Lithium, strontium, and neodymium isotopic compositions of oceanic island basalts in the Polynesian region: constraints on a Polynesian HIMU origin
Lithium, strontium, and neodymium isotopic compositions of oceanic island basalts in the Polynesian region: constraints on a Polynesian HIMU origin
复制标题
DOI：
10.2343/geochemj.39.91
复制
发表时间：
2005-02
影响因子：
0.8
通讯作者：
Y. Nishio;S. Nakai;T. Kogiso;H. G. Barsczus
中科院分区：
文献类型：
--
作者：
Y. Nishio;S. Nakai;T. Kogiso;H. G. Barsczus
为了探讨地幔中HIMU（高μ）储层的成因，我们测量了波利尼西亚地区几种大洋岛屿玄武岩（OIB）的Li、Sr和Nd同位素组成。我们使用了最近开发的多接收电感耦合等离子体质谱法，可以精确和准确的锂同位素测定。这项研究提出了第一个锂同位素数据的HIMU OIBs。波利尼西亚HIMU OIB（Mangaia、Tubuai和Rurutu）的全岩δ 7 Li值（δ 7 Li = [[7 Li/6Li]样品/[7 Li/6Li]L-SVEC标准-1] × 1000）在+5.0‰ ~+7.4‰之间，高于新鲜的正常洋中脊玄武岩（N-MORB）玻璃（约1000 μ m）。+3‰）。同时测量的K/Rb，Ba/Rb，和87 Sr/86 Sr的比值表明，分析的HIMU OIBs是免费的显着posteruption蚀变。这些结果表明，波利尼西亚HIMU源区的δ 7 Li值并不低于N-MORB源区的δ 7 Li值。在HIMU源起源的众多模型中，最广泛接受的模型是它涉及俯冲（脱水）洋壳。对于这个HIMU成因模型，我们新的Li同位素结果排除了高度蚀变的部分，即洋壳的最上部，因为俯冲高度蚀变的MORB的δ 7 Li值应该非常低（δ 7 Li <新鲜MORB）。基于这些原因，我们认为波利尼西亚的HIMU源是在高度蚀变的地壳之下的相对较少蚀变的洋壳。而铅，锶，钕同位素签名主要表明参与源沉积物，锂同位素签名是更敏感的玄武岩地壳所经历的蚀变程度，因此可以用来区分地壳的哪一部分被困在OIB岩浆。因此，它提供的信息补充了放射性同位素提供的信息。
To investigate the origin of the HIMU (high-μ) reservoir in the mantle, we measured Li, Sr, and Nd isotopic compositions of several oceanic island basalts (OIBs) from the Polynesian region. We used a recently developed multiple-collector inductively coupled plasma mass spectrometry method that allows precise and accurate Li isotopic determinations. This study presents the first Li isotopic data on HIMU OIBs. The measured whole-rock δ7Li values (δ7Li = [[7Li/6Li]sample/[7Li/6Li]L-SVEC standard - 1] × 1000) of the Polynesian HIMU OIBs (Mangaia, Tubuai, and Rurutu) range from +5.0‰ to +7.4‰, which are higher than those of fresh normal mid-ocean ridge basalt (N-MORB) glasses (ca. +3‰). The simultaneously measured K/Rb, Ba/Rb, and 87Sr/86Sr ratios indicate that the analyzed HIMU OIBs are free from significant posteruption alteration. These results suggest that the δ7Li value of the Polynesian HIMU source is never lower than those of the N-MORBs. Among the numerous models for the origin of the HIMU source, the most widely accepted model is that it involves subducted (dehydrated) oceanic crust. For this HIMU-origin model, our new Li isotopic results exclude the highly altered portion, that is, the uppermost part of the oceanic crust, because the δ7Li value of subducted highly altered MORB should be extremely low (δ7Li < fresh MORB). For these reasons, we propose that the Polynesian HIMU source is the relatively less-altered oceanic crust underlying the highly altered crust. Whereas Pb, Sr, and Nd isotopic signatures dominantly indicate the involvement of sediments in a source, the Li isotopic signature is more sensitive to the degree of alteration experienced by the basaltic crust and thus can be used to distinguish what part of the crust was trapped in the OIB magma. It therefore provides information complementary to that provided by the radiogenic isotopes.