Thorium isotope evidence for melting of the mafic oceanic crust beneath the Izu arc
Thorium isotope evidence for melting of the mafic oceanic crust beneath the Izu arc
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DOI：
10.1016/j.gca.2016.04.034
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发表时间：
2016-08-01
影响因子：
5
通讯作者：
Elliott, Tim
中科院分区：
文献类型：
--
作者：
Freymuth, Heye;Ivko, Ben;Elliott, Tim
我们解决的问题是否熔融的镁铁质洋壳下发生普通火山弧使用的限制，从U系列（U-238/Th-232，Th-230/Th-232和Ra-226/Th-230）测量。镁铁质地壳顶部几百米的蚀变导致铀的强烈富集。通过U-238衰变为Th-230，这导致在类似于350 ka的时间尺度上（Th-230/Th-232）升高（括号表示活动比率）。这一过程导致在伊豆-小笠原-马里亚纳岛弧附近ODP 801和1149站点采样的镁铁质蚀变洋壳（AOC）中出现高值（Th-230/Th-232），介于2.6和11.0之间。伊豆岛弧熔岩中Th活度比值为（Th-230/Th-232）= 1.2-2.0。这些值大大高于在伊豆海沟俯冲的大量沉积物中的值，也比大洋中脊玄武岩和马里亚纳弧中的值更高。我们发现，伊豆弧熔岩中Th同位素比值的范围与来自AOC和俯冲沉积物（AOC质量分数高达约80 wt.%）的混合源的板片熔体的存在一致添加到arc lava源的组件中。大洋板块俯冲在伊豆弧是比较冷的，因此，这表明温度高到足以流体饱和融化的AOC通常是在火山弧下实现。高比例的AOC/沉积物的板熔体成分建议伊豆弧熔岩需要优先熔融的AOC。这可以实现时，流体饱和的熔融板是由来自下面的俯冲蛇纹岩的流体触发。蛇纹岩脱水和流体迁移到上覆地壳导致AOC内开始熔融。没有一个显着的沉积物融化的组成部分表明，有足够的水通量AOC和上覆沉积物。(C)2016由Elsevier Ltd.出版
We address the question of whether melting of the mafic oceanic crust occurs beneath ordinary volcanic arcs using constraints from U-Series (U-238/Th-232, Th-230/Th-232 and Ra-226/Th-230) measurements. Alteration of the top few hundred meters of the mafic crust leads to strong U enrichment. Via decay of U-238 to Th-230, this results in elevated (Th-230/Th-232) (where brackets indicate activity ratios) over time-scales of similar to 350 ka. This process leads to the high (Th-230/Th-232), between 2.6 and 11.0 in the mafic altered oceanic crust (AOC) sampled at ODP Sites 801 and 1149 near the Izu-Bonin-Mariana arc. Th activity ratios in the Izu arc lavas range from (Th-230/Th-232) = 1.2-2.0. These values are substantially higher than those in bulk sediment subducting at the Izu trench and also extend to higher values than in mid-ocean ridge basalts and the Mariana arc. We show that the range in Th isotope ratios in the Izu arc lavas is consistent with the presence of a slab melt from a mixed source consisting of AOC and subducted sediments with an AOC mass fraction of up to approximately 80 wt.% in the component added to the arc lava source. The oceanic plate subducting at the Izu arc is comparatively cold which therefore indicates that temperatures high enough for fluid-saturated melting of the AOC are commonly achieved beneath volcanic arcs. The high ratio of AOC/sediments of the slab melt component suggested for the Izu arc lavas requires preferential melting of the AOC. This can be achieved when fluid-saturated melting of the slab is triggered by fluids derived from underlying subducted serpentinites. Dehydration of serpentinites and migration of the fluid into the overlying crust causes melting to start within the AOC. The absence of a significant sediment melt component suggests there was insufficient water to flux both AOC and overlying sediments. (C) 2016 Published by Elsevier Ltd.