Imbalance in the oceanic strontium budget

Palmer and Edmond [Earth Planet. Sci. Lett. 92 (1989) 11–26] indicated that thermally plausible oceanic hydrothermal inputs of strontium to the oceans are not sufficient to balance the riverine input. It has recently been suggested that off-axis low-temperature hydrothermal circulation may reconcile this discrepancy [e.g. Butterfield et al., Geochim. Cosmochim. Acta 65 (2001) 4141–4153]. Strontium isotope alteration profiles are compiled for sampled in situ ocean and ophiolite crust to calculate a sustainable cumulative hydrothermal flux to the oceanic strontium budget. High-temperature circulation contributes ∼1.8×109 mol yr−1 of basaltic strontium to the oceans. Enhanced hydrothermal systems in arc-related spreading environments (10% of the crust) may increase this to ∼2.3×109 mol yr−1. It is shown that low-temperature flow cannot supply the remaining flux required to reconcile the oceanic strontium budget (∼8.7×109 mol yr−1) because this would require 100% exchange of seawater strontium for basaltic strontium over an 820 m section of MORB-like crust. Currently sampled in situ ocean crust is not altered to this extent. The isotopic alteration intensity of 120 Myr crust sampled in DSDP Holes 417D and 418A indicates that off-axis low-temperature flow may contribute up to ∼8×108 mol yr−1 of basaltic strontium (9% of that required). The ocean crust can sustain a total basaltic strontium flux of ∼3.1±0.8×109 mol yr−1 (87Sr/86Sr ∼0.7025) to the oceans. This is consistent with hydrothermal flux estimates, but remains less than a third of the flux required to balance the oceanic strontium budget. The ocean crust cannot support a higher hydrothermal contribution unless the average ocean crust is significantly more altered than current observation.