The use of ~(87)Sr/~(86)Sr ratios to document physico-chemical changes in shallow-water hydrothermal systems

摘要

The dominant precipitates in the hydrothermal system at Ambitle Island are aragonite and hydrous ferric oxide (HFO). Their precipitation occurs due to a change in Eh, pH, temperature and chemical composition when the hydrothermal fluids discharge and mix with ambient seawater. The amount of sea-water-hydrothermal fluid mixing during their precipitation was calculated using strontium isotopes. Based on a ~(87)Sr/~(86) Sr-mixing model, aragonite precipitates up to a maximum seawater admixture of 11%, while above 11% and up to 57% HFO is the prominent precipitate. Thus, lamination of aragonite and HFO signals abrupt, short-term changes in the physical and chemical conditions at and near vents. These changes are most likely directly related to the rate at which Tutum Bay vents discharge and ultimately they are caused by changes in the hydrology of the hydrothermal system.