Carbon Enrichment in the Lithospheric Mantle: Evidence from the Melt Inclusions in Mantle Xenoliths from the Hainan Basalts

It is generally believed that the lithospheric mantle and the mantle transition zone are important carbon reservoirs. However, the location of carbon storage in Earth's interior and the reasons for carbon enrichment remain unclear. In this study, we report CO₂-rich olivine-hosted melt inclusions in the mantle xenoliths of late Cenozoic basalts from the Penglai area, Hainan Province, which may shed some light on the carbon enrichment process in the lithospheric mantle. We also present a detailed petrological and geochemical investigation of the late Cenozoic basalts and mantle xenoliths from northern Hainan Island. The collected samples of late Cenozoic Hainan Island basalts belong to both alkaline and subalkaline series, showing fractionated REE patterns with high (La/Yb)_N values of 3.52–11.77, which are typical for OIB. Based on Al-in-olivine thermometry, the temperatures estimated for the mantle xenoliths can be divided into two groups. One group has temperatures of less than 1050°C, and the other group has temperature ranging from 1050°C to 1282°C. Clinopyroxene (La/Yb)_N–Ti/Eu and clinopyroxene Ca/Al–Mg^# diagrams indicate that the mantle peridotite experienced metasomatism from both silicate and carbonate melts. Melt inclusions in the olivine of mantle xenoliths include (1) CO₂ bubble–rich melt inclusions; (2) multiphase melt inclusions (glass + CO₂ bubble + daughter minerals); (3) pure glass melt inclusions. Magnesite is a daughter mineral in the olivine-hosted melt inclusions, which could be interpreted as a secondary mineral formed by the interactions of CO₂-rich fluids with an olivine host, due to post-entrapment effects. The glasses in olivine-hosted melt inclusions have high SiO₂ contents (60.21–77.72 wt%). Our results suggest that a considerable amount of CO₂-rich melt inclusions are captured in the lithospheric mantle during metasomatism. The lithospheric mantle can therefore act as is a ‘carbon trap’, with much CO₂ being absorbed by the lithospheric mantle in this way.