Geochemistry of sediments beneath the Tonga-Kermadec arc

The fate of subducted sediment and the extent to which it is dehydrated and/or melted before incorporation into arc lavas has profound implications for the thermo-mechanical nature of the mantle wedge and models for crustal evolution. In order to address these issues, we have undertaken the first measurements of 10Be and light elements in lavas from the Tonga-Kermadec arc and the sediment profile at DSDP site 204 outboard of the trench. The 10Be/9Be ratios in the Tonga lavas are lower than predicted from flux models but can be explained if (a) previously estimated sediment contributions are too high by a factor of 2-10, (b) the top 1-22 m of the incoming sediment is accreted, (c) large amounts of sediment erosion are proposed, or (d) the sediment component takes several Myr longer than the subducting plate to reach the magma source region beneath Tonga. The lavas form negative Th/Be-Li/Be arrays that extend from a depleted mantle source composition to lower Th/Be and Li/Be ratios than that of the bulk sediment. Thus, these arrays are not easily explained by bulk sediment addition and, using partition coefficients derived from experiments on the in-coming sediment, we show that they are also unlikely to result from fluid released during dehydration of the sediment (or altered oceanic crust). However, partial melts of the dehydrated sediment residue formed at ~800 °C during the breakdown of amphibole +/- plagioclase and in the absence of cordierite have significantly lowered Th/Be ratios. The lava arrays can be successfully modelled as 10-15% partial melts of depleted mantle after it has been enriched by the addition of 0.2-2% of these partial melts. Phase relations suggest that this requires that the top of the subducting crust reaches temperatures of ~800 °C by the time it attains ~ 80 km depth which is in excellent agreement with the results of recent numerical models incorporating a temperature-dependent mantle viscosity. Under these conditions the wet basalt solidus is also crossed yet there is no recognisable eclogitic signal in the lavas suggesting that on-going dehydration or strong thermal gradients in the upper part of the subducting plate inhibit partialmelting of the altered oceanic crust.

俯冲沉积物的归宿，以及其在被纳入弧火山岩之前发生脱水和/或熔融的程度，对地幔楔的热力学-力学性质以及地壳演化模型具有深远意义。为解决上述问题，我们首次对汤加-克马德克弧的火山岩以及海沟外侧深海钻探计划（Deep Sea Drilling Project, DSDP）204站位的沉积物剖面中的10Be与轻元素开展了测量。汤加火山岩中的10Be/9Be比值低于通量模型的预测值，但可通过以下情形解释：(a) 此前估算的沉积物贡献量偏高2~10倍；(b) 即将俯冲的表层1~22米沉积物发生增生；(c) 存在大量沉积物侵蚀作用；(d) 沉积物组分到达汤加下方岩浆源区的时间比俯冲板块晚数百万年。这些火山岩形成了负相关的Th/Be-Li/Be阵列，其范围从亏损地幔端元组分延伸至低于整体沉积物的Th/Be与Li/Be比值。因此，这些阵列难以通过整体沉积物加入来解释，且结合入射沉积物实验得出的分配系数，我们证明它们也不太可能由沉积物（或蚀变洋壳）脱水过程中释放的流体所导致。然而，在约800℃、角闪石±斜长石分解且无堇青石存在的条件下形成的脱水沉积物残余物的部分熔融体，其Th/Be比值会显著降低。火山岩阵列可被成功建模为：亏损地幔被0.2%~2%的上述部分熔融体富集后，发生10%~15%的部分熔融。相平衡关系表明，这要求俯冲地壳顶部在到达约80千米深度时，温度达到约800℃，这与近期引入温度依赖性地幔黏度的数值模型结果高度吻合。在该条件下，湿玄武岩固相线也会被跨越，但火山岩中未识别出榴辉岩相信号，这表明俯冲板块上部持续的脱水作用或强热梯度抑制了蚀变洋壳的部分熔融。