Assimilation of sediments embedded in the oceanic arc crust: myth or reality?
Assimilation of sediments embedded in the oceanic arc crust: myth or reality?
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DOI：
10.1016/j.epsl.2014.03.038
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发表时间：
2014-06
影响因子：
5.3
通讯作者：
R. Bezard;J. Davidson;S. Turner;C. Macpherson;J. Lindsay;A. Boyce
中科院分区：
文献类型：
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作者：
R. Bezard;J. Davidson;S. Turner;C. Macpherson;J. Lindsay;A. Boyce
通常认为弧岩浆是由弧下地幔熔化形成的，弧下地幔因俯冲板片的成分而不同程度地富集。尽管到达地表的大多数岩浆都不是原始岩浆，但弧壳同化的影响常常被忽视，其结果是微量元素和同位素组成通常仅归因于地幔中存在的不同成分的不同贡献。这危及质量平衡回收计算的完整性。在这里，我们使用小安的列斯群岛圣卢西亚一系列火山岩矿物中的 Sr 和 O 同位素数据来表明大洋弧基底的同化作用可能非常重要。对来自四个苏弗里耶尔火山群（SVC；圣卢西亚）手部样品（87 Sr/86 Sr= 0.7089–0.7091）的单个斜长石斑晶中的 87 Sr/86 Sr 分析表明，手部样品之间的晶体同位素异质性范围为 0.7083 至 0.7094，单个斜长石斑晶中的差异高达 0.0008 手样品。 SVC 晶体中的 δ O 18 测量显示超出地幔范围的极端变化，斜长石 (n= 19) 为 + 7.5 至 + 11.1 ‰，石英 (n = 10) 为 + 10.6 至 + 11.8 ‰，角闪石 (n = 2) 为 + 9.4 至 + 9.8 ‰，辉石 (n = 3) 为 + 9 至 + 9.5 ‰。而较老的熔岩 （前苏弗里耶尔火山杂岩），具有较少放射成因的全岩 Sr 成分 (87 Sr/86 Sr= 0.7041–0.7062)，显示值更接近地幔范围：斜长石 (n= 4) 为 + 6.4 至 + 7.9 ‰，辉石 (n = 5) 为 + 6 至 + 6.8 ‰。我们认为 87 Sr/86 Sr 同位素不平衡和极端 δ O 18 值为弧壳内物质的同化提供了令人信服的证据。矿物 δ O 18 与整个岩石 87 Sr/86 Sr、143 Nd/144 Nd 和 206,207,208 Pb/204 Pb 之间的正相关性表明，同化作用似乎不仅是在圣卢西亚观察到的同位素异质性的原因，而且也是在整个小安的列斯群岛中观察到的同位素异质性的原因，因为圣卢西亚几乎涵盖了整个弧范围的同位素组成。这突出表明需要对海洋弧组进行详细的矿物规模调查，以量化同化作用，否则可能会导致对源成分和俯冲过程的误解。
Arc magmas are commonly assumed to form by melting of sub-arc mantle that has been variably enriched by a component from the subducted slab. Although most magmas that reach the surface are not primitive, the impact of assimilation of the arc crust is often ignored with the consequence that trace element and isotopic compositions are commonly attributed only to varying contributions from different components present in the mantle. This jeopardises the integrity of mass balance recycling calculations. Here we use Sr and O isotope data in minerals from a suite of volcanic rocks from St Lucia, Lesser Antilles arc, to show that assimilation of oceanic arc basement can be significant. Analysis of 87 Sr/86 Sr in single plagioclase phenocrysts from four Soufrière Volcanic Complex (SVC; St Lucia) hand samples with similar composition (87 Sr/86 Sr= 0.7089–0.7091) reveals crystal isotopic heterogeneity among hand samples ranging from 0.7083 to 0.7094 with up to 0.0008 difference within a single hand sample. δ O 18 measurements in the SVC crystals show extreme variation beyond the mantle range with+ 7.5 to+ 11.1‰ for plagioclase (n= 19),+ 10.6 to+ 11.8‰ for quartz (n= 10),+ 9.4 to+ 9.8‰ for amphibole (n= 2) and+ 9 to+ 9.5‰ for pyroxene (n= 3) while older lavas (Pre-Soufriere Volcanic Complex), with less radiogenic whole rock Sr composition (87 Sr/86 Sr= 0.7041–0.7062) display values closer to mantle range:+ 6.4 to+ 7.9‰ for plagioclase (n= 4) and+ 6 to+ 6.8‰ for pyroxene (n= 5). We argue that the 87 Sr/86 Sr isotope disequilibrium and extreme δ O 18 values provide compelling evidence for assimilation of material located within the arc crust. Positive correlations between mineral δ O 18 and whole rock 87 Sr/86 Sr, 143 Nd/144 Nd and 206,207,208 Pb/204 Pb shows that assimilation seems to be responsible not only for the isotopic heterogeneity observed in St Lucia but also in the whole Lesser Antilles since St Lucia encompasses almost the whole-arc range of isotopic compositions. This highlights the need for detailed mineral-scale investigation of oceanic arc suites to quantify assimilation that could otherwise lead to misinterpretation of source composition and subduction processes.