The importance of mantle wedge heterogeneity to subduction zone magmatism and the origin of EM1
The importance of mantle wedge heterogeneity to subduction zone magmatism and the origin of EM1
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DOI：
10.1016/j.epsl.2017.04.051
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发表时间：
2017-08
影响因子：
5.3
通讯作者：
S. Turner;C. Langmuir;M. Dungan;S. Escrig
中科院分区：
文献类型：
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作者：
S. Turner;C. Langmuir;M. Dungan;S. Escrig
为地幔楔提供补给的对流软流圈地幔的组成可以通过具有最小板块影响的弧后熔岩来研究。这种“周围地幔楔”的组成（在加入板块成分之前的楔的组成）在世界范围内和单个弧内都有很大的不同。受板块影响最小的弧后火山样品的143 Nd/144 Nd测量值与邻近火山锋层状火山的143 Nd/144 Nd测量值相似，表明弧前火山的143 Nd/144 Nd在很大程度上继承了周围地幔组成。Nd/ Nd与Th/U、Zr/Nb和La/Sm等比值相关，表明这些比值也受到环境楔块非均质性的强烈影响。在智利南部火山带（SVZ）的单个火山中也观察到同样的现象，在那里火山锋面的沿走向变化与弧后单成因火山的变化一致。贫地幔楔比富地幔楔受板源成分的影响更大。这导致全球数据集中出现了令人惊讶的微量元素相关性，例如Pb/Nb和Zr/Nb之间的相关性，而这些相关性无法用slab组分的可变组成或通量来解释。弧下存在贫外地幔，弧后扩张；相对富集的地幔与大陆相邻。环境地幔楔体的全球和区域非均质性形成了以高夫岛玄武岩为代表的贫地幔与em1型富集成分之间Sr、Nd和Hf的同位素混合轨迹。利用这种关系可以实现与SVZ数据的定量匹配。有人认为EM1型地幔储层是下大陆地壳再循环的结果，但这种模式没有考虑到某些微量元素的比例，如Ce/Pb和Nb/U，也没有考虑到EM1火山岩中惊人的均匀微量元素组成。在大陆弧中发现的EM1端元是由次大陆岩石圈地幔的低程度熔融交代作用产生的模型可能更合理。沿SVZ的143nd / 144nd最大值可能是两个古代岩石圈域的裂陷和碰撞或板块撕裂的结果。地幔楔体变化与EM1的对应表明，交代次大陆岩石圈可能在全球范围内形成EM1源。
The composition of the convecting asthenospheric mantle that feeds the mantle wedge can be investigated via rear-arc lavas that have minimal slab influence. This “ambient mantle wedge” composition (the composition of the wedge prior to the addition of a slab component) varies substantially both worldwide and within individual arcs. 143 Nd/144 Nd measurements of rear-arc samples that have minimal slab influence are similar to 143 Nd/144 Nd in the stratovolcanoes of the adjacent volcanic fronts, suggesting that 143 Nd/144 Nd of arc-front volcanics are largely inherited from the ambient mantle composition. 143 Nd/144 Nd correlates with ratios such as Th/U, Zr/Nb, and La/Sm, indicating that these ratios also are strongly influenced by ambient wedge heterogeneity. The same phenomenon is observed among individual volcanoes from the Chilean Southern Volcanic Zone (SVZ), where along-strike variability of the volcanic front tracks that of rear-arc monogenetic volcanics. Depleted mantle wedges are more strongly influenced by slab-derived components than are enriched wedges. This leads to surprising trace element correlations in the global dataset, such as between Pb/Nb and Zr/Nb, which are not explicable by variable compositions or fluxes of slab components. Depleted ambient mantle is present beneath arcs with back-arc spreading; relatively enriched mantle is present adjacent to continents. Ambient mantle wedge heterogeneity both globally and regionally forms isotope mixing trajectories for Sr, Nd and Hf between depleted mantle and EM1-type enriched compositions as represented by Gough Island basalts. Making use of this relationship permits a quantitative match with the SVZ data. It has been suggested that EM1-type mantle reservoirs are the result of recycled lower continental crust, though such models do not account for certain trace element ratios such as Ce/Pb and Nb/U or the surprisingly homogeneous trace element compositions of EM1 volcanics. A model in which the EM1 end-member found in continental arcs is produced by low-degree melt-metasomatism of the sub-continental lithospheric mantle may be more plausible. The 143 Nd/144 Nd maximum along the SVZ may be a consequence of either rifting and collision of two ancient lithospheric domains or a slab tear. The correspondence of mantle wedge variations with EM1 suggests a potential role for metasomatized sub-continental lithosphere in creating EM1 sources globally.