High 3He/4He ratios associated with the Yellowstone caldera reflect the presence of a magmatic helium component. This component is ultimately derived from a mantle plume capped by a cooling batholith underlying the caldera. In surface hot springs, fumaroles, etc., the 3He/4He ratio varies from ~1 to 16 tims the air ratio. The variations are produced by varying degrees of dilution of the magmatic component with radiogenic helium. The radiogenic helium is crustal-derived and is thought to be scavenged from aquifiers in which the hydrothermal fluids circulate. We determined the helium iosotopic composition in 12 different springs from the Lower Geyser Basin, a large hydrothermal basin with the caldera. The 3He/4He ratio was found to vary from ~2.7 to 7.7 times the air ratio. The variations correlate with variations in water chemistry. Specifically, the 3He/4He ratio increased with total bicarbonate concentration. The dissolved bicarbonate is from gas-water-rock interactions involving CO2 and Na silicates. The concentration of bicarbonate is a function of the availability of dissolved CO2, which, in turn, is a function of deep boiling with phase separation prior to CO2-bicarbonate conversion. The correlation of high 3He/4He ratios with high bicarbonate is interpreted as the result of deep dilution of a single thermal fluid with cooler water during ascent to the surface. The dilution and cooling deters deep boiling, and therefore both CO2 and 3He are retained in the rising fluid. Fluids that are not diluted with boil to a greater extent, losing a large proportion of 3He, as well as CO2, leaving a helium-poor residual fluid in which the isotopic composition of helium will be strongly affected by the addition of radiogenic helium. The helium isotopic compositions are also affected by the presence or absence of reactive volcanic glass in shallow reservoirs, the extent of these shallow reservoirs, and by the absorption of steam, derived from the early stages of deep boiling, into near-surface cold waters. ¿ American Geophysical Union 1987