EVIDENCE FOR EXTENSIVE DEGASSING OF THE HAWAIIAN MANTLE PLUME FROM HELIUM-CARBON RELATIONSHIPS AT KILAUEA-VOLCANO

We report helium and carbon isotope and abundance characteristics of s olfataras and steam fumaroles located within and around the central su mmit caldera of Kilauea volcano, Hawaii. Kilauea fluids are characteri zed by high-He-3 'hotspot' He-3/He-4 ratios of between 13.7 and 15.9 R -A (where R-A = air He-3/He-4) together with CO2/He-3 and delta(13)C(C O2) values of 4.6 - 8.4 (x10(9)) and -3.4 to -3.6 parts per thousand, respectively. We combine our measurements with CO2 flux estimates to r econstruct the He-3 characteristics of Kilauea parental magma allowing an estimate of the He-3 characteristics of the Kilauea mantle source. Derived He-3 contents of similar to 3.3 x 10(-11) cm(3)STP/g indicate that Kilauea magma sources are highly depleted in primordial He-3, co mpared to model estimates of magma sources supplying both spreading ri dges and ocean islands. Our results are consistent with the notion tha t the Hawaiian plume has undergone extensive degassing prior to incorp oration into the source region of Kilauea volcano. We suggest that deg assing of mantle plumes, at Hawaii and possibly elsewhere, can act as an important control on the range of He-3/He-4 ratios observed to char acterize ocean island basalts (OIBs); in turn, this can affect the rel ationship between helium isotopes and other tracers of mantle sources. Plume degassing can also explain the puzzling observation that the He -3 content of most OIBs is less than that of midocean ridge basalts (M ORBs).