Defining the Origin and Nature of the Tristan da Cunha Plume Source Using He Isotope and Pb Isotope Modelling

We present new He and Pb isotope and trace element data for Inaccessible Island and Tristan da Cunha, which confirms narrow ranges in He isotopes (5.5Ra+-0.6) and Pb isotopes (207Pb/206Pb=18.59+-0.10; 207Pb/204Pb=15.54+-0.01) and enrichment in incompatible elements (La/Yb=22-38) for the Tristan plume. We use this data to test models on the nature of the Tristan plume source. Helium isotope systematics (5.5Ra) indicate that the Tristan plume source is degassed relative to MORB-source mantle. This combined with distinctive Pb isotope systematics has lead to the theory that the Tristan plume constitutes recycled oceanic and continental lithosphere. Tristan's He isotope composition is more radiogenic than modelled degassed recycled oceanic crust (<0.0001Ra). A component of unradiogenic He is required in the source of the Tristan plume. There are two possible scenarios to explain Tristan's He isotope composition. (1)Subducted lithosphere retains sufficient He during subduction to remain relatively unradiogenic. (2)Degassed recycled material mixes with a unradiogenic He isotope component. If He is not perfectly incompatible than subducted oceanic lithosphere could lose U and He to similar degrees such that U/He ratios do not change significantly, which would lead to minor changes in the He isotope composition of subducted oceanic lithosphere. There is insufficient evidence that He and U have similar compatibilities during melting. T he unradiogenic He isotope component is likely to be primitive undegassed mantle material. Here we investigate two end-member possibilities. 1 A high Ra source analogous to Hawaii and Iceland: This requires a very small contribution from a source with high He concentration (9ppb He for 20ppb U) and unradiogenic He isotope composition (40 Ra). The contribution is sufficiently small (>1%) that it is not distinguishable using other geochemical tools. 2 A primitive mantle component with a Ra of 5.5: The Tristan plume has Pb, Nd and Sr isotope systematics very close to Bulk Earth. The Tristan volcanics were not considered to be melts of primitive mantle because of their low Ra. If, however, degassing of the lower mantle was not homogenous in the early Earth (i.e. primitive U/He ratios vary) it is possible that primitive mantle material could have remained isolated with a Ra lower than the MORB-source mantle. For Tristan 6.7ppb He and 20ppb U will produce a Ra of 5.5 over the lifetime of the Earth.