What is the source of magnesium in hydrothermal dolomites? New insights from coupling δ26Mg - ∆47 isotopes

Abstract

The occurrence of fault-controlled hydrothermal dolomitization (HTD) is ubiquitous across stratigraphic records and has been extensively studied due to its association with economic resources. The origin of HTD is often evaluated by combining carbonate geochemistry, fluid inclusion thermometry and reactive transport modelling. However, multiple diagenetic overprinting events can obscure original geochemical signatures. Here, we demonstrate the applicability of two combined isotope systems, magnesium and carbonate clumped isotopes (δ26Mg - ∆47), to trace the source of fluid and magnesium in basin-scale HTD from the Western Canadian Sedimentary Basin (WCSB) and Southern China. Extensive studies in these regions, providing tectono-stratigraphic information and dolomitization models, furnish a robust scenario to evaluate the new isotopic data. Our findings reveal that while the previous geochemical data (δ18Ofluid and 87Sr/86Sr) are partly compatible with dolomitization from seawater, the elevated δ26Mg and δ18Ofluid values (up to -0.3‰ and +11‰, respectively) and the high temperatures (up to 320 °C) are not consistent with dolomitization from seawater alone. Considering this, the uniform mean δ26Mg values of silicate rocks (-0.25‰) and the occurrence of basement-rooted faults in the study areas, the hypothesis from prior work that dolomitization was initially driven by seawater that progressively mixed with crustal fluids sourced from underlying basement rocks, was supported. As the δ26Mg values may reflect the isotopic signature of the fluid and the leached host rocks, when combined with ∆47-derived temperatures, it results in an effective tracer of fluid and Mg sources during dolomitization in tectonically complex basins that are otherwise challenging to unravel. A comparison of our results paints a global picture of isotopic imprints in hydrothermal dolomite and further demonstrate the ability of Mg isotopes to differentiate between dolomitization from seawater versus evaporated brines and also through Mg-rich crustal fluids.