Magnesium isotope signature of Middle Ordovician dolomites from the Ordos Basin, China

As a significant sink of seawater magnesium, δ²⁶Mg values of syndepositional marine dolomites can be utilized to reconstruct seawater Mg isotope composition in the past. This, in turn, may serve as a good tracer for the Mg cycle throughout Earth’s history. Furthermore, δ²⁶Mg values of ancient dolomites precipitated in different depositional environments may help in reconstructing the paleo-conditions that prevailed in these environments, in particular, the hydrology and restriction conditions of ancient sedimentary basins.

In this study, we collected Middle Ordovician marine dolomite samples from five profiles distributed in the different locations of the Ordos Basin, North China Plate. During the Ordovician, marine carbonates were deposited in the whole area of the Ordos Basin under different sedimentary settings that can be generally divided into open marine settings, a dolomite platform, and a saline gypsum lake. After systematic petrology observation (optic and cathode luminescence microscope), we microdrilled fine-crystallized dolomite to conduct chemistry and Mg isotope analyses. The dolomites from profile #1, which were deposited in an open marine environment, exhibit the lowest δ²⁶Mg values between -2.31‰ to -2.21‰. The δ²⁶Mg values of dolomite samples are generally heavier toward the more evaporitic center of the basin. The highest δ²⁶Mg values (-1.84‰ to -1.70‰) were measured in samples from profile #5 located in the saline gypsum lake area.

We suggest that this gradient of δ²⁶Mg values from the outer parts of the basin toward the center is resulting from a prior formation of dolomite in open versus gradually more restricted settings. Dolomite is enriched in the lighter isotope, ²⁴Mg, relative to its precipitating solution. Thus, under restricted settings, dolomite formation will increase the δ²⁶Mg value of the remaining dissolved Mg in the solution and consequently, also the δ²⁶Mg values of further precipitating dolomites. This further suggests that the Mg isotope composition of dolomite can be used to reconstruct the enclosure degree of ancient evaporitic basins. Together with the observation that δ²⁶Mg values of dolomite samples from profile #1 vary in a narrow range of 0.10‰ between different formations, this also suggests that profile #1 located in open marine settings can be utilized to reconstruct the Mg isotope signature of seawater during the Middle Ordovician.