Displaced helium and carbon in the Hawaiian plume

Abstract

High relative abundances of primordial 3He are commonly found in ocean island basalts (OIB) thought to be derived from mantle plumes, and high 3He/ 4He ratios have been used to distinguish plume-type from non-plume OIBs. In simple plume models, one expects to find the highest 3He/ 4He ratios in the axial part of the plume conduit, which is sampled during the shield building stage of the volcanoes. However, the actual locus of the highest 3He/ 4He ratios is sometimes significantly displaced. This is best documented for the Hawaiian plume, where the highest- 3He/ 4He basalts are found on Loihi, a volcano tens of kilometers ahead of the inferred plume center, and 3He/ 4He ratios decrease systematically toward MORB-type values during the main and late phases of eruption. We propose that this effect is caused by small amounts of carbonatite melt formed in plumes as they rise through the transition zone. If the plume conduit is tilted by plate-driven upper mantle flow, the carbonatite melt infiltrates more vertically due to its low density and viscosity and is thus displaced from the plume center. Helium, if partitioned into the carbonatite melt, will also be displaced from the plume center. To test this model we use a numerical simulation of the Hawaiian plume interacting with the fast-moving Pacific lithosphere. We obtain vertical separation velocities of the carbonatite melt on the order of a meter/year. Consequently, helium and carbon, initially located in the plume center at > 450 km depth, are laterally displaced by 50 to 80 km in the shallow mantle, depending on grain size, porosity and melt production rate. This can explain why the highest 3He/ 4HE ratios (R/Ra up to 39; R/Ra ≡ ( 3He/ 4He) sample/( 3He/ 4He) atmosphere) occur on pre-shield Loihi, why they decline during the shield phases of Mauna Loa, Mauna Kea and Haleakala, and why post-shield and rejuvenated Hawaiian volcanism delivers only low 3He/ 4He ratios (R/Ra = 8–10). Our results quantify the potential role of carbonatite liquids in transporting helium in the Hawaiian conduit, and they appear to apply also to other plumes tilted by upper-mantle ‘wind’.