Dating hydrothermal deposits using 228Th/228Ra, 228Ra/226Ra, 210Pb/226Ra & 226Ra/Ba

GNS Science is a world leader in the ability to date actively forming submarine ore deposits such as black smokers and VMS style mineralization that contain more than 1 wt. % barite.

New Zealand straddles a plate boundary which enables us to carry out valuable research on this active geological environment.

black smoker from Brothers Volcano, Kermadec Arc

Black smoker from Brothers Volcano, Kermadec Arc

At submarine volcanoes, hot acidic hydrothermal fluids selectively extract metal cations from rock, and when the solutions emerge into cold seawater, Cu, Fe, Zn and Pb precipitate as volcanic massive sulphides (VMS). Ba and chemically similar Ra are co-precipitated as barite but the 228Ra and 226Ra decay because the 232Th and 230Th that produces them remain in the rock. Thus VMS can be dated from the amount of 228Th that has grown from the decaying 228Ra, the 228Ra/226Ra or 226Ra/Ba values, provided that the initial ratios are known. An isochron method using 210Pb/226Ra is sometimes used when several samples are available from one large chimney.

Barium is measured by chemical means in our laboratory. The radium isotopes (or more specifically the isotopes into which they decay) are measured in a well type high resolution germanium gamma ray detector.

Isotopes used Age limits (years)
228Th/228Ra 0.3 – 12
228Ra/226Ra 3 – 35
210Pb/226Ra (isochron) 35 – 100
226Ra/[Ba] 500 – 15000

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