the mantle and is used as a marker of material of deep origin. Due to similarities in helium and carbon in magma chemistry, outgassing of helium requires the loss of volatile components (water, carbon dioxide) from the mantle, which happens at depths of less than 60 km. However, 3He is transported to the surface primarily trapped in the crystal lattice of minerals within fluid inclusions. Helium-4 is created by radiogenic production (by decay of uranium/thorium-series elements). The continental crust has become enriched with those elements relative to the mantle and thus more He4 is produced in the crust than in the mantle. The ratio (R) of 3He to 4He is often used to represent 3He content. R usually is given as a multiple of the present atmospheric ratio (Ra). Common values for R/Ra: Old continental crust: less than 1 mid-ocean ridge basalt (MORB): 7 to 9 Spreading ridge rocks: 9.1 plus or minus 3.6 Hotspot rocks: 5 to 42 Ocean and terrestrial water: 1 Sedimentary formation water: less than 1 Thermal spring water: 3 to 11 3He/4He isotope chemistry is being used to date groundwaters, estimate groundwater flow rates, track water pollution, and provide insights into hydrothermal processes, igneous geology and ore genesis. (U-Th)/He dating of apatite as a thermal history tool USGS: Helium Discharge at Mammoth Mountain Fumarole (MMF) Isotopes in actinide decay chains Isotopes in the decay chains of actinides are unique amongst radiogenic isotopes because they are both radiogenic and radioactive. Because their abundances are normally quoted as activity ratios rather than atomic ratios, they are best considered separately from the other radiogenic isotope systems. Protactinium/Thorium – 231Pa / 230Th Uranium is well mixed in the ocean, and its decay produces 231Pa and 230Th at a constant activity ratio (0.093). The decay products are rapidly removed by adsorption on settling particles, but not at equal rates. 231Pa has a residence equivalent to the residence time of deep water in the Atlantic basin (around 1000 yrs) but 230Th is removed more rapidly (centuries). Thermohaline circulation effectively exports 231Pa from the Atlantic into the Southern Ocean, while most of the 230Th remains in Atlantic sediments. As a result, there is a relationship between 231Pa/230Th in Atlantic sediments and the rate of overturning: faster overturning produces lower sediment 231Pa/230Th ratio, while slower overturning increases this ratio. The combination of δ13C and 231Pa/230Th can therefore provide a more complete insight into