Mantle sources of ocean islands basalts revealed from noble gas isotope systematics
Mantle sources of ocean islands basalts revealed from noble gas isotope systematics
复制标题
稀有气体同位素系统学揭示海岛玄武岩的地幔来源
DOI：
10.1016/j.chemgeo.2021.120626
复制
发表时间：
2022
期刊：
影响因子：
3.9
通讯作者：
Nicklas, Robert W.
中科院分区：
文献类型：
--
作者：
Day, James M.D.;Jones, Tim D.;Nicklas, Robert W.
稀有气体同位素系统学，特别是He的同位素系统学，已经基本上表明一些洋岛玄武岩（OIB）来自深地幔柱。夏威夷、萨摩亚、加拉帕戈斯和冰岛熔岩中的He、W、Os、Sr、Nd和Pb同位素之间的关系被认为反映了来自脱气较少的下地幔来源的贡献，甚至可能是来自核幔边界的平流物质。本文综述了OIB主要岩套的稀有气体（He-Ne-Ar-Ar）同位素系统学。重要的是在这个评估的空间和时间的变化，包括用于分析惰性气体的样品介质（玻璃，矿物，热液气体和流体），以及部分熔化的程度经历产生OIB的考虑。富气样品的有限可用性意味着OIB Ne、Ar、Ar同位素组成定义的覆盖范围不完整。此外，低度部分熔融将导致更易熔的、通常更富集的成分的优先取样，这可能影响惰性气体同位素系统学。OIB的3He/4He（8 ± 2RA）主要来自对流地幔结构域，也可能含有一些相对未脱气的（太阳）组分。它们的Sr-Nd-Os-Pb同位素组成范围反映了强烈的再循环地壳和/或岩石圈遗产。中等（> 10 RA）到高-3 He/4 He（> 25 RA）OIB（Loihi，夏威夷;冰岛; Fernandina，加拉帕戈斯; Ofu，萨摩亚）对一个储层进行采样，该储层自~45亿年以来一直相对孤立，如惰性气体（He、Ne、Ar、He）和W同位素所示。然而，该储层并不是原始的，这些OIB显示出包含Xe同位素以及亲石-亲铁放射性同位素系统的贫化和富集回收成分的证据。将最高的3He/4He OIB与假定的焦点区（FOZO）储层联系起来也是有问题的; FOZO不是受地壳再循环影响最小的储层，而是含有再循环成分（包括贫化岩石圈）并已被氧化的地幔储层。这种定义上的区别意味着，虽然FOZO储层几乎可以肯定是由最深、最热的地幔柱采样的，但它作为原始储层的地位是未经证实的。地幔对流的模型可以满足地震对地球深部地幔的约束，并揭示了广泛的混合与地球表面部分熔融过程形成的富集和贫化的岩性。古代深部孤立储集层可能是相对较小的现今地幔特征。更值得注意的是预测的强烈耐火地幔物质的丰度。为了更全面地了解地幔过程，需要关注这种难熔储层的惰性气体、放射成因和稳定同位素属性。
Noble gas isotope systematics, particular those of He, have been fundamental in showing that some ocean island basalts (OIB) were sourced from deep mantle plumes. Relationships between He, W, Os, Sr, Nd and Pb isotopes in Hawaiian, Samoan, Galapagos, and Icelandic lavas have been suggested to reflect contributions from less degassed lower mantle sources, and perhaps even materials advected from the core-mantle boundary. This study reviews the noble gas (He-Ne-Ar-Xe) isotope systematics of major OIB suites. Important in this evaluation are considerations of spatial and temporal variations, including the sample media (glass, minerals, hydrothermal gases and fluids) used for analyzing noble gases, as well as the degree of partial melting experienced to produce OIB. Limited availability of gas-rich samples means patchy coverage in definition of OIB Ne, Ar, Xe isotope compositions. Additionally, low-degree partial melting will lead to preferential sampling of more fusible, generally more enriched components which could affect noble gas isotope systematics. OIB with low- (<8RA) to MORB-like3He/4He (8 ± 2RA) dominantly sample convecting mantle domains and can also contain some relatively undegassed (solar) components. Their range in Sr-Nd-Os-Pb isotope compositions reflect a strongly recycled crustal and/or lithospheric heritage. Intermediate (>10RA) to high-3He/4He (>25RA) OIB (Loihi, Hawaii; Iceland; Fernandina, Galapagos; Ofu, Samoa) sample a reservoir that has been relatively isolated since ~4.5 billion years as shown by noble gases (He, Ne, Ar, Xe), and by W isotopes. However, this reservoir is not pristine and these OIB show evidence for containing depleted and enriched recycled components from Xe isotopes as well as lithophile-siderophile radiogenic isotope systematics. Linking the highest-3He/4He OIB to a putative Focus Zone (FOZO) reservoir is also problematic; FOZO is not a reservoir least affected by recycling of crust, rather a mantle reservoir that contains recycled components, including depleted lithosphere, and that has been oxidized. This distinction in definition means that, while the FOZO reservoir is almost certainly sampled by the deepest, hottest mantle plumes, its status as a primitive reservoir is unsubstantiated. Models of mantle convection can satisfy seismic constraints on Earth's deep mantle and reveal extensive mixing throughout with enriched and depleted lithologies formed by partial melting processes at Earth's surface. Ancient deep isolated reservoirs are likely to be relatively minor present-day mantle features. More remarkable is the predicted abundance of strongly refractory mantle material. Focus on the noble gas, radiogenic and stable isotope attributes of such refractory reservoirs are required for a fuller understanding of mantle processes.
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