Isotopic tracing of volcanic source regions from Hawaii: decoupling of gaseous from lithophile magma components

The well established geology and geochemistry of Hawaiian volcanism allows a detailed evaluation of the relationships between geochemically distinct tracers, such as the noble gases and the lithophile elements Sr, Nd and Pb. Four volcanic evolutionary stages show characteristic Sr-Nd-Pb isotopic progressions: early and late stages include depleted melts, whereas the intermediate shields tend to be less depleted, including source components similar to bulk earth values. In contrast, 3He/4He ratios gradually decrease from high values during the early stage to MORB-like values for the late stage. This lack of correlation between the lithophile and noble gas records may be accounted for by: (1) a unique ‘Loihi-like’ mantle component for Hawaii, or (2) a systematic offset between the two records. Whereas many studies have preferred the first scenario, we opt for the latter, and suggest that both systems are coupled in the deep mantle, but become decoupled when a gas-rich melt phase separates from the plume. gest that incipient CO2-dominated melts, carrying the noble gases, could separate in the plume flexure, and may buoyantly migrate ahead of the plume, carrying the bulk of its lithophile inventory. These melts permeate the lithosphere, which, upon sufficient heating by the plume, could generate the initial stage alkali basalts and tholeiites. The subsequent formation of the shield stage tholeiites occurs when the main plume, carrying the less depleted lithophile signature, has risen to shallower levels, and commences to melt on a large scale. The melts of this stage show lower 3He/4He ratios owing to asthenospheric entrainment by the plume, which has previously lost a major part of its volatile inventory, allowing for dilution of the isotopic signature by MORB-type noble gases. If this model is correct, the less depleted (Sr, Nd and Pb) and less degassed (noble gases) components are from one and the same source, and may provide the best estimate for the isotopic composition of the deep mantle.