Deep carbon recycling through oceanic subduction influences the atmospheric CO_2 budget and climate changes through geologic time. Sedimentary carbonate is the main light-delta~(26)Mg sink of Earth's reservoirs, and therefore its recycling into the mantle by plate subduction has potential to cause local mantle Mg isotopic heterogeneity. Therefore, Mg isotopes are potential traces for deep carbon recycling, based on a primary assumption that Mg isotopic systematics of the subducting rocks underwent little change during metamorphic dehydration and magmatic process. Previous studies demonstrated that magmatic processes do not significantly fractionate Mg isotopes. However, to date, the extent to which Mg isotopes fractionate during metamorphic process and origin of low delta~(26) Mg basalts remain largely unknown. Here we focus on these topics by introducing the stability and transformation of carbonates in high p-T condition, the behavior of Mg isotopes during plate subduction,and the possible influences on Mg isotopic compositions of the mantle by deep recycled carbonates.