Lower crustal assimilation in oceanic arcs: Insights from an osmium isotopic study of the Lesser Antilles
Lower crustal assimilation in oceanic arcs: Insights from an osmium isotopic study of the Lesser Antilles
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DOI：
10.1016/j.gca.2014.11.009
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发表时间：
2015-02
影响因子：
5
通讯作者：
R. Bezard;B. Schaefer;S. Turner;J. Davidson;D. Selby
中科院分区：
文献类型：
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作者：
R. Bezard;B. Schaefer;S. Turner;J. Davidson;D. Selby
我们提供了小安的列斯群岛弧沿线大部分镁铁质熔岩（MgO = 5–17 wt.%）以及俯冲玄武岩和沉积物的全岩 187Os/188Os 数据。弧熔岩中的 187Os/188Os 比率从 0.127 到 0.202 不等。 187Os/188Os 与 Os 浓度之间以及 187Os/188Os 与 MgO 等分化指数之间的负相关表明，同化作用（而不是来源变化）是观察到的 Os 同位素变化范围的原因。 87Sr/86Sr、La/Sm 和 Sr/Th 也因同化而改变，因为它们都与 187Os/188Os 相关。据推测，该同化剂具有类 MORB 的 87Sr/86Sr，具有高 Sr (>700 ppm)、低轻中重稀土元素 (L/M-HREE；La/Sm ∼2.5) 和 187Os/188Os > 0.2。这种成分特征可能对应于富含斜长石的早期弧堆积物。鉴于同化作用影响的是最后储存在5 kbar以上的熔岩，同化作用一定发生在中下地壳。只有来自格林纳达的高MgO苦辉石逃脱了明显的同化作用（MgO = 17% wt.%）并且可以反映地幔源成分。它具有很强的放射性成因 87Sr/86Sr (0.705)，但 187Os/188Os 比率与地幔范围重叠 (0.127)。沉积物和俯冲板片蚀变玄武岩的 187Os/188Os 和 87Sr/88Sr 比率从 0.18 到 3.52 和 0.708 变化。 0.714。因此，我们认为，与 Sr 不同，板片中的 Os 没有转移到母体岩浆中。 Os 可能保留在地幔楔中，甚至可能返回到俯冲板片的深地幔中。
We present whole rock187Os/188Os data for the most mafic lavas along the Lesser Antilles arc (MgO = 5–17 wt.%) and for the subducting basalt and sediments.187Os/188Os ratios vary from 0.127 to 0.202 in the arc lavas. Inverse correlations between187Os/188Os and Os concentrations and between187Os/188Os and indices of differentiation such as MgO suggests that assimilation, rather than source variation, is responsible for the range of Os isotopic variation observed.87Sr/86Sr, La/Sm and Sr/Th are also modified by assimilation since they all correlate with187Os/188Os. The assimilant is inferred to have a MORB-like87Sr/86Sr with high Sr (>700 ppm), low light on middle and heavy rare earth elements (L/M-HREE; La/Sm ∼2.5) and187Os/188Os > 0.2. Such compositional features are likely to correspond to a plagioclase-rich early-arc cumulate. Given that assimilation affects lavas that were last stored at more than 5 kbar, assimilation must occur in the middle-lower crust.Only a high MgO picrite from Grenada escaped obvious assimilation (MgO = 17% wt.%) and could reflect mantle source composition. It has a very radiogenic87Sr/86Sr (0.705) but a187Os/188Os ratio that overlaps the mantle range (0.127).187Os/188Os and87Sr/88Sr ratios of the sediments and an altered basalt from the subducting slab vary from 0.18 to 3.52 and 0.708 to 0.714. We therefore suggest that, unlike Sr, no Os from the slab was transferred to the parental magmas. Os may be either retained in the mantle wedge or even returned to the deep mantle in the subducting slab.