Continental subduction-triggered carbonate metasomatism of the lithospheric mantle: Implications for the deep carbon cycle

Carbonate melts play an essential role in the deep carbon cycle between Earth's surface and interior. Most previous studies proposed that the source of carbonate melt resulted from the recycling of subducted oceanic slabs. Recent studies on orogenic rocks show that continental subduction may also deliver carbonate sediments into the deep mantle. However, these rocks usually have undergone complex geological processes and strong modifications during exhumation. Here we report evidence of continental subduction-triggered carbonate metasomatism of the lithospheric mantle from a suite of mantle xenoliths entrained by the Cenozoic Qixia basalts from the southeastern North China Craton (NCC). Both petrological (the same variation of mineral modal content as in carbonate melt-peridotite reaction experiments, the replacement of olivine and orthopyroxene by clinopyroxene, and the presence of apatite) and geochemical (high Ca/Al, LaN/YbN, and low Ti/Eu ratios in clinopyroxene) evidence show mantle xenoliths record obvious signals of carbonated silicate melt metasomatism. The high 87Sr/86Sr ratios of 0.7094–0.7100 in clinopyroxene of these xenoliths indicate that the carbonated silicate melt was derived from subducted continental crust rather than oceanic crust and asthenosphere mantle. The carbonated silicate melt could be produced by carbonate dissolution during continental subduction or resulted from the partial melting of the diapiric carbonate-rich sediments. An estimation suggests a carbon input flux of 14 ± 7 Mt. yr−1 associated with the Yangtze continental subduction, approximately 20–50% of the estimated total carbon input flux of the global oceanic subduction. It is therefore important to include the contribution of continental subduction when deciphering deep carbon recycling and its effects.