Isotopic hyperbolas constrain sources and processes under the Lesser Antilles arc
Isotopic hyperbolas constrain sources and processes under the Lesser Antilles arc
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DOI：
10.1016/j.epsl.2010.07.018
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发表时间：
2010-09
影响因子：
5.3
通讯作者：
S. Labanieh;C. Chauvel;A. Germa;X. Quidelleur;E. Lewin
中科院分区：
文献类型：
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作者：
S. Labanieh;C. Chauvel;A. Germa;X. Quidelleur;E. Lewin
岛弧熔岩的化学和同位素特征通常归因于地幔楔体中俯冲物质和过程的综合作用。相对于MORB，大多数岛弧的成分都向大陆壳值移动，尤其是小安的列斯岛弧。来自小安的列斯岛弧的熔岩不仅含有很高比例的地壳物质，而且它们还显示出非常大范围的化学和同位素成分。马提尼克岛上的熔岩覆盖了整个弧区已知的大部分变化，在过去的25 Ma期间，火山活动几乎连续发生。因此，这是确定化学和同位素可变性的原因并评估是否存在长期关系的理想目标。我们测定了30个马提尼克岛年龄为25 Ma至今的样品的铅、锶、钕、氢同位素比值。选择的样本涵盖了从岛屿形成到最近一次喷发的所有火山阶段。我们的结果涵盖了由整个弧线定义的大部分同位素范围，从相当无放射性的到高度放射性的值；但更重要的是，他们在同位素空间定义了两条截然不同的双曲线。一组为古熔岩(25~7.1 Ma)，另一组为新近熔岩(5.1 Ma至今)。双曲线与简单的双组分混合物兼容，但不能被AFC模型重现。使用这些双曲线，我们可以限制混合端元的组成。这两种亏损源区都对应于受放射性成因锶组分污染的亏损地幔，而这两种富集组分类似于当地沉积物，几乎没有或几乎没有相对分馏的锶、钕、氢和铅。在此基础上，我们得出结论，沉积物是通过熔融而不是脱水的方式添加到地幔楔体中的。我们对马提尼克岛的解释可以外推到整个小安的列斯弧区，因为马提尼克岛和其他岛屿的同位素排列是无法区分的。更广泛地说，如果在大多数其他岛弧下也发生同样的情况，弧火山活动的化学和同位素特征可能与俯冲沉积物的数量和性质直接相关。
The chemical and isotopic characteristics of island arc lavas are usually attributed to the combined effects of subducted materials and processes occurring in the mantle wedge. Most island arcs have compositions displaced towards continental crust values relative to MORB, none more so than the Lesser Antilles arc. Not only do lavas from the Lesser Antilles arc contain a high proportion of crustal material but they also display a very large range of chemical and isotopic compositions. Lavas on Martinique Island cover most of the variability known for the entire arc, and volcanism occurred almost continuously for the last 25Ma. It is therefore an ideal target to establish the causes of the chemical and isotopic variability and to evaluate whether a secular relationship exists. We measured Pb, Sr, Nd and Hf isotopic ratios on 30 well-dated Martinique samples whose ages range between 25Ma and the present. Samples were chosen to cover all volcanic phases, from island formation to the most recent eruptions. Our results cover most of the isotopic range defined by the entire arc, from fairly unradiogenic to highly radiogenic values; but more importantly, they define two distinct hyperbolas in isotopic space. One array is defined by old lavas (25 to 7.1Ma), and the other by recent lavas (5.1Ma to present). The hyperbolas are compatible with simple two-component mixtures but are not reproduced by AFC models. Using these hyperbolas, we can constrain the composition of the mixing end-members. Both depleted sources correspond to depleted mantle contaminated by a radiogenic Sr component, and the two enriched components resemble local sediments with little to no relative fractionation of Sr, Nd, Hf and Pb. We conclude on this basis that sediments were added to the mantle wedge by melting and not dehydration. Our interpretation for Martinique can be extrapolated to the entire Lesser Antilles arc because the isotopic arrays of Martinique and other islands are indistinguishable. More generally, if the same occurs under most other island arcs, the chemical and isotopic characteristics of arc volcanism might be directly linked to the amount and nature of subducted sediments.