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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >The role of sediment recycling in EM-1 inferred from Os, Pb, Hf, Nd, Sr isotope and trace element systematics of the Pitcairn hotspot
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The role of sediment recycling in EM-1 inferred from Os, Pb, Hf, Nd, Sr isotope and trace element systematics of the Pitcairn hotspot

机译：从皮特凯恩热点的Os，Pb，Hf，Nd，Sr同位素和微量元素系统推断出沉积物再循环在EM-1中的作用

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摘要

We present comprehensive radiogenic isotope (Os, Pb, Hf, Nd, Sr) and trace element data on basaltic lavas from Pitcairn Island and the Pitcairn seamounts and examine the origin of the enriched mantle isotopic signature (EM-1) found in these lavas. The ~(187)Os/~(188)Os ratios of the lavas range from 0.131 to 0.254, while those of the high-Os concentration samples (>50 pg/g) lie between 0.131 and 0.148. All ~(187)Os/~(188)Os ratios are higher than the bulk silicate Earth reference value of 0.127. Since ancient subcontinental lithospheric mantle (SCLM) is expected to have a ~(187)Os/~(188)Os ratio less than 0.127, it appears that recycled SCLM plays no role in the Pitcairn source. Variations in ~(187)Os/~(188)Os ratios appear to be unconnected with those of ~(206)Pb/~(204)Pb ratios in Pitcairn lavas, suggesting that Pb and Os isotopic variations are controlled by different factors. Modeling shows that variations in Pb isotopic compositions may mainly reflect the proportion of recycled sediment in the source, while those of ~(187)Os/~(188)Os ratios may reflect the proportion of peridotite mantle versus recycled oceanic crust. The occurrence of negative Nb anomalies in some of the lavas, a correlation between Nb anomaly and ~(87)Sr/~(86)Sr ratios (0.7036-0.7051), and extremely unradiogenic and strongly correlated Nd and Hf isotopic compositions (ε_(Nd) of -5.9 to + 1.1 and ε_(Hf) of -5.3 to + 2.2) together suggest that the Pitcairn mantle source contains a recycled continental crustal component. The slope of the ε_(Hf) vs. ε_(Nd) correlation is shallower for Pitcairn Island than for the Pitcairn seamounts or the global OIB array, and may be due to a variable ratio of recycled mud to sand in the Pitcairn source. A trace element mixing model also indicates the presence of small amounts of recycled pelagic and terrigenous sediment and permits variable amounts of depleted components such as recycled MORB, gabbro and depleted mantle. The ~(206)Pb/~(204)Pb ratios of the Pitcairn lavas vary between 17.47 and 18.10 and are very unradiogenic compared to these of other ocean island basalts. By contrast, ~(208)Pb/~(204)Pb ratios are high and relatively homogeneous at values of ～39.0. This observation along with the measured Th/U ratios of the lavas, which range up to 14.1, indicate a long-term history of U loss in the Pitcairn source. In ~(207)Pb/~(204)Pb-~(206)Pb/~(204)Pb space, the data form a linear array that can be interpreted in terms of mixing between a minor recycled sediment end member and more depleted material. Lead isotopic compositions suitable for the recycled end member were investigated using a three-stage evolution model by Monte Carlo methods and suggest ages between 0.7 and 1.9 Ga for the recycled sediment. The relationships between measured Th/U and radiogenic ~(208)Pb~*/~(206)Pb~* ratios suggest that the isotopic arrays displayed by the lavas were produced by mixing, probably occurring during magma genesis.
机译：我们提供了皮特凯恩岛和皮特凯恩海山玄武岩熔岩的全面放射性同位素（Os，Pb，Hf，Nd，Sr）和微量元素数据，并研究了在这些熔岩中发现的富集地幔同位素特征（EM-1）的起源。熔岩的〜（187）Os /〜（188）Os比范围为0.131至0.254，而高Os浓度样品（> 50 pg / g）的比率在0.131至0.148之间。所有〜（187）Os /〜（188）Os的比率都高于硅酸盐的大地参考值0.127。由于预计古代亚大陆岩石圈地幔（SCLM）的〜（187）Os /〜（188）Os之比小于0.127，因此回收的SCLM在皮特凯恩源中似乎没有作用。皮特凯恩熔岩中〜（187）Os /〜（188）Os比的变化似乎与〜（206）Pb /〜（204）Pb比的变化无关，表明Pb和Os同位素变化受不同因素控制。模拟表明，铅同位素组成的变化可能主要反映了源中再生沉积物的比例，而〜（187）Os /〜（188）Os比的变化可能反映了橄榄岩地幔与再生海洋地壳的比例。某些熔岩中Nb负异常的发生，Nb异常与〜（87）Sr /〜（86）Sr之比（0.7036-0.7051）之间的相关性以及极不放射源且与Nd和Hf同位素组成高度相关的（ε_（ -5.9至+ 1.1的Nd）和-5.3至+ 2.2的ε_（Hf）一起表明皮特凯恩地幔源包含一个再循环的大陆地壳成分。皮特凯恩岛的ε_（Hf）与ε_（Nd）相关性的斜率比皮特凯恩海山或全球OIB阵列的斜率更浅，这可能是由于皮特凯恩源中的再生泥与砂之比可变。痕量元素混合模型还表明存在少量的再生中上层和陆源性沉积物，并允许可变数量的贫化组分，例如再生MORB，辉长岩和贫化地幔。皮特凯恩火山熔岩的〜（206）Pb /〜（204）Pb比在17.47至18.10之间变化，与其他大洋洲玄武岩相比，它们的放射原非常弱。相比之下，〜（208）Pb /〜（204）Pb的比率较高，并且在〜39.0时相对均匀。这项观测结果以及测得的熔岩的Th / U比值（最高可达14.1）表明了皮特凯恩源中U损失的长期历史。在〜（207）Pb /〜（204）Pb-〜（206）Pb /〜（204）Pb空间中，数据形成线性阵列，可以根据较小的再循环沉积物末端构件与更贫化的沉积物末端构件之间的混合来解释材料。通过蒙特卡洛方法使用三阶段演化模型研究了适用于再生末端成员的铅同位素组成，结果表明，再生沉积物的年龄介于0.7至1.9 Ga之间。 Th / U与放射源〜（208）Pb〜* /〜（206）Pb〜*比值之间的关系表明，熔岩显示的同位素阵列是通过混合产生的，可能发生在岩浆成因期间。

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