Evolution of an active sea-floor massive sulphide deposit

Hydrothermal circulation at oceanic spreading ridges causes sea water to penetrate to depths of 2 to 3 km in the oceanic crust where it is heated to ∼400 °C before venting at spectacular ‘black smokers’. These hydrothermal systems exert a strong influence on ocean chemistry1, yet their structure, longevity and magnitude remain largely unresolved2. The active Transatlantic Geotraverse (TAG) deposit, at 26° N on the Mid-Atlantic Ridge, is one of the largest, oldest and most intensively studied of the massive sulphide mounds that accumulate beneath black-smoker fields. Here we report ages of sulphides and anhydrites from the recently drilled3 TAG substrate structures — determined from 234U–230Th systematics analysed by thermal ionization mass spectrometry. The new precise ages combined with existing data4,5 show that the oldest material (11,000 to 37,000 years old) forms a layer across the centre of the deposit with younger material (2,300–7,800 years old) both above and below. This stratigraphy confirms that much of the sulphide and anhydrite are precipitated within the mound by mixing of entrained sea water with hydrothermal fluid6. The age distribution is consistent with episodic activity of the hydrothermal system recurring at intervals of up to 2,000 years.