Testing oceanic crust–mantle decoupling by Sr–Nd–Hf–Os isotopes of Neo-Tethyan ophiolites
Testing oceanic crust–mantle decoupling by Sr–Nd–Hf–Os isotopes of Neo-Tethyan ophiolites
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新特提斯蛇绿岩Sr-Nd-Hf-Os同位素测试洋壳-地幔解耦
DOI：
10.1016/j.lithos.2020.105757
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发表时间：
2020-12
期刊：
影响因子：
3.5
通讯作者：
Zhen-Yu Zhang
中科院分区：
文献类型：
--
作者：
Tong Liu;Fu-Yuan Wu;Chuan-Zhou Liu;Yener Eyuboglu;Di-Cheng Zhu;Chang Zhang;Wen-Bin Ji;Yang Xu;Zhen-Yu Zhang
人们认为洋壳和地幔具有相同的 Sr-Nd-Hf-Os 同位素组成。然而，越来越多的证据表明这两个单元之间的同位素差异，表明洋壳-地幔脱钩的存在。在这里，我们提供了克孜勒达（土耳其东南部）和日喀则（中国西藏南部）新特提斯蛇绿岩的最新全岩和矿物 Sr-Nd-Hf-Os 同位素数据，以测试海洋壳幔解耦是否是这些蛇绿岩的特征。克孜尔达蛇绿岩中的地幔橄榄岩在单斜辉石和斜方辉石 Hf 同位素组成 (εHf(t) = -27.7 至 +20.6) 和非放射性 Os 同位素比率 (187Os/188Os = 0.11853–0.12720) 方面存在很大差异。这些地幔橄榄岩经历了古代（高达约 1.6 Ga）熔体消耗事件，其古老的 Re 消耗年龄（TRD = 1632–357 Ma）证明了这一点，尽管 Hf 同位素已通过最近的熔体再受精而局部重置，这可能发生在弧前盆地环境中。日喀则蛇绿岩的地幔橄榄岩主要具有 DMM（贫化大洋中脊玄武岩地幔）类至超贫化单斜辉石 Hf 同位素组成（19 次分析中有 17 次分析中 εHf(t) = +9.1 至 +29.8），这些成分被硅酸盐熔体渗滤广泛覆盖。然而，日喀则地幔橄榄岩的锇同位素组成与克孜尔达蛇绿岩相似，再次表明早白垩世洋壳形成之前就存在古代熔体萃取。这些古老的超贫化地幔域在现代大洋岩石圈、弧前盆地和蛇绿岩中并不罕见，它们的密度低于肥沃的地幔，而且太难形成，无法对最近生成的洋壳做出贡献。在比较洋壳及其下面的岩石圈地幔时，采样偏差是不可避免的。因此，不能仅使用洋壳来估计其整个地幔源的同位素组成。
Oceanic crust and the mantle are thought to have identical Sr–Nd–Hf–Os isotopic compositions. However, there is increasing evidence of isotopic differences between the two units, suggesting the existence of oceanic crust–mantle decoupling. Here we present updated whole-rock and mineral Sr–Nd–Hf–Os isotope data for the Kızıldağ (southeast Turkey) and Xigaze (south Tibet, China) Neo-Tethyan ophiolites to test if oceanic crust–mantle decoupling characterizes these ophiolites. Mantle peridotites from the Kızıldağ ophiolite have large variations in clinopyroxene and orthopyroxene Hf isotopic compositions (εHf(t) = −27.7 to +20.6) and unradiogenic Os isotope ratios (187Os/188Os = 0.11853–0.12720). These mantle peridotites have experienced ancient (up to ~1.6 Ga) melt depletion events, as evidenced by their old Re depletion ages (TRD= 1632–357 Ma), although Hf isotopes have been locally reset by recent melt re-fertilization, which likely occurred in a forearc basin setting. Mantle peridotites from the Xigaze ophiolite mainly have DMM (depleted mid-ocean ridge basalt mantle)-like to ultra-depleted clinopyroxene Hf isotopic compositions (εHf(t) = +9.1 to +29.8 for 17 out of 19 analyses), which were extensively overprinted by silicate melt percolation. However, the Os isotopic compositions of the Xigaze mantle peridotites are similar to those of the Kızıldağ ophiolite, again suggestive of ancient melt extraction long before the Early Cretaceous formation of this oceanic crust. These ancient and ultra-depleted mantle domains are not uncommon in modern oceanic lithosphere, forearc basins and ophiolites, and are less dense than the fertile mantle, and also are too refractory to contribute to the recent generation of oceanic crust. Sampling bias is unavoidable when comparing the oceanic crust and its underlying lithospheric mantle. As such, the oceanic crust alone cannot be used to estimate the isotopic composition of its entire mantle source.
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