the Earth's surface and interior. It remains unclear if plate subduction has caused enrichment of carbon in the deep mantle. The Eastern China continent is underlain by the stagnant slab of the paleo-Pacific in the deep upper mantle, and the young intraplate volcanism in Eastern China provides a window to examine the carbon cycle associated with the slab materials. Here we report two olivine phenocryst types in nephelinites from Qixia, Shandong province, China: extreme-Mg olivine, with Fo contents up to 99 mol% and low Ca, Mn, and Ni contents (<500 ppm), and normal olivine (Fo <88 mol%). Crystallization modelling suggests that the extreme-Mg olivine crystallized from early primitive carbonated melts. Furthermore, the CO2-rich nature of the primary melts is also shown by Ca-Mg-rich and Si-poor carbonated melt inclusions hosted in olivine phenocrysts. The low-Ca contents (<1000 ppm) of the normal olivine can be explained by the low Ca partition coefficient between olivine and melts rich in CO2 (~20 wt%). The results of this study provide robust mineralogical evidence for a genetic relationship between the nephelinites and primary carbonated melts. Together with whole-rock geochemistry data of major and trace elements and Sr-Nd-Pb-Hf isotopes, we suggest that the primary carbonated melts are consistent with origin from the stagnant paleo-Pacific slab in the mantle transition zone underlying Eastern China.