The objective of this research is to better understand the fractionation processes of Mg and Sr isotopes and especially: (1) to identify and quantify the causes of 88Sr-enrichment in continental waters; (2) to follow and evaluate the variations of δ26/24Mg along the course of seawater evaporation up to the Mg-K facies.
Riverine Sr flux to the ocean is enriched in 88Sr relative to the continental rock sources. Three hypotheses for this fractionation were evaluated: (1) precipitation of continental carbonates; (2) gypsum precipitation; and (3) incongruent leaching of silicate minerals in soils. The results of the current study show that 86Sr is preferentially incorporated to the solid phase during precipitation of continental carbonates, and thus this process may be responsible for the 88Sr-enrichment of the riverine flux. Precipitation of gypsum, however, results in fractionation in the opposite direction implying that dissolution of ancient marine gypsum can also lead to 88Sr-enrichment of river waters. No evidence was found for Sr fractionation during weathering of silicates in soils.
Mg isotope fractionation during precipitation of Mg-salts was determined by controlled evaporation of seawater in the laboratory and analyses of natural Mg-salts. Bi-directional and mineralogy-dependent fractionation of Mg isotopes was found during the precipitation of kainite, carnallite, bischofite and Mg-sulfate minerals. These findings were corroborated by samples from natural evaporitic basins worldwide.
This study suggests that the oceanic δ88/86Sr may be affected by fractionation-producing processes on the continents (carbonate precipitation) and to a lesser extent by gypsum dissolution. In addition, precipitation of Mg-evaporites results in a complex evolution of the δ26/24Mg value in the residual brine and therefore affects the Mg isotopic composition of closed or semi-enclosed systems. However, on a global scale and over geological times, the precipitation of Mg salts has a limited impact on δ26/24Mg of the ocean.