Water Content and Oxygen Isotopic Composition of Alkali Basalts from the Taihang Mountains, China: Recycled Oceanic Components in the Mantle Source
Water Content and Oxygen Isotopic Composition of Alkali Basalts from the Taihang Mountains, China: Recycled Oceanic Components in the Mantle Source
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DOI：
10.1093/petrology/egv013
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发表时间：
2015-04-01
影响因子：
3.9
通讯作者：
Feng, Min
中科院分区：
文献类型：
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作者：
Liu, Jia;Xia, Qun-Ke;Feng, Min
中国东部新生代大陆玄武岩的成因一直存在较大争议。越来越多的人认为俯冲洋壳（最有可能是太平洋板块）参与了它们的地幔来源。太行山位于地球物理研究所揭示的太平洋板块停滞俯冲的西缘，为研究太平洋板块俯冲对中国东部地幔特征影响的空间范围提供了机会。在以往的研究中，太行山碱性玄武岩被认为是软流圈源熔体与古老富集的岩石圈地幔相互作用的产物，其镁同位素特征表明大洋碳酸盐的贡献。为了探讨再循环洋壳在太行山玄武岩成因中的可能贡献，我们采用基于斑晶的方法（C-水（熔体）= C-水（矿物质）/D-水（矿物质/熔体））估算了岩浆的水含量，并采用二次离子质谱法测量了与以往研究相同样品组的单斜辉石斑晶的氧同位素组成。母岩浆的水含量（H2O）在0.20 ~ 1.07wt%之间，相应的H2O/Ce比值与（Nb/La）（n）、（Ba/Th）（n）和Nd同位素有较好的相关性。氧同位素比值（δ O-18（SMOW））的范围从5.8%到7.4%，每个样品的平均值范围从6.6%到7.0%。这些新的数据表明，极端脱水的再循环洋壳以及夹带的沉积物有很大的贡献。太行山玄武岩中橄榄石斑晶的微量元素组成（Ca、Mn、Ni含量）表明，该玄武岩可能是洋壳源熔体与环境地幔橄榄岩相互作用形成的辉石岩源区。
The genesis of Cenozoic continental basalts in Eastern China is highly debated. Subducted oceanic crust (most probably the Pacific oceanic slab) has been increasingly suggested to be involved in their mantle source. The Taihang Mountains are located at a surface position corresponding to the western edge of a stagnant subducted Pacific slab revealed by geophysical investigations, and thus provide an opportunity to study the spatial extent of the effect of Pacific subduction on the mantle characteristics of Eastern China. In previous studies, alkali basalts from the Taihang Mountains were considered to be the products of interaction between asthenosphere-derived melts and the old, enriched lithospheric mantle; their magnesium isotope signature suggested the contribution of oceanic carbonate. To explore the possible contribution from recycled oceanic crust in the genesis of the Taihang basalts we estimated the water contents of the magmas by a phenocryst-based approach (C-water(melt) = C-water(mineral)/D-water(mineral/melt) water) and measured the oxygen isotope compositions of clinopyroxene phenocrysts by secondary ion mass spectrometry for the same sample suite as used in previous studies. The calculated water contents (H2O by weight) of the parental magmas range from 0.20 to 1.07 wt %, and the corresponding H2O/Ce ratios correlate well with (Nb/La)(n) (where n represents primitive mantle normalization), (Ba/Th)(n) and epsilon Nd. The oxygen isotope ratios (delta O-18(SMOW)) range from 5.8 to 7.4%, and the average value for each sample ranges from 6.6 to 7.0%. These new data indicate a large contribution from extremely dehydrated recycled oceanic crust together with entrained sediments. The minor element compositions (Ca, Mn and Ni contents) of olivine phenocrysts in the Taihang basalts suggest a pyroxenite source that was probably formed by the interaction between oceanic crust-derived melt and ambient mantle peridotite.