Subduction erosion of forearc mantle wedge implicated in the genesis of the South Sandwich Island (SSI) arc: Evidence from boron isotope systematics
Subduction erosion of forearc mantle wedge implicated in the genesis of the South Sandwich Island (SSI) arc: Evidence from boron isotope systematics
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DOI：
10.1016/j.epsl.2010.11.008
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发表时间：
2011-01
影响因子：
5.3
通讯作者：
S. Tonarini;W. Leeman;P. Leat
中科院分区：
文献类型：
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作者：
S. Tonarini;W. Leeman;P. Leat
南桑威奇火山弧位于年轻的洋壳上，喷发低钾拉斑玄武岩，其特点是下降板片上有奇异的远洋和火山沉积物，构造环境简单，是评估俯冲带元素迁移的理想选择。作为量化俯冲相关流体从板块转移到地幔楔的过程的一种手段，确定了沿弧线的代表性熔岩的硼浓度和同位素组成。样品显示出可变的流体可移动/流体不可移动元素比率以及 B/Nb（2.7 至 55）和 B/Zr（0.12 至 0.57）的高富集度，与在西太平洋弧中观察到的类似。 δ11B 值是迄今为止报告的地幔熔岩中最高的值之一；这些值在弧的中部最高（+15 至 +18‰），向南端和北端逐渐减小（+12 至 +14‰）。 δ11B 与 B 浓度和 87Sr/86Sr 比率大致呈正相关，但与其他流体移动元素（如 Rb、Ba、Sr 和 U）耦合性较差。从弧前沟槽中挖出的橄榄岩也具有较高的 δ11B（约 +10‰）和较高的 B 含量（38-140ppm）。在 ODP 站点 701 采样的传入中上层沉积物显示 δ11B 范围很广（+5 至 -13‰；平均值=-4.1‰），负值最常见。南桑威奇地幔楔推断出的异常高的 δ11B 值不能轻易归因于俯冲板片材料或直接源自其的流体的直接掺入。相反，岩浆源的重B同位素特征更合理地解释为流体的进入，该流体源自与马里亚纳弧前类似的蚀变额弧地幔楔形材料的俯冲侵蚀。我们提出，高 δ11B 和高 B 蛇纹岩（可能由弧地壳和火山碎屑沉积物点缀）的多阶段回收可以在火山弧下方的板片深度产生极其富含 11B 的流体。这些流体渗入地幔楔可能解释了该弧推断出的不寻常的岩浆来源。
The South Sandwich volcanic arc is sited on a young oceanic crust, erupts low-K tholeiitic rocks, is characterized by unexotic pelagic and volcanogenic sediments on the down-going slab, and simple tectonic setting, and is ideal for assessing element transport through subduction zones. As a means of quantifying processes attending transfer of subduction-related fluids from the slab to the mantle wedge, boron concentrations and isotopic compositions were determined for representative lavas from along the arc. The samples show variable fluid-mobile/fluid-immobile element ratios and high enrichments of B/Nb (2.7 to 55) and B/Zr (0.12 to 0.57), similar to those observed in western Pacific arcs. δ11B values are among the highest so far reported for mantle-derived lavas; these are highest in the central part of the arc (+15 to +18‰) and decrease toward the southern and northern ends (+12 to +14‰). δ11B is roughly positively correlated with B concentrations and with87Sr/86Sr ratios, but poorly coupled with other fluid-mobile elements such as Rb, Ba, Sr and U. Peridotites dredged from the forearc trench also have high δ11B (ca. +10‰) and elevated B contents (38–140ppm). Incoming pelagic sediments sampled at ODP Site 701 display a wide range in δ11B (+5 to −13‰; average=−4.1‰), with negative values most common. The unusually high δ11B values inferred for the South Sandwich mantle wedge cannot easily be attributed to direct incorporation of subducting slab materials or fluids derived directly therefrom. Rather, the heavy B isotopic signature of the magma sources is more plausibly explained by ingress of fluids derived from subduction erosion of altered frontal arc mantle wedge materials similar to those in the Marianas forearc. We propose that multi-stage recycling of high-δ11B and high-B serpentinite (possibly embellished by arc crust and volcaniclastic sediments) can produce extremely11B-rich fluids at slab depths beneath the volcanic arc. Infiltration of such fluids into the mantle wedge likely accounts for the unusual magma sources inferred for this arc.