Extracting Mg isotope signatures of ancient seawater from marine halite: A reconnaissance

Abstract

The Mg/Ca ratio of seawater fluctuated remarkably over the Phanerozoic. The processes that controlled the temporal variation in seawater Mg/Ca ratio may also have caused the secular changes in Mg isotope ratios of seawater. Therefore, the Mg isotope composition of seawater may be useful to understand the long-term variation in seawater chemistry. Different sedimentary carbonates have been proposed as archives of ancient seawater Mg isotope signatures, but each type of carbonate has its limitations. Halite is a common mineral in evaporite deposits and their fluid inclusions may be used to infer the Mg isotope composition of contemporary seawater. In this study, we developed a method to measure the isotope composition of Mg in halite with an accuracy of better than ±0.1‰ in δ26Mg. The accuracy of the method was verified systematically, using synthetic NaCl solutions, as well as Holocene lacustrine halite samples from the Dead Sea. The δ26Mg values of marine halite samples show a large variation, ranging from −0.1‰ to −1.6‰ relative to the DSM3 international Mg isotope standard. The δ26Mg values of some ancient salt samples are lower than a published model curve for δ26Mg of seawater, and such inconsistency could be explained by 1) isotopic evolution of brines due to mineral precipitation, 2) entrapment of evaporite minerals into halite during halite precipitation or post-depositional deformation, 3) recrystallization and dissolution of pre-existing Mg minerals, and 4) limitations of the model curve of seawater δ26Mg values. Collectively, we suggest that Mg isotopes in halite are a new geochemical proxy to study the secular variations in seawater chemistry, syndepositional and post-depositional processes of evaporite deposits. Our data highlight the importance of understanding the sedimentary background and textural details of evaporites for appropriate interpretations of Mg isotope data from halite.