Strontium mass balance

Strontium isotope ratios and abundances in samples from the oceanic crust may be used to determine the complete chemical mass balance of strontium exchange between seawater and basalt, including the loss of basaltic strontium to hydrothermal solutions, and uptake of basaltic or seawater strontium from hydrothermal solutions. A mass balance of this exchange can be made in four steps, (i) The relative amount of basaltic and seawater strontium in altered basalt can be determined from the measured Sr/ Sr of an altered sample as a (linear) mixture of strontium from the two end members, the (contemporaneous) seawater and basalt, (ii) The inventory of the basaltic and seawater strontium in an altered sample (in mg/kg) may then be determined from the above ratio of seawater and basalt in the sample and the total strontium abundance measured for this sample, (iii) Seawater strontium addition to the basalt is given directly by the seawater strontium inventory calculated in Step (ii). (iv) The determination of flux of basaltic strontium in or out of an altered sample is more complicated because it has to be related to the original inventory of strontium. It is determined as the difference between the original basaltic inventory and the basaltic strontium present in the altered sample. 

The increase in the Sr/ Sr ratio of the beachrock cement with decreasing age is not only related to regional tectonics. The evolution of the ratio of strontium-87 to strontium-86 in beachrock cements reflects changes in the global strontium isotope mass balance. 

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