FABRIC-SPECIFIC ISOTOPIC VARIABILITY IN LATE EDIACARAN DOLOMITES, SOUTH CHINA: ELUCIDATED BY MAGNESIUM, SULPHUR AND CLUMPED ISOTOPES (Invited Presentation)

Dolomite (CaMg(CO₃)₂) as a major carbonate mineral commonly distributes on the shallow-water platform throughout Earth history. Due to its thermodynamic stability, stoichiometric dolomite is less susceptible to the chemical alterations and often regarded as a reliable archive to reconstruct past environmental and ocean chemistry conditions. This is particularly important during the Precambrian time as it lacks of biological proxies. However, using dolomite as a geochemical proxy is challenging due to potential exposure to various diagenetic realms, such as burial or subaerial condition, which can partly alter and, in some cases, completely overprint the original signatures. Such phenomenon will generate large isotopic variations depending on the different dolomite fabrics. To examine the fidelity of geochemical records in dolomite and understand the governing processes of fabric-specific isotopic variations, the dolomite in the Dengying Formation of Ediacaran was selected as an example. Five fabrics (abiogenic and biogenic micrites, dolomitized ooids and early cement, and saddle dolomite) were identified in this Ediacaran-aged formation and they show a large variation in δ¹³C values (from 0 to 6‰). Based on clumped isotopes (Δ₄₇) and strontium contents, this study calculated the degree of fluid-rock interaction (W/R ratio) and established a diagenetic framework for each fabric. Both the abiogenic and biogenic micrites were characterized by the rock-buffered system (low W/R ratio = 0.1-0.2), while the fabrics of the ooid, early cement and saddle dolomite were buffered by the hydrothermal fluids in the late diagenesis (high W/R = 1.4-5). Furthermore, this interpretation is further supported by high δ²⁶Mg values, Sr and Mn contents and elevated δ³⁴S_CAS values in biogenic micrite indicating a closed or partly closed diagenetic system which allow the Rayleigh distillation enriching ²⁶Mg and enhanced ³⁴S of residual SO₄^2- by bacterial sulphate reduction. In contrast, the fabrics (ooids, early cement and saddle dolomite) in the hydrothermal-buffered system showed an extremely low values inδ³⁴S_CAS, which can be interpreted as a result of oxidation of sulphides (e.g. pyrite). The outcome of this study highlights the utility of systemically petrographic analysis and multiple geochemical proxies to decipher the diagenetic processes of dolomite and caution on interpreting the geochemical records in dolomite successions.