U–Th isotopes in Hainan basalts: Implications for sub-asthenospheric origin of EM2 mantle endmember and the dynamics of melting beneath Hainan Island
U–Th isotopes in Hainan basalts: Implications for sub-asthenospheric origin of EM2 mantle endmember and the dynamics of melting beneath Hainan Island
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DOI：
10.1016/j.lithos.2010.01.010
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发表时间：
2010-04
期刊：
影响因子：
3.5
通讯作者：
H. Zou;Q. Fan
中科院分区：
文献类型：
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作者：
H. Zou;Q. Fan
在过去的100万年里，中国南部的海南岛上爆发了大量（约5000km3）的玄武岩岩浆。测定了海南岛马安岭火山和雷虎岭火山最年轻火山岩的U-Th不平衡数据，以及Nd、Sr、Pb同位素和主微量元素浓度。海南全新世玄武岩均呈现轻稀土元素（LREE）富集模式和洋岛玄武岩（OIB）型不相容元素分布。它们的ε nd值在+4.1 ~ +4.8之间，87Sr/86Sr比值在0.7039 ~ 0.7042之间，206pb /204Pb比值在18.63 ~ 18.71之间。海南火山岩具有贫Sr-Nd同位素组成和dupal型富集地幔2 (EM2) Pb同位素特征，具有高Th/U和235u /Pb的时间积分特征。马陵橄榄橄榄拉斑岩表现出18 ~ 20%的230度过剩，雷虎岭碱性橄榄玄武岩表现出22 ~ 32%的230度过剩。全新世玄武岩中明显的230度过量表明，全新世海南熔岩是由石榴石稳定场（bbb75 km）地幔源熔融形成的。由于海南岛下方岩石圈厚度较薄（55km），海南玄武岩的石榴石橄榄岩地幔源不在岩石圈地幔中。Nd同位素组成不显示一个高度枯竭的软流圈地幔源。因此，我们认为海南年轻玄武岩的EM2地幔源位于地幔过渡带或更可能是下地幔，这与地幔柱起源一致。显著的230过量也表明缓慢（<1cm/年）上升流，可能表明浮力较弱的地幔柱。海南、南海盆地及邻区EM2新生代老玄武岩也可能源于海南上升地幔柱中下地幔物质的部分熔融作用。
Extensive (about 5000km3) basaltic magmas erupted on the Hainan Island, south China, mostly during the past 1millionyears. 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 εNdvalues range from +4.1 to +4.8,87Sr/86Sr ratios vary from 0.7039 to 0.7042, and206Pb/204Pb 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 and235U/Pb. The olivine tholeiites from Maanling display 18–20%230Th excesses and the alkali olivine basalts from Leihuling show 22–32%230Th excesses. The pronounced230Th excesses in the Holocene basalts indicate that the Holocene Hainan lavas were produced by melting of a mantle source in the garnet stability field (>75km). Since the lithosphere thickness beneath the Hainan Island is thin (55km), 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 significant230Th excesses also suggest slow (<1cm/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.