Volcanism on the flanks of the East Pacific Rise: Quantitative constraints on mantle heterogeneity and melting processes
Volcanism on the flanks of the East Pacific Rise: Quantitative constraints on mantle heterogeneity and melting processes
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DOI：
10.1016/j.chemgeo.2011.12.015
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发表时间：
2012-03-02
期刊：
影响因子：
3.9
通讯作者：
Galer, Stephen J. G.
中科院分区：
文献类型：
--
作者：
Brandl, Philipp A.;Beier, Christoph;Galer, Stephen J. G.
我们目前的主要和微量元素和Sr，Nd和三峰铅同位素数据为17个新鲜的火山玻璃从海山6，一个10公里直径的海山位于东太平洋隆起以东140公里，12度45 'N。地质和年代学证据表明，在海山1-2 Ma的活动期内，岩浆成分由拉斑玄武岩演化为碱性玄武岩和玄武粗安岩。6号海山熔岩的主量元素和微量元素组成随同位素比值的变化而系统地变化;具有最高不相容微量元素浓度的最年轻熔岩具有最高的La/Yb、Nb/Zr、K2 O/TiO 2、Sr-87/Sr-86、Pb-206/Pb-204和最低的Nd-143/Nd-144、MgO和CaO。海山6号熔岩中的元素浓度、不相容元素比率和同位素组成范围超过了在相邻海脊轴喷发的熔岩中观察到的范围，与迄今为止研究的所有近海脊海山报告的熔岩组成范围相当。所观察到的范围内的熔岩成分是一致的富集和亏损之间的混合熔体在地壳中的浅层次。这些混合端元之间的成分的推断差异不能用单一源成分的不同熔化程度来解释，并要求上地幔在单一海山下熔融区尺度上是极不均匀的，我们可以证明EPR海山熔岩的成分范围不可能是由富集和亏损物质贡献相等的非均匀地幔熔融产生的源混合（Source Mixing）相反，微量元素和同位素组成的海山熔岩可以复制的熔融模型，其中更丰富，肥沃的地幔岩性优先熔融地幔上涌。在熔融程度逐渐降低时，喷发的熔岩因此更富含不相容的微量元素，具有较高的La/Yb、K/Ti、Sr-87/Sr-86比值和较低的Nd-143/Nd-144比值。如果这是一个常见的过程，那么幔源岩浆不太可能继承其地幔源的平均不相容微量元素和同位素组成，这可能是显着更贫化，也不会显示在地幔熔融区存在的全部组合物。这些结果对大洋玄武岩可用于推断上地幔成分的方式具有意义。(C)2011爱思唯尔有限公司版权所有。
We present major and trace element and Sr, Nd and triple-spike Pb isotope data for 17 fresh volcanic glasses from Seamount 6, a 10-km diameter seamount located 140 km east of the East Pacific Rise at 12 degrees 45'N. Geological and geochronological evidence show that magma compositions evolved from tholeiitic basalts to alkalic basalts and basaltic trachyandesites during the 1-2 Ma active lifetime of the seamount. Major and trace element compositions in Seamount 6 lavas vary systematically with isotope ratios; the youngest lavas with the highest incompatible trace element concentrations have the highest La/Yb, Nb/Zr, K2O/TiO2, Sr-87/Sr-86, Pb-206/Pb-204 and the lowest Nd-143/Nd-144, MgO and CaO. The range in element concentrations, incompatible element ratios, and isotope compositions in Seamount 6 lavas exceeds that observed in lavas erupted at the adjacent ridge axis, and is comparable to the range in lava compositions reported from all near-ridge seamounts studied to date. The observed range in lava compositions is consistent with mixing between enriched and depleted melts at shallow levels in the crust. The inferred difference in composition between these mixing endmembers cannot be explained by variable degrees of melting of a single source composition, and requires that the upper mantle is extremely heterogeneous on the scale of the melting region beneath a single seamount.We can show that the range in composition of EPR seamount lavas cannot be generated by melting of variably heterogeneous mantle in which enriched and depleted materials contribute equally to melting (source mixing). Instead, the trace element and isotope compositions of seamount lavas can be reproduced by melting models in which more enriched, fertile mantle lithologies are preferentially melted during mantle upwelling. At progressively lower degrees of melting, erupted lavas are thus more enriched in incompatible trace elements, have higher La/Yb, K/Ti, Sr-87/Sr-86 ratios and lower Nd-143/Nd-144 . If this is a common process, then mantle-derived magmas are unlikely to inherit the average incompatible trace element and isotope composition of their mantle source, which is likely to be significantly more depleted, nor will they display the full range of compositions present in the mantle melting region. These results have implications for the way in which oceanic basalts can be used to infer the composition of the upper mantle. (C) 2011 Elsevier B.V. All rights reserved.