Geochemistry of Lesser Antilles lavas

We present new U-series disequilibrium and radiogenic isotope data for 7 mafic lavas from the Lesser Antilles arc. These are combined with published data in an internally consistent model that quantitatively estimates the amount of sediment and fluid added to the source of the Lesser Antilles arc system. Some lavas form an array consistent with bulk sediment addition (0.2-2%) whereas others appear to require addition of 0.4-2% sediment melt, particularly in the south of the arc. Evidence for both bulk sediment and sediment melt addition can be found within both the northern and central sections of the arc suggesting a thermal structure whereby the upper portions of the subducted sediment pile lie close to their solidus beneath much of the arc. Addition of up to 5% fluid derived from altered oceanic crust to these sediment enriched mantle wedge source regions can simulate the majority of the lavas on a plot of 207Pb/204Pb versus Ce/Pb. By taking into account the range in calculated wedge compositions and allowing for some mobility of Th in the fluid, the same model can also account for much of the observed range of U-Th-Ra disequilibria, especially if the eclogitic residue contains trace amounts of rutile. The implication of this more complex model is that the time scales for fluid addition and differentiation could be significantly shorter than those estimated in some previous studies.

本研究报道了采自小安的列斯群岛弧（Lesser Antilles arc）的7件基性熔岩（mafic lavas）的全新铀系不平衡（U-series disequilibrium）与放射成因同位素（radiogenic isotope）测试数据。我们将该数据集与已发表的同类数据整合，构建了一套内部一致模型，可定量估算小安的列斯群岛弧系统源区所接收的沉积物与流体添加量。部分熔岩所形成的演化序列符合整体沉积物加入（0.2%~2%）的地球化学特征，而其余熔岩则似乎需要添加0.4%~2%的沉积物熔融产物，这一现象在弧南部尤为显著。在弧的北部与中部区域，均可同时观测到整体沉积物加入与沉积物熔融加入的证据，这表明俯冲沉积物堆的上部在弧内绝大多数区域之下均接近其固相线（solidus）的热结构特征。向这些富沉积物的地幔楔源区加入最高可达5%的蚀变洋壳（altered oceanic crust）来源流体后，该模型可在以207Pb/204Pb与Ce/Pb为坐标轴的判别图上复现绝大多数熔岩的分布趋势。若进一步考虑计算得到的楔体组分变化范围，并允许流体中Th元素存在一定迁移性，该模型同样可以解释大部分观测到的U-Th-Ra不平衡（U-Th-Ra disequilibria）分布范围，尤其是当榴辉岩相残余（eclogitic residue）含有微量金红石（rutile）时。这一更完善的复杂模型表明，流体加入与岩浆分异的时间尺度可能显著短于此前部分相关研究的估算结果。