U-Th isotopes in Hainan basalts: Implications for sub-asthenospheric origin of EM2 mantle endmember and the dynamics of melting beneath Hainan Island

Extensive (about 5000 km(3)) basaltic magmas erupted on the Hainan Island, south China, mostly during the past 1 million years. U-Th disequilibrium data as well as Nd, Sr, Pb isotopes and major and trace element concentrations were measured on the youngest lavas from Maanling volcano and Leihuling volcano of the Hainan Island All the Holocene Hainan basalts display light rare earth element (LREE) enriched patterns and ocean island basalt (OIB)-type incompatible element distributions Their epsilon(Nd) values range from +41 to +4.8, Sr-87/Sr-86 ratios vary from 0.7039 to 0.7042. and Pb-206/Pb-204 ratios range from 18.63 to 18.71 The Hainan lavas are characterized by their depleted Sr-Nd isotopic compositions and Dupal-like EM2 (enriched mantle 2) Pb isotope signatures with time-integrated high Th/U and U-235/Pb The olivine tholeutes from Maanling display 18-20% Th-230 excesses and the alkali olivine basalts from Leihuling show 22-32% Th-230 excesses The pronounced Th-230 excesses in the Holocene basalts indicate that the Holocene Hainan lavas were produced by melting of a mantle source in the garnet stability field (>75 km) Since the lithosphere thickness beneath the Hainan Island is thin (55 km), the garnet pendotite mantle source for the Hainan basalts is not located in the lithospheric mantle The Nd isotopic compositions do not indicate a highly depleted asthenospheric mantle source We thus suggest that the EM2 mantle source for the young Hainan basalts is in the mantle transition zone or more likely lower mantle, which is consistent with a plume origin. The significant Th-230 excesses also suggest slow (<1 cm/year) upwelling, possibly indicative of a weakly buoyant mantle plume The older EM2 Cenozoic basalts from Hainan, South China Sea Basin and adjacent areas may also originate from partial melting of lower mantle materials in the rising Hainan plume. (C) 2010 Elsevier B V. All rights reserved