Magnesium Isotopes Archive the Initial Carbonate Abundances of Metasedimentary Rocks Prior to Thermal Decarbonation

Investigating the carbonate preservation efficiency (CPE) of continental crust is crucial to understand the global carbon cycle, which requires constraints on initial carbonate abundances (ICAs) of crustal rocks. To link Mg isotopes to ICAs, we present elemental and Mg isotopic data for Himalayan carbonate‐bearing and carbonate‐free metasedimentary rocks. Given no evident melt extraction or external‐fluid infiltration, ICAs of these samples can be independently estimated by elemental data. Despite different carbonate species in the protoliths, all the samples show congruent relationship between their δ26Mg and ICAs, owing to the elevated carbonate δ26Mg and Mg/Ca in protoliths of calcite‐rich samples resulting from diagenetic processes. When collated with literature data, we suggest the observed correlation here can be applied to most carbonate‐bearing (meta‐)sedimentary rocks. Based on a steady state box‐model, we constrained the modern net carbonate accretion flux (9.50−5.56+9.50 ${9.50}_{-5.56}^{+9.50}$ Tmol/year) and the average time‐integrated CPE (∼80−43+20 ${80}_{-43}^{+20}$%) for continental crust. Plain Language Summary Investigating the fate of carbonate preserved in continental crust is fundamental for understanding its role playing in the global carbon cycle, but is hindered by the lack of knowledge about the initial carbonate abundance of metasedimentary rocks prior to modification (e.g., anatexis). By analyzing Himalayan metasedimentary rocks, here we show a congruent relationship between their δ26Mg and initial carbonate abundances, irrespective of their protolith carbonate species, and suggest it is applicable to most carbonate‐bearing (meta‐)sedimentary rocks. Based on this relationship, the carbonate preservation in continental crust was simulated using a steady state box‐model. The results indicate a very high carbonate accretion influx to the continental crust, seven times higher than the C degassing flux in the mid‐ocean ridge. Considering explosive degassing of the accreted carbonates during episodic tectonomagmatic events, the continental crust could have been an important driving force for regulating the atmospheric CO2 during Earth's history. Key Points A strong correlation between bulk δ26Mg of carbonate‐bearing metasedimentary rocks and their initial carbonate abundances was established Despite different carbonate species in protoliths, both the calcite‐rich and dolomite‐rich samples show nearly identical correlations Steady stat