MARGINS: The Slab Component in Central America: Isotopic Evidence For Both Hydrous and Silicic Fluxes in Nicaragua

[905167] carr边际项目的俯冲工厂倡议努力通过火山弧系统了解通量。具体目标包括揭示是什么调节了岩浆的产生。我们的工作将通过澄清中美洲板状熔剂的性质来实现这一目标。我们最近在中美洲的熔岩中发现了不寻常的微量元素系统。数据最好通过两个端元之间的混合来解释，因此至少有两种通量可以识别。俯冲于中美洲之下的沉积物部分也有两个端元；下部为碳酸盐软泥，上部为半深海泥。这些岩性具有截然不同的微量元素特征，总的来说，沉积物的化学特征与熔岩中看到的两个端元相匹配。我们确定两个地球化学端元的起源为俯冲沉积物，但沉积物如何成为触发熔融和火山作用的助熔剂？它们是以含水流体的形式进入地幔还是以熔体的形式？我们提议的项目可以通过采用非常精确的同位素测量来回答后一个问题的一半。中美洲熔岩的Sr-Nd同位素数据定义了一个明显包含微量元素数据中所见的相同端元的阵列。一旦有了足够的数据，更为精确的同位素数据有可能严重限制解释。中美洲同位素混合线的几何形状表明，来自上层半深海沉积物的通量可能是熔体，但不可能是含水流体。为了确定这一点，我们计划扩展数据集，目前数据集过于稀疏，无法让我们将其作为一个确定的结论

9905167CarrThe Subduction Factory Initiative of the MARGINS Program strives to understand fluxes through volcanic arc systems. Specific goals include uncovering what regulates the production of magma. Our work will address this goal by clarifying the nature of the slab fluxes in Central America. We recently discovered unusual trace element systematics in lavas from Central America. The data are best explained by mixing between two end-members, thus there are at least two fluxes to identify. The sediment section, subducting beneath Central America, also has two end-members; a lower carbonate ooze and an upper hemipelagic mud. These lithologies have hugely different trace element signatures and, in general, the chemical signatures of the sediments match up with the two end-members seen in the lavas. We identify the origin of the two geochemical end-members as the subducted sediments, but how do sediments become fluxing agents that trigger melting and volcanism? Do they move into the mantle as hydrous fluids or as melts? Our proposed project can answer half of the latter question by employing very precise isotopic measurements. The Sr-Nd isotopic data for Central American lavas define an array apparently involving the same end members seen in the trace element data. The far more precise isotopic data have the potential to severely restrict interpretations once sufficient data are available. The geometry of the isotopic mixing lines for Central America is such that a flux from the upper, hemipelagic sediments can be a melt but can not be a hydrous fluid. To establish this, we plan to expand the data set, which at present is too sparse to allow us to state this as a firm conclusion