Widespread Os-isotopically ultradepleted mantle domains in the Paleo-Asian oceanic upper mantle: evidence from the Paleozoic Tianshan ophiolites (NW China)
Widespread Os-isotopically ultradepleted mantle domains in the Paleo-Asian oceanic upper mantle: evidence from the Paleozoic Tianshan ophiolites (NW China)
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DOI：
10.1007/s00531-019-01759-x
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发表时间：
2019-07
影响因子：
2.3
通讯作者：
X. Gong;Ji‐Feng Xu;Xijun Liu;Xiao-Xiao Huang-Xiao;Zhiguo Zhang;Hai-Long Zhou;Zheng-Yu Yang;Wenbin Zha
中科院分区：
文献类型：
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作者：
X. Gong;Ji‐Feng Xu;Xijun Liu;Xiao-Xiao Huang-Xiao;Zhiguo Zhang;Hai-Long Zhou;Zheng-Yu Yang;Wenbin Zha
古亚洲洋壳地幔同位素不均一性的规模和程度仍然缺乏限制。在这里，我们报告的地幔橄榄岩的Os同位素数据从古生代干沟和Kumishi蛇绿岩，这是保存在中国西部天山缝合线（新疆，中国），并认为采样古亚洲大洋上地幔。这些严重蛇纹橄榄岩显示Al 2 O3，Ti和Yb含量之间的正相关性，表明不同程度的熔融提取肥沃的地幔来源。然而，橄榄岩中富集的轻稀土元素模式与简单的部分熔融残留物不一致，从而反映了熔融后熔体渗滤的效果。两个蛇绿岩中的橄榄岩显示出很大的Os同位素组成变化，其中一些具有高度非放射性的187 Os/188 Os值（0.117-0.118），反映了元古代熔体的亏损。这一特征在现代大洋橄榄岩和大多数古生代蛇绿岩中普遍存在，这意味着Os同位素超亏损域在古亚洲洋壳地幔中广泛存在。天山古生代蛇绿岩和现代大洋橄榄岩的锇同位素分布特征基本相似，表明它们来自同一个非均质上地幔储集层。这可能支持的假设，同位素超贫化域的对流上地幔的固有组成部分，并没有充分均匀的对流。另外，再贫化年龄（TRD）的这些古老的域大致符合地壳锆石年龄的主要高峰，被认为是反映在相邻的伊犁-中天山微大陆的陆壳生长的情节。这表明，天山蛇绿岩中的超亏损地幔域可能来自古老的陆下岩石圈地幔，这些地幔被剥离和折返形成洋盆基底。
The scale and extent of isotopic heterogeneities in the Paleo-Asian oceanic mantle remain poorly constrained. Here, we report Os isotopic data for mantle peridotites from the Paleozoic Gangou and Kumishi ophiolites, which are preserved in the western Chinese Tianshan sutures (Xinjiang, China) and considered to sample the Paleo-Asian oceanic upper mantle. These heavily serpentinized peridotites display positive correlations between Al2O3, Ti, and Yb contents, indicating variable degrees of melt extraction from fertile mantle sources. However, enriched light rare-earth element patterns in the peridotites are inconsistent with simple residues of partial melting, thus reflecting the effect of post-melting melt percolation. Peridotites from both ophiolites show wide variation of Os isotopic compositions, several of which are characterized by highly unradiogenic187Os/188Os values (0.117–0.118), reflecting Proterozoic melt depletion. Such feature is common in modern oceanic peridotites and most Phanerozoic ophiolites, meaning that Os-isotopically ultradepleted domains should be widespread in the Paleo-Asian oceanic mantle. The overall similarity of Os-isotope distribution between the Paleozoic Tianshan ophiolites and modern oceanic peridotites suggests that they derive from the same heterogeneous upper mantle reservoir. This might support the hypothesis that isotopically ultradepleted domains are inherent components of the convecting upper mantle and have not been sufficiently homogenized by convection. Alternatively, Re-depletion ages (TRD) for these ancient domains are broadly coincident with the major peaks in crustal zircon ages that are thought to reflect episodes of continental crust growth in the adjacent Yili–Central Tianshan microcontinents. This suggests that the ultradepleted mantle domains in some of the Tianshan ophiolites may derive from ancient subcontinental lithospheric mantle, which detached and were exhumed to form the substrate of ocean basins.