U–Th isotopes in Hainan basalts: Implications for sub-asthenospheric origin of EM2 mantle endmember and the dynamics of melting beneath Hainan Island Haibo Zou a, ⁎, Qicheng Fan b a Department of Geology and Geography, Auburn University, Auburn, AL 36849, USA b Institute of Geology, China Earthquake Administration, Beijing 100029, China abstract article info Article history: Received 1 November 2009 Accepted 16 January 2010 Available online 25 January 2010 Keywords: Basalts U–Th disequilibrium Hainan Island Hotspot Plume 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 ε Nd values range from +4.1 to + 4.8, 87 Sr/ 86 Sr ratios vary from 0.7039 to 0.7042, and 206 Pb/ 204 Pb 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 235 U/Pb. The olivine tholeiites from Maanling display 18–20% 230 Th excesses and the alkali olivine basalts from Leihuling show 22–32% 230 Th excesses. The pronounced 230 Th 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 peridotite 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 230 Th 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. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The volcanic eruptions in the northern Hainan Island (20°N, 110°E), south China, produced about 5000 km 3 basaltic magmas (Fig. 1). Although the eruptions may have initiated more than 1 million years ago, most of the basalts were produced over a time interval of 200,000 to 500,000 years (Flower et al., 1992). The average magma supply of 0.1 to 0.25 km 3 /year approaches that of a major flood basalt episode (Flower et al., 1992). The Hainan eruptions are part of a regional magmatic episode surrounding the South China Sea Basin and nearby regions, such as Vietnam (Hoang and Flower, 1998), Thailand (Zhou and Mukasa, 1997) and SE China (Zou et al., 2000). These volcanic rocks surrounding the South China Sea Basin, and those from seamounts within the South China Sea Basin, are younger than the South China Sea sea-floor extension, i.e., postdating the opening of the South China Sea Basin (at 30 to 16 million years ago). The Hainan lavas, located at the northern edge of the South China Sea Basin, may present a microcosm of the volcanic activity surround- ing the South China Sea Basin (Flower et al., 1992). It has been recognized (Tu et al., 1991; Flower et al., 1992) that (1) chemically the Hainan basalts resemble the oceanic island basalts (OIB) with enriched OIB-type incompatible element distributions, and (2) isotopically the Hainan basalts are characterized by the intriguing Dupal-like Pb isotopic signatures and depleted Sr–Nd isotopic compositions. There are still debates about the origins of the Hainan volcanoes, with implications for the origins of the ‘Southern Hemisphere’ Dupal anomaly in the Northern Hemisphere Hainan basalts (Tu et al., 1991; Tu et al., 1992; Liu, 1999; Zhao, 2007). On a global scale, it is not clear whether such Dupal mantle reservoirs are mainly derived from shallow subcontinental lithospheric mantle (Hawkesworth et al., 1990; Tu et al., 1991) or from deep mantle (e.g., lower mantle) (Hart, 1984; Castillo, 1988; Hart et al., 1992). To provide new insights into the origins of the Hainan basalts, additional geological tools (in addition to Nd, Sr and Pb isotopes and seismic images) are needed. Young basalts erupted ∼ 9000 years (9 ka) ago at Maanling volcano and Leihuling volcano (Fan et al., 2004) in Lithos 116 (2010) 145–152 ⁎ Corresponding author. Tel.: +1 334 844 4315; fax: +1 334 844 4486. E-mail addresses: haibo.zou@gmail.com, haibo.zou@auburn.edu (H. Zou). 0024-4937/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2010.01.010 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos