Radiogenic isotopes document the start of subduction in the Western Pacific
Radiogenic isotopes document the start of subduction in the Western Pacific
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DOI：
10.1016/j.epsl.2019.04.041
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发表时间：
2019-07
影响因子：
5.3
通讯作者：
Hongyang Li;R. Taylor;J. Prytulak;M. Kirchenbaur;J. Shervais;J. Ryan;M. Godard;M. Reagan;
中科院分区：
文献类型：
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作者：
Hongyang Li;R. Taylor;J. Prytulak;M. Kirchenbaur;J. Shervais;J. Ryan;M. Godard;M. Reagan;
俯冲起始是板块构造学中最不为人所知的方面之一。为了获得俯冲开始的第一个现场岩浆记录，国际海洋发现计划第352远征队在小笠原海沟的内沟壁的四个地点钻探，以恢复在几百万年内增生的1.22公里的海洋上地壳。俯冲的开始。靠近海沟的两个地点（U1440和U1441）产出轴向和离轴弧前玄武岩（FAB），而靠近海沟的两个地点产出轴向和离轴弧前玄武岩（FAB）。距离海沟15 km处（U1439和U1442）产生了轴向低硅玻安岩和高镁安山岩，其上覆有离轴高硅玻安岩。本研究采用Hf-Nd-Sr-Pb同位素分析方法，50个具有地层代表性的岩心样品，以追踪俯冲开始后FAB-通过玻安岩岩浆作用的短暂时期内地幔源的演化。结果表明：1）FAB具有相对于εNd高的εHf，并且源自“印度”物源的可变亏损地幔，没有可检测到的俯冲输入; 2）轴向玻安岩遵循残余FAB地幔源和浅层俯冲成分之间的混合趋势（角闪岩相）“太平洋”物源洋壳熔融;离轴玻安岩定义了混合地幔楔（残余地幔+板片熔体）与具有较低εNd和较高207 Pb/204 Pb的俯冲分量之间的混合趋势，这需要远洋沉积物的显著贡献。随着俯冲作用的发展，远洋沉积物的进入可能意味着从增生边缘到非增生边缘的变化。因此，结果表明，在地球动力学，岩浆成因和地壳增生俯冲启动后立即迅速演变的系统。
Subduction initiation is one of the least understood aspects of plate tectonics. In an effort to obtain the firstin situmagmatic record of subduction initiation, the International Ocean Discovery Program Expedition 352 drilled at four sites in the inner trench wall of the Bonin Trench to recover 1.22 km of oceanic upper crust accreted within a few m.y. of subduction initiation. The two sites nearer to the trench (U1440 and U1441) yielded axial and off-axis fore-arc basalts (FAB), while those c. 15 km further from the trench (U1439 and U1442) yielded axial low-silica boninites and high-Mg andesites overlain by off-axis high-silica boninites. This study uses Hf–Nd–Sr–Pb isotope analyses from c. 50 stratigraphically representative core samples to trace the evolution of the mantle source during the brief period of FAB-through-boninite magmatism immediately following subduction initiation. Results show that: 1) the FAB have highεHf relative toεNd and were derived from variably depleted mantle of ‘Indian’ provenance with no detectable subduction input; 2) the axial boninites follow mixing trends between a residual FAB mantle source and a subduction component derived from shallow (amphibolite facies) melting of oceanic crust of ‘Pacific’ provenance; and 3) the off-axis boninites define mixing trends between a hybrid mantle wedge (residual mantle + slab melt) and an additional subduction component with lowerεNd and higher207Pb/204Pb that requires a significant contribution from pelagic sediment. This incoming of pelagic sediments may signify a change from an accretionary to non-accretionary margin as subduction evolves. The results thus indicate a rapidly evolving system in terms of geodynamics, magma genesis and crustal accretion immediately following subduction initiation.