Alkali elements as geothermometers for ridge flanks and subduction zones

Hydrothermal experiments were conducted on basaltic ash and smectite, with a water/rock mass ratio of ~5-8, at 600 bar pressure. The reaction temperature was gradually changed from 35 to 350°C at 20-50°C increments, and at each temperature steady-state was reached. The results indicate that each alkali metal has a distinct temperature-dependant partition profile between fluid and the solid phase. The data indicate significant fractionations among alkali metals from 35-350°C and a strong influence by the starting solids on the mobility of the alkali metals. The reactivity of the solid for Li isotopes increases dramatically with temperature. Cesium exhibits significant affinity for the fluid at temperatures as low as 150°C, suggesting that ridge flanks may be an important source of Cs to the ocean. The results also suggest that alkali element ratios together with Li isotope ratios may be used as approximate geothermometers for deep-sourced fluid at subduction zones.