Late diagenetic alteration of carbonate evidenced by carbonate-deficient siliciclastic laminae

Abstract

The alteration of carbonate, caused by organic acid release during hydrocarbon generation, is usually neglected in carbonate chemistry, since the unaltered carbonate, which may reflect the (syn)depositional environments, can dilute and conceal the modification signals (e.g., δ13Ccarb and elemental compositions). In carbonate-deficient black shales, however, active hydrocarbon generation may provide sufficient organic acid to dissolve the original minerals, and the geochemical signals may remain evident in the altered carbonate. Moreover, laminae represent the smallest phase in sedimentary rock and can be distinguished by their chemical and mineral compositions, which reflect the alteration reactions and products. Thus, microscale investigation on laminae would help clarify alteration pathways during burial diagenesis and its geochemical effects. In order to understand the diagenetic reactions and organo-modifications of carbonate minerals, we examined both the mineral and geochemical characteristics of the carbonate-deficient, organic-rich, laminated black shales from the Yanchang Formation of the Ordos Basin, North China. Our study demonstrates that the carbonate content is negatively correlated with the ion concentrations within the carbonate lattice (e.g., Srcarb), providing evidence that carbonate phases have undergone considerable late diagenetic alteration during deep burial. This is consistent with the argument that stronger alteration indicators for carbonate should be found in rocks with lower carbonate content. In addition, the (re)precipitation of carbonate minerals, accompanied by corrosion of siliciclastic minerals, is also observed in thin sections, supporting the deep alteration scenario. By confirming the presence of deep diagenetic alteration of carbonate during deep burial and investigating this alteration in detail, this study provides new insights into interpretations of carbonate chemistry.